An animal study published in Cancer Prevention Research found that eating walnuts could modify gut bacteria in a way that is beneficial to colon health, and may be associated with colon tumor suppression.³ In the study, researchers incorporated walnuts into two different diets, a standard mouse diet supplemented with 0, 15, 22.5, or 30.2 percent of calories from walnuts, and a Western diet, representing typical American intake, supplemented with 0, 5.2, 10.5, or 21.4 percent of calories from walnuts. Calories from fat sources were proportionally lowered in each diet to compensate for the addition of walnuts. Male mice fed the Western diet with 10.5 percent of total calories from walnuts, which translates to just over one ounce of walnuts in a human diet, showed a significant reduction in the number and size of tumors.

Animal research conducted at the Beth Israel Deaconess Medical Center and Harvard Medical School demonstrated that walnuts in the diet may inhibit colorectal cancer growth in mice by suppressing angiogenesis.⁴ In this study, mice with human colon cancer cells were randomized to diets containing approximately 19 percent of total energy from corn oil (control diet), flaxseed oil or ground walnuts (approximately 2 ounces of walnuts in a human diet). Compared with the corn oil diet, mice fed the walnut diet (equivalent to 2 servings of walnuts per day in humans) or flaxseed diet exhibited significantly slower tumor growth rates and lower tumor weights. The differences between walnut and flaxseed diets did not reach statistical significance. However, compared to the control-fed mice, consumption of walnuts significantly decreased angiogenesis which may be beneficial against the progression of colorectal cancer.