A Review on the Potential Human Health Benefits of the Black Walnut: A Comparison with the English Walnuts and Other Tree Nuts

Abstract

The aim of this review was to evaluate the potential human health benefits of the Black walnut by comparing its compositional similarities and differences with other nuts, particularly the English walnut. The Black walnut contains higher levels of monounsaturated fatty acids and antioxidants, such as the polyphenols and γ-tocopherol, compared to English walnut. These components have been correlated with the prevention and/or attenuation of several types of diseases, including cancer, diabetes, and neurodegenerative conditions. Other nutrients that are also present in the Black walnut include dietary fiber, folate, phytosterols, protein, melatonin, etc., which have been linked with multiple human health promoting properties. Based primarily on its phytochemical composition, the Black walnut is a potentially potent, yet critically understudied, dietary system for promoting human health.

Keywords:

• Black walnut
• English walnut
• Health benefits
• Compositional profiles
• Human health

Introduction

The walnut is the second highest produced edible tree nut in the United States (504,000 tons produced in 2010) positioned behind almonds (820,000 tons).^[1,^2] In particular, the English walnut, or common walnut (Juglans regia L.), well exceeds production of its other walnut counterparts (Juglans nigra L.). English walnuts, frequently referred to only as “walnuts,” originated in Persian and expanded to the United States by English settlers. Currently, California accounts for 90% of the production and processing of English walnuts.^[3] Alternatively, the Black walnut, the second highest produced walnut in the United States, is native to the U.S. Midwest and Northeastern regions with Missouri being the highest ranking producer.

Walnuts (Juglans regia) are consumed throughout the world succeeded only by almonds (Prunus amigdalis), and hazelnuts (Corylus avellana), but proceeding pistachios (Pistachia vera), pine nuts (Pinus pinea), cashews (Anacardium occidentale), pecans (Carya illinoiensis), macadamias (Macadamia integrifolia), and Brazil nuts (Bertholletia excelsa).^[4] In the United States, walnuts, almonds, and pecans are the three most highly consumed nuts, i.e., listed in descending order of intake.^[3] Black and English walnuts are usually sold as cooking ingredients for candies, cereals, baked goods, and other snacks. Still, 90% are sold in-shell as most consumers prefer to crack the nuts.^[5] However, the commercial value of the Black walnut is lower than the English walnut due in part to its small kernels and hard thick shells.

Both epidemiologic and clinical studies have shown a link between tree nut consumption and various protective benefits against multiple diseases, including cardiovascular diseases (CVDs), diabetes, inflammation, oxidative stress, cancer, and neurodegenerative conditions.^[6–14] These health promoting properties have been attributed mainly to the presence of unsaturated fatty acids and other bioactive nutrients, including phenolic compounds, tocopherols, phytosterols, high-quality protein, fiber, and minerals.^[15–18] Despite the Black walnuts richly diverse phytochemical composition, the majority of health-related research has focused on the English walnut. Therefore, the aim of this review is to examine the broad range of potential benefits that the Black walnut may have on human heath based upon its compositional profile. This undertaking will be completed based on comparing the Black walnut with the English walnut and other types of tree nuts, as more health studies have been accomplished with these natural systems. The rationale of this work is that compositional comparisons with the English walnut and other tree nuts will show the nutrient diversity of this interesting dietary system, which is expected to stimulate future human health related research on Black walnuts.

Walnut Composition

Walnuts are categorized as tree nuts because they contain one seed where the ovary wall becomes hard at maturity. These edible seeds are nutrient dense containing high levels of protein (~15% of the energy), lipids, fiber, vitamins, minerals (Table 1), and other bioactive molecules, such as the phenolic compounds.

