Plasma Fatty Acid Composition in Continuous Ambulatory Peritoneal Dialysis Patients: An Increased Omega-6/Omega-3 Ratio and Deficiency of Essential Fatty Acids

Abstract

Patients with end-stage renal disease, including those treated with peritoneal dialysis, have a high risk for death, particularly from cardiovascular causes. Plasma fatty acid (FA) composition is used as an indicator of disease risk, because its alteration has been related to metabolic disease and cardiovascular disease. For this purpose, we have measured plasma FA composition in continuous ambulatory peritoneal dialysis (CAPD) patients and compared them with those of healthy subjects. This study was performed on 51 (21 M, 30 F) CAPD patients at least 6 months under dialysis, aged 20–75 years (mean 47.81 ± 11.8 years) and 45 (25 M, 20 F) healthy control subjects aged 20–60 years (mean 38.62 ± 12.9 years). Plasma 10-cis-pentadecanoic acid, 10-cis-heptadecanoic acid, heneicosanoic acid, tricosanoic acid, nervonic acid, saturated fatty acid, and monounsaturated FA levels and delta 9 desaturase activity were significantly higher whereas linoleic acid, linolenic acid, 11,14-eicosedienoic acid, arachidonic acid, docosahexaenoic acid, and omega-3 FA levels were significantly lower in the CAPD group than those in the healthy group. Our results show that there are FA abnormalities and especially a depletion in essential FA levels and a high level of omega-6/omega-3 ratio in CAPD patients, the underlying mechanism of which is not known and needs to be investigated. Therefore, we believe that essential FA supplementation should be encouraged for CAPD patients.

• essential fatty acids
• omega-6 fatty acids
• omega-3 fatty acids
• peritoneal dialysis
• elongase activity
• desaturase activity

INTRODUCTION

Peritoneal dialysis (PD) is a well-established modality of treatment for patients with end-stage renal disease (ESRD), giving excellent short-term patient and technique survival rates.¹ Patients with ESRD, including those treated with PD, have a high risk for death, particularly from cardiovascular causes.² Statistics show that 50–60% of all deaths among dialysis patients are secondary to cardiovascular complication.³ It has been reported that compared with hemodialysis, PD patients generally have a more atherogenic serum lipid profile which may be related partially to peritoneal glucose absorption.⁴ Plasma fatty acid (FA) composition is used as an indicator of disease risk, because its alteration has been related to metabolic disease and cardiovascular disease.⁵ Excess saturated fatty acid (SFA) and a relative deficiency in unsaturated FAs, especially omega-3 FAs (ω-3 FAs), have been identified as cardiovascular disease contributing factors.⁶

There are limited and inconclusive data, which deal with changes in FA composition of plasma in patients with ESRD. Also, to our knowledge, the plasma-free FA levels in continuous ambulatory peritoneal dialysis (CAPD) patients have not been investigated yet. Therefore, in this study, we determined the plasma FA composition in CAPD patients and compared them with those in healthy subjects.

SUBJECTS AND METHODS

Study Population

This study was performed on 51 (21 M, 30 F) CAPD patients (four exchanges of 2 L dialysate/day containingvarying glucose concentrations to obtain adequate ultrafiltration) at least 6 months under dialysis, aged 20–75 years (mean 47.81 ± 11.8 years) and 45 (25 M, 20 F) healthy control subjects aged 20–60 years (mean 38.62 ± 12.9 years). The primary causes of renal diseases were chronic pyelonephritis (2 patients), diabetes mellitus (6 patients), hypertension (7 patients), adult polycystic kidney disease (2 patients), nephroangiosclerosis (4 patients), glomerulonephritis (4 patients), and unknown (26 patients). Exclusion criteria for the study included malignant disease, chronic liver disease, infectious disease, and taking a supplement fish oil tablet (ω-3 or ω-6 fish oil). None of patients received lipid-lowering drugs, l-carnitine, or β-blockers in the 6 months prior to the study. However, 21 patients with hypertension received treatment with angiotensin-converting enzyme inhibitors. Dietary patterns of the subjects were estimated using a food questionnaire. All patients maintained the usual dialysis diet and PD regimen. All anthropometric measurements were made with participants wearing light clothing and no shoes. Body mass index (BMI) was measured in all participants. BMI was calculated as weight (in kilograms) divided by height (in meters) squared. The study protocol was approved by the Ethics Committee of Meram Medical School, University of Selcuk, Konya, Turkey. All patients were informed of the details of the study and the written consent of each patient was received.

