Emerging research has found that certain foods contribute positive changes to the gut microbiome. Evidence suggests walnuts may be one food to consider due to their prebiotic potential, which may promote the growth of beneficial bacteria, as well as their possible role in a variety of other associated health benefits.
You’ll find more information below about the scientific research completed so far.
A study from the USDA and University of Illinois, published in The Journal of Nutrition, found that walnut consumption was associated with positive changes to the gut microbiome. 1 A small sample of 18 healthy adults (ages 35-68) who ate 42 grams (about 1.5 ounces) of walnuts each day for 3 weeks experienced a decrease in secondary bile acids, which may play a role in colon cancer, inflammation, and gastrointestinal diseases. The study also found that eating walnuts resulted in an increase in gut bacteria that is thought to be beneficial for health.
Another study, published in Nutrients, found that consuming a walnut-enriched diet positively impacted the gut microbiome by enhancing good probiotic- and butyric acid-producing bacteria.2 Butyric acid is thought to be useful for digestive health by helping to maintain the health of the colon. The study included 194 healthy German adults (mean age of 63 years old) randomized into two different diet phases, each lasting for eight weeks. One group followed a walnut-enriched diet that included 43 grams (about 1.5 ounces) of walnuts per day and then switched to a nut-free diet. The other group followed the diets in reverse order. During the walnut diet, participants were also randomized to reduce their intake of carbohydrates, fat, or both under the advisory of a nutritionist.
Larger and longer-term studies are needed to clarify these effects in a broader population, and future research is needed to understand how specific bacterial species may be associated with favorable health effects, such as heart health.
Evidence from two animal studies also suggest an association between walnut consumption and a healthier gut.
A study published in The Journal of Nutritional Biochemisty found that walnut consumption may be beneficial for digestive health by increasing the amount of probiotic-type bacteria in the gut.3 In this study, rats were randomly assigned to a diet containing ground walnuts, equivalent to about two ounces per day in humans, or a diet without walnuts for up to 10 weeks. Calorie and nutrient intake was similar between the two groups. Compared to those that did not consume walnuts, rats that ate a walnut-enriched diet saw an increase in beneficial bacteria including, Lactobacillus, Roseburia, and Ruminococcaceae.
Another animal study published in Cancer Prevention Research found that eating walnuts may modify gut bacteria in mice to support colon health, which is considered when looking at the ability to protect against colon tumors.4 Researchers incorporated walnuts into two different diets: a standard mouse diet which is widely used in animal studies, and a Western diet representing typical American intake. Male mice fed the Western diet with 10.5 percent of total calories from walnuts, translating to just over one ounce of walnuts in a human diet, showed a significant reduction in the number and size of tumors. There was also a modest reduction in the number and volume of tumors in mice that consumed the standard diet with 15 percent of total calories from walnuts.
These animal studies provide background that can be used to inform future studies needed to understand the effect on humans. Researchers studied mice that had colon cancer, which may have altered the normal function of the gut microbiome.
1 Holscher HD, Guetterman HM, Swanson KS, et al. Walnut Consumption Alters the Gastrointestinal Microbiota, Microbially Derived Secondary Bile Acids, and Health Markers in Healthy Adults: A Randomized Controlled Trial. J Nutr. 2018;148(6):861-867. Doi: 10.1093/jn/nxy004.
2 Bamberger C, Rossmeier A, Lechner K, et al. A Walnut-Enriched Diet Affects Gut Microbiome in Healthy Caucasian Subjects: A Randomized, Controlled Trial. Nutrients. 2018;10(2): 244. doi:10.3390/nu10020244.
3 Byerley LO, Samuelson D, Blanchard E, et al. Changes in the Gut Microbial Communities Following Addition of Walnuts to the Diet. J Nutr Biochem. 2017. doi: 10.1016/j.jnutbio.2017.07.001.
4 Nakanishi M, Chen Y, Qendro V, et al. Effects of walnut consumption on colon carcinogenesis and microbial community structure. Cancer Prev Res (Phila). 2016;9(8):692-703. doi: 10.1158/1940-6207.
