Vitamins in dialysis: who, when and how much?

Abstract

Despite the significant technical evolution of the blood purification methods, cardiovascular morbidity and mortality in dialysis patients is still several times higher than that observed in the general population. Vitamins are playing a crucial role in multiple key metabolic pathways. Due to multiple factors, dialysis patients present very often hypo- or hypervitaminosis for a broad range of vitamins. Dialysis in the context of renal replacement therapy is associated with a non-physiological potassium-sparing dietetic regime. Additionally, there is a non-selective intradialytic loss of micro- and macronutrients, deranged intracellular kinetics and gastrointestinal malabsorption due to uratemia. Frequent treatment with antibiotics due to infections associated with the acquired uremia-related immunosuppression may derange the vitamin-producing intestinal microflora. Certain agents prescribed in the context of renal failure or other conditions may reduce the absorption of vitamins from the gastrointestinal tract. These factors may deplete a dialysis patient from vitamins, especially the ones with antioxidant activity that may be associated with cardioprotective properties. In other cases, vitamins metabolized and excreted by the kidneys may be accumulated and exert toxic effects. The scope of this paper is to describe the main issues on vitamin therapy in dialysis patients in view of the ever contradictory opinions and practices.

• Dialysis
• hemodialysis
• peritoneal dialysis
• vitamins

Introduction

Malnutrition–inflammation–atherosclerosis syndrome and oxidative stress are associated with increased cardiovascular disease, the commonest cause of morbidity and mortality in dialysis patients.1 Renal failure and hemodialysis may stimulate the generation of reactive oxygen species and the oxidation of low-density lipoproteins appears to be a necessary prerequisite for the development of atherogenesis. In that context, antioxidant treatment, including antioxidant vitamins and carotenoids, may exert protective properties.2

A vitamin is an organic compound that is required as a nutrient in small amounts but cannot be synthesized in sufficient quantities, and therefore must be obtained from the diet.3 It was in 1912 when Funk coined the term vitamine, from the Latin vita for “life” and amine, for the prominent chemical reactive group. Vitamin adequacy in dialysis patients, 100 years after their discovery, is still an issue of debate with conflicting opinions and recommendations. Apart from protein-energy malnutrition commonly observed in dialysis patients, a deficiency of micronutrients—particularly trace elements and vitamins—may occur. The most common vitamin deficiencies observed in dialysis patients include those for vitamin C (ascorbate), folate, vitamin B₆ (pyridoxine), and 1,25-dihydroxycholecalciferol (calcitriol). The abnormally high prevalence of antioxidant deficiency in dialysis patients may be due to the coupled association of low protein-energy intake with inadequate ingestion of antioxidant vitamins (i.e., vitamins E, C, and carotenoids).4

Multiple factors may affect the action of vitamins in dialysis patients (Table 1).

Table 1. The factors affecting the action of vitamins in dialysis patients.

Factors affecting the action of vitamins in dialysis patients
• The “non-physiologic” dietary restrictions seen in dialysis patients leading to inadequate quantities of vitamins received with the nutrients
• The gastrointestinal disorders associated with chronic kidney disease that may affect the intestinal absorption of water-soluble and lipid-soluble vitamins
• The perturbated metabolism and the affected biochemical reactions controlled or facilitated by the vitamins in the uremic milieu
• The unknown quantities of vitamins cleared with the various methods of hemodialysis, i.e.. low or high flux hemodialysis, hemodiafiltration, or peritoneal dialysis, i.e., chronic ambulatory or the various types of automated dialysis
• The difference in the racial and ethnic characteristics of the studied patients’ populations who, despite the homogeneity of dietary restrictions, consume a totally different type of foods according to the local culinary identity and availability2

There is a large load of data accumulated on the effects of different vitamins administered in various forms, but the relative paucity of big randomized placebo-controlled studies and the ever contradictory results do not give space for clear decisions. This conflicting output is reflected by the lack of unanimity over the use of vitamin supplements in dialysis patients. In a prospective observational study with data from DOPPS I trial, there was a large variation by region in the percentage of patients administered water-soluble vitamins: Europe ranged from a low of 3.7% in the United Kingdom to a high of 37.9% in Spain, 5.6% in Japan, and 71.9% in the United States.5

There is also a significant discrepancy concerning their effects on important primary outcomes. In the same study by Fissel et al.,5 the use of water-soluble vitamins was associated with a substantially and significantly lower risk for mortality but the authors admit that only a randomized trial could prove that water-soluble vitamins improve outcomes. In another non-randomized trial, the patients treated with multivitamins during a 4-year follow-up or until death had a significantly lower mortality risk than the non-treated ones. These associations remained significant after adjustment for age, cardiovascular disease, albumin, and cardiac troponin T at baseline.6

In an elaborate review-meta-analysis by Coombs et al., 37 out of the 53 studied trials demonstrated a reduction in biomarkers of oxidative stress following antioxidant therapy, whereas 15 showed no effect and in 8 of them there was a paradoxical increase. In 20 out of these 37 positive studies, the prescribed agent was α-tocopherol.7

Vitamins: what they do

Of the 13 known vitamins, four are fat-soluble, namely vitamins A, D, E, and K. The others are water-soluble: vitamin C and the B-complex, consisting of vitamins B₁, B₂, B₆, B₁₂, niacin, folic acid, biotin, and pantothenic acid. The vitamins are essential nutrients. They may be required in small amounts, but have important and specific functions. The water-soluble vitamins (Table 2) circulate freely in the organism and any excess is excreted by the kidneys. They can be retained for shorter periods so they must be consumed more often than fat soluble vitamins. The fat-soluble or hydrophobic ones (Table 3) have specific roles in growth and maintenance of the body functions. They are found in oils and fats and enter the lymphatic system when absorbed. Many of them require protein carriers in order to be transported. Since they can be stored, toxic levels can be reached faster than with the water-soluble vitamins.

Table 2. Characteristics of water-soluble vitamins.

