On the flip side, if you go for a tough run, then your body starts to burn carbs instead of just fat. “When you go higher in intensity, your body can’t use fat fast enough, so it burns some fat and some carbohydrates,” says Speaker.
That doesn’t mean slow and steady wins, though. Remember that your body only burns more fat during the actual workout when you slow your roll. Numerous studies have shown that high-intensity interval training stokes your metabolism and keeps burning fat long after your routine is done, so depending on how hard you worked, tough sessions may win out after you hit the showers.
But What if You’ve Got No Energy?
It’s clear that busting out cardio before breakfast can be beneficial. But what if you feel like you don't have enough energy to exercise before you've eaten? Well, that’s nonsense, according to Speaker. “You have plenty of energy in your muscle and fat stores to give your body what it needs to exercise,” she says. “Your liver provides glucose for your body, and you don’t really start tapping into that until you deplete the glycogen stores in your muscles. Depending on how fed you were the day before and how trained you are, you have about an hour-and-a-half to two hours before you even dip into your liver glucose.”
In other words, you’re good to do about an hour of cardio, max, while you’re fasted, says Speaker.
Of course, if you’re upping the intensity or going any longer than that, it’s important to start fueling, perhaps with energy chews or gels. But refueling post-workout is what's really important, so your muscles can start repairing the tiny tears they endure during exercise, says Speaker. One cool upside: Fasting primes your body for the nutrients you’re about to give it. “You’ll better absorb protein, and your body is going to know right where to put it because you’ve used energy, creating space for new energy to go in,” she says. “So protein and carbohydrates will go back into the muscles to help them refuel, as opposed to going into fat stores.”
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When It's an Issue to Sweat Before You Eat
If you’re generally in good health, there aren’t too many downsides to consider so long as you’re doing cardio (it’s a whole different ballgame for strength training), says Speaker. The main issue: Those on a low-carb diet, like Paleo diet followers, may struggle to bust out their best cardio workout because their livers don’t have a lot of fuel readily available, and it’s likely that they’re not fully recovering in between workouts because they’re not refueling their glycogen stores, she explains. But still, that’s not a serious health threat. “You’ll just hit a wall and your body will signal you to stop,” says Speaker.
Your appetite post-workout may be affected, too. One study found that when runners hit the treadmill for an hour without eating first, they were more hungry than those who chowed down ahead of exercise.
It All Comes Down to Personal Preference
Working out in a fasted state is a big mental game, says Speaker. “If someone believes they’re going to pass out if they haven’t eaten and they really think they have to get something down before they work out, then I’m going to suggest that they eat,” she says. “It’s not that they physically need the food—they psychologically do. And that’s just as valid.”