Kristi Storoschuk, BSc (Hons.) is a science communicator with a research focus on ketogenic diets, metabolic therapies, and fasting for health optimization. She currently works alongside the world’s leading ketogenic researchers providing scientific education for the mainstream audience. Outside of her research, you will find her traveling the world, doing CrossFit, and optimizing her health through diet and lifestyle.

A woman’s menstrual cycle is a sign of vitality and overall health, so an absent period is definitely something worth your attention. If you haven’t already, we recommend reading our articles on how fasting interacts with the menstrual cycle, part 1 and part 2. These articles will help premenopausal women sort out the nuances of fasting and reproductive health.

Addressing the question at hand, as far as we are aware, no research has explicitly looked at the effects of intermittent fasting on menstruation. Though we do have research that suggests ovulation is regulated in relation to energy balance (calories in versus calories out). In animal models, the female reproductive system is more sensitive to reduced Caloric intake than males. In humans, through a combination of exercise and Caloric restriction, the greater the energy deficit the greater the frequency of menstrual disturbances has been found. Specifically, a greater frequency occurred with a deficit of ~470 Calories per day and beyond over the course of one menstrual cycle. Though this particular study jumped from a deficit of ~162 to ~470 Calories, so it’s possible some women may have experienced changes in their menstrual cycle between these ranges.

The working hypothesis is that women need extra reserves in order to support the life of their offspring and reproduction in females requires greater energy output. So, a repeated signal of negative energy balance due to a long-term Caloric deficit or increased energy expenditure may appear threatening to this system. Indeed, research suggests a Caloric deficit, rather than the restriction of any specific macronutrient, is the main driver of menstrual irregularities.

The potential impact fasting can have on your menstrual cycle will likely be influenced by the degree and frequency of energy restriction. For example, acute restriction (e.g., 3-day fast) during women’s follicular phase does not appear to suppress reproductive function in normal-weight women, but 6-weeks of ~800-1000 calories/day, or 1kg weight loss per week, in normal-weight women led to anovulatory cycles in six out of the nine subjects. While daily intermittent fasting (e.g., time-restricted feeding) does not inherently imply you are restricting Calories, if you lose your period after adopting an IF regimen, there is a high probability you are sustaining a Caloric deficit. It would be wise to periodically increase Calories by either eating more during your feeding window or extending it to keep up with your body’s energy demands. Unfortunately, this may take some trial and error to determine the right eating pattern for your body, as not all women in the study mentioned previously experienced menstrual changes with a deficit of ~470 Calories per day. Due to the impact of body fat and activity level on the menstrual cycle, it is fair to assume that the lower your body fat and higher your activity levels, the more often you’ll need to periodically increase Calories to mitigate potential side effects. 

Martin B, et al. Sex-dependent metabolic, neuroendocrine, and cognitive responses to dietary energy restriction and excess. Endocrinology. 2007;148(9):4318-4333.

Williams NI, et al. Magnitude of daily energy deficit predicts frequency but not severity of menstrual disturbances associated with exercise and caloric restriction. Am J Physiol Endocrinol Metab. 2015;308(1):E29-E39.

Bringer J., et al. Deficiency of energy balance and ovulatory disorders. Human Reproduction. 1197;12(suppl_1):97-109. 

Olsen BR, et al. Short-term fasting affects luteinizing hormone secretory dynamics but not reproductive function in normal-weight sedentary women. J Clin Endocrinol Metab. 1995;80(4):1187-1193.

Pirke KM, et al. The influence of dieting on the menstrual cycle of healthy young women. J Clin Endocrinol Metab. 1985;60(6):1174-1179. 

Thanks to Vidyia B. and Ali H. for submitting similar questions! 

Should I Be Concerned If I Lose My Period While Fasting?

Should I Be Concerned If I Lose My Period While Fasting?

Thanks to Clodagh P. for this question.

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