A systematic review and meta-analysis from the Harvard T.H. Chan School of Public Health examined 25 years of evidence for the role of walnut consumption on cardiovascular risk factors, including cholesterol, triglycerides, blood pressure, and weight.5 A meta-analysis was done on 26 randomized controlled trials representing 1059 individuals (ages 22-75), including those with a variety of conditions such as high cholesterol, type 2 diabetes, metabolic syndrome, overweight or obesity, as well as those that were healthy. A diet supplemented with walnuts in amounts varying from 5-24 percent of total calories per day (equivalent to 0.5-3.9 ounces per day) showed a significantly greater reduction in total cholesterol on average with the walnut-enriched diets – specifically, this difference represents a 3.25% greater decrease in total cholesterol concentration in walnut-enriched diets compared with control diets. Greater reductions were also observed for LDL cholesterol (3.73%), triglycerides (5.52%), and apolipoprotein B (4.19%) when compared to the control diets, including low-fat, Mediterranean, American or Japanese. In addition, incorporating walnuts into the diet had no adverse effects on body weight or blood pressure, according to the studies included in the meta-analysis. A meta-analysis offers a comprehensive look at findings among patients of various backgrounds, however, it can be limited by the methods, reported outcomes and quality of the individual studies involved.
In a separate epidemiological study by researchers from the Harvard T.H. Chan School of Public Health, they found higher walnut consumption – both in terms of the amount and frequency – may be associated with a lower risk of death and an increase in life expectancy among older adults in the U.S., compared to those who do not consume walnuts.6 Study participants included over 67,000 women of the Nurses’ Health Study (1998–2018) and some 26,000 men of the Health Professionals Follow-up Study (1998–2018) who had their dietary assessment taken via self-reported food frequency questionnaire. Both groups were on average around 63 years old and free of cancer, heart disease, and stroke at baseline. Researchers found that participants who reported eating five or more servings of walnuts per week (one serving = one ounce) had a 14% lower risk of death (from any cause), 25% lower risk of dying from cardiovascular diseases, and a gain in about 1.3 years of life expectancy, compared to participants who reported no walnut consumption. Results from the prospective observational study does not prove cause and effect, but it does shed light on how walnuts may support an overall healthy lifestyle that promotes longevity. Future research is needed to confirm the longevity benefits of eating walnuts in more diverse groups of individuals and these results cannot be applied to the general population.
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Research from the landmark PREvención con DIeta MEDiterránea (PREDIMED) study further demonstrated the potential heart health benefits of walnuts. The study was conducted among more than 7,000 Spanish individuals (ages 55-80) at high risk for cardiovascular disease and found that a Mediterranean diet supplemented with mixed tree nuts (primarily walnuts) was associated with a lower risk of cardiovascular events, including cardiovascular death, myocardial infarction (heart attack), and stroke, when compared to a low-fat control diet.7 It is important to note that the amount of walnuts the participants in this study ate was relatively large and might be tough to be consistent with in a real-world setting. In the PREDIMED study, it is also difficult to precisely define what part of the Mediterranean diet was associated with cardiovascular benefits.
