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Here's What Happens When You Work Out on an Empty Stomach

A dietitian reveals how working out in a fasted state impacts your energy levels and overall performance.

Many of us have been there before: It's early morning, the sun is rising, and you have yourself the ideal backdrop to squeeze in an invigorating run or workout. You lace up your sneakers and leave breakfast plans for after your sweat session's done. But what actually happens to your body when you work out on an empty stomach? Can it help speed up or impair your progress?

Research from sports scientists at Nottingham Trent University showed that healthy individuals who exercised on an empty stomach experienced a 70% greater fat burn. However, their performance was lacking. We're diving into exactly how an empty stomach can affect your workout and energy levels, so read on to learn the scoop.

Working out on an empty stomach could make you feel fatigued and decrease performance.

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"During a workout, your muscles use glucose, which is the simplest form of carbohydrates, and fuel for your body to fuel a workout," explains Chrissy Arsenault, RDN at Trainer Academy. "However, when you work out on an empty stomach, your bloodstream is usually low on glucose, so your muscles are forced to use other macronutrients as a source of energy or draw from glycogen reserves. Because [you start] out with less energy, you may experience lower levels of performance due to lower blood glucose levels and fatigue."

Some research supports that working out on an empty stomach—aka in a fasted state—can promote fat loss. That being said, Arsenault points out that the fat-loss process isn't so clear-cut; it's typically determined based on your daily caloric balance and the kinds of workouts you're doing.

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Is it safe to work out on an empty stomach?

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When it comes to whether or not working out on an empty stomach is safe, Arsenault says it really depends on your goals.

"If you work out for muscle gain or strength, an empty stomach workout will not be a good idea because you may experience early fatigue, decreased endurance, and may risk increased protein breakdown in muscles," she tells us. "Generally, for my athlete clients, I wouldn't recommend working out on an empty stomach because the negative impacts on your workout will outweigh the benefits. Those engaging in light exercise may be able to do so on an empty stomach without repercussions."

In addition, Arsenault stresses that those with diabetes shouldn't exercise in a fasted state to avoid blood glucose levels from becoming too low. "For these individuals, I recommend a light snack with carbs and protein beforehand to support their workouts," she says.

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Aim to eat a light meal or snack at least an hour before working out.

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Your goal should be to consume a light meal or snack at least an hour before you start exercising. Make sure it contains protein and carbs to fuel your body with energy and help avoid muscle loss.

Arsenault recommends the below recipes for quick and seamless mornings:

  • A fresh fruit smoothie with Greek yogurt: If you favor early-morning workouts and usually feel weighed down by breakfast, consider whipping up a fresh fruit and veggie smoothie with protein powder or Greek yogurt. (The addition of creamy Greek yogurt will give you a stellar protein boost!)
  • Sliced bananas with peanut butter on whole-grain toast: "Bananas are one of the best sources of energy and electrolytes and can be a fantastic fuel for your workout, especially if you eat a short time before your workout," explains Arsenault. Pair them with protein-packed peanut butter and slow-digesting carbs from whole-grain toast, and you'll be ready to take on your workout.
Alexa Mellardo
Alexa is the Mind + Body Deputy Editor of Eat This, Not That!, overseeing the M+B channel and delivering compelling fitness, wellness, and self-care topics to readers. Read more about Alexa
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