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June 29, 2016 | 3 minute read

Walnuts May Alter Microbiome, Suppress Colon Tumors in Mice

A new study conducted in mice finds that eating walnuts appears to alter their gut bacteria and suppress colon tumors, especially in male mice at certain concentrations.

Published in Cancer Prevention Research, the study was supported in part by the American Institute for Cancer Research and California Walnut Commission.

Earlier research has suggested that the Mediterranean diet — which includes nuts — and walnuts by themselves may play some role in lowering cancer risk. Walnuts contain polyunsaturated fatty acids omega-3s and high levels of gamma-tocopherol, a form of vitamin E well studied in lab cancer research.

In one experiment, researchers added different concentrations of ground walnuts to the diet of a group of mice. A comparison group ate a standardized diet — no walnuts — and fat content was equalized among the diets.  Here, the group consuming the lowest concentration of walnuts — 15 percent of calories from the nuts — showed a slight decrease in colon tumor growth compared to the non-walnut eating group. The effect was most pronounced in male mice. Higher concentrations of walnuts did not show added tumor suppression.

A second mouse experiment added ground walnuts to mice fed a diet similar to a Western Diet.  The male mice consuming 7 percent walnuts by weight — 10.5 percent total calories — had 2.3 times fewer tumors when compared to mice fed no walnuts. That’s similar to a person eating about an ounce of walnuts a day. There was little effect on tumor size, suggesting the effect may relate to tumor initiation. Again, higher concentration of walnuts made no difference.

When the researchers tested DNA in fecal samples from the mice, they saw increased levels and variety of certain groups of bacteria compared to the non-walnut eating group. At each concentration of walnuts tested, there was a marked increase in the diversity and richness of the gut microbiota, the study reports. This reshaping of the microbiome may potentially play a role in suppressing tumors.  Previous research has shown that certain gut bacteria metabolize phytochemicals and fiber into compounds that have tumor-suppressing properties.

Here again, there were differences between the genders. Males on walnut-free diets tended to have less bacterial diversity than females. Adding walnuts to the diets of male mice brought increased diversity to their microbiomes, making them closer to those of female mice on either of the diets.

Whether this change contributes to the protection seen in male mice is unknown. More research is needed both to confirm these findings and better understand the mechanisms.

Source: Nakanishi M, Chen Y, Qendro V, Miyamoto S, Weinstock E, Weinstock GM, Rosenberg DW. Effects of walnut consumption on colon carcinogenesis and microbial community structure. Cancer Prev Res (Phila). 2016 May 23. [Epub ahead of print]

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