Other nuts mostly contain monounsaturated fatty acids, but walnuts are primarily composed of polyunsaturated fatty acids (PUFA) and contain significant amounts of alpha-linolenic acid (ALA), the plant-based source of omega-3 fatty acid.
Walnuts are also rich in other nutrients, such as folate, which helps reduce the risk of central nervous system defects in unborn babies, and vitamin E, which helps form hemoglobin, the protein that carries oxygen in the blood.
However, they are also a high-calorie food, prompting concerns that long-term intake may lead to weight gain. In this article, we will investigate a number of studies demonstrating the pros and cons of a diet that includes walnuts.
Here are some key points about walnuts. More detail and supporting information is in the main article.
Walnuts, cholesterol, and heart health
Some evidence infers that walnuts might improve heart health.
According to the Centers for Disease Control and Prevention (CDC), an estimated 73.5 million adults in the United States (31.7 percent of the adult population) have high levels of LDL cholesterol.
But, less than half (48.1 percent) are receiving treatment to lower their levels, putting themselves at increased risk of heart disease and other cardiovascular conditions.
Preliminary findings from an ongoing study presented in April at the Experimental Biology 2016 meeting in San Diego, CA, indicates that the daily consumption of walnuts has a positive effect on cholesterol levels, without increasing body weight.
The researchers studied data on 514 older adults, with an average age of 69, participating in the Walnuts and Healthy Aging (WAHA) study.
Around half the participants were asked to add a handful of walnuts to their daily diet, representing 15 percent of their daily calorie intake, while the other half were asked to consume their usual daily diet without nuts.
After 1 year, the team found that participants on the walnut diet had significantly lower levels of LDL cholesterol than those consuming their usual diet. There were no significant differences between the groups regarding changes in body weight and levels of "good" HDL cholesterol.
"Acquiring the good fats and other nutrients from walnuts while keeping adiposity [body fat] at bay and reducing blood cholesterol levels are important to overall nutritional well-being of aging adults. It's encouraging to see that eating walnuts may benefit this particular population."
Dr. Emilio Ros, lead researcher, Hospital Clinic of Barcelona, Spain
A number of other studies have also indicated that a daily helping of walnuts can benefit cardiovascular health.
Research, published in BMJ Open Diabetes Research & Care, looked at individuals, aged 25-75, who were at high risk of diabetes. One group of participants were asked to include 56 grams of walnuts in their daily diet; the second group excluded walnuts.
After 6 months, the team found that the functioning of the blood vessel wall (endothelium) had significantly improved, and LDL cholesterol was reduced in the walnut-eating group.
However, the team also reported that consuming walnuts had no significant effect on blood pressure, blood glucose, or HDL cholesterol levels. It was also reported that when the walnut-eating group did not receive dietary counseling on how to adjust calorie intake, it led to significant increases in body fat.
Further insight comes from a review published in 2015 that used data from 61 trials involving more than 2,500 people. Published in the American Journal of Clinical Nutrition, the authors concluded that the consumption of tree nuts, including walnuts, was associated with a reduction in cardiovascular risk factors, such as LDL cholesterol and triglycerides levels.
"Our study results further support the growing body of research that tree nuts, such as walnuts, can reduce the risk of cardiovascular diseases."
Dr. Michael Falk, review author, Life Sciences Research Organization, Bethesda, MD.
More health benefits
As well as the potential cardiovascular benefits of a diet rich in walnuts, eating these tree nuts in moderation may be associated with a variety of other beneficial health outcomes, according to research results.
A study, published in Biology of Reproduction, found that eating walnuts was associated with an improvement in several indicators of sperm health.
In total, 117 healthy men, aged 21-35, who consumed a Western-style diet, were divided into two groups. The first group ate 75 grams of walnuts a day for 12 weeks while the second group was asked to avoid walnuts. At the end of the study period, the researchers found that men assigned to eat walnuts had significantly increased sperm vitality, motility, and morphology compared with those in the nut-free group.
However, it is not known whether eating walnuts would benefit young men with fertility problems, said lead researcher Wendie Robbins, from the University of California.
Walnuts might reduce the risk of pancreatic cancer.
Research, published in the British Journal of Cancer, took data from more than 75,000 women in the Nurses' Health Study. The findings suggest that a 28 gram serving of nuts, including tree nuts such as almonds, Brazil nuts, and walnuts, at least twice a week, is linked to a significant reduction in the risk of pancreatic cancer.
Lead author, Ying Bao of Brigham and Women's Hospital and Harvard Medical School, Boston, MA explains:
A 2015 study conducted in mice, and published in The Journal of Nutritional Biochemistry, suggests that a diet containing walnuts may slow colon cancer growth by causing beneficial changes in cancer genes.
In 2014, Dr. Lenore Arab, from the David Geffen School of Medicine at The University of California, published a study in The Journal of Nutrition, Health & Aging, indicating that walnut consumption may improve mental ability.
The team used data from multiple National Health and Nutrition Examination (NHANES) surveys of U.S. adults aged 20-90. They found that an increased daily intake of walnuts was associated with improved performance on six tests of mental ability including memory, concentration, and information processing speed. The authors conclude:
"These significant, positive associations between walnut consumption and cognitive functions among all adults, regardless of age, gender, or ethnicity suggest that daily walnut intake may be a simple, beneficial dietary behaviour."
The possible beneficial effects of eating walnuts on brain function were also identified in a 2014 study on mice specifically bred to model the progression of Alzheimer's disease.
Dr. Abha Chauhan, from the New York State Institute for Basic Research in Developmental Disabilities, found that animals fed a walnut-rich diet showed significant improvements in learning skills, memory, anxiety levels, and other measures of brain function compared with mice that were not fed walnuts.
These findings follow results from a previous cell study by Dr. Chauhan, which suggested that walnut extract may protect against the damaging effects of amyloid beta protein. This protein is the main constituent of amyloid plaques, which form in the brains of people who develop Alzheimer's disease.
"These findings suggest that dietary supplementation with walnuts may have a beneficial effect in reducing the risk, delaying the onset, or slowing the progression of, or preventing AD [Alzheimer's disease]," the researchers comment in the Journal of Alzheimer's Disease.
Although walnuts are a nutritionally-rich food, they are also high in calories and have the potential to cause weight gain if eaten in large quantities. Therefore, they should be consumed in moderation as part of a healthy lifestyle to avoid any possible adverse effects.
Also, as with most types of nut, walnuts have the potential to cause severe, or even fatal, allergic reactions and should be strictly avoided by people with nut allergies.
There is a growing body of research suggesting that a healthy plant-based diet comprising moderate amounts of walnuts has beneficial effects on cholesterol levels and heart health, and may improve brain ability and protect against a variety of conditions.
However, it should also be pointed out that many of the studies reporting the beneficial effects of walnuts were supported by interested parties, such as the California Walnut Commission, and that further research is needed to confirm these possible health benefits.