Whether imposed by diet, strenuous exercise, or both, your body is able to sense when you’re in an extreme calorie deficit. If your body fat drops below a certain level or you remain in this state of negative energy balance for too long, your menstrual cycle may be disrupted.
Losing your period is an adaptive mechanism that exists to prevent you from getting pregnant in an environment of low energy. Without enough food to fuel basic functions, your body will attempt to preserve itself by shutting down non-essential processes such as menstruation and reproduction.
Losing your period for an appreciable amount of time can have serious side effects, including the development of osteoporosis—where your bones become porous, brittle, and susceptible to fractures—as well as skeletal muscle disturbances and negative changes in blood lipid (cholesterol) levels. If you experience such disruptions in your menstrual cycle while dieting, this should be taken as a sign that it’s time to change your goal from fat loss to reverse dieting or muscle gain.