October 2013 Issue
Walnuts — They’re Good for the Heart and Offer Other Health Benefits
By Linda Antinoro, JD, RD, LDN, CDE
Vol. 15 No. 10 P. 74
The adage “everything old is new again” seems to apply to walnuts. They’re one of the oldest tree foods known to man, dating back to 7,000 BC. They were considered the most important nuts from a health standpoint in the ancient Mediterranean world. Now, fast-forward to the 21st century and research steadily mounts regarding their nutritional properties and implications for promoting health.
While all nuts are nutrient dense, the walnut shines as being the nut highest in alpha-linolenic acid (ALA), a plant-based omega-3 source, and polyphenols, an important group of antioxidants.
“Walnuts are ‘polyphenomenal,’” says Wendy Bazilian, DrPH, RD, author of The SuperFoodsRx Diet. “Most likely it’s the synergy of all the substances in walnuts—namely the phytonutrients, micronutrients, ALA, and fiber—that allow walnuts to provide an array of health benefits.” These benefits include improved risk factors associated with cardiovascular disease, diabetes, certain cancers, cognitive impairment, and perhaps even male infertility.
This year marks the 20th anniversary of landmark clinical research from Loma Linda University in California that shows the cardioprotective effects of walnuts.1 That study directly linked walnut consumption to significantly lowering LDL cholesterol levels in a small group of healthy men. Since then, research has shown cardiovascular benefits in other populations and by other mechanisms. Beyond lowering cholesterol levels, researchers have observed decreases in blood pressure, oxidative stress, and inflammatory markers as well as improvement in endothelium-dependent vasodilation.
A crossover study published in June in The Journal of Nutrition was the first of its kind to begin identifying which parts of the walnut may provide protection against cardiovascular disease.2 Each of the 15 participants with high cholesterol levels received four treatments: 85 g (3 oz) of whole walnuts; 5.6 g of walnut skin; 34 g of defatted nut meat; or 51 g (3.5 T) of walnut oil. Then participants underwent several biochemical and physiological tests, both before consuming these treatments and at varying time intervals thereafter.
The results showed that a one-time consumption of whole walnuts was associated with a 3.3% increase in cholesterol efflux, which enhances HDL transport and the removal of excess cholesterol from the body. Walnut oil, on the other hand, was linked to favorable effects on endothelial or blood vessel function.
“The high amount of walnuts and walnut oil was used in this study so outcome effects could be observed with acute consumption,” says Claire Berryman, lead author and a PhD candidate in nutritional sciences at Penn State University. “Substituting a more modest quantity of walnuts—28 to 42 g, or 1 to 1.5 oz—in the diet each day for less healthful foods also will provide cardiovascular benefits.”
Evidence also has shown that eating walnuts may reduce the risk of type 2 diabetes, including when researchers investigated the association between walnut intake and the incidence of type 2 diabetes in two large cohort studies: the Nurses’ Health Study and the Nurses Health Study II.3 Researchers followed nearly 138,000 women over a 10-year period and found that those who ate two or more 1-oz servings of walnuts per week were 24% less likely to develop diabetes compared with those who ate fewer or no walnuts.
The women who ate walnuts also tended to weigh less, exercise more, and consume more fish, whole grains, fruits, and vegetables, but researchers controlled for these factors. One possible reason for the diabetes risk reduction is the high level of polyunsaturated fats (PUFAs) walnuts contain, which may favorably influence insulin resistance.
Numerous substances in walnuts, such as gamma tocopherol, carotenoids, phytosterols, and polyphenols (especially ellagitannins), may offer protection against cancer development. Various animal studies have shown that walnuts provide some protective effects against colon, breast, and prostate cancers. The different mechanisms by which they offer protection include curbing the proliferation of cancer cell growth, inducing apoptosis (cancer cell death), and stopping the angiogenesis (blood vessel growth) of cancer cells.
Human studies are lacking; however, a small human clinical trial is under way at Marshall University in West Virginia. “We are obtaining additional biopsies at the time a woman has her initial biopsies for pathology and diagnosis of a breast lump,” explains researcher Elaine Hardman, PhD. “At this time, we know that there is a lump but not if the lump is cancer. She will be asked to consume 2 oz/day of walnuts—an equivalent amount used in some animal studies—until the time of surgery if the lump is cancer. We will obtain an additional specimen from the surgically removed tissue. New technology—Next Generation RNA sequencing—allows us to assess the expression of all RNAs in the specimen. Our hypothesis is that if the walnuts are doing anything to alter growth characteristics of the cancer, this should be reflected in the changes in messenger RNA expression and ultimately in the proteins that control cell proliferation and death.”
