If you don't eat enough carbs, you may experience irregular menstrual cycles or amenorrhea. Amenorrhea is defined as a woman's menstrual cycle being absent for 3 months or more.
The most common cause of amenorrhea is hypothalamic amenorrhea, caused by too few calories, too few carbs, weight loss, stress or too much exercise. Amenorrhea occurs due to the drop in levels of many different hormones, such as gonadotropin-releasing hormone (GnRH), which starts the menstrual cycle.
This results in a domino effect, causing a drop in the levels of other hormones such as luteinizing hormone (LH), follicle-stimulating hormone (FSH), estrogen, progesterone and testosterone. These changes can slow some functions in the hypothalamus, the region of the brain responsible for hormone release.
Low levels of leptin, a hormone produced by fat cells, is another potential cause of amenorrhea and irregular menstruation. Evidence suggests that women need a certain level of leptin to maintain normal menstrual function. If your carb or calorie consumption is too low, it can suppress your leptin levels and interfere with leptin's ability to regulate your reproductive hormones. This is particularly true for underweight or lean women on a low-carb diet.
However, evidence on amenorrhea on low-carb diets is scarce. Studies that report amenorrhea as a side effect were usually only done in women following a predominately low-carb diet for a long period of time. One study followed 20 teenage girls on a ketogenic (very low-carb diet) diet for 6 months. 45% experienced menstrual problems and 6 experienced amenorrhea.