Intermittent fasting has become a popular way to lose weight. Also known as time-restricted eating, this diet plan is more about when you eat and less about what you eat. There are many variations, but all intermittent-fasting diets increase the amount of time you don’t eat during the day. This allows your body to use up transient sugar for energy, then switch to stored fat.

But what effect, if any, does regular fasting have on hormonal cycles? One concern is that intermittent fasting may have a negative impact on female reproductive hormones. However, a new study finds little evidence to support that concern.

The new study is from obesity researchers at the University of Illinois Chicago. Fertility issues have been a concern for both men and women trying time-restricted eating, but this small study focused only on pre- and postmenopausal women. Twelve premenopausal and 11 post-menopausal women were randomly assigned to an eight-week diet plan that restricted eating to either four or six hours per day.

During fasting, study subjects could only drink water. During the eating time periods, they could eat whatever they wanted. These women were compared to a control group of age-matched women who ate as they would normally, with no changes. All the women in the study were obese. Obesity is determined by body mass index (BMI), which is a weight to height ratio of 30 or higher, according to the Centers for Disease Control and Prevention.

Weight Loss Is a Plus for Fertility

Body weight was measured before and after the study. Blood testing of reproductive hormones was done at the beginning and end of the study. Hormones tested were…

  • Testosterone and androstenedione (male hormones that can be converted into estrogen)
  • Dehydroepiandrosterone (DHEA), a hormone that improves ovarian function and egg quality
  • Sex hormone binding globulin (SHBG), a protein that transports sex hormones
  • Female hormones estradiol, estrone and progesterone (tested only in postmenopausal women)

These were the key findings from the study, which were reported in the journal Obesity

  • There were no significant weight changes in the control group.
  • There was no significant difference between the four and six-hour dieters.
  • Premenopausal women dieters lost about three percent of their body weight.
  • Postmenopausal women dieters lost about four percent of their body weight.
  • The only change in blood testing was DHEA which decreased by 13 to 14 percent in both the pre- and postmenopausal dieters.

The research team judged the dropped DHEA in the premenopausal women to be a minor fertility risk compared to the proven benefit for fertility gained by weight loss. For the postmenopausal women, the drop in DHEA could be a concern because it might increase loss of estrogen that occurs after menopause, but none of the postmenopausal women had any signs or symptoms of low estrogen.

Eat and Sleep Like a Warrior

The type of intermittent fasting used in this study is sometimes called the “warrior” diet. In prehistoric times, people went without food for long periods and engaged in more exercise, similar to warriors. People went to sleep when it became dark and got up at light, increasing the time of nighttime fasting. Today people stay up long into the night watching TV or using the internet. During this time, they often snack. This type of eating does not allow for any intermittent fasting and may increase the risk of obesity.

The belief that intermittent fasting might have a negative effect on menstrual cycles and reproductive health comes from a study done in young rodents. In human years, these rodents would be about nine years old. This study received a lot of coverage in the media, raising concerns about the increasingly popular intermittent fasting diet trend. Other studies have found that intermittent fasting may reduce the risk of type 2 diabetes, high blood pressure and obesity.

The research team in this study also found that the women dieters had a drop in insulin resistance and oxidative stress after the diet. Insulin resistance can lead to type 2 diabetes, and oxidative stress has been linked to many long-term diseases. The team is encouraged by the results of their study, but due to the small test size, they call for more research in both women and men.

Intermittent fasting is not recommended for people under age 12, or for people on insulin, pregnant or breast feeding, or with any history of an eating disorder.

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