TABLE 1 Black and English walnut nutrient composition (USDA)^[20]

Nutrient	Unit	Value per 100 g*
macronutrients		Black walnut	English walnut
Water	g	4.56	4.07
Energy	kcal	618	654
Protein	g	24.06	15.23
Total lipid (fat)	g	59.00	65.21
Carbohydrate, by difference	g	9.91	13.71
Fiber, total dietary	g	6.8	6.7
Sugars, total	g	1.10	2.61
Minerals
Calcium, Ca	mg	61	98
Iron, Fe	mg	3.12	2.91
Magnesium, Mg	mg	201	158
Phosphorus, P	mg	513	346
Potassium, K	mg	523	441
Sodium, Na	mg	2	2
Zinc, Zn	mg	3.37	3.09
Vitamins
Vitamin C, total ascorbic acid	mg	1.7	1.3
Thiamin	mg	0.057	0.341
Riboflavin	mg	0.130	0.150
Niacin	mg	0.470	1.125
Vitamin B-6	mg	0.583	0.537
Folate, DFE	µg	31	98
Vitamin A, RAE	µg	2	1
Vitamin A, IU	IU	40	20
Vitamin E (alpha-tocopherol)	mg	1.80	0.70
Vitamin E (gamma-tocopherol)	mg	28.48	20.85
Vitamin K (phylloquinone)	µg	2.7	2.7
Lipids
Fatty acids, total saturated	g	3.368	6.126
Fatty acids, total monounsaturated	g	15.004	8.933
Oleic acid (C18:1)	g	14.533	8.7
Fatty acids, total polyunsaturated	g	35.077	47.174
Linoleic acid (C18:2)	g	33.072	38.2
α-linolenic acid (C18:3)	g	2.006	9.08

*Component value refers to edible portion.

