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For as long as I can remember — from the first time I set foot in a gym, actually — I’ve been listening to folks debate the question: When is the best time to exercise?

Now a new study may finally shine some light on the matter. Researchers found that exercising before eating has several beneficial effects, including preventing weight gain and maintaining insulin sensitivity.

Researchers in Belgium took 27 healthy young men and fed them all a horrible diet high in sugar and fat and calories. The particular diet was chosen because it was just about guaranteed to create both weight gain and a reduction in insulin sensitivity. Insulin sensitivity is something good — it’s when the cells respond well to insulin, meaning that insulin does an excellent job of removing excess sugar from the bloodstream and getting it into the cells where it can be “burned” for energy. When someone is said to be insulin resistant, on the other hand, the system doesn’t work well, and the person winds up with high blood sugar and high insulin — a path to either metabolic syndrome or diabetes. Most diabetics are insulin resistant, and most people who are insulin resistant are overweight, since insulin “shuts down” the fat-burning process.

In the Belgian study, the researchers divided the men into three groups. One group did nothing but eat the terrible diet. The second and third group exercised and did the exact same workout, but the second group exercised after breakfast and the third group exercised before breakfast.

The results were both surprising and dramatic. The control group gained a lot of weight and also saw their insulin sensitivity plunge (meaning they became much more insulin resistant — not a good outcome). The group who exercised after eating also gained weight but not nearly as much as the control group. And the group’s insulin sensitivity went down, just as with the control group.

But the group who exercised before eating was a whole different story. This group, despite eating a horrible, weight gain-inducing diet, did not gain weight. Not only that, but the group’s insulin sensitivity remained high and the bad diet did not make the group insulin resistant. “This study for the first time shows that fasted (empty stomach) training is more potent than fed training to facilitate adaptations in muscle and to improve … glucose tolerance and insulin sensitivity,” said the study’s authors.

Conventional wisdom holds that it’s always best to eat something before working out. Proponents point out that you need energy for working out — energy that comes from carbohydrates. “Fat burns in a flame of carbohydrate,” they say.

It’s interesting that back in the days of “Stay Hungry” and “Pumping Iron,” when the big meccas of bodybuilding like World Gym and Gold’s Gym in Venice were home to such legendary bodybuilders as Arnold Schwarzenneger and Franco Columbo, everybody trained on an empty stomach. Bodybuilders of that era believed that you were more likely to mobilize your fat stores for fuel if you didn’t have to burn off a whole bunch of carbs that you just scarfed down for breakfast. We now know that they were mostly right.

So should you forgo eating before working out? Not necessarily. “If you’re interested in performing better — like if you’re training for an event — you might want to eat first,” said exercise physiologist and That’s Fit expert Liz Neporent. “But for weight loss, evidence does seem to be trending towards not eating before working out.”

Neporent pointed out that as a practical matter, there are going to be a fair amount of people who don’t do well when they don’t eat before working out. “They get dizzy, sick and even faint sometimes,” she said. “But I’ve also had people eat right before working out that have felt exactly the same way!”

Bottom line: It’s an individual thing, and no one prescription is going to work for everyone.

But for those who want to try it (and who don’t get lightheaded or dizzy), working out first thing in the morning on an empty stomach might be just the thing to stop weight gain in its tracks. It did in the Belgian study, and those folks were purposely eating a really bad diet. It should work even better if you also couple it with a diet designed to help you reach your goals.

via When Is the Best Time to Exercise? – That’s Fit.