Working Out Hungry Is Bad For You

A new study tested cyclists who worked out after eating and while fasting. It turns out they burned the same amount of fat either way, but there were two downsides to working out on an empty stomach:

  • About 10% of the burned calories came from protein, not fat, including muscle. So you actually lose more muscle by working out hungry, but you don’t burn more fat.
  • Since you’re hungry and you don’t have fuel to burn, the intensity of the workout and therefore the number of calories burned is lower on an empty stomach.

So if you’re trying to lose fat, eat something good for you before working out. Cliff bars are pretty filling energy bars that are natural and kinda good for you and come in at about 240 calories — just make sure you burn more than that working out.

Photo by Les Chatfield


Update: A particularly astute reader pointed out a possible contradiction between this study and another study from last year, which found that working out strenuously in the morning before eating breakfast (i.e., on an empty stomach) led to lower weight gain, more efficient fat burning and less insulin resistance. There are a few differences between the two studies though, namely that in the breakfast one:

  • they fed the healthy subjects a really bad high-fat diet: 50% more fat and 30% more calories than they were consuming before. The breakfast was rich in carbs and they had sports drinks during the workouts. The cyclists in this study on the other hand, were probably on a decent diet.
  • they focused on breakfast. It could be there’s something special going on right after we wake up: e.g., maybe the body has been turning fat into carbs all night to prepare fuel for the morning.
  • all of the subjects gained weight due to the awful diet, but the ones that exercised after breakfast gained a negligible amount. So it could be that they actually lost muscle mass and replaced it with fat, which would actually fit in with the cyclist study’s findings; it found that 10% of calories came from protein, and some unspecified part of that was muscle mass — that still leaves 90% that came from carbs and fat.
  • it didn’t test for hunger. Unless they hadn’t eaten in a long time, many people don’t wake up being hungry right away. In the cycling study, the group had been fasting, so they were presumably pretty hungry. It’s reasonable to assume that the body won’t signal hunger until it’s out of fuel, at which point it starts cannibalizing muscles. Therefore, if the people in the breakfast study weren’t hungry in the morning, it’s also reasonable to assume their bodies didn’t cannibalize muscle.

Perhaps one tell-tale sign is the intensity of the workout. The cyclist study found that the ones fasting didn’t have as intense of a workout. So if in the morning, you feel like you can easily have as intense of a workout before breakfast as after breakfast, then you probably have enough fuel in your system to not worry about burning muscle mass.

From The New York Times, via Lifehacker

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