TABLE 2 Summary of studies on walnuts and health promotion

Stress-disease	Model	Treatment	Dosage	Main Outcomes	References
CVD	7447 subjects (57% women) with high cardiovascular risk	Mediterranean diet supplemented with mixed nuts (50% of walnuts)	30 g of mixed nuts per day	Experimental diet containing walnuts led to a reduction of the major cardiovascular events (myocardial infarction, stroke, or death from CHD)	Estruch et al.^[^61^]
CVD	20 hypercholesterolemic subjects	a. American diet (control): 8.7% energy from PUFA (7.7% LA, 0.8% ALA); b. LA diet: 16.4% energy from PUFA from whole walnuts and walnut oil (12.6% LA, 3.6% ALA); c. ALA diet: 17% energy from PUFA from walnuts, walnut oil, and flax oil (10.5% LA, 6.5% ALA)	ALA and LA diets: 37 g and 15 g of walnuts and walnut oil, respectively, per day. In addition, the ALA diet contained 19 g of flaxseed oil	Experimental diets (ALA and LA) reduced diastolic blood pressure and total peripheral resistance. Flow-mediated dilation and arginine-vasopressin were increased after intake of the ALA diet	West et al.^[^67^]
CVD	36 Proatherogenic Apoe(-/-) mice	a. sunflower oil diet (control); b. high fat diet supplemented with walnuts (rich in omega-3 PUFA and antioxidants); c. walnut oil (with omega-3 PUFA only)	5 g of indicated diets daily during 8 weeks (1.2 g of English walnuts in the walnut diet)	The consumption of whole walnuts led to a 55% reduction in atherosclerotic plaque formation compared to the control group; triglycerides, cholesterol and prothrombin serum levels decreased in this group by 36, 23 and 21%, respectively	Nergiz-Ünal et al.^[^68^]
CVD	18 hypercholesterolemic patients (9 women and 9 men)	Sequences of diets containing 40% fat from walnuts, virgin olive oil or almonds.	35–50 g (olive oil), 65 g (walnuts), 50–75 (almonds). Four weeks consumption of each diet	LDL-c was significantly decreased by 7.3, 10.8, and 13.4% after the olive oil, walnut and almond diets, respectively. Total cholesterol and LDL/HDL ratios were significantly reduced, but only after nut diets	Damasceno et al.^[^9^]
CVD	87 individuals (38 men and 49 women) with normal to moderate high plasma total cholesterol	Walnut-supplemented diet (approximately 12% of total daily energy intake) or habitual diet (control group)	28–64 g walnuts/day for 6 months (randomized crossover study)	Walnuts supplementation decreased total cholesterol and triglyceride levels, more significantly in the high plasma total cholesterol group (at baseline)	Torabian et al.^[^69^]
CVD	29 participants (13 men and 16 women)	Diets supplemented with Black or English walnuts	30 g walnut daily for 28–30 days (after 12-week washout period subjects switched to the other type of walnut)	Walnuts intake led to a 2.7% reduction of total cholesterol and 4.2% of LDL-cholesterol. Black walnut consumption resulted in more improvement of blood lipids than English walnuts.	Fitschen^[^72^]
Oxidative process	in-vitro (induction of LDL protein oxidation) and ex-vivo (36 subjects)	Black and English walnuts extracts (in-vitro assay). Diets supplemented with Black or English walnuts (ex-vivo assay)	In-vivo assay: 1.00 or 0.10 μg/mL walnut extracts. Ex-vivo assay: random consumption of either 30 g portions of English or black walnuts daily for 28 days	Both walnuts improved markers of oxidation, with English walnuts exhibiting a higher antioxidant capacity via in vitro assay. However, the consumption of walnuts was not sufficient to improve protection of LDL oxidation.	Rorabaugh et al.^[^42^]
Cancer	Human renal cancer cell lines A-498 and 769-P, and the colon cancer cell line Caco-2	English walnut extract	31.25, 62.5, 125, 250, and 500 µg extract/mL	Walnuts extract was able to inhibit the growth of cancer cells in a dose-dependent manner, which was not correlated with the total phenolic contents in the extracts tested, suggesting that specifics phytochemicals may be responsible for their antiproliferative activities	Carvalho et al.^[^10^]
Cancer	HT-29 human colon cancer cells injected in 48 6-wk-old female mice	Diets supplemented with walnuts, flaxseed oil, or corn oil (control group)	19% of total energy from walnuts, flaxseed oil, or corn oil (control group) daily for 25 days	Walnut and flaxseed diets were responsible for slowing tumor development by 27 and 43%, respectively, while tumor weight was also reduced. In addition, plasma expression levels of angiogenesis factors were significantly reduced in mice fed on experimental diets. Angiogenesis was significantly decreased only in walnut diet.	Nagel et al.^[^75^]
Cancer	21 men at risk for prostate cancer	Usual diet with or without English walnuts	75 g of English walnuts per day during 8 weeks	English walnuts supplementation led to an increased plasma γ-tocopherol and increased ratio of free prostate specific antigen (PSA): total PSA compared to the control diet	Spaccarotella et al.^[^76^]
Neurodegenerative diseases	45 male Fischer 344 rats, 19-month-old	Two English walnut supplemented diets: a. 6% diet: 60 g walnuts/kg and 40 g corn/kg, and b. 9% diet: 90 g walnuts/kg and 10 g corn/kg. Control diet: 100 g/kg corn.	Ad libitum feeding during 15 weeks	English walnut consumption led to a significantly reduced aggregation of polyubiquitinated proteins in brain. Also, rats fed with walnut diet exhibited up-regulation of autophagy.	Poulose et al.^[^11^]
Neurodegenerative diseases	BV-2 microglial cells (In-vitro study)	English walnut extracts and LPS stimulation	Walnut extract concentrations: 0.00001 to 10 mg of walnuts/mL of solution	Cells treated with English walnut extract produced lower amounts of NO and reduced expression of iNOS. Walnut supplementation also induced a reduction in tumor necrosis-alpha (TNF-α) generation and internalization of the LPS receptor TLR-4.	Willis et al.^[^14^]

NO: nitric oxide; iNOS: inducible nitric oxide synthase; TLR-4: toll-like receptor 4.