Vitamin	Effects	RDA	Clearance	Supplementation
Thiamin/B1	• Conduction of nerve impulses • Muscle function • Stimulation of appetite	M: 1.2 mg F: 1.1 mg	• HD: 6%9 • Low flux= high flux10 • PD< urine11	0.6–1.5 mg/d8
Riboflavin/B2	• Release of energy from nutrients • Supports normal vision • Healthy skin	M: 1.3 mg F: 1.1 mg	• HD: 7%10	20 mg post-HD 3/w17
Nicotinamide/B3	• NAD+/NADP+: oxidation-reduction reactions • Improve lipid profile • Hyperphospatemia19,20	M: 16 mg F: 14 mg	• Rapid metabolic clearance • Not cleared by dialysis
Biotin/B8	• Energy metabolism: tricarboxylic acid cycle • Gluconeogensis • Metabolism of fatty acids • Breakdown of amino acids	30 mcg	• Partially cleared in high flux HD	30 mcg/d
Pantothenic acid	• Synthesis of lipid, neurotransmitters, steroid hormones and haemoglobin • Part of Coenzyme A	5 mg		5 mg/
Pyridoxine/B6	• Metabolism of amino acids and fatty acids cognitive development immune function • Steroid synthesis erythropoietic activity • Peripheral neuropathy	1.3 mg	• PD<HD • Clearance 28–48% High eff/cy HD > 50%24,25	50–300 mg i.v. post-HD 60–100 mg/d per os26–29,32
Folate	• DNA synthesis/cell division • B12 conversion • Interconversion of aminoacids	400 mcg Pregnancy: 600 mcg	• HD=PD11,33,34 • Clearance 37%10,17,24	5–10 mg/d for hyperhomocysteinaemia 1 mg/d in dialysis
Cobalamin/B12	• DNA and RNA synthesis • Homocystein reduction	2.4 mcg	• Not cleared in HD and PD	<1000 mg/d i.v.26,28,29,48,49
Ascorbic Acid/vit C	• Antioxidant • Formation of collagen • Matrix to form teeth and bone • Wound healing • Production of Norepinephrine and Thyroxine • Iron absorption • Resistance to infections	M: 90 mg F: 75 mg	• Clearance: 30–53% • Losses: 80–280 mg per session8,55,56 Diffusion: 2/3 of loss • Convection 1/3 of loss56	60 mg/d per os

Table 3. Characteristics of fat-soluble vitamins.

Vitamin	Effects	RDA	Clearance	Supplementation
Vitamin A (retinol, beta-carotene)	• Function of retinal cells • Epithelial cells differentiation • Immunity • Growth • Strong antioxidant	M: 900 mcg F: 700 mcg	• HD and PD: hypervitaminosis A74,75 • No clearance11	Not needed
Vitamin D (calciferol)	• Cell differentiation • Calcium availability • Bone mineralization and growth	15–20 mcg 800 IU	•	Follow 25-vitD3 levels
Vitamin K (tocopherol)	• Blood coagulation • Inhibitor of vascular calcification83 • Bone matrix protein	M: 120 mcg F: 90 mcg	• ?	Not needed3
Vitamin E (phylloquinone)	• Potent antioxidant7 • Anti-atherosclerotic89,93 • Hypolipidaemic92 • Recurrent muscle cramps57,97 • Erythropoietin dose reduction94	15 mcg	• No clearance11	400–800 IU/d9

Water soluble vitamins

Thiamine (B₁)

Thiamine (B1) is a part of the coenzyme thiamine pyrophosphate that promotes the conversion of pyruvate to acetyl CoA. It is useful in many activities such as the conduction of nerve impulses, muscle function or stimulation of appetite and its deficiency leads to Beriberi. In modern times, hypovitaminosis is mainly associated with excessive alcohol use. It is mainly contained in vegetables and fruits.8 Long-term use of phenytoin, penicillins, cephalosporins, aminoglycosides, tetracycline derivatives, loop diuretics, fluoroquinolones, sulphonamide derivatives, and trimethoprim may deplete vitamin B₁ 9 (Table 4).

Table 4. Interactions of vitamins with drugs.

Vitamin	Interacting medication	Effect of interaction
Interactions of vitamins with drugs
Vitamin A	- Tetracyclines - Antacids - Anticoagulants - Bile acid sequestrants - Statins - Doxorubicin - Orlistat - Retinoids	- Risk of intracranial hypertension - Increase of their efficacy - Risk of bleeding - Reduction of vitamin A absorption - Increase vitamin A levels in the blood - Vitamin A enhances its action - Reduces vitamin A absorption - Toxicity (nausea, vomiting, blurred vision, etc.)
Vitamin D	- Estrogens, isoniazid, thiazides - Antacids, calcium ch. blockers - Cholestyramine, orlistat, sevelamer - Phenobarbital, phenytoin - Mineral oil - Doxorubicin	- They increase vitamin D levels in the blood - May decrease the production of vitamin D - Interferes with the absorption of vitamin D - May accelerate the body's use of vitamin D - Interferes with absorption - Vitamin D enhances its effects
Vitamin E	- Cyclosporine - Cytochrome P450 3A4 (CYP3A4) substrates - Anticoagulants/antiplatelets - Statins	- Vitamin E increases Cyclosporine absorption - Vitamin E reduces their effectiveness through acceleration of their breakdown - Vitamin E enhances their effects - Vitamin E decreases their effectiveness
Vitamin K	- Cephalosporins, sevelamer - Phenytoin - Warfarin	- Reduce the absorption of vitamin K - Interferes with metabolism of vitamin K - Its activity is antagonized by vitamin K
Vitamin B1	- Phenytoin, penicillins, cephalosporins, aminoglycosides, tetracycline derivatives, loop diuretics, fluoroquinolones, sulphonamide derivatives, trimethoprim	- Long-term use depletes vitamin B1
Vitamin B2	- Anticholinergic drugs - Tetracycline - Doxorubicin - Tricyclic antidepressants, phenothiazines phenytoin, methotrexate - Probenecid - Thiazide diuretics	- Inhibit absorption of riboflavin - Vitamin B2 reduces absorption/effectiveness - Vitamin B2 deactivates and gets depleted by doxorubicin - Inhibits riboflavin’s action - Decreases riboflavin absorption from GI tract and increases urine excretion - Increase urine riboflavin excretion
Nicotinamide (Niacin)	- Statins	- Risk of myopathy and rhabdomyolysis
Biotin	- Antibiotics - Anticonvulsant	- Reduce biotin producing microflora - Reduces biotin levels
Pantothenic acid (B5)	- Tetracycline - Cholinesterase inhibitors	- Vitamin B5 interferes with absorption/effectiveness - Vitamin B5 may increase their effects
Pyridoxine (B6)	- Levodopa - Phenytoin	- Decreased efficacy/Parkinsonian symptoms - Risk of seizure
Folic acid	- Methotrexate	- Folic acid supplementation reduces toxicity without affecting efficacy
Vitamin B12 (cobalamin)	- Anticonvulsants, chemotherapy medications, colchicine, bile acid sequestrants, H2 blockers, metformin, proton pump inhibitors	- They reduce levels of B12
Vitamin C (ascorbic acid)	- Aspirin/non-steroidal anti-inflammatory drugs - Acetaminophen - Aluminum-containing antacids - Barbiturates - Chemotherapy drugs - Nitrates - Oral contraceptives/hormone replacement therapy - Protease inhibitors - Tetracycline	- Increase urine excretion of vitamin C - Vitamin C increases their levels - High vitamin C doses reduce urine excretion - Vitamin C increase aluminum absorption - They decrease the effects of vitamin C - Vitamin C interferes with chemotherapy agents - Vitamin C reduces nitrates tolerance - Vitamin C raises estrogen levels. Oral estrogens decrease vitamin C effects - Vitamin C lowers their levels - Vitamin C increases its levels

