Researchers found that people in the study lost 3.1 ounces (89 grams) of fat per day, on average, when they followed a low-fat diet for six days. In comparison, the same people lost 1.9 ounces (53 grams) of fat per day while following a low-carb diet for the same amount of time.
"A lot of people have very strong opinions about what matters for weight loss, and the physiological data upon which those beliefs are based are sometimes lacking," study author Kevin Hall, a metabolism researcher at the National Institute of Diabetes and Digestive and Kidney Diseases, said in a statement. "I wanted to rigorously test the theory that carbohydrate restriction is particularly effective for losing body fat, since this idea has been influencing many people's decisions about their diets."
For the study, the researchers recruited 19 obese people whose average age was 35. During the first five days of the study, the participants ate a baseline diet of 2,740 calories per day, with 50 percent of the calories coming from carbohydrates, 35 percent from fat and 15 percent from protein. Then, for six days, the people followed this diet but cut 30 percent of the total calories by reducing their intake of carbohydrates. [ Dieters, Beware: 9 Myths That Can Make You Fat ]
After taking a break for a few weeks, the people again followed the baseline diet for five days. Then, for the next six days, they followed a low-fat diet, cutting 30 percent of the total calories by reducing their intake of fat.
While they were following the diets, the people in the study stayed at the laboratory, and the researchers closely monitored and controlled what the people ate.
Some proponents of low-carb diets have claimed that cutting carbs reduces the production of insulin, which in turn increases the breakdown of fat, leading to greater fat loss than low-fat diets, the researchers said.
But in the new study, the researchers found that, although following a low-carb diet did reduce insulin production and increase the breakdown of fat, these changes did not translate into an increased loss of body fat, compared with the amount of fat the participants lost while following a low-fat diet.
It is too early to make specific recommendations based on the study as to which diet people should choose to lose weight, the researchers said. The diet that the people in the study followed was strictly controlled and it did not emulate normal, real-world dieting, they said.
Moreover, the researchers noted that staying on any diet over time is important for weight loss. "What [the new study] doesn't say is whether or not, for example, a low-carb diet might be easier to stick to" than a low-fat diet, Hall said.
Previous research has shown, for instance, that people may lose more weight on low-carb diets than low-fat diets over six months of dieting. "Maybe it is because it is easier to stick to, maybe you feel full, maybe you don't feel as hungry," Hall told Live Science. But more research is needed to look at these factors, he added.
The new study was published today (Aug. 13) in the journal Cell Metabolism.
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