As gut health continues to be a topic of conversation surrounding health and wellness, consumers are looking for simple ways to support their gut and ultimately their overall health. While there is still so much we don’t know about the gut microbiome, one thing is relatively certain: the foods we eat have an impact on the health and diversity of the community of beneficial bacteria living in our gut otherwise known as the gut microbiome.
When it comes to food, research has shown that consuming walnuts may help promote gut health. A study from the USDA and University of Illinois found that a small sample of 18 healthy adults (ages 35-68) who ate 42 grams (about 1.5 ounces) of walnuts each day for three 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 seemed to result in an increase in gut bacteria that is thought to be beneficial for health.1 While these results are promising, more research is needed to explore the relationship between cause and effect. Walnuts also provide a variety of other nutrients per one ounce serving. They are the only nut to provide an excellent source of the anti-inflammatory omega-3 alpha-linolenic acid, or ALA (2.5g/oz), plus 2 grams of fiber and a mix of tocopherols (5.91mg/oz) and antioxidant polyphenols (69.3 ± 16.5 µmol catechin equivalents/g).2,3,4,5,*
Now new evidence suggests that walnuts may have prebiotic potential.6 Prebiotics have been shown to promote the growth of beneficial gut bacteria.7 This evidence was reported in two recently published review papers in Nutrients and Antioxidants, which explored the relationship between walnuts and a compound called urolithin A (UA). UA is produced by gut bacteria and may play a role in the gut microbiome and personalized, precision health benefits.
In a 2023 review published in Nutrients, researchers summarized findings surrounding nut consumption and its impact on the gastrointestinal system. Specific to walnuts, the review showed a beneficial role this nut may play in supporting beneficial gut bacteria, related to composition and diversity. While the findings are promising, the review ultimately showed inconsistent results associated with improved health benefits related to critical gut bacteria such as microbial alpha- and beta-diversity.8
A second 2023 review, published in Antioxidants and comprised of 33 studies, investigated the evidence supporting the benefits of walnuts in relation to inflammation and overall disease risk. The findings suggest that walnuts, as part of a healthy diet, may help reduce inflammation and promote normal digestive function, due in part to their unique nutrient matrix and metabolites produced during digestion. This paper suggests much of the beneficial properties of walnuts associated with anti-inflammatory effects are linked to ellagitannins, or polyphenols found in walnuts which are ultimately metabolized by gut microbiota to bioactive, anti-inflammatory urolithins as discussed above.6
Researcher and Professor of Medicine at the University of Connecticut School of Medicine, Dr. Daniel W. Rosenberg, Ph.D. shares, “Evidence that microbial metabolism of walnut-derived ellagitannins is highly patient-dependent has prompted our NIH-funded clinical trial to study ellagic acid metabolism in subjects at elevated risk of colorectal cancer. Our goal is to better understand how two ounces per day of walnuts added to the regular diet may affect an individual’s microbiome and to define the specific microbes responsible for these metabolic reactions required to form urolithins.”
While most of the research efforts have largely focused on Western populations, there are some studies assessing Type 2 diabetes microbiota associations in Middle Eastern communities where Type 2 diabetes prevalence is over 20%.9 The rates of Type 2 diabetes in the Middle East have risen dramatically in the last several decades due to socio-economic changes resulting in changes in dietary and sedentary lifestyles. This emergence has grown more rapidly and affects larger proportions of the population with estimates of Type 2 diabetes prevalence impacting 25% of the population. Walnuts are easy to fit in Middle Eastern meals and daily consummate of walnuts may help reduce to risk of diabetes and improve gut health in the region.
Walnuts offer a simple, cost-effective (and delicious!) strategy for providing a wide range of potential health benefits that may improve gut bacteria and overall gut health. As research continues to evolve in this space, it can’t hurt to give your gut some love with walnuts!
For simple ways to incorporate walnuts in your eating pattern try gut health-promoting recipes like this fiber-rich Toasted Walnut, Black Bean, Corn and Tomato Salad or this Steak Sandwich with Walnut Kimchi Slaw.
For more information on how walnuts can contribute to overall gut health check out this Gut Health Research Summary or this Digest the Latest on Gut Health and Walnuts blog post and be sure to read our Cancer Research Summary for information surrounding that topic.
California Walnut Commission
The California Walnut Commission, established in 1987, is funded by mandatory assessments of the growers. The CWC represents over 4,500 growers and over 90 handlers (processors) of California walnuts in export market development activities and conducts health research. The CWC is an agency of the State of California that works in concurrence with the Secretary of the California Department of Food and Agriculture (CDFA).
1Holscher 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, 861
2Dietary reference intakes for energy, carbohydrate, fiber, fat, fatty acids, cholesterol, protein, and amino acids (Macronutrients) (2005) NAS. IOM. Food and Nutrition Board.
3Prior R. Oxygen radical absorbance capacity (ORAC): New horizons in relating dietary antioxidants/bioactives and health benefits. Journal of Functional Foods. 2015;18(B):797-810
4U.S. Department of Agriculture and U.S. Department of Health and Human Services. Dietary Guidelines for Americans, 2020-2025. 9th Edition. December 2020. Available at DietaryGuidelines.gov.
5Halvorsen 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.
6Fan N, Fusco JL, Rosenberg DW. Antioxidant and anti-inflammatory properties of walnut constituents: Focus on personalized cancer prevention and the microbiome. Antioxidants. 2023;12(5):982.
7Davani-Davari D, Negahdaripour M, Karimzadeh I, Seifan M, Mohkam M, Masoumi SJ, Berenjian A, Ghasemi Y. Prebiotics: Definition, Types, Sources, Mechanisms, and Clinical Applications. Foods. 2019 Mar 9;8(3):92. doi: 10.3390/foods8030092.
8Mandalari G, Gervasi T, Rosenberg DW, Lapsley KG, Baer DJ. Effect of nuts on gastrointestinal health. Nutrients. 2023;15(7):1733.
9Amein Al Ali, Alexa Dodwell, Abdulmohsen Al Elaq, Fahad Abdulaziz Al-muhanna; gut microbiota analyses of Saudi populations for type 2 diabetes related phenotypes, October 2021
* 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.
~ Animal studies are valuable for providing background information and can be used as a basis for additional research. Since mice have different microbiota than humans, more research is needed to determine how these results may translate to humans.