Black walnuts are particularly high in unsaturated fatty acids,^[15] monounsaturated fatty acids (MUFAs) and polyunsaturated fatty acids (PUFAs) contributing ~80% of the total lipid content. The MUFA profile consists of oleic acid (C18:1n-9) and omega-6 linoleic acid (LA 18:3n-6), whereas omega-3 α-linolenic acid (ALA; 18:3n-3) is the major PUFA present in both the Black and the English walnut (Table 1). The PUFA (LA and ALA) account for more than half of the total lipid content, which is an important attribute considering that these fatty acids are not synthesized in the human body.^[19]

Epidemiologic studies have shown overall improvement in individuals afflicted with CVD in response to ALA intake.^[20] After absorption into a cell, LA is elongated and desaturated to arachidonic acid (AA) while ALA is converted to eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) by a series of elongation and desaturation reactions.^[21–24] The metabolites derived from AA, such as leukotrienes, prostaglandins, and thromboxanes, are generally pro-inflammatory and proaggregatory agonists, while those derived from the ALA inhibit platelet aggregation and inflammation. The latter mechanisms are involved in the onset of CVD, hypertension, type 2 diabetes, chronic obstructive pulmonary disease, among others.^[25] Based on the ALA content in the Black walnut (Table 1), increased consumption is likely to exert potent heart health benefits.

Walnuts also contain considerably high levels of non-cholesterol sterols known as plant sterols or phytosterols.^[17] Black walnuts contain higher levels of phytosterols (109 mg/100 g) compared to the English walnut (72 mg/100 g).^[26] Phytosterols are able to reduce cholesterol absorption in the intestinal lumen due their high affinity for forming micelles. Consequently, cholesterol is displaced from micelles resulting in lower cholesterol absorption and reduced plasma cholesterol.^[27,^28]

Additionally, walnuts are a rich source of the tocopherols with γ-tocopherol being the predominant Vitamin E isomer. Although present in both the English and the Black walnut, the latter nut contains even higher amounts of γ-tocopherol (Table 1). Gamma-tocopherol has not been investigated as intensively as α-tocopherol for its health properties, but recent studies indicate that γ-tocopherol may be more efficacious as increased intake has been correlated to lower risks for cancer and CVD.^[29]

The chemically diverse phenolic compounds are also present in walnuts.^[30,^31] Phenolic acids are a subclass of the phenols that contain at least one aromatic ring and one hydroxyl group.^[32] Condensed tannins or proanthocyanidins are polymeric flavonoids composed of at least two linked catechin units.^[33] Flavonoids (e.g., catechin) are the highest consumed subclass of polyphenols in the human diet. These compounds are characterized by two or more aromatic rings containing at least one hydroxyl group bonded to each ring.^[32] Phenols have been linked to several human health benefits due primarily to their antioxidative or anti-inflammatory properties.^[34–38] (Figure 1 illustrates the basic chemical structure of the phenolic compounds.)

FIGURE 1 Examples of phenolic acids, condensed tannins, and flavonoids structure (adapted from).^[32,^38]

Phenolic compounds present in walnuts are mainly concentrated in the hull (the skin that covers the kernel), and this fraction has been linked to improved human health effects. ^[39] Importantly, reports have shown that the walnut phenols are highly bioavailable.^[40] Karamac^[41] showed that the total phenolic content of English walnuts (550 ± 11 mg catechin equivalents per gram of tannin fraction) was higher than hazelnut (329 ± 7) and almonds (83 ± 2). Analysis of Black walnuts performed in the Schlegel laboratory, Food Science and Technology, University of Nebraska, showed a phenolic content of 2.45 ± 0.01 mg trans-cinnamic acid equivalents per gram of kernel (data not published), which was slightly higher than values obtained from English walnuts (2.14 mg gallic acid equivalents per gram of kernel) reported by Carvalho et al.^[10] It must be noted that only one solvent system was used by both laboratories, which cannot possibly account for total levels due to the chemical diversity of phenols. However, Rorabaugh et al.^[42] used two solvent systems (80% acetone and 80% methanol in water) to extract flavonol from both the English and Black walnut. These analyses showed that English walnuts contained higher levels of phenolic acids and flavonols, compared to Black walnuts. The total flavonol profile for the acetone extract was 166.1 and 24.2 μg/g for English and Black walnut, respectively, and the total profile for the methanol extract was 147.6 and 4.1 μg/g for the English and Black walnut, respectively. The specific compounds identified in both types of walnuts included 5-caffeoylquinic acid, 4-caffeoylquinic acid, quercetin-3- rutinoside, quercetin-3-galactoside, quercetin-3-pentoside, quercetin-3-arabinoside, and quercetin-3-rhamnoside (Fig. 2). However, in our study (data not published), anthocyanins were also detected in the Black walnut at 80.62 μg/g when extracted with 50:50 water:methanol. Caffeoylquinic acid and quercetin are powerful phenolic compounds that have been linked to protect against several types of conditions, including, CVDs,^[43] cancer,^[44] Alzheimer’s,^[45] gastric disorders,^[46] hyperglycemia,^[47] and dyslipidemia.^[48] To the best of the authors’ knowledge, the only report on anthocyanins present in English walnut (27.10 γg/g) was provided by the U.S. Department of Agriculture (USDA) database,^[49] but the extraction methods were not cited.

