Intermittent Fasting: a mechanism for altering body composition?
There has been a lot of talk these days about intermittent fasting and the effects it may have on changing body composition (i.e. decreasing fat mass). Let’s look at what the research says about this.
Intermittent fast is actually a broad term that encompasses a variety of fasting methods; however, there are 3 forms that are most common: alternate day fasting (alternating between a fasting day and a day eating ad libitum – as much as one wants), whole day fasting (fasting 1-23 days a week and eating ad libitum the remaining days), and time-restricted feeding (fasting 16 hours each day and eating ad libitum the other 8 hours a day). Keep in mind that both with alternate day fasting and whole day fasting, typically individuals are permitted to consume up to 25% of the energy they require, or around 400 to 600 calories. The ‘fast diet’ is an example of whole day fasting, for instance. What I have seen more attention paid to recently has been time-restricted feeding as a fasting approach.
I must say there is promise in the research. The majority of the research on fasting has been done on whole day fasting and alternate day fasting, and primarily has been conducted on overweight and obese individuals. I have attached a review article here that summarizes the findings from this research. Overall, weight loss appears to be similar to regular calorie restricting (such as eating 250-500 calories less a day than what one needs), though there have been studies where fasting approaches showed greater weight loss results.
Challengingly, very little research has been conducted on time-restricted feeding and even fewer studies done on healthy weight individuals, and the research that includes resistance training is even sparser. I did find one study that looked at time-restricted feeding (TRF) in young men conducted resistance training, and while I was not able to get access to the full article (yet – I have requested it and will get it in a day or two), there were promising results. TRF – consuming ad libitum calories for 4 hours a day in this case, 4 days a week and following a normal eating schedule on the other 3 days – reduced caloric intake by ~650 calories/day. Body composition did not change, of note, but upper and lower body strength increased in both groups (it seems there was a placebo group) and lean body mass was retained.
Another study looked 34 resistance trained males, either consuming a TRF diet of 16 hours fasting and ate 100% of their calorie/energy needs, versus the normal diet group that still ate at regular times of the day. Of note here is that unlike the previous study which was ad libitum (and resulted in a 650 kcal deficit for the TRF group), this diet had each group consume 100% of their recommended daily calorie amount. Participants completed 8 weeks of the diets and included resistance training. The TRF group significantly lost more fat mass while fat free mass and muscle strength was maintained in both groups. A negative outcome was that testosterone and IGF-1 (anabolic hormone) levels decreased in the TRF group which currently cannot be explained by study results.
So what does this mean? So far, this is all considered very preliminary. Before I will change my recommendations more broadly to athletes I will wait for more research (essentially, a lot of it) before I feel comfortable changing from current best practices. That being said, for those who are interested in intermittent fasting and specifically TRF, there do not seem to be any significant adverse effects for this approach and there may be some benefits to altering body composition – that is, decreasing fat mass and maintaining lean body mass.
Those who know me should expect to hear my regular cautions – it is totally up to each person to try what works best for them, and this could be worth experimenting with. However, please hear my voice in the back of your head if you do decide to try it and ask yourself is this a realistic approach for you? The research studies looked at 8-week intervals, and so at the very least I would recommend trying it for that period of time. I know for myself, I get hungry every couple of hours and I would not be very happy utilizing this approach; other people may find this very realistic and sustainable. So I encourage you to ask yourselves these questions in evaluating this approach.
Let me know if you have any questions. And if you try it let me know what you find – I am curious to hear your experience! Below are some research articles in case you’d like to read more, including the 2nd article I referenced on resistance training and TRF: