07/12/2021 20:23
One of the biggest nutrition tactics to dominate social media and the literature in the last few years is intermittent fasting or ‘IF’.
The concept of fasting goes back thousands of years but it has become somewhat ‘popular’ over the last ten years or so with more studies and books widely available on the subject.
With the popularity on social media, it almost comes across as something like a ‘magic pill’ for weight loss and is a popular talking point for people who are interested in losing weight. As we know however there are no ‘magic pills’ when it comes to weight loss - but how effective is intermittent fasting as a tactic?
We’ve seen intermittent fasting pop up in many different forms; as the 5:2 diet, the ‘warrior’ diet, or fasts of different lengths ranging upwards from 14 hours without eating.
Before we get into the topic of whether intermittent fasting may have a different effect for women, let’s just go over what exactly intermittent fasting involves, and what the studies say about the effects for the general population.
WHAT IS INTERMITTENT FASTING?
Fasting is a common practice dating back thousands and thousands of years and is used in religious and cultural practice as well as for health benefits. For example throughout the month of Ramadan in the Muslim calendar, in which worshippers will not eat or drink between the hours of sunrise and sunset.
Intermittent fasting is therefore an eating pattern of abstaining from consuming food for a certain period of time. There is no limit on how much you can eat and what you eat in your eating window, and the times in which you choose to eat may vary from day to day. You might decide to fast from 6pm to 12 midday the next day, or even fast for a whole 24 hour period.
Most studies have shown that intermittent fasting has lowered bodyweight and led to improved cardiovascular health - however, is this due to being in a calorie deficit or the actual effects of going without food for an extended period of time?
When I looked at the studies on fasting, those that advocated for positive metabolic adaptations were all done on men.
I wanted to find out if there was any evidence which would promote intermittent fasting as something beneficial for female metabolic health aside from as a weight-loss tool.
STUDIES ON FASTING AND WOMEN
One of the only studies I found was on women in 2013 attempted to find out whether improved health was down to this caloric restriction or the hormonal effects of fasting. It was done on women between the ages of 35-70, and all of these women were classed as ‘obese’ by their BMI (body mass index) rating. They were also mainly sedentary and were at their maintenance calories before starting the study.
The women were split into four groups:
Group 1 ate in a deficit at 70% of their maintenance calories, and followed an IF schedule.
Group 2 ate their usual maintenance calories, but on an IF schedule.
Group 3 ate in a deficit at 70% of their maintenance, but with no IF schedule.
Group 4 continued to eat at their maintenance calories with no change.
For their fast, the IF groups fasted for 24 hours after breakfast, on 3 non-consecutive days per week.
Group 1 and 3, those in the deficit, both lost weight. The IF group in the deficit (G1), actually lost more weight than those who were just eating at a deficit (G3). However! Those who were practising IF while in their deficit actually reported taking in less calories than they were meant to and so this could actually be due to just a bigger deficit than originally intended. This was due to hunger levels being lower after IF, and not having that much time available in the day to get the calories in.
The group who ate at maintenance calories but also practised IF did not see any changes to metabolic health apart from very slight weight loss - due to the lack of time available to eat all the food required and hunger levels being lower.
This can show us how IF can be a tool to help keep us in a caloric deficit when we are intentionally trying to lose weight.
INSULIN SENSITIVITY
One of the biggest claims for intermittent fasting is that it helps to improve insulin sensitivity.
Insulin sensitivity: insulin is a hormone secreted by the pancreas released in response to glucose levels in our blood. Insulin helps shuttle glucose from our bloodstream to be used by the body for energy or to store it as glycogen. We want insulin to respond to blood sugar levels efficiently. Insulin resistance (think of this as the opposite to insulin sensitivity) can occur when our blood sugar levels are constantly too high, and our insulin does not work as effectively, which can lead to health concerns such as obesity and type 2 diabetes.
However, in this study done entirely on women, there was no conclusive evidence that IF promoted better insulin sensitivity, and after 24 hours fasting, there were markers which pointed towards slightly increased insulin resistance.
Other studies which have seen improved insulin sensitivity in women after a fast have noted that the improved hormonal response to blood glucose levels is most likely due to weight loss after being in a deficit induced by the fast.
Studies on men, however, have shown that insulin sensitivity improved after longer periods of fasting. One study I looked at set out to mimic the hunter-gatherer lifestyle which we would have lived thousands of years ago - going prolonged periods of time without food and then having a large feast when food became available to see if there was a genetic adaptation. The study subjected men to 20 hours of fasting every other day for 15 days to mimic this lifestyle and found that insulin sensitivity did indeed increase throughout this longer fast.
If this is a genetic adaptation to survival, then perhaps this can explain some of the difference in the hormonal response of women to fasting.
Evidence seems to suggest that in fact, due to the female reproductive system, intermittent fasting and caloric restriction for anything over 12 hours can actually be detrimental to our hormone balance and our menstrual cycles. As we know from our other articles on the site, that lower than sufficient intake of energy can lead to RED-S (relative energy deficiency in sport), which in turn can cause irregularity with the menstrual cycle. Remember I spoke before about the hypothalamus monitoring the feedback loops within our body in order to regulate hormones to maintain energy balance?
HORMONES
As women’s bodies are much more sensitive to changes in energy balance, it seems that intermittent fasting or calorie restriction for a prolonged period can disrupt our HPA axis (that system of feedback loops) and lead to hormone imbalances which may both increase our output of cortisol due to calorie restriction being a stress on the body, and lead to disruption of our cycle. Our estrogen levels for example are very sensitive to low levels of energy, and a drop in estrogen can lead to increased hunger levels and higher rates of fat storage.
Combine this with fasting for 12-16 hours, then being let loose on your fridge… I think we are going to have the opposite results to what we were originally aiming for!
It was very difficult to actually find any studies which were done on general population women in the subject of IF and the effects, and once again this has demonstrated to me just how underrepresented we are!
There were plenty of studies on men and IF, female mice, rats, (even some on female seals…!), and I came across just one on pregnant women. I was very excited after trawling through articles for what felt like a long time to find a study done in 2001 where 7 women underwent a 72 hour fast to see the hormonal effects.
This study did see raised cortisol levels due to the fast, and another interesting outcome was that fasting had an effect on the circadian rhythm of the women. For a bit of background on the importance of our internal body clocks and the circadian rhythm, check out my blog article about sleep here.
Our body works in harmony with our biological clock, with certain hormones releasing at certain times on that clock, known as our circadian rhythm. The women who fasted saw melatonin (a hormone which tells our body it’s time to wind down for sleep) being released nearly 90 minutes earlier than usual.
A similar study found that leaner women were more likely to have elevated cortisol levels after a 72 hour fast, and thyroid stimulating hormone, which naturally elevates in the evening, was suppressed. TSH is responsible for stimulating hormone release from the thyroid gland, and is an essential part of the regulation of hormone levels in accordance with our biological clock.
FINAL THOUGHTS
I started this post in the hope that I would find some evidence for positive effects of intermittent fasting for women… but so far I have yet to find concrete evidence from studies that suggest IF is not just another way of restricting calories and aiding weight loss as a tool.
It looks like as female hormones are so much more sensitive to changes in energy balance, IF may actually be detrimental to our hormonal health.
If you’re a female looking to lose weight, IF is a tool which can be used to aid caloric restriction (as we can see that it blunts hunger levels and gives you less time to actually eat), but bear in mind to keep those fasts relatively short (12-16 hours) so as not to contribute to any hormonal imbalances!