Thiamine plasma concentration may not reflect its biological activity. The recommended dietary allowance (RDA) for thiamine is 1.2 mg/d for adult males and 1.1 mg/d for adult females. The predialysis levels of thiamine fall by 6% post-dialysis.10 There is no difference in the clearance of water-soluble vitamins between low-flux and high-flux hemodialysis membranes.11 It is less excreted with the peritoneal dialysate than with the native kidneys.12

The activity of the thiamine-dependent enzyme transketolase in erythrocytes (ETKo) is found insufficient or marginal in half of the hemodialysis patients, while whole blood thiamine may be within the normal range. Therefore, it could be suggested that the uremia-associated insufficient ETKo activity may be related to inhibition of the enzymatic system rather than to true vitamin deficiency.13

It has been shown that thiamine deficiency can be a—reversible if treated—cause of unexplained encephalopathy in dialysis patients. As the clinical output of thiamine’s deficiency can mimic the uremic symptoms, it is often difficult to recognize the patients who will benefit from vitamin B₁ supplementation.14,15

No study that measured the antioxidant effects of vitamin B₁ supplementation on markers of oxidation has shown any benefit so far. In a study by Nascimento et al.,16 thiamine given in a dose of 250 mg/d did not significantly affect the serum levels of albumin, plasma high sensitivity C-reactive protein, interleukin-6, advanced oxidation protein products, and pentosidine as well as 8-hydroxy-2′-deoxyguanosine.16

According to the European Best Practice Guidelines, thiamine intake in hemodialysis patients can range from 0.6 to 1.5 mg/d depending on individual food consumption.8 In patients on thrice weekly high flux hemodialysis who were receiving a multivitamin tablet containing 100 mg of thiamine hydrochloride post-dialysis, the serum levels of vitamin B₁ in the form of ETKo activity was normal.10 The oral administration of benfotiamine leads to higher thiamine diphosphate concentrations in erythrocytes accompanied with a significant improvement of the ETKo activity in dialysis patients compared with the administration of thiamine nitrate.17

Riboflavin (B₂)

Riboflavin (B₂) is necessary for the release of energy from nutrients and supports normal vision and healthy skin. Hypovitaminosis clinically presents as angular stomatitis and painful tongue with purple discoloration. The RDA for riboflavin is 1.3 mg/d for adult males and 1.1 mg/d for adult females. Seven per cent of the predialysis serum riboflavin is removed during hemodialysis and a dose of 20 mg per os post-dialysis thrice weekly can conserve its metabolite’s alpha erythrocyte glutathione reductase (alpha-EGR) levels in a normal range.11,18 Anticholinergic drugs inhibit the absorption of riboflavin; tetracycline interferes with its absorption and effectiveness whereas tricyclic antidepressants, phenothiazines, phenytoin, and methotrexate inhibit riboflavin’s action. Riboflavin deactivates and gets depleted by doxorubicin and probenecid decrease riboflavin absorption from the gastrointestinal tract and increases urine excretion as thiazide diuretics increase riboflavin excretion only (Table 4).

In a study on peritoneal dialysis patients, it has been found that riboflavin availability, as measured by alpha-EGR, is a determinant of fasting serum homocysteine.19 This association has not been accompanied by randomized intervention studies with riboflavin treatment as a method of hyperhomocysteinanemia correction neither by clinical outcome studies investigating the influence of riboflavin treatment of cardiovascular outcomes.

Nicotinamide

Nicotinamide is the amide of nicotinic acid (vitamin B₃/niacin) that can also be synthesized from the amino acid tryptophan. Nicotinic acid, also known as niacin, is converted to nicotinamide in vivo, and, although the two are identical in their vitamin functions, nicotinamide does not have the same pharmacologic and toxic effects as niacin. In the intracellular level, niacin is incorporated into nicotinamide adenine dinucleotide (NAD) and nicotinamide adenine dinucleotide phosphate (NADP), although the pathways for nicotinamide and nicotinic acids are very similar. NAD+ and NADP+ are coenzymes in a wide variety of enzymatic oxidation–reduction reactions. If taken in extremely large doses, it can be toxic and its deficiency causes Pellagra. Niacin interacts with the statins (increased risk of myopathies or rhabdomyolysis) (Table 4).

