Men and women don’t experience the same effects.
There’s a theory that exercising when fasted will force your body to tap into its fat stores rather than relying on carbohydrates eaten for energy. However, a recent study in the Journal of the International Society of Sports Medicine found no difference in body composition between women who fasted and those who drank a meal replacement shake prior to exercising. That could be because females are designed to put on fat when fasted in order to supply their body’s own calorie source, explains Stacy Sims, Ph.D., exercise physiologist and nutrition scientist at the University of Waikato in Hamilton, New Zealand.
Working out on an empty stomach is detrimental. “Without available blood glucose from carbs, it’ll be harder to hit the intensities you want during exercise,” says Sims. What’s more, it could impair your recovery, which can stand in your way of building lean mass and encourage your body to put on fat instead. “The stress hormone cortisol naturally rises when you sleep so it’s higher in the morning,” explains Sims. Eating before going to the gym increases your blood sugar to help bring cortisol back to baseline, but continuing to fast keeps it high. Exercise may be a good stressor, but it further elevates cortisol levels. Over time, chronic high cortisol levels can drive inflammation, predisposing you to anemia and hormonal problems that throw off your menstrual cycle, Sims adds.
While cortisol is elevated in both men and women in the morning, it doesn’t have as big of an effect on guys, says Sims. “From a biological standpoint, when there isn’t much food available, the natural response for men is a spike in both testosterone and growth hormone, which boosts their metabolism,” says Sims. Guys can afford to get away with skipping the pre-gym snack without the same consequences—but it doesn’t mean they should. “When you look at the data comparing men who exercise fasted and those who don’t, there is no performance improvement,” says Sims.
Man or woman, taking the time to fuel your body in the morning can set the stage for feeling strong during your workout and for the rest of the day. Don’t worry too much about the carb to protein ratio, just opt for something that contains both macros and that your stomach tolerates. Try almond milk mixed with protein powder and a shot of espresso (for an added caffeine boost) 30 minutes before a workout. Sims likes almond milk because it’s easily digestible, so you don’t need much lag time between eating and going to the gym. Toast with a bit of nut butter works, too.