With intermittent fasting becoming increasingly popular for weight control, some people are concerned about its impact on overall health, particularly hormone levels.
Studies have suggested that intermittent fasting may reduce levels of reproductive hormones in females, leading to potential problems with fertility.
Now, a small study of individuals with obesity at the pre-and postmenopausal stage has suggested that these fears may be unfounded.
The researchers stress the need for more investigations into the effects of intermittent fasting in different populations.
Intermittent fasting, in which periods of normal eating are interspersed with periods of fasting or restricted calorie intake, is becoming increasingly popular as a method of weight control.
The three methods people may follow are:
- the 5:2 diet, where people eat normally 5 days a week and have a very low-calorie intake for 2 nonconsecutive days each week
- alternate-day fasting, where normal eating days are alternated with days of restricted intake
- time-restricted eating—During an eating window of 4–12 hours (most commonly 8–10 hours), people eat normally, with only water and energy-free drinks allowed outside this period.
Time-restricted eating (TRE), which does not involve calorie restriction, is the most popular of the three methods. However, studies have shown varying results regarding the health impact of this diet type.
Is this diet type effective?
One review of studies looking at individuals with overweight or obesity showed that TRE was as effective as calorie restriction for weight loss. Another study, however, found no significant difference in weight loss between those following TRE and a control group.
While it may help some people maintain a healthy weight, are there other health effects
Some studies have suggested that intermittent fasting may affect hormones, particularly reproductive hormones, although most studies have been conducted in rodents, not people. There is a specific lack of data on the effect of TRE on postmenopausal individuals with obesity.
A new study by researchers at the University of Chicago has shown that TRE had little effect on most reproductive hormones in women with obesity.
However, dehydroepiandrosterone (DHEA) concentrations decreased in the participants, which may affect reproductive health.
What the study did
The study, which appears in the journal Obesity, was a secondary analysis of females with obesity who had followed an 8-week TRE program. Of the 23 participants, 12 were premenopausal and 11 were postmenopausal. All had body mass indexes (BMIs) between 30 and 49.9.
All participants followed 4-hour or 6-hour TRE. Those on the 4-hour TRE ate whatever they wanted between 3 p.m. and 7 p.m., with only water or energy-free drinks for the other 20 hours. Those on the 6-hour TRE ate freely between 1 p.m. and 7 p.m., with an 18-hour fasting window.
Prof. Tim Spector, scientific co-founder of ZOE and professor of epidemiology at King’s College London, expressed some reservations, telling Medical News Today: “The study used a small sample size of 23 women over 8 weeks. The eating window is only 4–6 hours, which is not sustainable for most people in real-life settings.”
Participants gave 12-hour fasting blood samples at baseline and 8 weeks. Researchers instructed them to avoid exercise, alcohol, and coffee for 24 hours before the blood tests.
The researchers then measured the concentration of testosterone, androstenedione, SHBG, DHEA, estradiol, estrone, and progesterone in the participants at the postmenopausal stage.
The researchers did not measure estradiol, estrone, and progesterone in participants at the premenopausal stage, as levels of these hormones change throughout the menstrual cycle.
The effect on weight loss
Both groups lost weight during the study. Participants at the premenopausal stage lost an average of 3% of their body mass; the postmenopausal group lost 4%.
Drop in DHEA levels
Of all the hormones measured, only one, DHEA, was significantly affected by the change in eating patterns. In both groups, DHEA concentration dropped after 8 weeks of TRE.
DHEA is a steroid hormone made in the body by the adrenal glands, the gonads, and the brain. It is important for creating estrogen and androgen hormones, such as testosterone.
In females, low levels of DHEA are associated with reduced libido, vaginal dryness, and osteoporosis. Studies have also shown that supplementing DHEA may improve success rates in females undergoing fertility treatment.
Although it is widely believed that low DHEA might reduce fertility, little peer-reviewed research supports these claims. However, studies have shown that obesity is linked to fertility issues, so weight loss alone may improve a person’s chances of having a baby.
High levels of DHEA are associated with an increased risk of breast cancer, particularly estrogen receptor-positive and progesterone receptor-positive tumours, in both premenopausal and postmenopausal individuals.
However, Prof. Spector cautioned that longer-term effects were unknown: “In the long term, this type of TRE, with a very narrow eating window, could impact sex hormone function, but this study shows that in the short term at least the sex hormones themselves are not impacted.”
More studies needed
This study was carried out on obese females. Dr Varady stressed that more work is needed to see whether the results apply to other populations.
“I don’t know if these results can be extrapolated to women of a healthy weight,” she cautioned. “I think we need several more studies looking at the effect of intermittent fasting on reproductive hormones in varying populations. We will also need to see how intermittent fasting affects fertility in men and women.“
Prof. Spector agreed: “Obesity causes changes to metabolism and hormone levels, so these results can’t be extrapolated to women without obesity—it would be necessary to replicate the study in women without obesity and in bigger numbers to give the trial more statistical power to make conclusions.”
Several studies are currently investigating the benefits of intermittent fasting. Dr. Varady is researching whether intermittent fasting may be beneficial for those with polycystic ovary syndrome (PCOS). ZOE is running a large intermittent fasting study, the big IF study.
Intermittent fasting: Is it all it’s cracked up to be?
Intermittent fasting is an increasingly popular weight loss dieting strategy. Beyond weight loss, however, the diet has promising benefits that may reduce the risk of developing some chronic, lifestyle diseases. In this Honest Nutrition feature, we explain all you need to know about intermittent fasting and whether it is worth the hype.
