The effect of intermittent fasting on hormones
A 2022 literature review of human trials on the effect of intermittent fasting (IF) on reproductive hormones in females, and males suggests that IF may be a more favourable intervention for premenopausal women with obesity and or PCOS*, than it is for lean, athletic young men.
Lets take a quick deep dive into the studies, what to conclude from them, and whether IF is a suitable intervention for men beyond weight loss!
The review took into account 5 human trials of time restricted eating (TRE) of which there was one female study, four male studies.
The effect of time restricted eating on female hormones in overweight/obese women with polycystic ovarian syndrome
The female TRE study contained 18 women, aged 18-31yrs. All participants were asked to consume their daily calories between 8am-4pm over a 5 week period.
Results for this study were as follows:
Body weight fell by 2%
Total Testosterone came down by 9%
Sex Hormone Binding Globulin (SHBG)* increased by 2%.
The free androgen index (FAI)* fell by 26%.
*SHBG binds and transports testosterone and oestrogen to target tissues - the availability of these hormones in the body is largely influenced by levels of SHBG. Only the amount of sex hormones that are free or bound to albumin are considered biologically active.
*A high FAI value indicates there is too much biologically active testosterone in the blood.
The effects of time restricted eating on sex hormones in men
In one study, 26 lean, physically active men, aged 18-35 years were split into 2 group.
The first group consumed their calories in an 8 hour window combined with a 25% reduction of calories and resistance training 3 x week for a period of 4 weeks.
Group 1 results:
Body weight fell by 1%
Fat Mass fell by 9%
Total Testosterone came down by 1%
The second group in that study weren’t following time restricted eating plan. They combined a 25% reduced calorie diet alongside resistance training 3 x week for a period of four weeks.
Group 2 results:
Body weight fell by 2%
Fat Mass fell by 9%
Total Testosterone came down by 1%
Another small study of 16, male, elite cyclists aged 19yrs+- compared the outcomes of 15hrs of time restricted overnight (group 1), versus 10 hours overnight while adhering to their regular diet (group 2). The results for testosterone in group 2 were more favourable
Group 1 results:
Body weight fell by 2%
Free (active) Testosterone fell by 27%!!!
Group 2 results:
No changes were observed to body weight.
Free (active) Testosterone fell by 8% (compared to 27% in the first group.)
Other male intermittent fasting studies where time restricted eating was set at 8 hours in combination with resistance training 3 x week, or without, showed free testosterone drop by 21% and 27% respectively.
The effect of calorie restriction on sex hormones in premenopausal, overweight/obese women
The review also included 1 human trial of the 5:2 diet, a restricted calorie diet where participants observe 2 x 500 calorie day a week.
Study participants were 107 women, aged 30-45 years.
They were split into 2 groups as follows;
Group 1 combined 2 x 500 cal fast days weekly with 5 feast days, over 24 weeks
Their results were:
Body weight fell by 7%
Sex Hormone Binding Globulin increased by 14%.
Free androgen index fell by 6%.
Group 2 observed a calorie restricted diet of 1,500 calories/day over the same time period
Group 2 results were:
Body weight fell by 5%
Sex Hormone Binding Globulin increased by 6%.
Free androgen index fell by 10%.
DHEA fell by 6%
The effect of meal timing on sex hormones in premenopausal women with PCOS
The final intermittent fasting study included in the literature review examined the effect of meal timing on reproductive hormone levels
The meal timing study had 60 female participants, aged 25-39 yrs. They were overweight or of normal weight, and had a PCOS diagnosis.
They were split into 2 groups as follows;
Group 1 consumed more than half their calories at breakfast
Group 2 consumed more than 50% of their calories at dinner.
Group 1 results were:
Free testosterone decreased by 50%
Total testosterone decreased by 47%
Group 2 results were:
Oestradiol increased by 35%
These results suggest that eating a large amount of food later in the day may increase estrogen levels in women with PCOS, which may worsen an already oestrogen dominant state and interfere with ovulation.
A comparison of the findings:
Overall, the studies suggest that time restricted eating reduces free and total testosterone levels in young, physically active men. Sex hormone binding globulin (SHBG) however did not change. Interestingly, there were no changes to muscle mass, and muscular strength in spite of reduced testosterone levels.
The various fasting studies reviewed for the female cohorts suggest that generally intermittent fasting shows a lot of promise as a dietary intervention for overweight, premenopausal women and those with high levels of androgens as seen in conditions like polycystic ovarian syndrome (PCOS)
The positive effects on weight loss, menstrual cycles and fertility were more likely when calories consumed were limited to the earlier part of the day (before 4pm).
Conclusions
It’s difficult to draw any conclusion based on the above because very few studies have examined how intermittent fasting effects sex hormones.
There were several limitations in each of the studies; Sample sizes were small, there weren’t always controls, the duration of the trials were often short, and given that measuring hormones was not the primary purpose of the studies, it would be wrong to assume that the outcomes bore any real statistical significance!
Currently the only studies examining how intermittent fasting affects sex hormones are limited to those above. More studies are needed to properly assess the potential intermittent fasting has on influencing hormonal health in both women and men.
Some thoughts
While I was looking at this review, and trying to figure why testosterone fell so much in the male studies , I thought about 4 key players that are known to negatively impact testosterone levels.
Cortisol - too much cortisol in the blood (exercise induced or other) stops testosterone production, reduces testosterone synthesis and accelerate cell death of the Leydig cells where 95% of testosterone is made.
High Insulin - most athletes use carbohydrates as their primary fuel source to meet their energy demands during vigorous exercise because carboydrates break down quickly into glucose. Insulin’s job it to move glucose from the blood into cells for energy. If blood glucose levels remain chronically high, insulin levels will also remain high. Insulin levels are inversely proportional to testosterone, meaning high insulin, low testosterone.
Oestrogen - cortisol exaggerates aromatase which converts testosterone to oestrogen.
Oxidative stress - from exercise induced over heating of the testicles during cycles, vigorous exercise
I’ve seen IF work for both male and female hormones, but certain hormones. It can be a particularly effective intervention for insulin resistance, leptin and ghrelin (the satiety hormones), and weight loss.
But this review is a great reminder that there’s nothing like a bespoke approach - especially important with hormones where so many other variables come into play!
It’s why I love to work with clients 1:1 over a few months where I have the luxury of time so I can test and address the client’s needs where they’re at and adapt them as needed.
If you’re interested in getting help for your hormones or weight, book a free call and lets see if we’d be a good fit!!
P.S. Full study can be accessed here: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9182756/