Figure 2 Main types of phenolic compounds present in walnuts (adapted from ChemSpider database).^[50]

Other nutrients abundant in both Black and English walnuts are folate, melatonin, pectin, and various types of minerals (Table 1). These components have been associated to improved lipid profiles, endothelial function, increased plasma antioxidant capacity, and neuroprotective action.^[17,²⁴,^50–55] In particular, the predominant B vitamins in walnuts (folate [B9], cobalamin [B12], pyridoxal phosphate [B6]) play an important role in detoxifying homocysteine; a methionine derived amino acid with atherothrombotic properties.^[56] Finally, the Black walnut contains slightly higher levels of arginine (3.62 g per 100 g) compared to the English walnut (2.28 g per 100 g).^[26] It has been reported that arginine lowers blood pressure by acting as a precursor for the endogenous vasodilator, nitric oxide (NO).^[57]

Black Walnuts and Potential to Health Outcomes

Black walnuts contain constituents that have shown potent health promoting properties as isolated components or as part of other natural systems. Yet, there remains a critical gap in knowledge that directly links the Black walnut to the prevention of cellular stressors or diseases. Several studies have shown positive correlations between English walnut consumption and multiple health impacts, the Black walnut will again be compared to the English walnut as a means to elucidate the potential of Black walnuts as a dietary system for human health purposes. It must be noted that many studies presented herein did not specify the type of walnut studied, but it was assumed that English walnut was the type used due to its wide consumption and denomination as the “common walnut.” Table 2 summarizes the main studies presented in this report.

CVD

Epidemiological research accumulating throughout decades has shown strong correlations between nut consumption and reduced risks for heart disease. In particular, four seminal studies showed a positive impact on chronic heart diseases in response to the consumption of several foods (including nuts). These studies include: The Adventist Health Study,^[58] The Iowa Women’s Health Study,^[59] The Nurses’s Health Study,^[8] and The Physicians’ Health Study.^[60]

For the Adventist Health Study,^[58] 27,000 California Seventh-day Adventists provided information on their dietary and lifestyle habits during six years while coronary heart disease (CHD) data were also obtained. Among the foods consumed, nut intake resulted in the greatest inverse relationship for the risk of non-fatal myocardial infarction (MI) or death from CHD. Individuals who ate nuts five or more times per week presented with 48 and 38% reduced risk of death from MI and CHD, respectively, compared to the individuals who consumed nuts less than once per week.

The Iowa Women’s Health Study^[59] correlated CHD risks with the consumption of 127 foods, which included nuts. A food frequency questionnaire was again administrated to the participating women and their lifestyle habits were monitored. After 7 years of data collection, 19,411 women who were not taking vitamin supplements but were consuming nuts more than 4 times per month resulted in a 40% reduction of fatal CHD risk compared to the group with the lowest consumption (rarely or never).

