If you've ever tried to lose weight, you've probably gotten drawn into the argument over whether it's better to cut carbs or fat from your diet. A new study doesn't completely resolve that question, but it does provide an important insight.
Some proponents of the low-carb diet insist that you must cut carbs to burn off body fat. Their reasoning goes that when you cut carbs, your body's insulin levels drop, and that's essential in order to burn fat.
To put that question to the test, Kevin Hall at the National Institute of Diabetes and Digestive and Kidney Diseases and colleagues recruited 19 obese volunteers (average weight over 230 pounds) to participate in a rigorous study.
For two weeks they were kept in a lab around the clock, where scientists could provide them a precise diet. One group got a low-carb diet that reduced their total calories by 30 percent. Another group went on a low-fat diet that also reduced their total calories by 30 percent. Then, after a few weeks of rest, the two groups switched diets.
As Hall now reports in the journal Cell Metabolism, cutting carbs did work.
"We cut the carbohydrates, insulin went down, and fat burning went up, exactly the way that theory predicts, and people lost fat," Hall says.
The average participant lost about a pound of fat over two weeks, and about 4 pounds of weight total (the rest was probably water).
But Hall's study also showed that the low-carb, low-insulin conditions were not necessary to shed body fat. In fact, the low-fat diet also led to the loss of about 1 pound of body fat. So it was just as good.
Hall says, so much for the idea that only low-carb diets can help people shed fat. "That theory, as it stands — that very strong claim — is certainly not true," he says.
Instead, his evidence favors those who say if you want to lose body fat, total calories matter most.
"All calories weren't exactly equal when it came to losing body fat ... but they were pretty close," he says.
"I love this paper!" says Susan Roberts, a professor of nutrition at Tufts University, who was not involved with the research. She says it cuts through an amazing amount of noise that surrounds diet advice.
"There are too many commercial interests, there's too many bad research studies ... that get too much attention. And this a really good, high-quality study that helps get rid of some of the confusion," she says.
She agrees with Hall that it dispels the notion that fat loss absolutely requires a low-carb diet. "They've shown that that's bogus," she says.
Roberts wrote a commentary that accompanies the research paper. She says with this bit of physiology settled, researchers can step back to ask some more practical questions, such as what is in fact the best diet to reduce body fat. That's not just a matter of body chemistry.
"This [paper] doesn't say anything about how easy it is to stick with a high-carb or a high-fat diet," she says. And it's no use having a diet that people can't maintain.
Hall says some people might find it's easier to cut calories by limiting fat. Other experiences suggest that some dieters do better on a low-carb diet. He cites some studies that show that, over a six-month period, people do tend to lose more weight on a low-carb diet compared with a low-fat diet.
"We would suggest that that's probably because they end up eating [fewer] calories in total," he says, not because physiology favors one approach over another.
ROBERT SIEGEL, HOST:
To lose weight, exercise and eat a healthy diet. But what is a healthy diet - less fat, fewer carbohydrates? Well, NPR's Richard Harris reports on the study that provides some hard facts on a hotly-debated question.
RICHARD HARRIS, BYLINE: Cut out the carbs. Cut out the fat, or just cut the calories. That's not simply a question for dieters. Obesity researcher Kevin Hall at the National Institute of Diabetes and Digestive and Kidney Diseases wished he knew the answer, too.
KEVIN HALL: People have very strong opinions and have their theoretical reasons why one diet might work better than another diet. And we wanted to test those theories in a very strictly-controlled fashion.
HARRIS: So Hall and his colleagues sequestered 19 obese individuals at the NAH over two-week spans to control what they ate and to measure their body chemistry precisely. The researchers wanted to test the very popular belief that the only sure way to lose weight is to cut out the carbs to make your insulin levels drop. And true, cutting carbs did work.
HALL: We cut the carbohydrates. Insulin went down. Fat burning went up just exactly as that theory predicts, and people lost fat.
HARRIS: But then the same volunteers were also subjected to a much different diet. Here, they left carbs alone and cut out fat instead. Insulin levels weren't affected by this diet. But it was, in fact, every bit as effective as the low-carb diet. Hall says so much for the idea that only low-carb diets can help people shed fat.
HALL: That theory, as it stands, that very strong claim, is certainly not true.
HARRIS: Instead, the best evidence favors those who say if you want to shed body fat, it's the total calories that matter most.
HALL: All calories weren't exactly equal when it came to losing body fat with respect to carbs versus fat in the diet, but they were pretty close.
SUSAN ROBERTS: I love this paper.
HARRIS: Susan Roberts is a professor of nutrition at Tufts University. She says it cuts through an amazing amount of noise that surrounds diet advice.
ROBERTS: There's too many commercial interests. There's too many bad research studies on, you know, like, six mice that get too much attention. And this is a really good, high-quality study that helps get rid of some of the confusion.
HARRIS: It certainly doesn't answer all questions about the optimal weight-loss diet, but Roberts says it does disprove that popular notion that fat loss absolutely requires a low-carb diet.
ROBERTS: They find that that's bogus.
HARRIS: Roberts wrote a commentary that accompanies the paper, which is published in the journal Cell Metabolism. She says with this bit of physiology settled, researchers can step back to ask more practical questions, like, what is the best diet to reduce body fat?
ROBERTS: This doesn't say anything about how easy it is to stick with a high-carb or a high-fat diet.
HALL: That's the key question.
HARRIS: And that's a question of human behavior, not simply body chemistry. Hall says some people might find it easier to cut calories by limiting fat. Other experiences suggest that dieters do better on a low-carb diet.
HALL: Over the first six months, people do tend to lose more weight and fat on a reduced-carb diet. But we would suggest that that's probably because they end up eating less calories in total.
HARRIS: His study didn't get at that question directly since people were only on restricted diets for two weeks at a time. And it's not an easy question to answer with high confidence because it's not practical to lock up many people for months on end. And in the end, it might simply be a matter of personal preference to find the most sustainable approach. Richard Harris, NPR News. Transcript provided by NPR, Copyright NPR.