Estrogen levels are directly related to the risk of developing breast cancer, a relationship we've known about for more than 100 years, according to a January 2001 article in the “New England Journal of Medicine.” Research investigating what causes relatively high or low levels of estrogen ranges from ethnic diversity to diet, including vegetarianism and veganism.
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One study, published in the “British Journal of Cancer” in 1999, dismissed correlations between vegetarian or vegan diets and lower estrogen levels because the data had not been corrected for differences in BMI, or body mass index. The researchers stated that the statistically significant lower body weight – and therefore BMI – of vegetarians and vegans explained the lower estrogen levels, rather than any specific nutrients either eaten or not eaten by vegetarians and vegans. That being said, regardless of whether it was something specific to the diet or simply the fact that vegans tend to have lower body weights, there was clearly a lower level of estrogen in vegetarians and especially in vegans.
At least two studies have investigated the impacts of insoluble fiber on reducing estrogen. Research published in November 1988 in “Nutrition Research” and May 1991 in the "Journal of Steroid Biochemistry and Molecular Biology” both found that insoluble fiber from both natural fiber like oat bran or wheat bran and purified fiber like cellulose or lignan removed an average of 73 percent of estrogen from the test solution, with linseed fiber removing as much as 91 percent. Although it is arguable that vegans benefit more than omnivores from this action because they eat more plant foods and therefore more fiber, omnivores who actively add more fiber to their diet should benefit as well.
Vegetarian Vs. Vegan
Overall, vegans have statistically lower body weight and BMI than vegetarians, who are statistically lower than omnivores. Vegans also have statistically lower estrogen levels and incidence of breast and other cancers than vegetarians, who, again, are statistically lower than omnivores. It seems likely that both the higher fiber intakes and lower body weights that are common among vegetarians and especially among vegans are protective against high estrogen levels as well as cancer.
Dietary Fat and Cholesterol
It bears mentioning that diets high in either or both dietary fat and/or cholesterol are associated with increased risk of nearly every type of cancer as well as heart disease. Excess body weight, which is associated with a high-fat diet, is another risk factor for cancer and heart disease, as well as elevated estrogen. Vegetarians tend to eat a diet that is lower in dietary fat and cholesterol than a typical omnivorous diet, and the typical vegan diet is even lower in fat while being completely free of cholesterol.