While fasted workouts can be very effective for some people, they're not for everyone. "In some cases, it could be a bad thing to introduce," Chag says. For example, if you're someone who usually works out in the evenings, then attempting to fast all day would be difficult and potentially dangerous. Or if you're training for a marathon and have to run for many hours a day, then fasting beforehand wouldn't be a wise choice because your body needs more fuel. At first, working out on an empty stomach is going to make you feel exhausted, because you're just not used to it, Chag says. "If you're doing a super hard workout, you're pushing your body to an area that it's not prepared for," she says. "That transition period can be tricky and challenging."