Some women report having a change in their menstrual flow when they go on a low carb diet. What causes this?
A woman's period is regulated by her hormones. The time it begins, the time it ends, the thickness of the lining, all are controlled by hormones which are trying to create the ideal situation for a new birth.
Therefore, any change you make to the foods that you eat that could have an affect on your hormones will show up in changes in your monthly period.
For example, many low carbers (and many vegetarians) turn to soy products as a substitute for other foods. Soy contains phytoestrogens, which would affect your estrogen levels. Soy is linked to fertility problems in fact because of its affect on a women's menstrual cycle.
If you take out the soy effect, weight loss in general causes fluctuations in hormones as your body adjusts to how much hormone it needs to put out to reach your current body shape. This could also cause changes in your menstrual cycle.
Studies have shown that low carb diets help women who have polycystic ovaries (PCO). The lower carbs help create a more even hormone flow and therefore more regular cycles. Some PCO patients even report a lessening of side effects from PCO, such as acne and facial hair.
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