A new study has investigated how an eight-week intermittent fasting (IF) regimen affects the levels of circulating hormones in pre- and post-menopausal women.1

Intermittent fasting and health IF has gained traction over recent years as an eating approach for fat mass loss and to protect against diseases such as Alzheimer’s disease, arthritis and multiple sclerosis (MS).

Fasting has likely been practiced by humans for thousands of years, due to its roots in ancient cultures and religious groups. Scientists exploring the effects of calorie restriction in mouse models during the early 20th century paved the way for the highly productive research field we see today.

What is IF? IF is an umbrella term referring to diets that involve restricting food consumption for specific periods of time, which varies depending on the regimen adopted. Example regimens include:

5:2 fasting – where a normal diet is consumed five days of the week, and two days are spent fasting.

Alternate-day fasting – where a normal diet is consumed one day, followed by a complete fast or restricted calories the following day.

Time-restricted eating (TRE) – where a normal diet is consumed within a specific timeframe, such as 12pm–8pm. No food is consumed outside of that time period.

The effects of IF continue to be explored in human subjects, but initial concerns have emerged regarding its impact on reproductive health. “Some women are skeptical about IF because they believe it may negatively affect levels of estrogen and other reproductive hormones, leading to menstrual cycle irregularities and fertility issues,” Dr. Sofia Cienfuegos, a postdoctoral research fellow at the University of Illinois (UIC), Chicago, and colleagues wrote in a 2022 review on IF. A 2013 study, which uncovered an adverse effect of IF on young rat models’ reproductive health, is likely the root of this skepticism, the review authors suggest. However, the rats used in the experiment were three months old, corresponding to a human age of nine years; therefore, the data cannot be translated into how IF might impact hormone levels in adult females.

Exploring how eight weeks of IF impacts hormone levels Cienfuegos is the co-author of a new paper published by UIC researchers in the journal Obesity. In this study, researchers led by Dr. Krista Varady, professor of nutrition at UIC and IF expert, explored how eight weeks of TRE affected the levels of sex hormones in a group of 12 pre-menopausal and 11 post-menopausal women, compared to a control group.

The TRE regimen followed by the women in the study is referred to as the “warrior diet”. It involves eating food (without calorie counting) within a four-hour window each day. A water fast is then adopted until the following day.

Blood samples and weight measurements were obtained from all participants at baseline and after the eight-week regimen. The researchers analyzed the blood samples to obtain measurements of sex hormone binding globulin (SHBG) and dehydroepiandrosterone (DHEA). Levels of estradiol, estrone and progesterone were measured only in post-menopausal females, as the level of these hormones circulating throughout the body can vary during pre-menopausal women’s menstrual cycle.

A guide to SHBG and DHEA

Sex hormone binding globulin (SHBG)

Is a protein produced by the liver that binds to the reproductive hormones estrogen, dihydrotestosterone (DHT) and testosterone, transporting them around the body.

Dehydroepiandrosterone (DHEA)

A hormone that supports the production of other hormones, such as estrogen and testosterone. DHEA might be prescribed by fertility clinics to enhance the function of the ovaries and egg quality.

A reduction in DHEA, but not SHBG Women undergoing the “warrior diet” regimen lost between three and four percent of their baseline weight. In contrast, there was effectively no weight loss observed in the control group.

When analyzing hormone levels, Varady and colleagues discovered that SHBG levels did not change throughout the course of the study in either group. Levels of DHEA, however, were significantly lower in pre- and post-menopausal women after eight weeks of TRE, reducing by ~14%.

“This suggests that in pre-menopausal women, the minor drop in DHEA levels has to be weighed against the proven fertility benefits of lower body mass,” Varady said. “The drop in DHEA levels in post-menopausal women could be concerning because menopause already causes a dramatic drop in estrogen, and DHEA is a primary component of estrogen. However, a survey of the participants reported no negative side effects associated with low estrogen post-menopause, such as sexual dysfunction or skin changes.”