Biochemical Analyses

Blood samples were obtained after an overnight fasting in empty vacuum tubes and in tubes containing EDTA. Plasma and serum samples were obtained after suitable centrifugation and samples were stored frozen at –80°C until the day of plasma FAs analysis. Serum lipids, high-sensitivity C-reactive protein (hsCRP), albumin, uric acid, hemoglobin, parathyroid hormone (PTH), serum calcium (Ca), and phosphate (P) levels were measured immediately.

Analysis of Plasma FAs

Plasma FAs concentrations were measured using a modification of the method described by Lagerstedt et al.⁷ and using an internal standard mixture (Larodan-Lipids KT 90-1100, Malmö, Sweden). Analysis was performed on a Shimadzu Qp 2010 (Kyoto, Japan) gas chromatography electron-capture negative-ion mass spectrometry. The plasma FAs concentration values are reported in milligrams per liter (mg/L).

Calculations

The enzymatic activities were estimated from the product precursor ratios of individual FAs as described previously,^5,8 that is, the activity of elongase as the ratio of C18:0–C16:0, delta 9 desaturase (D9D) as the ratio of C16:1–C16:0, and delta 6 desaturase (D6D) activity as the ratio of C18:3–C18:2. Total ω-3 FAs were given as the sum of eicosapentaenoic acid (EPA), docosahexaenoic acid (DHA), and linolenic acid. Total ω-6 FAs were given as the sum of arachidonic and linoleic acid.

Analyses of other Analytes

Serum total cholesterol, triglycerides, high-density lipoprotein cholesterol (HDL cholesterol), albumin, uric acid, hemoglobin, Ca, P, and hsCRP levels were measured by commercially available kits based on routine methods on the SYNCHRON LX System (Beckman Coulter, Fullerton, CA, USA). Low-density lipoprotein cholesterol (LDL cholesterol) was calculated using the formula by Rifai et al.⁹ PTH level was determined by routine chemiluminesans method on E170 analyzer (Roche Diagnostics, Mannheim, Germany).

Table 1. Clinical characteristics and biochemical variables of the PD patients and control subjects.

	Control	PD patients	p-Value
	(n = 45)	(n = 51)
Gender (M/F)	(25/20)	(21/30)	–
Age (years)	38.62 ± 12.9	47.81 ± 11.8	0.001
Duration (years)	–	5.45 ± 5.0	–
BMI (kg/m^2)	22.32 ± 3.3	26.92 ± 5.5	<0.001
Hypertension (yes/no)	(−/45)	(21/30)	–
Diabetes (yes/no)	(−/45)	(11/40)	–
Total cholesterol (mg/dL)	181.80 ± 32.2	197.01 ± 52.1	0.103
Triglycerides (mg/dL)	89.93 ± 44.5	187.95 ± 91.3	<0.001
LDL cholesterol (mg/dL)	101.78 ± 25.6	119.05 ± 30.4	0.005
hsCRP (mg/L)	4.65 ± 3.4	13.22 ± 12.9	<0.001
Albumin (g/dL)	4.34 ± 0.2	4.06 ± 0.3	0.002
Uric acid (mg/dL)	4.68 ± 1.7	7.89 ± 1.6	0.023
Hemoglobin (g/dL)	13.79 ± 1.2	12.19 ± 1.3	<0.001
PTH (pg/mL)	51.06 ± 19.5	326.32 ± 224.8	<0.001
Serum Ca (mg/dL)	9.42 ± 0.4	9.64 ± 1.0	0.178
Serum P (mg/dL)	3.26 ± 0.5	5.24 ± 1.5	<0.001

Notes: All values (except gender) are mean ± standard deviations. BMI, body mass index; LDL cholesterol, low-density lipoprotein cholesterol; hsCRP, high-sensitivity C-reactive protein; PTH, parathyroid hormone; Ca, Calcium; P, phosphate; PD, peritoneal dialysis.

Statistical Analyses

All data are expressed as mean ± standard deviations (SD). Statistical analyses were done using SPSS v. 16.0 (SPSS Inc., Chicago, IL, USA). The normal distribution of variables was examined with independent-samples t-test, and non-normally distributed variables were examined by Mann–Whitney U-test. Differences were considered significant at a probability level of p < 0.05.

RESULTS

Clinical characteristics and biochemical parameters of the subjects are presented in Table 1. As seen from the table, BMI, triglycerides, hsCRP, LDL cholesterol, uric acid, PTH, and serum P levels of the CAPD patients were significantly higher (p < 0.05 for uric acid, p < 0.01 for LDL cholesterol, and p < 0.001 for the other parameters) whereas albumin (p < 0.01) and hemoglobin (p < 0.001) levels were lower than those of the healthy group.

Table 2. Plasma fatty acid profile, estimated desaturase activities in PD patients and healthy subjects (mg/L).

