Exercise, Then Starve? Not Necessary

Feb. 3, 2010 — -- Even a very small change in diet in the hours after vigorous exercise can have a significant impact on how the human body metabolizes food, according to new research by a team of scientists seeking to understand exactly how the body processes vital resources, like sugar.

The focus of the research, published in the current issue of the Journal of Applied Physiology, is on diabetics, so the findings could have profound implications for persons with insulin resistance, but the research also sheds some light on the symbiotic relationship between eating and exercising.

The study suggests, for example, that it isn't necessary to starve yourself after working out in order to reap the full benefits of exercise. You probably can replace those calories that you burned off on the treadmill and profit just as much as the fanatics who go hungry, at least when it comes to processing the day's supply of sugar.

Nine "healthy young men" participated in a grueling research project so scientists could measure exactly how they benefited from exercise and diet.

It's been known for years, of course, that exercise helps in many ways, but it's less clear how what we eat helps, or hinders, the final results. That is one of the goals of this research, which is still in its early stages at the University of Michigan.

The participants, who lived at a hospital during the research so their activities and diet could be strictly controlled, all benefited from a reduced carbohydrate diet in the hours after exercising.

What Happens After Exercise

Specifically, their insulin sensitivity was enhanced, making it easier for their bodies to take up sugar from the blood stream and concentrate it in tissues like muscles, where it can be stored or used as fuel.

All these guys were healthy subjects, in their late twenties, and none of them were obese or diabetic, making the results all the more impressive.

"If you see a little bit of an increase (in insulin sensitivity) in a healthy person, you are going to see a profound result in an obese person," Jeffrey Horowitz, the study's senior author, said in a telephone interview. Horowitz and his colleagues have just started testing obese persons.

In Study, Reducing Calories Not as Effective as Lowering Carbs

The research, at least at this early stage, suggests that overweight people with insulin resistance should see an enormous improvement just by eating a diet that is lower in carbohydrates after they exercise. And it doesn't have to be all that much lower.

The participants took part in four separate experiments. In one test they did not exercise at all; in the other three they spent 90 minutes exercising. The tests differed only in the meals ingested after exercise -- a nutrient-balanced diet; a lower-carbohydrate diet, and a reduced-calorie diet that did not fully replace the calories that had been burned during the exercise.

The greatest improvement occurred during the low-carb test, so reducing calories was not as effective as lowering the carbs.

The lost calories from the reduction in carbohydrates were replaced by an increase in fat calories, but it didn't take much.

Carbs Not Necessarily a Bad Thing; Body Needs Sugar They Produce

"They get more salad dressing, or a little butter with their bread," Horowitz said.

During the tests a small chunk of muscle tissue, "about the size of a grain of rice," was extracted from each participant, so the scientists could determine just how much sugar the muscles had taken up from the bloodsteam. That's how they knew the reduced-carbohydrate diet had been the most effective in enhancing insulin sensitivity.

Horowitz cautions that the results are significant in terms of understanding insulin resistance, but they may have little or no application for persons with no diabetic problems. Carbs are not necessarily a bad thing, because the body needs the sugar they produce.

"We only store about a day's worth of carbohydrates in our body, so it makes sense that we would have evolved to have these highly regulated systems to protect against low blood sugar because that's what your brain is feeding on," Horowitz said. "If you run out of carbohydrates, you're done."

That's why marathon runners, for example, load up on carbs "to have the fuel in their muscles for the next day when they need to do their next training session," he said.

Should You Cut Carbohydrates After Exercise?

But what about the rest of us? Should we be watching our carbs after exercising for a few minutes?

Not necessarily, Horowitz said. A healthy person with no insulin resistance is not likely to need to enhance his or her insulin sensitivity, so the results of reducing carbs is debatable.

But much more needs to be learned before we really understand the relationship between diet and exercise. What this research shows is that diet did make a difference. How much remains to be seen.