The Nurses’s Health Study^[8] also reported a cardio-protective effect in response to nut consumption. During 14 years, 86,016 nurses were monitored in terms of their intake of 61 foods, including nuts, lifestyle, and occurrence of CHD. The risk of fatal and non-fatal MI decreased by 39% and 32%, respectively, in the group who consumed the highest intake of nuts (≥5 times per week), compared to the group who consumed nuts less than once per month. Additionally, the Physicians’ Health Study followed male physicians (22,017) for 17 years while monitoring their nut consumption. The group who consumed nuts more than once per week presented with 20% less risk of sudden cardiac death, but there was no reduction in non-fatal MI. These results were attributed to the overall low frequency of nut consumption by the studied population compared to those monitored in the previous studies.^[60]

Consistent with those studies, Estruch et al.^[61] published a recent report where 7447 subjects (57% women) with high cardiovascular risk were randomly assigned to one of the following three diets: (1) a Mediterranean diet supplemented with extra-virgin olive oil, (2) a Mediterranean diet supplemented with mixed nuts (50% walnuts, 25% hazelnuts,and 25% almonds), and (3) a control diet. The participants were then monitored for several types of cardiovascular risks over a 4.8-year period. Both supplemented diets reduced the risk of major cardiovascular events, including MI, stroke, or death from CHD, compared to the control diet.

The reduced risk of CVDS via nut consumption can be mainly attributed to their rich omega-3 PUFAs and antioxidants, as demonstrated by several epidemiologic studies using these dietary agents in isolation.^[48,⁶²,^63] Additionally, overall improvements in plasma lipids and endothelial function have been reported in response to omega-3 fatty acids intake.^[64,^65] English walnuts contain the highest amount of ALA among all edible plants, and their antioxidant polyphenols content is noteworthy.^[39,^66]

Accordingly, several studies have been performed with the purpose of investigating the cardio-protective effect of walnuts. West et al.^[67] assessed the effects of ALA on the vascular endothelial function in 20 hypercholesterolemic subjects using a randomized crossover study. Individuals were fed an average American diet, providing 8.7% energy from PUFA (7.7% LA, 0.8% ALA). The predominantly LA diet was provided by whole walnuts and walnut oil, which resulted in 16.4% of energy (12.6% LA, 3.6% ALA). Finally, a diet high in ALA was based on walnuts, walnut oil, and flax oil (10.5% LA, 6.5% ALA), which provided 17% of energy. Although flow-mediated dilation and arginine-vasopressin increased only after intake of the ALA diet, the experimental diets (ALA and LA) significantly reduced diastolic blood pressure (–2 to –3 mm Hg) and total peripheral resistance (–4%). The results from this study suggest novel mechanisms for the cardio-protective effects of walnuts, such as reduction of blood pressure and systemic vascular resistance, i.e., by reduction in C-reactive protein and increase in flow-mediated dilatation.

Nergiz-Ünal et al.^[68] demonstrated that English walnut consumption positively influences plasma lipids levels and atherosclerotic plaque formation using a proatherogenic Apoe(-/-) mice model. The mice were fed a high fat diet supplemented with English walnuts (rich in omega-3 PUFA and antioxidants), walnut oil (omega-3 PUFA only), or sunflower oil (control group) for 8 weeks. The consumption of whole walnuts, but not walnut oil, led to a 55% reduction in atherosclerotic plaque formation in the aortic arch compared to the control group. In addition, triglycerides, cholesterol and prothrombin serum levels decreased in the first group by 36, 23, and 21%, respectively. Moreover, increased plasma antioxidant capacity was detected. The atheroprotective effect of English walnuts was attributed to the combination of n-3 PUFA along with other antioxidants, most likely the polyphenols.

In a crossover study, the effects on serum lipids and other markers of CVD risk were evaluated in 18 hypercholesterolemic patients (9 women and 9 men) randomized into a sequence of diets containing 40% fat from walnuts, virgin olive oil or almonds, during a 4-week duration for each food system.^[9] Low-density lipoprotein-cholesterol (LDL-c) significantly decreased by 7.3, 10.8, and 13.4% in individuals who consumed olive oil, walnuts, and almond diets, respectively, relative to the baseline. Similarly, total cholesterol and LDL/high-density lipoprotein (HDL) ratios were significantly reduced but only after consumption of the nut diets.