Niacin undergoes a rapid metabolic clearance and does not seem to be cleared by dialysis. Higher niacin doses improve lipid profile by increasing serum HDL and reducing LDL cholesterol fraction and serum triglycerides. The RDA for niacin is 16 mg/d for adult males and 14 mg/d for adult females. There is a growing interest on the efficacy of nicotinamide for the treatment of hyperphosphatemia not as a phosphate binder, but rather as a direct inhibitor of the Na–Pi–2b sodium-dependent transporter in the gastrointestinal tract. There is no hard evidence on the efficacy of this approach, and there are still serious safety concerns as the effective doses are very high (500–1750 mg/d).21,22

Biotin

Biotin (vitamin H–B₈) participates in energy metabolism as a coenzyme that carries CO₂ and participates in the tricarboxylic acid cycle, in gluconeogenesis, in the metabolism of fatty acids, and the breakdown of amino acids. It can be synthesized by bacteria in the gastrointestinal tract. The RAD for biotin is 30 mcg/d for healthy adult males and females dialysis patients. Long-term antibiotic treatment may deplete the biotin producing microflora and anticonvulsant medication may reduce its levels (Table 4). Just like most of the other water-soluble vitamins, biotin can be found in the restricted for dialysis patients, potassium-containing foods; its absorption is deranged in the gastrointestinal tract, and it is partially cleared during a high-flux dialysis session.

Pantothenic acid

Pantothenic acid (B₅) takes part in the synthesis of many lipids, neurotransmitters, steroid hormones, and hemoglobin. It is a part of the coenzyme A and its deficiency rarely occurs as it is contained in a large range of nutritional categories. The RAD for pantothenic acid is 5 mg/d for both adult males and females. A supplementation with the daily dose of 5 mg/d is considered adequate as there are no clinical trials on the gastrointestinal absorption or the dialysis kinetics for this molecule. Vitamin B₅ interferes with absorption/effectiveness of tetracycline and may increase the effects of cholinesterase inhibitors (Table 4).

Pyridoxine (B₆)

Pyridoxine (B₆) is a family of compounds that, unlike other water-soluble vitamins, can be stored in muscles. It is important for the metabolism of amino acids and fatty acids and influences cognitive development, immune function as well as steroid synthesis. The symptoms of B₆ deficiency include weakness, irritability, insomnia and, in advanced stages, failure to grow, motor function impairment, convulsions, and immunosuppression. Toxic symptoms include neuromuscular disorders and nerve damage leading to numbness and muscle weakness. Alcohol promotes disintegration and loss of vitamin B₆. Concerning the interactions with other substances, pyridoxine decreases the effects of both phenytoin and levodopa when the later is not prescribed in combination with carbidopa (Table 4).

In dialysis patients, it has been shown that pyridoxine supplementation with 300 mg i.v. three times a week may significantly correct the high levels of total cholesterol, triglyceride, and LDL and the low HDL. The RDA for vitamin B₆ is 1.3 mg/d for adult males and females through age 50. This daily dose of pyridoxine should be higher in hemodialysis patients as they present increased erythropoietin activity associated with the use of erythropoietin and there are some drugs and other substances that interfere with pyridoxine metabolism. In a study on anaemic dialysis patients, the addition of pyridoxine in the conventional iron treatment has led to a more solid and sustainable correction of hemoglobin levels.23

In a recent meta-analysis, vitamin B₆ deficiency was shown to be marked in 24–56% of dialysis patients. The clearance of pyridoxine is much lower in peritoneal dialysis than in hemodialysis during which the mean serum levels may fall by 28–48% depending on the dialyzer used.11,12,24,25

High-efficiency hemodialysis is leading to an even higher clearance of pyridoxine (over 50% compared to low flux dialysis) and low serum levels even if they have been supplemented.26,27

Fifty milligrams of pyridoxine hydrochloride per os three times a week post-dialysis seem to correct the serum levels of the B₆ metabolite glutamic oxaloacetic transaminase of the erythrocytes18 and a daily dose of 100 mg/d along with supraphysiologic doses of vitamin B₁₂ and folic acid have led to the normalization of the serum homocysteine levels in hemodialysis as well as peritoneal dialysis patients.26–29 In contrast, there is no evidence that high thiamine (250 mg/d) and pyridoxine (200 mg/d) supplementation may influence the plasma levels of advanced glycation end products and other oxidative stress markers in hemodialysis patients.16

A daily dose of 30 mg/d in a group of elderly peritoneal dialysis patients has also led to a significant amelioration of their peripheral neuropathy symptoms.32 The same positive outcome was noted in a study of hemodialysis patients with normal serum levels of pyridoxine but with a significant amelioration of the symptoms of peripheral neuropathy after supplementation with 60 mg/d concluding that this result may be associated with a relative resistance of the peripheral nervous system to pyridoxine under uremic conditions.33 A recommended supplementation of 10 mg/d is considered as the lowest pyridoxine hydrochloride dose that has consistently normalized pyridoxine deficiency in dialysis patients.34

Folate

Folate–folic acid is important in DNA synthesis/cell division and helps to convert vitamin B₁₂ into its coenzyme form. It is also essential for the interconversion of amino acids (i.e., homocysteine to methionine). It is mainly contained in green vegetables, fruits, and meat. The RDA for folate is 400 mcg/d for adult males and females. Pregnancy will increase the RDA for folate to 600 mcg/d. Folic acid supplementation reduces toxicities of methotrexate without affecting its efficacy35 (Table 4).

In dialysis patients, it is cleared significantly during on-line hemofiltration–hemodiafiltration and high-flux as well as low flux hemodialysis but also with peritoneal dialysis.12,36,37 It has been estimated that the serum levels fall by 37% post-dialysis and an oral supplement containing 6 mg of folate can restitute the serum levels.11,18,26

As the intestinal absorption is inadequate, the recommended dose for dialysis patients is 1 mg/d in order to prevent deficiency and 5–10 mg/d for the potential treatment of hyperhomocysteinanemia.38 In a study by Koyama,39 15 mg of folic acid per day combined to 500 mg of vitamin B₁₂ i.v. post-dialysis led to the normalization of the significantly increased pre-dialysis serum homocysteine levels.39 This reduction in serum homocysteine levels has not been accompanied by an improvement in endothelial dysfunction parameters in vivo.40 A dose comparison study showed no difference in the effectiveness between 15 mg and 60 mg of folate per day.41