Fatty acids	Control (n = 45)	PD patients (n = 51)	p-Value
Lauric acid (C12:0)	0.22 ± 0.3	0.25 ± 0.2	0.211
Myristic acid (C14:0)	0.51 ± 0.2	0.55 ± 0.4	0.612
Myristioleic acid (C14:1)	0.25 ± 0.2	0.30 ± 0.5	0.569
Pentadecanoic acid (C15:0)	0.09 ± 0.3	0.09 ± 0.06	0.930
10-cis-pentadecanoic acid (C15:1)	0.10 ± 0.1	0.11 ± 0.1	0.011
Palmitic acid (C16:0)	10.9 ± 5.7	17.2 ± 7.2	0.055
Palmitoleic acid (C16:1)	0.60 ± 0.3	1.17 ± 2.5	0.362
Heptadecanoic acid (C17:0)	0.19 ± 0.1	0.28 ± 0.3	0.972
10-cis-heptadecanoic acid (C17:1)	0.13 ± 0.1	0.30 ± 0.2	<0.001
Stearic acid (C18:0)	3.49 ± 1.8	6.42 ± 8.5	0.902
Oleic acid (C18:1)	4.69 ± 2.6	7.35 ± 8.5	0.106
Linoleic acid (C18:2, ω-6)	22.39 ± 7.8	18.87 ± 5.2	0.025
Linolenic acid (C18:3, ω-3)	0.85 ± 0.9	0.64 ± 0.8	0.017
Arasidic acid (C20:0)	0.49 ± 0.2	0.74 ± 0.8	0.950
11-Eicosenoic acid (C20:1)	0.49 ± 0.3	0.71 ± 0.7	0.622
11,14-Eicosedienoic acid (C20:2, ω-6)	3.70 ± 2.3	1.69 ± 2.4	<0.001
Heneicosanoic acid (C21:0)	0.33 ± 0.1	0.54 ± 0.4	0.018
Arachidonic acid (C20:4, ω-6)	5.07 ± 1.0	3.64 ± 1.6	<0.001
11,14,17-Eikosatrienoic acid (C20:3, ω-9)	1.45 ± 1.5	1.58 ± 1.2	0.153
Eicosapentaenoic acid (C20:5, ω-3)	0.70 ± 0.8	0.59 ± 0.8	0.238
Behenic acid (C22:0)	0.33 ± 0.2	0.32 ± 0.4	0.912
Erucic acid (C22:1)	0.28 ± 0.4	0.46 ± 0.4	0.075
Docosahexaenoic acid (C22:6, ω-3)	1.24 ± 1.57	0.70 ± 1.0	0.031
Tricosanoic acid (C23:0)	4.52 ± 2.4	6.70 ± 2.7	<0.001
Lignoceric acid (C24:0)	0.19 ± 0.2	0.27 ± 0.5	0.082
Nervonic acid (C24:1)	0.24 ± 0.5	0.34 ± 0.1	<0.001
SFA	20.08 ± 16.2	27.52 ± 9.6	<0.001
MUFA	15.82 ± 7.6	21.90 ± 14.6	0.018
PUFA	33.33 ± 7.8	30.25 ± 10.3	0.121
Total ω-3 fatty acids	3.75 ± 3.5	2.10 ± 2.1	0.010
Total ω-6 fatty acids	25.17 ± 5.7	22.75 ± 5.4	0.064
ω-6/ω-3 ratio	15.51 ± 9.9	19.5 ± 11.3	0.128
Elongase activities (stearic acid/palmitic acid)	0.27 ± 0.3	0.38 ± 0.3	0.217
D9D (oleic acid/stearic acid)	1.12 ± 2.2	1.18 ± 0.4	0.001
D6D (linolenic acid/linoleic acid)	0.051 ± 0.07	0.038 ± 0.06	0.072

Notes: All values are mean ± standard deviations. SFA, saturated fatty acid; MUFA, monounsaturated fatty acid; PUFA, polyunsaturated fatty acid; D9D, delta 9 desaturase activities; D6D, delta 6 desaturase activities; ω-3, omega-3 fatty acids; ω-6, omega-6 fatty acids.

The plasma levels of FA and elongase and desaturase activity of the groups are presented in Table 2. As seen, the levels of 10-cis-pentadecanoic acid (p < 0.05), 10-cis-heptadecanoic acid (p < 0.001), heneicosanoic acid (p < 0.05), tricosanoic acid (p < 0.001), and nervonic acid (p < 0.001) were significantly higher and the levels of linoleic acid (p < 0.05), linolenic acid (p < 0.05), 11,14-eicosedienoic acid (p < 0.001), arachidonic acid (p < 0.001), and DHA (p < 0.05) were significantly lower in the CAPD group than in the healthy group. We also observed a significant increase in the concentration of SFA (p < 0.001) and monounsaturated fatty acid (MUFA) (p < 0.05) in the CAPD group. No differences were found in plasma concentration of polyunsaturated fatty acid (PUFA) between the CAPD and healthy groups. Also, significantly higher level of D9D activity was observed in the CAPD group (p < 0.01) whereas plasma ω-3 FA levels were lower than those of the healthy group (p < 0.01). There were no significant differences between elongase activities, D6D activity, plasma ω-6 FA levels, and ω-6/ω-3 ratios of the groups.