Torabian et al.^[69] performed a randomized crossover study to investigate the effects of extended consumption of walnuts by 87 individuals (38 men and 49 women) with normal to moderate high plasma total cholesterol. The participants were assigned a walnut-supplemented diet (approximately 12% of total daily energy intake) or habitual diet (control group) for 6 months each. The results showed that walnut supplementation decreased total cholesterol and triglyceride levels, which was more significant in the high plasma total cholesterol group (at baseline). A high degree of dietary compliance was also detected among the participants (95%), indicating a walnut-rich diet can be maintained for long periods. The effects of English walnuts on lipid profiles were consistent with the previous cited studies and others that have not been discussed herein, but are provided in the references.^[53,⁷⁰,^71]

Studies involving Black walnuts and their effect on CVDs or other disease states are limited despite their high phytochemical composition. However, Fitschen^[72] investigated the cardiovascular effect of Black versus English walnuts consumption over 28–30 days. The 29 participants (13 men and 16 women) were randomly selected to consume either Black or English walnuts (30 g daily). After a 12-week washout period, the subjects were switched to the other type of walnut and were instructed to again to consume 30 g daily for 28–30 days. Walnut intake resulted in lower total cholesterol (2.7%) and LDL-cholesterol (4.2%). Importantly, CVD markers were significantly improved in response to Black walnut intake compared to the English walnut.

In another study completed by Rorabaugh et al.,^[42] the antioxidant capacity of English and Black walnuts was evaluated by their ability to prevent LDL-c oxidation. Using an in-vitro assay and walnut extracts, the results revealed that both walnuts improved markers of oxidation, with English walnuts exhibiting higher antioxidant capacity. However, human subjects randomized to consume their habitual diets but supplemented with either English or Black walnuts (30 g daily/28 days) were not protected against LDL oxidation by either system. Based on these limited studies and the richly chemically diverse bioactive compounds present in Black walnuts, it is expected that Black walnuts will provide protective benefits against CVDs similar to or even exceeding the English walnut and other types of nuts.

Cancer

Several bioactive compounds present in nuts have been reported to possess potent antioxidant, anti-inflammatory, i.e., cell stressors that can lead to cancer. Such dietary agents include omega-3 fatty acids tocopherol, phytosterols, folate, and polyphenols.^[7,¹⁷,^74] As cited previously, both Black and English walnuts are excellent sources of these components (Table 1). Walnuts also contain the highest levels of antioxidant compounds compared to other nuts, mainly the polyphenols and tocopherols.^[30]

Carvalho et al.^[10] investigated the antiproliferative and antioxidant activities of English walnut in the human renal cancer cell lines, A-498 and 769-P, and the colon cancer cell line, Caco-2. The walnut extracts were able to inhibit proliferation in a dose-dependent manner for all the cell types. As this inhibition did not correlate with total phenols levels in the extracts tested, a specific phytochemical or combination of phytochemicals was most likely responsible for the health benefit. The walnut extracts were also able to protect membranes of human erythrocytes from induced oxidative hemolysis in a time- and concentration-dependent manner. Data from this work indicate that walnuts are an excellent source of antioxidants and anti-cancer agents.

Nagel et al.^[75] demonstrated that dietary walnuts inhibited colorectal cancer by suppressing angiogenesis. The research was completed by injecting HT-29 human colon cancer cells in female mice. The mice were then randomized according to diets containing approximately 19% of total energy from walnuts, flaxseed oil, or corn oil (control group). After a 25 day of feeding, the mice were evaluated for tumor growth. The walnut and flaxseed diets slowed tumor development by 27 and 43%, respectively, while tumor weight also decreased in both treatment groups. Alternatively, plasma analysis of angiogenesis factor expression showed significantly lower levels in the animals fed a walnut diet only. As a result, the authors recommended that more studies were needed to verify if walnuts supplementation could exert a similar response in humans with colon cancer.

