Can soy increase the length of a woman's menstrual cycle or affect PMS?

By Monique N. Gilbert, B.Sc. Bachelor of Science degree, Certified Personal Trainer/Fitness Counselor, Health Advocate, Recipe Developer, Soy Food Connoisseur and the author of: Virtues of Soy: A Practical Health Guide and Cookbook. This is an archived Q&A. The following information might not be up-to-date.

Question: Hi there, I was reading the info on vegweb about the soy and I am also wondering about the estrogen effects of soy. I eat soy a lot, I'd say I eat tofu about once a day, soy milk twice a day, I eat steamed soy beans twice a week...you get the picture. I've noticed lately (past two years) that my periods are off and my cycle has gone from 28 to almost a 40 day cycle. I'm 26 years old and have never had this sort of problem, not to mention the PMS ... its never been so horrible and painful. Could this be related to my increase in soy? How much soy is too much? Thanks, Colleen

Answer: Yes, a daily intake of soy protein can produce a significant change in the length of a woman's menstrual cycle. A British study found that women who were given 60 grams of soy a day had longer cycles. Asian women, who ingest more soy on a daily basis, generally have longer menstrual cycles than American and European women. The longer a woman's cycle, the better.

Soy foods contain phytoestrogens (isoflavones) that have similar properties to human estrogen, but are much weaker. Soy's natural plant estrogen helps regulate hormone levels. Isoflavones bind to the body's estrogen receptors and supplement the effects of estrogen when levels are low. This can help prevent breast cancer and relieve many premenstrual symptoms.

Estrogen is produced during the menstrual cycle. Longer cycles lead to less exposure to estrogen during a woman's lifetime, putting her at a lower risk of breast cancer than women with shorter cycles. The more often a woman menstruates, the greater her exposure to estrogen. This is why women who start menstruating early and stop late, who have short menstrual cycles, or never had children are believed to have an increased risk of breast cancer.

Premenstrual syndrome (PMS) is generally a result of the fluctuating estrogen levels each month in the days just before the start of the menstrual cycle. Symptoms generally begin from 7 to 14 days before menstruation, and end within 24 hours after the cycle has begun. As the estrogen level drops in the second two weeks of a woman's cycle, the body starts accumulating fluid. This usually leads to weight gain and breast tenderness. To reduce the fluid buildup, nutritionists suggest to drink plenty of water, reduce the amount of salt intake, and increase the amount of fruits, vegetables and soy products in the diet.

The best forms of soy are those with the highest amount of isoflavones in them, like whole soybeans, tempeh, textured soy protein, soynuts and some soy protein powders. The next best would be tofu, soymilk, miso and various ready-made products. However, the actual isoflavone content has to be high enough to produce positive effects. Some processed soy foods made from soy protein concentrate, like soy hot-dogs, have very little isoflavones due to their processing method. Other products, such as soybean oil and soy sauce, contain no isoflavones in them at all.

Consuming soy products that are high in calcium and magnesium, like tofu and soymilk, can also help alleviate many PMS symptoms. The American Journal of Obstetrics and Gynecology reported a study which found that 1,200 milligrams of calcium taken daily reduced the physical and psychological symptoms associated with PMS by almost 50 percent. Another study reported in the Journal of Women's Health found that 200 milligrams of magnesium a day produced a 40 percent reduction in fluid retention, breast tenderness and bloating.

I would also suggest that you keep track on a calendar, the day you start your period. After several months to a year you will see a pattern forming in your cycle. Your cycle will vary from 28 days (usually in the winter) to 32 days (usually during the summer time). Menstrual cycles tend to follow the lunar cycles, so don't be surprised if you occasionally get 2 periods during one calendar month. With this you can begin to predict when your period will come. This in turn, can help you better prepare for and cope with the effects of PMS.