Intermittent Fasting | Woman and Hormone Imbalance
One of the revolving concerns is how IF can mess up women’s hormones causing all kinds of issues from altered menses, mood swings, low energy, and even possible difficulties in conceiving.
Backstory: our reproductive system is controlled by hypothalamic pituitary gonadal axis (HPG axis), where hypothalamus (in the brain) releases series of hormones creating ovulation conditions including pregnancy. [8,9] The concern is that women’s hormonal pathways are more sensitive to environmental factors which can be negatively impacted during IF.
Facts: to tackle this important subject, we’ve extensively searched the medical research data bases, and found NO HUMAN studies that backed up this concern.
Majority of sites and people take information from two animal papers regarding female’s hormonal imbalances using IF protocols. These studies discuss negative effects on rats that were exposed to every other day of fasting for 12 weeks ; and sheep that fasted for 5 days during their reproductive (estrous) cycle .
3 Facts that Debunk IF & Hormone Imbalance
1. IF and Calorie-restriction
The first thing we want to mention is that there is a difference between IF and calorie restriction diets. Intermittent fasting can inherently reduce calories, but that is not the goal of these protocols, as meals during an eating window should make you full and satisfied. 
Studies that withheld a full day or 5-days worth of nutrients [10,11] likely moved those animals into a calorie restriction territory. Both papers even explain how food restriction can cause fertility issues. Restricting calories can produce semi-starvation states which diverts energy from processes like reproduction to essential functions for survival. 
2. The type of Fasting also matters
It’s hard to communicate to an animal, to not worry about food, as it will be available tomorrow or in 5 days time. Not knowing, creates that calorie restriction / semi-starvation environment. By placing animals on a 24-hour and 120-hour fasts likely produced a greater stress response, more than just nutrient restriction. [10,11]
This would not be the case with people, in terms of knowing meal times and fasting protocols. Majority of women often choose a gradual adaptation of a 10-14 hour fasting windows. Such windows remain flexible and adjusted based on personal life schedule and factors, thus reducing stress level on the body, the mind and the endocrine system.
3. Rats and Sheep are not Human
By the end of each experiment, rats ovaries shrunk a bit and sheep had a decrease in ovarian follicles. Like humans, sheep are few of the animals that have a luteal phase. Fasted sheep maintained follicle amounts for majority of the protocol including days 3 and 4.  The estrogen levels also varied between studies, increasing in rats,  but reducing in sheep. 
Another big difference is that many mammals can pause pregnancy due to environmental factors such as extreme temperatures, or scarcity of food.  Human females don’t have that option, thus producing a different hormonal response and moving forward with pregnancy.
The exact reproductive mechanism is still unknown, but scientists believe that overall energy balance is an important indicator. Burning more energy than consumed creates a negative energy balance, and vice versa.  The energy balance influences HPG axis which coordinates same series of hormones that part take in pregnancy (like: GnRH, LH, FSH, and Kisspeptin). 
Energy balance also affects metabolic hormones such as insulin, insulin-like growth factors (IGF-I, and IGF-II), leptin, ghrelin, neuropeptide Y (NPY) and growth hormone (GH)—all important in various reproductive stages.