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Low-carb diets do not improve metabolism, one study finds


Over the last several decades, Americans have been searching for the ultimate diet that will result in the greatest weight loss for everyone. Low-carbohydrate diets are currently among the most popular, with proponents saying they are the ticket to losing weight. But how do low-carb diets actually compare to low-fat diets?

This week on “Take Care,” we interview a researcher who studied that very question. Kevin Hall is a senior investigator at the National Institute of Diabetes & Digestive & Kidney Diseases, one of the National Institutes of Health. His main research interests are the regulation of food intake, macronutrient metabolism, energy balance, and body weight.

Kevin Hall and his team want to find out whether low-carb or low-fat diets led to more fat loss in the body.

“One of the things I noticed, is that nobody had ever done a study where they did head-to-head comparisons where you cut just carbs or just fat from somebody’s baseline diet, keeping the other nutrients the same,” said Hall.

So they took 19 adults with obesity and confined them to a NIH clinical center for a pair of two-week stays. The researchers fed them all of their food, and the participants had to eat all of their food. Hall says his team used sophisticated metabolic measurements to calculate when they cut carbs from the participants’ diet how many grams of fat were lost from their body every day, versus cutting the same number of calories of fat.

The main takeaway from the study was that the results were very close, says Hall. Both diets led to fat loss from the body. Hall says the differences between the two diets were pretty miniscule. But in differences that are statistically significant, the reduced-fat diet tended to win.

Hall says another purpose of the study was to test the theory that many low-carb diets claim – that cutting carbs does something beneficial to a body’s metabolism. And those results are what Hall says he finds particularly interesting.

“When we cut fat from people’s diets, almost nothing happened to their metabolism. In other words, they just kept burning the same amount of fat as before. Nothing happened to the amount of this hormone called insulin that’s secreted. And they lost fat by a very simple mechanism -- they cut the fat that came into their body, and their body didn’t care. It just kept burning the same amount,” said Hall.

Something different happened within the body for those participants on a reduced-carb diet.

“They reduced the amount of this hormone insulin that’s being secreted, that tends to cause the body to release fat from fat cells and burning extra fat. And that also led to fat loss, just not quite as much fat loss as cutting the fat in the first place,” said Hall.

Hall says their research showed no evidence for one of the theories of low-carb diets – that it would increase metabolism. In fact, when people cut the carbs in their diet, their metabolism actually slowed down slightly, by about 100 calories per day.

But, Hall says, it is true that when people try to do these diets on their own and they’re not locked up at NIH, the reduced-carb diets tend to work better.

“We think that the reason why people tend to do better on low-carb diets in the real world is because they chose to eat less food, less calories. Why that is, I think is a very interesting question. But what we’ve basically shown in this study is it’s not some fancy metabolic advantage of a reduced-carb diet, if anything, the advantage seems to belong to the reduced-fat diet.”

Hall says he and his team are still analyzing hunger levels during experienced by the participants during the two diets, and the satisfaction they with the two different kinds of meals. The researchers did brain scans when the subjects were eating different-tasting kind of food. Hall says maybe this will help explain why one diet works better than another.

“What’s probably more likely to be paying a role is that you may feel less hungry on a low-carb diet. You may feel more full. You may not want to eat as much. Or it’s just because you’re targeting a nutrient that happens to have the highest percentage of calories of the food that you’re eating then you tend to eat less. We don’t know the reason why yet, but it’s probably not because of these metabolic changes.”