In an elaborate study by Ingrosso et al., it was shown that the toxic action of homocysteine can be mediated by a macromolecule hypomethylation affecting the epigenetic control of gene expression. Folate therapy has been found to restore DNA methylation to normal levels and corrected the patterns of gene expression.42 In another study by Nakamura H et al., it was shown that folate and vitamin B₁₂ supplementation led to homocysteine levels normalization, reductions in alpha symethyl arginine serum levels, and improvement of arterial stiffness indices.43 Despite the improvements concerning the biological markers, supplementation with folate in physiologic or supraphysiologic doses did not result in a subsequent amelioration of cardiovascular or overall morbidity or mortality.44–46

In contrast, when pooling seven clinical trials, Qin X et al. concluded that folic acid therapy reduces the risk of cardiovascular disease by 15%, especially among those with treatment duration over 24 months and a reduction in serum homocysteine levels over 20%.47 A later meta-analysis of 11 trials reporting on 4389 patients showed that folic acid-based reduction of serum homocysteine levels does not reduce the incidence of cardiovascular events in people with kidney disease.48

Vitamin B12

Vitamin B₁₂ (cobalamin) is closely related to folate. This vitamin is necessary for DNA and RNA syntheses. The RDA for vitamin B₁₂ is 2.4 mcg/d for adult males and females, including the dialysis patients. Deficiencies occur mainly because of inadequate absorption caused by atrophic gastritis (lack of HCl) or a lack of the intrinsic factor (pernicious anemia). Inadequate absorption may be caused by drug interactions such as those taken during chemotherapy, salicylates (aspirin and antacids), and oral contraceptives. The deficiency of folate and vitamin B₁₂ can lead to megaloblastic anemia. Anticonvulsants, chemotherapy agents, colchicine, bile acid sequestrants, H₂ blockers, metformin, and proton pump inhibitors may reduce the levels of vitamin B₁₂ (Table 4).

Vitamin B₁₂ is hardly detectable in the spent peritoneal dialysis fluid.12 The hemodialysis and on-line hemofiltration/hemodiafiltration procedure do not affect the serum cobalamin levels.37

In an editorial by de Koning et al. published in circulation on 2010, it is stated that there is no definite opinion on whether there is an association between homocysteine lowering and reduction of cardiovascular events.49,50 Indeed, there are studies showing impressive homocysteine reductions (from 11 to 30%) that may even include normalization of its serum levels39 in hemodialysis and peritoneal dialysis patients receiving various but usually supraphysiologic doses of vitamin B₁₂—up to 1000 mg/d in a parenteral form, alone or in combination with folic acid and/or vitamin B₆ supplements for a period of 1–6 months.28,30,31,51,52 Nevertheless, the clinical efficacy studies do not show any serious benefit on clinical cardiovascular outcomes after treatment with vitamin B₁₂, B₆, and folic acid. In a randomized double-blind multicenter outcome study from Germany, it was shown that increased intake of folic acid, vitamin B₁₂, and vitamin B₆ did not reduce total mortality and had no significant effect on the risk of cardiovascular events in patients with end-stage renal disease.53 It is therefore evident that the pathophysiological chain of mechanisms that include vitamin prescription-homocysteine lowering-reduction in cardiovascular events has to be fully validated with more and bigger randomized controlled studies.

In dialysis patients and especially in those who are Helicobacter Pylori positive, a functional resistance in vitamin B₁₂ therapy has been recognized necessitating the implementation of supraphysiologic doses and serum levels in order to achieve the desired effects.54,55 Persistently abnormal nerve conduction seen in some dialysis patients may be reversed with large doses of parenteral vitamin B₁₂.56

Vitamin C

Vitamin C (ascorbic acid) is an antioxidant that participates in the formation of collagen, serves as a matrix to form teeth and bone, is important in wound healing and participates in the production of norepinephrine and thyroxin. It facilitates the iron absorption and increases the resistance to infections. Its deficiency causes scurvy and toxicity symptoms include nausea, vomiting, and diarrhoea. RDA for vitamin C is 90 mg/d for adult males and 75 mg/d for adult females.

Ascorbic acid interacts with multiple pharmacological agents (Table 4). It is found in fruits and vegetables, the foods that are usually restricted in dialysis patients in the context of a low potassium diet. The patients on hemodialysis and on peritoneal dialysis usually present with low serum vitamin C levels.57,58

During a single dialysis session, the estimated serum reduction ratio is 30–40% and the losses are estimated between 80 and 280 mg per session.8,58,59 Diffusive transport is responsible for two-thirds whereas the mechanism of convection accounts for only one-third of this loss.59 In a study by Fehrman-Ekholm et al.,37 p-ascorbate concentrations were lowered by 51% and 53% in the conventional hemodialysis and on-line hemodialysis groups, respectively, after treatment, and this reduction was significant while concentrations below the reference values were found in the vast majority of non-supplemented patients. A dose of 500 mg per os post-dialysis thrice weekly has led to sustainably normal levels of vitamin C in hemodialysis patients.18 A daily dose of 250 mg can significantly protect the hemodialysis patients from muscle cramps and its combination with vitamin E has an additional effect on that matter.60

A recent meta-analysis showed conflicting results on the antioxidant effects of vitamin C that was mostly notable with doses between 250 mg/d and 1 g/d orally for a period between 3 months and 1 year and intravenous doses of 300 mg/d–1 g/d for 2 months.7 The antioxidant effect of vitamin C administration has been associated with functional improvements in the microvascular reactivity. In a study by Cross et al.,61 intra-arterial infusion of vitamin C in predialysis patients and intravenous infusion in hemodialysis patients, resulted in a 100-fold increase (intra-arterial studies) and a 4.5-fold increase (intravenous studies) in serum antioxidant activity and an improvement in NO-mediated resistance vessel dilatation.

