Intermittent fasting (IF) first came on my radar almost a decade ago from several big names in the fitness and nutrition industry. Proponents talked about all the benefits which went beyond fat loss, like reduced blood lipids and blood pressure, reduced markers of inflammation, increased cellular turnover and repair, increased growth hormone and metabolic rate, etc.
So of course this piqued my interest not only for me but also my clients. And we all spent time trying variations of IF, including the common 16-hour fast with an 8-hour window of eating. We also did 24-hour fasts and 12-hour fasts.
I wanted to believe in this strategy, and for some it's still valid. But after giving it a fair shot, I found some fundamental issues with its effectiveness and practicality. Hunger suppression was one area that made me change my mind on IF.
Hungry, Hungry Hormones
A number of hormones that regulate hunger, appetite, and satisfaction are in play after a meal. For example, both leptin and insulin decrease hunger, giving your brain the message of satisfaction and "turning off" the need to eat.
A side effect of fasting is that, for some, these hormones get out of balance, leading you to become unresponsive to cues that tell you whether you're full and should stop eating. And if people alter their hunger hormones, they find themselves battling an uncontrollable appetite. Once they eat, the satiation cues telling them to stop won't register. Their appetites are insatiable. I know a number of people who have done IF and found that they've binged following fasting.
Stress and Insomnia
Another negative is the excess stress and insomnia from fasting. Any time you go without food for long periods you activate the flight or fight sympathetic nervous system and increase cortisol secretion in order for the body to mobilize energy stores.
For people who already have a lot of stress in their daily life (including training) fasting may increase it further, upping the body's cortisol levels, which has a number of negative effects. Cortisol is catabolic – it can break down muscle tissue and make it harder to build muscle. The combination of fasting and cortisol can produce obsessive thoughts about food which raises anxiety, causing a further release of cortisol.
High cortisol and the activation of the hypocretin neurons incite wakefulness, leading to insomnia. I experienced this and heard from others who did too.
Finally, a study by Bogdan looking at the effect of Ramadam fasting found significant alterations in testosterone release, suggesting an alteration in circadian function.
A Drop in Testosterone
A lot of IF advocates will promote studies that show no drop in strength or lean mass from their fasting protocols. One 8-week study with natural bodybuilders did show that the test group and control group achieved similar results in strength and muscle mass. However, a big point is that testosterone and IGF-1 did decrease significantly in the fasting group.
I also struggled to be proactive both physically and mentally during specific points in the day when fasting. But this isn't uncommon; even IF advocates schedule meetings and other appointments during the feeding window.
Although there's evidence to support this protocol as a fat loss strategy, for me there are too many factors that make IF unsuitable because of how it affects stress, sleep, and testosterone production – all vital components in building muscle. Also, the inability to control your appetite and nutritional choices during the feeding windows can produce more problems than benefits.