High levels of serum estrogens and androgens promote premature menarche, premenstrual symptoms , menstrual pain , polycystic ovary syndrome, hirsutism, menopausal syndrome, and reproductive system (breast, ovarian, etc.) cancers in women .
Asian women and women eating plant-based diets excrete more estrogens via the stool than omnivores, and can have 15-30% lower estrogen levels than women eating omnivorous diets, along with lower risks of all of the conditions above [4 , 5 , 6 ].
The Diet and Androgens (DIANA) trial produced a reduction of insulin resistance, a 4 kg weight loss, a small decrease in estrodiol, and a 20% decrease in serum testosterone in women who adhered to a very low fat, plant-based diet with high intake of whole grains, flaxseed and legumes, and minimal intake of animal products over the course of 4.5 months .
Nowak et al reported a dramatic decrease in testosterone levels and hirsutism in a PCOS patient who consumed 30 G daily of flaxseed for 4 months [8 ].
Barnard et al reported a decrease in body weight, duration and intensity of premenstrual symptoms, water retention, and duration and intensity of menstrual pain in 33 women who ate a low fat, vegetarian diet for two menstrual cycles [9 ].
Bagga et al reported a significant decrease in serum estrogens with no adverse effects on menstrual cycling among women who ate an ad libitum diet supplying 10% of energy from fat and 25-35 g of fiber daily [10 ].
Excess stored iron appears to play a role in PCOS and insulin resistance , and we have evidence suggesting a role in breast cancer . Meat provides more bioavailable iron (heme iron) than plant foods, and oral contraceptives reduce iron losses; each of these could contribute to excessive stored iron.
I think the literature gives a pretty clear picture. Women eating diets low in fat and animal products, and high in whole plant foods, generally have lower estrogen and testosterone levels and lower incidences of hormone-related diseases, compared to women eating diets high in fat and animal products. Certain plant foods, e.g. flaxseed, legumes, whole grains, appear particularly effective at increasing elimination of excess estrogens and androgens. Clinical trials show that reducing fat and animal products and increasing plant foods rich in fibers will for many women result in reduced hormone levels. A reduction in hormone levels will generally result in reduced premenstrual discomfort and dysphoria, menstrual pain, fibrocystic changes, and risk of hormone-related cancers.
From an evolutionary perspective, women performed most of the gathering, and while gathering, they probably nibbled on what they were gathering, so I would suspect that they consumed considerable amounts of plant foods on a daily basis, including various seeds, nuts, fruits, and even fresh legumes (wild peas and beans) containing phytoestrogens, just like other primates.
So here's my hypothesis: I think it probable that human metabolism is adapted to regular intake of phytoestrogens, meaning that our hormone production pattern proceeds on the genetically built-in premise that phytoestrogens and other plant chemicals will be part of the diet, altering hormone levels. In other words, our hormone production proceeds with the assumption that phytochemicals will be draining out some of our hormones; its production is adapted to a drain that was constant in the millions of years that our ancestors ate plant-based diets. When we adopt diets that lack these compounds, while simultaneously increasing intake of other compounds (animal protein and fat) that feed the cholesterol and steroid production system, our own production of hormones becomes excessive, producing disease.