The pros and cons of fasting for women of childbearing age
Virtually all of the clinical studies on intermittent fasting have been carried out with test subjects who are men or post-menopausal women. Studies generally do not include pre-menopausal women due to concerns that fasting of any kind may negatively affect hormonal balance and the monthly cycle.
Some pre-menopausal women use intermittent fasting for weight loss, and with great success—you can search the internet to see countless before and after pictures. But there are an equal number of women out there who report that intermittent fasting had significant negative effects on their hormonal balance and monthly cycle. Since controlled studies with pre-menopausal women haven’t been done, we can only guess at the factors that could affect why some women can safely fast while others can’t. They likely include: overall health, how much excess weight they have to lose, how long they fast each day, and how much they restrict their overall caloric intake.
My guess is that overall caloric intake is a big factor. Intermittent fasting and caloric reduction are, in fact, two different things. You can intermittent fast every day while still eating your normal amount of calories during your feeding window. This allows you to get some benefits of intermittent fasting, like insulin reduction and training your body to burn fat, without dramatic weight loss. When we restrict our caloric intake for too long, our body can go into “starvation mode,” which can potentially upset hormonal balance and monthly cycle.
On the flip side, there some potential benefits of fasting for pre-menopausal women when it comes to fertility. First, intermittent fasting is becoming popular among women with polycystic ovary syndrome (PCOS) as a way to lower their insulin levels, lose weight, reduce inflammation, reduce stress, and alleviate depression and anxiety—all symptoms of PCOS. Some of these positive effects have now been proven in studies.
Second, intermittent fasting can lead to weight loss. Being overweight or obese negatively impacts fertility in a number of ways: by raising insulin and blood sugar levels, by increasing the risk of PCOS, by decreasing the sex hormones necessary for conception, and by increasing the risk of anovulation (when no egg is released by the ovaries).
Third, fasting may improve fertility in older women. A 2008 study carried out at Harvard Medical School found that caloric restriction significantly extended fertility in adult female mice. When the calorie-restricted mice reached 12 months of age—advanced age when it comes to mouse fertility—their eggs had far fewer abnormal chromosomes than the eggs of mice whose eating had been unrestricted. The calorie-restricted mice also produced more eggs than normal mice when their ovaries were artificially stimulated, their eggs were more likely to develop into embryos upon fertilization, and their offspring survival rates were dramatically higher than the control mice.
Another study showed that when worms are starved, they put reproduction on hold—not surprising. But in worms, their bodies destroy existing eggs during starvation and regenerate a new crop of healthy eggs from stem cells once they begin eating again. In the study, starvation allowed the worms to reproduce when they were 15 times older than normal reproductive age. The starved worms also lived three times longer than worms that were not starved.
This brings us to a topic that is now hotly debated: Do ovarian stem cells exist in humans?
Since 1951, we’ve believed that women are born with all the eggs they’ll ever have. However, a growing body of research suggests that mammals, including humans, do possess ovarian stem cells that are capable of generating new eggs as we age. If this is true, it could explain why women who have been through radiation treatment or bone marrow transplantation, which damage or destroy mature and immature eggs, can regain their fertility.
In the same way that fasting renews the immune system, it may renew women’s egg supply. Fasting induces autophagy, potentially triggering the body to recycle old, damaged eggs. Subsequent refeeding may then stimulate ovarian stem cells to regenerate new, healthy eggs. That’s what happened in the worm study—could a similar process happen in humans? It remains to be seen.
Experts recommend, and common sense dictates, that if you want to use fasting to improve your fertility you should do it before trying to conceive. Fasting while trying to conceive is counterproductive, because the body is much less likely to support a pregnancy when calories are scarce.