What is fasted cardio (and can it help me lose weight faster)?

If we're going to change into leggings, slather on sunscreen and get all sweaty, it better be worth the effort. Which is why we were intrigued to hear about how fasted cardio—basically, working out on a completely empty stomach—could maximize our fat-burning potential. We checked in with Denise Pate, doctor of internal medicine at Medical Offices of Manhattan, to learn more about it—including whether it’s actually safe.

What are the benefits of fasted cardio? “Fasted cardio is exercise done in a fasted state, in which your body is no longer processing or digesting food. Benefits include increased lipolysis, fat oxidation and decreased insulin levels. Increased lipolysis is when the body breaks down fatty cells to use them as energy, and fat oxidation is when the body burns the energy from fat cells when the body's glycogen (the body's storage form of energy) is low. When the body is resting, especially during sleep, blood insulin levels begin to drop; this means the body turns to using fat as energy instead of carbohydrates. These benefits all stem from increased body fat loss.”

So, is fasted cardio safe? “Yes, if it’s done carefully. Exercising or doing cardio during a fasted state can cause low blood sugar, which could lead to feelings of lightheadedness or lethargy. Because our body derives energy from a different source when we do fasted cardio, this energy can come from muscle, hence some may experience muscle loss, especially in the elderly. If you have other metabolic diseases or health concerns such as diabetes, it is best to contact your doctor prior to doing fasted cardio to prevent low blood sugar.”

So it’s not harmful to work out on an empty stomach? “While it’s not necessarily harmful to the body, there are pros and cons. One pro: You can lose up to more than 20 percent body fat when you exercise on an empty stomach. The con of working out on an empty stomach is that your body eventually kicks into 'survival mode,' where it will eventually burn fewer calories as your body will try to prevent too much fat from being burned; this can make it more difficult to lose weight. You may also feel ill or lacking in energy after.”

Can habitual fasted cardio lead to any health conditions? “If you’re generally in good health, fasted cardio may not be an issue. However if you have a health condition, like diabetes, contact your doctor before trying it out.”

Will you burn more fat or calories from fasted cardio? “There are several studies regarding fasted cardio and body fat (like this one from Lehman College) that determine you can lose up to 20 percent more body fat doing fasted cardio than fed cardio (eating prior to working out). When your body is in a fasted state, your body turns to fat for energy instead of stored carbohydrates from the meal prior—but it becomes more difficult if fasted cardio is done for long of a period of time, as your body will eventually need to refuel.”

So there you have it: If you’re in good health, it might be worth giving fasted cardio a try. (You just might need to ignore a few stomach rumbles during spin class.)

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