While some people believe that soy triggers weight gain stemming from its plant-based estrogen content, very little is known about how much of an impact soy estrogen has on human estrogen levels. In fact, there's some evidence that including soy in your diet might actually prevent weight gain. If you're concerned about including soy in your diet, consult a professional for personalized diet recommendations.
Soy estrogens, or soy isoflavones, have weak estrogenic activity, which means their chemical structure mimics that of human estrogen, so they're "recognized" as estrogen-like in your body. However, it's not clear exactly how these estrogens affect your cells; they might boost the effects of estrogen, or they may block estrogen from communicating with your cells, essentially reducing estrogen function. Soy estrogens offer several health benefits, including bone health, possible protection against breast cancer, relief from menopausal symptoms and reduced risk of heart disease. The level of soy estrogens you get from your diet doesn't pose a health risk, according to the Linus Pauling Institute. However, it's not clear if soy estrogen supplements pose a health risk; if you plan to take them, first consult your doctor.
Soy Estrogen and Weight Gain
The soy estrogen weight-gain theory may be related to the the belief that the plant-based estrogen affects thyroid hormone levels that can lead to hypothyroidism, an illness associated with weight gain. However, according to LPI, high intakes of soy estrogen do not increase your risk of hypothyroidism. But people with hypothyroidism may need to eliminate soy foods from their diet to prevent interactions with medication.
Estrogen is stored in fat; higher body fat may cause higher estrogen levels, so maintaining a healthy weight, which you could achieve by including low-calorie soy in your diet, will help balance your estrogen levels.
Soy and Healthy Weight
Soy foods and soy estrogen may help you maintain a healthy weight; epidemiological evidence indicates people with a higher intake of soy have lower body mass index, according to a 2008 study published in the European Journal of Nutrition. A 2015 animal study, also published in the European Journal of Nutrition, investigated the effects of a soy estrogen-rich diet on weight in rats. The researches found that the soy reduced the risk of obesity in the rats and preserved lean body mass. Of course, human studies are necessary to determine a correlation.
Get the benefits of soy with a few tweaks to your diet. Use soy milk for your morning cereal or coffee, add edamame to your salad at lunch, and snack on roasted soy nuts in the afternoon. Use tofu in your next stir-fry.
Weight Gain Basics
Although genetics plays a role, diet and activity level also help determine weight. Eating more calories than your body burns causes weight gain, not a high intake of soy estrogen. Poor-quality foods -- soda, fast food, cake, cookies and fried foods -- lead to weight gain because they increase overall caloric intake. Fruits, vegetables, whole grains and healthy sources of protein such as soy are filling and help you lose weight by assisting you in eating fewer calories.