An updated systematic review from the Harvard T.H. Chan School of Public Health examines 25 years of evidence for the role of walnut consumption on cardiovascular risk factors, including cholesterol, triglycerides, blood pressure, and weight.1 The original meta-analysis, “Effects of walnut consumption on blood lipids and other cardiovascular risk factors: a meta-analysis and systematic review,” was published in 2009 and reviewed 13 trials representing 365 individuals.2 Researchers evaluated clinical trials that have published since then and released an updated review that now includes twice the number of trials and represents about three times the number of individuals, compared to the initial publication. Findings from the meta-analysis suggest that walnut-enriched diets may lead to significantly greater reductions in total cholesterol, LDL cholesterol, triglycerides, and apolipoprotein B, compared to control diets.
Researchers examined 26 randomized controlled trials representing 1059 participants (22-75 years old), 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. Walnut-enriched diets varied in amounts ranging from 5-24 percent of total calories per day (equivalent to 0.5-3.9 ounces per day) and were compared to control diets, including low-fat, Mediterranean, or a traditional American or Japanese diet. When compared to control diets, a diet supplemented with walnuts resulted in a significantly greater percent decrease in total cholesterol (3.25%), LDL cholesterol (3.73%), triglycerides (5.52%), and apolipoprotein B (4.19%). (Apolipoprotein B is the primary protein found in LDL cholesterol.) 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.
Walnuts have been investigated for their potential benefits on a variety of health outcomes, including cancer, gut health, diabetes, cognitive function, and male reproductive health, but the strongest evidence exists for cardiovascular benefits. Walnuts are recognized by the American Heart Association and U.S. Food and Drug Administration as a heart-healthy3 food, and there are a number of properties in walnuts that may be responsible for health benefits. Walnuts are a rich source of recommended polyunsaturated fat (13 grams per ounce), which includes an excellent source of the essential omega-3 fatty acid alpha-linolenic acid (2.5 grams per ounce). They also offer a variety of antioxidants (3.721 mmol/oz), including polyphenols (69.3 ± 16.5 μmol catechin equivalents/g) and gamma tocopherol (5.91 mg/ounce).4,5
As with any scientific research, some study limitations should be considered. Most of the trials in this review had relatively small sample sizes, which could limit the ability to determine significant effects. Additionally, in some cases, the amount of walnuts consumed in the trials was relatively large and might be difficult to maintain in a non-research setting. However, researchers still saw significant benefits when lower amounts of walnuts were consumed (less than 28 grams per day), particularly with total and LDL cholesterol.
1 Guasch-Ferré M, Li J, Hu FB, et al. Effects of walnut consumption on blood lipids and other cardiovascular risk factors: an updated meta-analysis and systematic review of controlled trials [published online ahead of print June 21, 2018]. Am J Clin Nutr. doi: 10.1093/ajcn/nqy091
2 Banel HK, Hu FB. Effects of walnut consumption on blood lipids and other cardiovascular risk factors: a meta-analysis and systematic review. Am J Clin Nutr. 2009;90(1):56-63.
3 Supportive but not conclusive research shows that eating 1.5 ounces of walnuts per day, as part of a low saturated fat and low cholesterol diet, and not resulting in increased caloric intake may reduce the risk of coronary heart disease. One ounce of walnuts provides 18g of total fat, 2.5g of monounsaturated fat, 13g of polyunsaturated fat, including 2.5g of alpha-linolenic acid, the plant-based omega-3.
4 Halvorsen BL, Carlsen MH, Phillips KM, et al. Content of redox-active compounds (ie, antioxidants) in foods consumed in the United States. Am J Clin Nutr. 2006;84:95-135.
5 The data for antioxidant capacity of foods generated by test-tube methods cannot be extrapolated to human effects. Clinical trials to test benefits of dietary antioxidants have produced mixed results.
