Fasting and intermittent fasting has become very mainstream in the last decade. Many people report to feel better, build more muscle, look better, lift more, and generally improve their lives just by not eating food for certain amounts of time.
One of the explanations for these effects is that fasting supposedly increases human growth hormone (HGH) and thus boosts metabolism, energy, muscle mass, strength and more due to its higher levels.
There is a lot of mystery surrounding this HGH increase. There are many people who say it’s entirely negligible while others who say they can feel the boost. It seems to come down a lot to how each individual responds and what works for people. Since there is such a wide range of anecdotal evidence both supporting and “myth-busting” an HGH increase, your own experience may be one of the best sources of information that you can rely on.
One study that has been done has found a 5x increase in HGH from fasting for 24 hours. There are other studies which say that longer fasting has even more of an effect, up to 1000% increase. One study also has seen an increase in cortisol when fasting, however, since fasting puts stress on the body which then results in higher levels of cortisol, a catabolic hormone. It is possible that the lack of protein and higher amounts of cortisol while fasting will offset the HGH increase, leaving no major benefits for lifting.
This leaves the choice up to you to decide based off experience and the studies available, what benefits there are for you from fasting. If you are on a cut, fasting 16 hours at a time could help with fat loss while giving the benefit of a HGH boost. If you are bulking though, trying to fast every day is probably going to hinder your progress and will not be optimal for muscle recovery if you are training every day since the protein supplied to muscles will be less consistent.
For an increase an HGH, based off the research, fasts that last at least 20 hours are going to have more an effect than fasts lasting 16 hours. If you are fasting every day however, a 4 hour eating window is likely too short to get enough food in so a 16 hour fast could be opted for.
24 Hour Fast
The 24 hour fast is the type of fasting that I would recommend for weightlifters. This is because the studies which show a HGH increase have found that boost from fasts that last at least 24 hours.
To do a 24 hour fast while still lifting and progressing in the gym, the fast needs to be done so that it fits into the workout routine you have. If you are following the push-pull-leg routine and are hitting every muscle group twice a week, that leaves one rest day. This day could then be utilized for a 24 hour fast to boost metabolism, “reset” the body, and increase levels of HGH.
To pull this off, calories for that day will need to be spread out to other days of the week in order to adhere to your diet. That means you will have to consume an extra 300-500 calories per day so that you can fast without being in a deep caloric deficit every week.
If you are bulking you may have to eat even more than 500 extra calories per day in order to fast, so the difficulty of that alone may skew the work-reward balance towards not even bothering.
Fasting is far better suited for cuts since the main benefits of fasting which are fat burning and a higher metabolism are more advantageous when trying to lose fat than when trying to gain muscle. If you are on a bulk, fasting is not worth the extra energy and time since the focus should be on optimizing your training and getting enough food in to grow. If you are on a bulk and set on fasting then less than 24 hours is going to be optimal.
16 Hour Fast
The other common type of fasting is to do 16 hour fasts. The advantage of fasting for 16 hours is that it can be done every day.
When fasting for 16 hours every day, there is an 8 hour time window to eat food. For some people, this helps with time management and productivity, since the first 6-8 hours of the day they are not eating any food and therefore have less distractions.
Another benefit to a shortened amount of time eating is more energy. Digesting requires energy and if you give the body time to run without investing energy into constantly digesting foods, energy levels may increase and you’ll have more mental focus to do other things.
This increase of focus is backed up by studies which have shown higher levels of the neurotransmitters norepinephrine and epinephrine when in a fasted state. These neurotransmitters are responsible for the flight/ fight response and increase levels of alertness.
Ultimately, if you can work in a 16 hour fast around your routine while on a cut there are most likely more benefits than not fasting at all. It is also possible that your expectations are not met. Since HGH isn’t increased as much from 16 hour fasting as it is from 24 hour fasting, the decrease in protein supplied in muscles along with only a minimal change in HGH may mean that there are little to no impacts on muscle retention although fat loss may be helped.
If you are on a bulk, fasting is not ideal since it will have a negative impact on your ability to train optimally and eat enough. If you are able to fast one full day a week, that has been shown to have more benefits than frequent fasting in 16 hour intervals but will still require an increase of calories other days of the week which will be hard to do while bulking.
If you are not losing enough fat on a cut, a better strategy than fasting is to lower your caloric intake. After lowering calories, if you are still not satisfied with the rate of fat loss, then 16 hour fasts could help.