Mind & Body

Should You Exercise on an Empty Stomach?

If you ask the internet, there are a lot of rules when it comes to nutrition and exercise — listicles abound laying out the foods you should never eat, the exercises you're definitely doing wrong, and the hot new heart-rate metric that only this expensive smartwatch will measure. Some tips become popular enough to rise above the noise, and the practice of "fasted cardio" — exercising before you eat breakfast — is one of them. Is it the magic bullet for burning fat? Science may have an answer.

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Burnin' for You

Here's the basic idea behind fasted cardio: When you sleep, your metabolism keeps on ticking. When you wake up and exercise before eating, you've got less glucose in your muscles to use as fuel so your body shifts gears to burn the next best thing: fat. Studies seem to support this: A 2015 meta-analysis in the British Journal of Nutrition looked at 27 studies and concluded that exercising fasted does burn more fat than exercising "fed." Other studies have found that exercising fasted helps the body rev up its fat-burning abilities on a molecular and genetic level, which could make it easier to burn more fat over time.

But that's not the whole story. For one thing, you can't fool your body for too long; it'll eventually adapt to your wily ways and shift the rate at which it burns fat and glucose, or carbs. And according to some experts, there's also a risk that your body will shift its fuel source too far from glucose and start burning protein, which will make exercise recovery harder. The problem is that most of the studies looking into fasted cardio study its effects over the short term, and some don't control for calorie intake (and, surprise surprise, someone who eats breakfast consumes more calories in a day than someone who skips an entire meal).

When you address those shortcomings, the results are very different. A 2013 study in the journal Obesity that had women do either fasted or fed high-intensity interval training workouts over a period of six weeks found that both groups lost the same amount of fat by the end. A 2014 study from Brad Schoenfeld and Alan Aragon put women on an identical calorie-restricted diet — the kind you might follow if you were trying to lose weight — and had them do one hour of steady-state cardio three times a week for four weeks, either before or after eating breakfast. Both groups lost weight, much of it fat, and there were no other differences between the groups.

Still, pros have sworn by fasted cardio for years as a way to cut that little bit of extra fat before a big bodybuilding competition. A study from the 1990s did find that training fasted can boost the amount of fat you burn around your midsection. But these are already-ripped fitness models we're talking about; when your body fat percentage is that low, a tiny change can produce big results. The same isn't likely to happen for your average joe.

You Do You

Overall, there's little evidence to support the idea that exercising on an empty stomach helps you burn any more fat than exercising after breakfast. The key, like with any exercise regimen, is to do whatever makes you more likely to stay with it. If eating before you exercise makes you feel weighed down, wait to eat until you're done with your workout. If you can't imagine exercising without a little food in your stomach, go for it — just keep it small and try to give yourself at least 30 minutes between your last bite and the start of your workout. When it comes to exercise, there are a lot of rules, but there's really only one you need to follow: If it works for you, do it.

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Written by Ashley Hamer November 27, 2018

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