It's the worst in the beginning, but should taper off after your body gets used to this new way of eating, especially with the leangains plan, which is a more regular eating and fasting schedule.
It’s not ideal for sedentary people
IF has been called the lazy person's diet, but that's not entirely accurate. Getting some exercise is important if you want to reap the benefits of intermittent fasting. "If you are sedentary, it’s going to be difficult for you to lose weight because you’ve programmed your body to store whatever you’re eating," Dr. Splaver says. So make sure you're scheduling moderate exercise (ideally during your feeding window, so you have the energy to workout).
It’s also not great for super-active people
"[For] someone who is consistently very active, a diet like this may be detrimental because you will break down muscle," Dr. Splaver explains. People who are incredibly active -- think serious athletes, extreme Crossfitters, etc. -- need extra protein, carbs, and fat to actively build muscle, which they may not get with intermittent fasting. The long stretches of time without eating could also combat the gains you're getting at the gym.