In a group of hemodialysis patients with microcirculatory disturbance but without peripheral vascular disease, a treatment with a combination of vitamin C (200 mg daily) and vitamin E (600 mg daily) was administered for 6 months. Transcutaneous partial O₂ pressure values remarkably increased and serum levels of thrombomodulin, a marker of endothelial injury, and thiobarbituric acid reactants, a marker of lipid peroxidation, were significantly reduced.62 At the same time, total vitamin C levels were independently associated with adverse cardiovascular outcomes but not with all-cause mortality.63

The pleiotropic effects of vitamin C treatment have also been studied in dialysis patients. Serum vitamin C levels are negatively associated with serum parathyroid hormone levels64 and a two months treatment with 500 mg vitamin C i.v. three times-a-week post-hemodialysis has led to a significant reduction in phosphorus, CRP level and Ca×P product.65 In what concerns the lipidemic profile, after 3 months of treatment with 250 mg vitamin C daily, LDL-c, and total cholesterol levels as well as the ratios of LDL-c to HDL-c and cholesterol to HDL-c were significantly reduced.66

In a number of different studies, ascorbate (300–500 mg intravenously after hemodialysis, 1–3 times a week for 2–3 months) not only facilitated the iron release from reticuloendothelial system but also increased iron utilization circumventing the resistance to erythropoietin in iron-overloaded hemodialysis patients with functional iron deficiency.67–72 Ascorbate administration can significantly reduce serum concentrations of soluble transferrin receptors and increase the percentage of transferrin saturation, probably through alterations in intracellular iron metabolism.73 In a meta-analysis on this matter, vitamin C supplementation was found to significantly reduce the erythropoietin required dose and improve the transferrin saturation with no change in Ferritin concentration.74 The potential benefits of restored vitamin C status and improved erythropoiesis may be entirely overruled by the adverse consequences of excessive dose-associated oxidative tissue injury. In a number of studies, it has been found that supplementation with high doses of vitamin C may increase the lipid peroxidation in hemodialysis patients.75,76 As the ascorbic acid is converted to oxalic acid, some authors do not recommend these high doses and propose a low dose of 75–90 mg/d as a permanent treatment.7

Fat soluble vitamins

The fat soluble vitamins are the A, D, E, and K.

Vitamin A

Vitamin A (retinol, beta-carotene) in nature is found in the water-soluble precursor form of beta-carotene or in the lipid soluble form of retinol. Foods of animal origin contain retinol and those from plants provide beta-carotene. Vitamin A is important for the function of retinal cells; it participates in the differentiation of epithelial cells, in the immunity and growth and is considered a strong antioxidant. The symptoms of hypovitaminosis A include night blindness, xerophthalmia, keratinization of cells, anemia, kidney stones, poor bone growth and deranged tooth enamel formation. The symptoms of hypervitaminosis A include bone decalcification, hypocoagulation, amenorrhea, skin rashes, nausea, vomiting, blurred vision, and appetite loss. The RDA for vitamin A is 900 mcg/d for adult males and 700 mcg/d for adult females. It is mainly contained in the liver, dairy products, and fish. In fruits and vegetables, it is found in the form of beta-carotene. Retinol is bound to retinol-binding protein-4 (RBP4) and secreted from the liver in complex with prealbumin. It interacts with tetracycline antibiotics (risk of intracranial hypertension), antacids (increase of their efficacy), anticoagulants (risk of bleeding), bile acid sequestrants cholestyramine and colestipol (reduction of vitamin A absorption), statins (increase vitamin A levels in the blood), doxorubicin (vitamin A enhance its action), orlistat (reduces vitamin A absorption), and retinoid (contraindication of simultaneous vitamin A supplementation) (Table 4). In patients with chronic kidney disease, the catabolism of RBP4 is impaired, leading to increased levels of unbound RBP4. It is, therefore, not necessary to prescribe supplements containing retinol in this group of patients. A series of studies is indicating a tendency of hypervitaminosis A in hemodialysis and peritoneal dialysis patients. In these patients, their levels are usually found to be elevated two-fold or greater compared to the controls.77,78

In peritoneal dialysis, vitamin A metabolites are hardly detectable in the dialysate.12 Nevertheless, serum retinol, alpha-, and beta-carotene concentrations are reduced in patients on peritoneal dialysis for a longer period, while they are inversely related to interleukin-6 and C-reactive protein levels.79

RBP4 is elevated in dialysis patients. Its levels are associated with the patients’ survival and its clearance is considered to affect the complications of the uremic state, in view of the similarities in the symptoms of the uremic state and hypervitaminosis A.80

In a prospective observational cohort study with 261 patients, it was shown that lower retinol levels are an independent predictor of overall and cardiovascular mortality in hemodialysis patients but it is unknown whether this effect is attributed to a better coexistent nutritional status or to retinol's specific effects.81 These results were confirmed in a larger cohort study with 1177 German diabetic hemodialysis patients that revealed a strong association of low retinol and RBP4 concentrations with cardiovascular and all-cause mortality.82 There are no intervention studies measuring the clinical outcomes of vitamin A supplementation in dialysis patients.

Vitamin D

Vitamin D (calciferol) can be synthesized in the body when exposed to sunlight. It promotes cell differentiation and its chief role is the increase of calcium availability for bone mineralization and growth. Deficiencies may occur as a result of hepatic or renal failure and the symptoms are those of a calcium deficiency. Clinically, in children, it appears as rickets and in adults as osteomalacia. Hypervitaminosis D causes an increase in calcium absorption that may trigger the production of kidney stones and the calcification of blood vessels.

It interacts with estrogens isoniazid, thiazides (they increase vitamin D levels in the blood), or antacids, calcium channel blockers-such as verapamil- (may decrease the production of vitamin D by the body), sevelamer, cholestyramine and orlistat (interferes with the absorption of vitamin D as well as other fat-soluble vitamins), phenobarbital, phenytoin, and other anticonvulsant medications (may accelerate the body's use of vitamin D), mineral oil (interferes with absorption), and doxorubicin (vitamin D enhances its effects).83

For healthy individuals from 12 months to age 50, the RDA is set at 15 mcg whereas 20 mcg of cholecalciferol (or 800 IU) is the recommendation for maintenance of healthy bone for adults over 50 years old. For the dialysis patients, it is necessary to measure serum 25-hydroxy-vitamin D₃ regularly in order to substitute as needed. Vitamin D is one of the most important players on bone metabolism in health but also in the context of chronic kidney disease. Its pleiotropic effects are also important.

