Intermittent fasting, an eating plan that involves restricting eating times rather than specific foods, is becoming increasingly popular.
"There have been a couple of animal studies that I don't think are very translatable to humans," Krista Varady, Ph.D., professor of kinesiology and nutrition at the University of Illinois, Chicago, told TODAY. "But the second they're up on Instagram, everyone finds out about them."
People are also frequently interested in questions about whether the timing of their meals has an impact on their health, Dr. Adrian Dobs, professor of medicine and oncology at Johns Hopkins Medicine, told TODAY.
So Varady and her team set out to study the potential effects in humans and set the record straight.
How does intermittent fasting affect hormones?
For the study, published last month in the journal Obesity, the researchers looked at two groups of women who went on time-restricted eating plans for eight weeks. The participants also provided blood samples to measure their sex hormone levels.
The data were originally collected for a previous study, which included more than 50 participants. But, after the researchers excluded men from this study, that left them with just 12 premenopausal women in one group and 11 postmenopausal women in the other.
Over the eight-week period, both groups lost about the same small amount of weight. And neither group saw major differences in levels of certain male sex hormones (testosterone and androstenedione) or sex hormone-binding globulin, a protein that controls the amount of other hormones in your blood.
In postmenopausal women, there was no change in their levels of estradiol, estrone or progesterone. The researchers didn't measure female sex hormone levels in premenopausal women because it would have required extensive tracking of their menstrual cycles and precise timing, Varady explained.
Overall, "these findings are reassuring," Dr. Reshmi Srinath, director of the Mount Sinai Weight and Metabolism Program, told TODAY. As someone who frequently discusses intermittent fasting options with her patients, Srinath said it was nice to see that most hormones in the study seemed to be unaffected by the eating plan.
The study addressed "a very important question," Dr. Sobia Khan, internal medicine specialist and women's health expert at Cleveland Clinic, told TODAY. And the finding that hormone levels "were not fluctuating drastically" was a good start, she said.
One puzzling finding
The only hormone that changed significantly during the study period was dehydroepiandrosterone, also called DHEA, which helps the body make male and female sex hormones, according to Mayo Clinic. DHEA, which has a similar structure to testosterone, can naturally rise and fall with weight changes, said Dobs, whose research focuses on male sex hormones and endocrine disorders.
Both groups of women in the study saw a statistically significant decrease in DHEA levels over the eight weeks they were on the time-restricted eating plan. But Varady emphasized that, even with the drop, participants' DHEA levels were still within the normal range — and they didn't show any worrying side effects in other tests.
"Going into this, I honestly thought nothing would change (over the study period)," Varady said. Looking at the DHEA data, "I was shocked," she said.
What the change actually means isn't quite clear yet. On the one hand, elevated DHEA levels have been linked to an increased risk for breast cancer, Varady said. Overly high DHEA levels can also be related to irregular menstrual cycles, excessive acne and changes in hair growth, Srinath added.
So, the small decrease in DHEA seen in this study may be beneficial for some people. It could also simply be a side effect of losing weight, Srinath said.
Elevated DHEA may also be linked to polycystic ovarian syndrome, which is where Varady’s team is taking this research next.
Overall, the DHEA finding shouldn't be a major cause of concern. And it will take more research — ideally with a longer study period and larger age range — to really understand, the experts agreed.
“With such small sample sizes, it could just be a fluke,” Dobs said.
The groups were, indeed, quite small and did not have an even representation of ethnic groups, Kahn noted. For instance, the 11-person postmenopausal group consisted of 10 Black women and one white woman.
To her, it wasn't surprising that the researchers saw few changes in most of the hormone levels because the study was only eight weeks long. "If it was extended to three months minimum, we would be able to appreciate (any changes) better," she said.
If you're thinking of trying intermittent fasting...
If you're someone who's interested in trying intermittent fasting, you should be reassured by the results of this study that trying a time-restricted eating plan isn’t likely to have a major negative impact on your hormone levels or fertility.
Remember that there are lots of intermittent fasting-style plans that might work for you, Srinath said. The time-restricted eating models in this study, which only allowed participants to eat for either four or six hours per day, is on the stricter side, she added.
"This was an extreme," Dobs agreed. "In the long run, there's going to be poor compliance." Other intermittent fasting schedules allow for longer eating windows, like eight hours, or only require followers to fast every other day.
If you're curious about intermittent fasting but aren't sure where to start, Varady recommends taking a look at her Instagram page, which she uses to address frequently asked questions.
And intermittent fasting can be dangerous for some people, Dobs said. As always, it’s a good idea to chat with your doctor before taking on a drastic change in your diet. In this case, that’s especially important for people taking certain medications and those with certain health conditions, like diabetes, the Mayo Clinic explains.
This article was originally published on TODAY.com