Results from the study are expected in the next one to two years.
Walnuts may not only protect against cancer but also may help improve memory. Researchers observed this finding after examining 447 individuals from the PREDIMED trial cohort, a long-term nutritional intervention study aimed at evaluating the efficacy of the Mediterranean diet in the primary prevention of cardiovascular diseases.4 The researchers evaluated the intake of various foods, including walnuts, and performed neuropsychological tests to assess cognitive function related to diet. Increased walnut consumption, but not other nuts, was associated with better working memory scores. Investigators believe walnuts’ high polyphenol content may be the key factor in helping preserve cognition, and they concluded that at least a 1-oz serving is needed to obtain the benefit.
Other research has suggested that walnuts may protect against dementia. An in vitro study using walnut extract found that it counteracted oxidative stress and cell death caused by amyloid beta-protein, a major component of amyloid deposits and senile plaques in the brains of individuals with Alzheimer’s disease.5 The combination of antioxidants and ALA in walnuts may provide antiamyloidogenic, antioxidative, and anti-inflammatory properties, thereby protecting brain cells.
Walnuts also have been linked to a positive effect on sperm development. In one study, 117 healthy men aged 21 to 35 were randomized either to continue eating their usual diet or adding 2.5 oz of walnuts to their usual diet.6 Before the study started and at the end of 12 weeks, researchers obtained blood and semen specimens. Men who consumed the walnuts experienced improvement in sperm shape, movement, and vitality as well as fewer chromosomal abnormalities in their sperm. The control group experienced no changes.
It’s thought that the abundance of PUFAs in walnuts and the accompanying increased serum levels of omega-3 and omega-6 fatty acids seen in the men eating walnuts may have contributed to the changes in sperm quality. Whether the changes seen in this study would apply to men with fertility problems currently is unknown, yet these findings show another potential benefit of walnut consumption.
When recommending that clients incorporate walnuts and/or walnut oil into their diet, suggest they do the following:
• Adjust their diet as needed to account for the calories from walnuts (185 kcal per 1-oz serving) or walnut oil (120 kcal per 1 T). This is especially important if the amount consumed is higher or closer to the quantities used in some studies.
• Use walnut oil for dipping, blending into a dressing, or drizzling on food. Heating the oil may not appeal to everyone, as it can have a bitter taste.
• Purée or grind walnuts and incorporate them into recipes such as dips and chili or use as a coating for fish. This is particularly helpful for individuals who can’t chew walnuts.
• Store walnuts in an airtight container in the refrigerator or freezer to maintain freshness.
• Consult a physician if they’re taking thyroid medication because walnuts may decrease its absorption and the dose may need to be altered.
— Linda Antinoro, JD, RD, LDN, CDE, is a freelance writer and member of the Nutrition Consultation Service at Brigham and Women’s Hospital in Boston.
Nutrition Facts About Walnuts
Based on a 1-oz portion (14 halves):
• Calories: 185
• Protein: 4.3 g
• Fiber: 1.9 g
• Alpha-linolenic acid: 2.5 g
• Linoleic acid: 11 g
— Sources: California Walnut Commission; USDA National Nutrient Database For Standard Release, Release 25
1. Sabaté J, Fraser GE, Burke K, Knutsen SF, Bennett H, Lindsted KD. Effects of walnuts on serum lipid levels and blood pressure in normal men. N Engl J Med. 1993;328(9):603-607.
2. Berryman CE, Grieger JA, West SG, et al. Acute consumption of walnuts and walnut components differentially affect postprandial lipemia, endothelial function, oxidative stress, and cholesterol efflux in humans with mild hypercholesterolemia. J Nutr. 2013;143(6):788-794.
3. Pan A, Sun Q, Manson JE, Willett WC, Hu FB. Walnut consumption is associated with lower risk of type 2 diabetes in women. J Nutr. 2013;143(4):512-518.
4. Valls-Pedret C, Lamuela-Raventos RM, Medina-Remon A, et al. Polyphenol-rich foods in the Mediterranean diet are associated with better cognitive function in elderly subjects at high cardiovascular risk. J Alzheimers Dis. 2012;29(4)773-782.
5. Muthaiyah B, Essa MM, Chauhan V, Chauhan A. Protective effects of walnut extract against amyloid beta peptide-induced cell death and oxidative stress in PC12 cells. Neurochem Res. 2011;36(11):2096-2103.
6. Robbins WA, Xun L, Fitzgerald LZ, Esguerra S, Henning SM, Carpenter CL. Walnuts improve serum quality in men consuming a Western-style diet: randomized control dietary intervention trial. Bio Reprod. 2012;87(4):101.