Treatment with vitamin D2 or D3 derivates allows for the reduction of vitamin D deficiency, better control of mineral metabolism with less use of active vitamin D, attenuation of inflammation, reduced dosing of erythropoiesis-stimulating agents, and possibly improvement of cardiac dysfunction.84

In recent studies vitamin D treatment was associated with decreased risk of all-cause and cardiovascular mortality in patients with CKD not requiring dialysis and patients with end stage renal disease (ESRD) requiring dialysis85 and low-serum 25(OH)D levels were found to be associated with the severity of coronary artery stenosis86 In a recent meta-analysis. it was shown that higher 25(OH)D levels are associated with significantly improved survival in patients with CKD.87

The administration of either native or active vitamin D has been associated with an improvement of anaemia and reduction in erythropoietin stimulating agents’ requirements.88

In what concerns the substitution regimen, vitamin D(3) is more effective than vitamin D(2) in providing adequate 25(OH)D serum levels in hemodialysis patients.89

Vitamin K

Vitamin K’s (tocopherol) chief function is its role on blood clotting. It is involved in the synthesis of 4 out of the 13 proteins of the blood coagulation cascade. Deficiencies may occur if fat absorption is impaired. Toxicities are generally uncommon. RDA for the males is 120 mcg and for the females 90 mcg/d.

In what concerns the drug interactions, cephalosporins may reduce the absorption of vitamin K (elimination of vitamin K activating microbial flora by the antibiotherapy), phenytoin interferes with the body's ability to use vitamin K, Warfarin’s activity is antagonized by vitamin K and orlistat as well as bile acid sequestrants may reduce the overall absorption of dietary fats and fat-soluble vitamins (Table 4). There are two main forms of vitamin K: vitamin K₁ (phylloquinone, found in vegetables) and vitamin K₂ (menaquinone, produced by bacteria in the intestine and in fermented foods). Vitamin K₁ is principally transported to the liver, regulating the production of coagulation factors. Vitamin K₂, instead, is also transported to extra-hepatic tissues, such as bone and arteries, regulating the activity of matrix Gla-protein (MGP) and osteocalcin. MGP is a central calcification inhibitor of the arterial wall; its activity depends on vitamin K-dependent γ-glutamate carboxylation. Inactive MGP levels can be decreased markedly by daily vitamin K₂ supplementation.90 Vitamin K is essential for the activity of γ-carboxyglutamate (Gla)-proteins including matrix Gla28 protein and osteocalcin; an inhibitor of vascular calcification and a bone matrix protein, respectively. Insufficient vitamin K intake leads to the production of non-carboxylated inactive proteins that may contribute to the high risk of vascular calcification in hemodialysis patients. The intake of vitamin K is low in hemodialysis patients.91 One-third of hemodialysis and 46% of peritoneal dialysis patients present with a low level of vitamin K without any clinical indices associated with it.92,93 The dialysis patients do not need vitamin K supplementation with the exception of deranged coagulation and long-term antibiotherapy. The lipid bounding renders vitamin K into a molecule potentially hard to clear during dialysis although this has never been studied.3 Vitamin K is involved in the mechanisms antagonizing the vascular calcification. In a recent study conducted in rats, it was shown that vitamin K improves the indices of vascular calcification counteracting the effects of warfarin.94

In patients with chronic renal failure and renal replacement therapy, there is no correlation between vitamin K levels and osteocalcin, PTH or other biochemical parameters of bone metabolism.95 In an intervention study, vitamin K treatment (45 mg/d) in hemodialysis patients with low intact PTH levels (<100 pg/mL) led to a significant increase of the serum intact osteocalcin, PTH, bone alkaline phosphatase, and cross-linked N-terminal telopeptide of type I collagen level after 1–12 months of treatment.96

Vitamin K is a cofactor for gamma-glutamyl carboxylase, the enzyme responsible for the formation of matrix-carboxyglutamate residues that confer calcium-binding properties to vitamin K-dependent proteins. In a recent elaborate controlled trial, hemodialysis patients had 4.5-fold higher dephosphorylated–uncarboxylated matrix carboxyglutamate protein, a vitamin K-dependent central calcification inhibitor of the arterial wall and 8.4-fold higher uncarboxylated osteocalcin levels, compared with controls. Daily vitamin K in escalating doses for 6 weeks induced a dose- and time-dependent reduction in circulating dephosphorylated–uncarboxylated Matrix Gla protein, uncarboxylated osteocalcin, and uncarboxylated prothrombin levels.97

This study is opening the discussion for a systematic treatment with vitamin K agents in the prophylaxis and treatment of vascular calcifications. As the latter and vitamin K status are related to bone turnover, the question arises on whether supplementation would benefit all hemodialysis patients or only certain subgroups, depending on their bone turnover status.98,99

Vitamin E

Vitamin E (phylloquinone) is a very potent antioxidant that stabilizes cell membranes and regulates oxidation reactions. Vitamin E deficiency is usually associated with diseases that cause fat malabsorption, such as cystic fibrosis. The clinical features of vitamin E deficiency include hemolysis, neuromuscular dysfunction of the spinal cord, and retinal lesions. At the cellular level, it interferes with reactions implied in the progression of atherosclerosis such as smooth muscle cell proliferation, platelet aggregation, monocyte adhesion, oxidized low-density lipoproteins uptake, and cytokine production.100

Food and supplement labels list alpha-tocopherol in International Units (IU), not in milligrams (mg). One mg of alpha-tocopherol equals to 1.5 IU. RDA for males and females over the age of 14 is 15 mcg of alpha-tocopherol per day. The major components of vitamin E are alpha- and gamma-tocopherol. It interacts with cyclosporine (vitamin E increases cyclosporine absorption), with cytochrome P450 3A4 (CYP3A4) substrates (vitamin E reduces their effectiveness through acceleration of their breakdown), anticoagulants/antiplatelet drugs (vitamin E enhances their effects), and statins (vitamin E decreases their effectiveness) (Table 4).