Spaccarotella et al.^[76] evaluated the effects of English walnuts supplementation on markers of prostate cancer. Twenty-one men at risk for prostate cancer were randomized into their usual diet with or without 75 g of walnuts during an 8-week period. The walnuts based diets resulted in higher plasma γ-tocopherol levels and increased ratio of free prostate specific antigen (PSA): total PSA compared to the control diet.

As previously discussed, Black walnuts contain relatively high levels of γ-tocopherol (Table 1), a nutrient linked to lower risks for prostate cancer.^[77,^78] For example, γ-tocopherol inhibited tumor cells growth for a variety of cancer cells, including prostate cancer cells.^[79] Similar to the heart health benefits, Black walnuts is expected to be a promising dietary system to protect against cancer prevention considering that the potential anti-cancer agents present, most notably the polyphenolic compounds and γ-tocopherol vitamin E (Table 1).

Neurodegenerative Diseases

The beneficial effects promoted by walnuts intake also extends to brain health. Recent studies have demonstrated that animals fed diets rich in walnuts presented with improved indices of memory, cognition, and motor functions.^[80–82] Omega-3 acid rich foods, namely EPA and DHA, play a crucial role in brain health by maintaining synaptic plasticity, neuronal membrane stability, gene expression, mitigation of oxidative stress and regulation of immune function.^[24,^52] In addition, vitamin E, folate, phenolic compounds, melatonin, and other micronutrients have been linked to neuroprotective effects.^[52,^82]

Poulose et al.^[11] investigated whether an English walnut rich diet could activate autophagy function as a means to maintain protein homeostasis and thus prevent polyubiquitinated protein accumulation in brain of aged rats (19 months). These toxic proteins increase in the brain with age due in part to increased oxidative and inflammatory stresses. After consuming a diet supplemented with 6–9% walnuts, the rats presented with significantly reduced aggregation of polyubiquitinated proteins while autophagy activity increased.

Willis et al.^[14] examined the effects of an English walnut extract on lipopolysaccharide (LPS)-induced activation in BV-2 microglial cells. Microglial activation can lead to the production of cytotoxic intermediates, which in turn are associated with a variety of age-related and neurodegenerative diseases.^[83] Cells treated with the walnut extract produced lower amounts of NO and reduced the expression of inducible NO synthase. Walnut supplementation also resulted in reduced tumor necrosis-alpha production and internalization of the LPS receptor toll-like receptor 4. This first study showing the anti-inflammatory effects of walnuts in microglia indicates that walnuts may play an important role in the prevention/treatment of neurodegenerative conditions. The authors attributed the essential fatty acids as the primary agents responsible for this benefit. Similar fatty acids present in the Black walnut in addition to other bioactive components support the hypothesis that Black walnuts will positively affect neurodegenerative conditions through comparable mechanisms as cited for the English walnut.

Conclusion

In this work, we addressed the English nut and other tree nuts ability to protect or treat multiple cellular stress or diseases, and then tied the bioactive agents most responsible for these benefits to the Black Walnut nutrient content. This work thus supports the premise that the Black Walnut will promote multiple health benefits similar to or even greater than the English walnut and other tree nuts based on its compositional profile. However, much more research on the Black Walnut is needed in order to provide a direct link in its ability to prevent or remediate these conditions. As a U.S. grown nut, research most likely should be directed at diseases currently afflicting our culture, such as cancer, diabetes, and heart diseases. The ease in consuming the Black walnut will also have to be addressed, as most walnuts are consumed raw from cracking the shells. This is difficult to do for the Black Walnut due to its thick outer shell. It is expected that once this type of research is completed, the Black nut will be consumed as a whole food ingredient within a different foods thereby eliminating the cracking feature. However, field-to-fork processing should be minimal, which is expected to maintain the compositional integrity of the Black Walnut.