DISCUSSION

Lipid abnormalities are common in patients with renal disease, probably contributing to the high incidence of cardiovascular diseases in this population.¹⁰ In our study, plasma FA composition was altered in CAPD patients compared to the control subjects. For example, plasma linoleic acid, linolenic acid, and arachidonic acid levels of the CAPD patients were significantly lower than those of the controls. This finding is in accordance with the findings of other investigators. Nakamura et al.¹¹ who investigated chronic kidney disease patients and Dasgupta et al.¹² who investigated chronic hemodialysis patients have found that linolenic acid levels of the patients were significantly lower than that of the controls. Also, Girelli et al.¹³ have found significantly lower levels of linoleic acid and arachidonic acid of red blood cell membrane in CAPD patients. Friedman et al.¹⁴ have found significantly lower levels of linoleic acid and arachidonic acid of red blood cells and plasma of hemodialysis patients.

It has been reported that depletion of essential FA is important because these abnormalities may play an important role in the pathogenesis of some common clinical conditions associated with uremia, such as skin diseases, fragility of erythrocytes, lipid anomalies, and hormonal aberrations.¹²

These abnormalities could be related to the abnormal FA profile and resulting abnormal prostaglandin synthesis.¹⁵ In this study, we have found a significant decrease in plasma DHA levels in CAPD patients whereas no significant difference between plasma EPA levels of the groups was noticed.

There are some conflicting findings concerning plasma EPA and DHA levels of kidney disease patients. For example, Girelli et al.¹³ have found no difference between EPA and DHA levels of red blood cell membranes in CAPD patients. Friedman et al.¹⁴ have found no significant difference between EPA levels of plasma and red blood cells and but significantly lower level of DHA in hemodialysis patients. However, Ristić et al.³ have found that EPA and DHA levels were significantly lower in hemodialysis patients than in the control subjects. Also, Peck et al.¹⁵ have found that EPA level was significantly lower in hemodialysis patients than in the control subjects but they have found no difference between DHA levels of the groups.

The underlying mechanism of this finding is not known and needs to be investigated. It has been reported that one possible cause for the depletion of essential FAs might be their increased lipid peroxidation. The high level of lipid hydroperoxides is well documented in chronic renal failure patients. The reaction rate of free-radical-induced lipid peroxidation is in the milliseconds range, meaning a much shorter half-life time than that of unesterified FA and other lipids in plasma.¹⁶

In our study, ω-3 FA levels were significantly lower in the CAPD patients. Furthermore, ω-6/ω-3 ratio was 20:1 in the CAPD patients. Higher ratios of ω-6/ω-3 promote the pathogenesis of many diseases, including cardiovascular disease, cancer, and inflammatory and autoimmune diseases, whereas increased levels of ω-3 FAs (a lower ω-6/ω-3 ratio) exert suppressive effects.¹⁷ The American Heart Association and various international health organizations recommend that persons at high cardiovascular risk, such as chronic kidney disease patients, maintain a dietary ratio of ω-6/ω-3 as low as 4:1.^18,19

In this study, SFA and MUFA levels of the CAPD patients were significantly higher than those of the controls. In the literature, plasma MUFA levels of the hemodialysis patients were found to be significantly higher than that of controls in three studies ^3,14,20 and plasma SFA levels were found to be significantly higher than that of controls in another study.¹⁶ These findings suggest that in CAPD patients, the higher levels of SFA and MUFA might be influenced by the severity of hyperlipidemia. In addition, D9D activity of our CAPD patients was significantly higher than that of controls. It has been reported that higher activity of D9D, which is strongly stimulated by insulin and glucose, the continuous hyperglycemic stress of CAPD patients mediated by the constant absorption of glucose from dialysate, may play a role.¹³

In conclusion, our results show that there are FA abnormalities and especially a depletion in essential FA levels and a high level of ω-6/ω-3 ratio in CAPD patients, the underlying mechanism of which is not known and needs to be investigated. Therefore, we believe that essential FA supplementation should be encouraged for CAPD patients.

Declaration of interest: The authors report no conflicts of interest. The authors alone are responsible for the content and writing of the paper.