The major pathway of tocopherol metabolism is via phytyl side chain oxidation, leaving carboxyethyl–hydroxychromans (CEHC) as metabolites. Alpha and gamma CEHC are water soluble, excreted by the kidneys, and retain potent anti-inflammatory and antioxidative properties. Serum alpha and gamma CEHC accumulate in uremic patients and supplementation with tocopherols dramatically increases serum CEHC levels in both healthy subjects and hemodialysis patients.101 A lower retinol level is an independent predictor of overall and cardiovascular mortality in hemodialysis patients.81

In a study from USA, serum vitamin E metabolite concentrations, after having increased 10-fold within 30 d of supplementation (400 IU alpha-tocopherol per os), did not increase any more with further treatment, suggesting the presence of alternative non-renal metabolic excretion routes.102

In peritoneal dialysis vitamin E, metabolites are hardly detectable in the dialysate.12 The hypolipidemic properties of vitamin E have also been studied in a clinical level. In a study by Mafra et al.103 with 19 stable hemodialysis patients, 120 d of supplementation by alpha-tocopherol (400 UI/d) led to a significant reduction of the mean electronegative low-density lipoprotein concentration.

Vitamin E antioxidant effect

In a recent meta-analysis studying the substrate actions on biomarkers and the clinical effects of antioxidant vitamins in dialysis patients, the doses of α-tocopherol used were 200–800 mg/d in studies showing no antioxidant effect. Consequently, within this dose range, α-tocopherol appears to have limited effect on oxidative stress in hemodialysis patients. A daily dose of 1000 mg for a period of 8 weeks would be subsequently recommended for this group of patients.7

The European Best Practice Guidelines on Renal Nutrition recommend a daily supplement of 400–800 IU for the secondary prevention of cardiovascular events and recurrent muscle cramps based on a solitary trial in 196 patients presenting a secondary cardioprotective effect.10 In a recent systematic review, it was concluded that alpha-tocopherol may reduce oxidative stress.7 Apart from that, there are no hard data suggesting a beneficial effect of vitamin E supplementation as an antioxidant or as a metabolic nutritional agent.

The most cited randomized controlled trial on the antioxidant effects of vitamin E treatment was secondary prevention with antioxidants of cardiovascular disease in end-stage renal disease (SPACE). Ninety-seven patients were treated with 800 IU of α-tocopherol/day for 500 d, with 99 patients receiving placebo. The major statistically significant findings were a 54% reduction in cardiovascular risk, a 40% reduction in composite cardiovascular end points, and a 70% reduction in total myocardial infarction.104

As oxidative stress could be one of the resistance factors to erythropoietin response in hemodialysis patients, vitamin E supplementation was found to have a sparing effect on erythropoietin dosage requirement without a change in the hemoglobin concentration, in parallel to an improvement in oxidative markers.105 Nevertheless, prolonged or high doses of alpha-tocopherol administration in hemodialysis patients may induce a paradoxical pro-oxidant effect due to the reduction of antioxidant defense system components. Vitamin E is an antioxidant agent, but it is also known to have pro-oxidant properties under certain conditions.106

In an Italian study from 2002, reported data suggest that the 5-lipoxygenase branch of the arachidonate cascade is only responsible for membrane peroxidation, oxidative stress and apoptosis of leucocytes in hemodialysis patients and, regardless of the administration route, vitamin E may partially control the lipid peroxidation and oxidative stress through direct inhibition of 5-lipoxygenase.107

HOPE study was a large primary prevention study comparing the effects of 400 UI of vitamin E received from natural sources and ramipril in a group of healthy adults at risk for cardiovascular events. After 4.5 years of treatment with vitamin E, there were no apparent effects in the cardiovascular outcomes.108 The same results applied in a subgroup of patients with mild-to-moderate renal failure.109

In what concerns the muscle metabolism, 400 mg of vitamin E can significantly reduce the prevalence of muscle cramps in hemodialysis patients.60,110

Vitamin-E-coated dialyzers

The lipophilic antioxidant vitamin E was used as a surface modifier (or coating agent) of hollow-fiber dialyzer membranes with the aim of increasing their biocompatibility and preventing oxidative stress.111,112 There are a large number of studies that have investigated the effects of vitamin E-coated dialyzers in oxidation, inflammation, atherosclerosis, and other parameters.113–117 In a controlled study by Baragetti et al.118 with 16 hemodialysis patients, vitamin E-coated membrane has shown to reduce the levels of advanced glycation end products that were found to be negatively correlated with brachial artery flow-mediated dilatation, a marker of endothelial function. In another Italian study, treatment with vitamin E-coated dialyzers resulted in a reduction of reactive oxygen species, prevention of monocyte activation, resetting of IL-12 and IL-18 release, and restoration of Th1/Th2 balance.119 Vitamin E-bonded dialyzers may also improve the functional capacity of white blood cells population. In a study by Kojima et al., hemodialysis with this type of dialyzers was found to ameliorate eosinophilia through a mechanism mediated by a decrease in IL-5 secretion by CD4-positive lymphocytes.120 In a meta-analysis of 14 other studies by Sosa et al., excebrane-type vitamin-E-coated dialyzer treatment was associated with a significant reduction of lipid peroxidation biomarkers in plasma.121

In a controlled trial by Kobayashi et al., 1 year of hemodialysis treatment with vitamin E-coated dialyzers led to the significant reduction of the percentage of dysmorphic red blood cells, intima-media thickness in the carotid arteries and to the improvement of blood viscosity. Dialysis with conventional membranes was associated with the worsening of all the above-mentioned studied parameters. Weekly erythropoietin dose was significantly reduced in the vitamin-E coated group.122

In conclusion, the issue of vitamin therapy in dialysis patients has not been fully elucidated. Serum vitamin levels should be included in the list of the routine blood tests in the follow-up of dialysis patients. Excess doses should be avoided, even for the hydrophilic vitamins, as they may exert paradoxical toxicity actions. It is better to provide the vitamin treatment in small doses more frequently in order to avoid excesses in the serum levels. Interindividual and international differences in local dietary practices may necessitate a special approach concerning the vitamin treatment. Regarding the favorable treatment strategies, the therapeutic community at large is divided between the ones who “substitute what is below the normal” and the ones who “treat based on clinical outcomes”.

A large volume of data shows in vitro beneficial effects but this has not been reflected in positive clinical outcomes. There is a serious lack of well-designed large clinical outcome trials that would effectively study the effects of certain vitamins in certain dosing regimens, and the ones that have already been done were—in general—inconclusive or negative.

Declaration of interest

The authors report no conflicts of interest.