New results from a National Institutes of Health diet-and-weight-loss trial strike a blow against the theory that people gain weight because they eat too many carbohydrates.

That theory, championed by popular author Gary Taubes, Harvard Medical School professor David Ludwig, Ph.D., and others, contends that a high-carb diet spikes insulin levels, leading to increased fat absorption by cell. Proponents of the theory say the way to lose fat is to eat a low-carb/high-fat diet.

The NIH study found the opposite: Subjects on a low-fat but relatively high-sugar diet achieved more fat loss than those on an equal-calorie, low-carb and low-sugar diet. “We can definitely reject the claim that carbohydrate restriction is required for body fat loss,” wrote lead author Kevin Hall, a star math modeler of nutrition and weight loss at the NIH, in Cell Metabolism.

Unlike observational studies that can’t strictly control the behavior of subjects, the new experiment was carried out in a metabolic ward. In other words, researchers could measure every calorie consumed and burned by subjects. The research followed 19 obese adults on either a high-carb/low-fat or a low-carb/high-fat diet during six days of living in the ward. After a “wash-out” period, the subjects followed the same procedure with the opposite diet for an additional six days. Both diets reduced daily calorie intake by about 800 calories/day per subject, and both provided subjects with the same number of calories/day (1,918). Protein contributed the same amount, roughly 21 percent of total calories, to both diets.

After six days, the high-carb group lost an average of 89 grams of fat a day, compared to 53 grams per day for the low-carb group. The low-carb group lost more body weight—4.07 pounds versus 2.86—probably as a result of increased water loss at the beginning of a low-carb diet. But, “Fat loss is a more important goal than weight loss,” Hall wrote.

Importantly, the high-carb group slightly increased its total consumption of simple sugars during the six-day trial. Still, the group lost more weight than the low-carb and low-sugar group. If simple sugars are the trigger for fat gain, that wasn’t apparent in this study.

Rather, the low-carb/high-fat diet group lost less fat than the high-carb group. This remained true despite a large increase in fat burning among the low-carb subjects. It appears that the high-fat burn is insufficient to overcome the high-fat consumption. The authors used the word “remarkable” to describe the very small decline in fat burning among the low-fat eaters.The NIH research team measured a number of other outcomes among subjects. As expected, total cholesterol and HDL cholesterol dropped more among the low-fat dieters, while triglycerides dropped more in low-carb dieters.

Low-carb fans will likely criticize the new paper for the modestly high carbohydrate consumption of the low-carb group. Some critics would like to see subjects consuming fewer than 50 carb grams a day (rather than 140). Hall agreed that such very-low-carb diets would “result in fat losses comparable to low-fat diets.”

However, he said his mathematical model predicts only small differences in results with equal-calorie diets that include a wide range of fat and carbohydrate ratios. “The body acts to minimize such differences,” he said, “and the total calories in the diet is the primary driver of body fat loss.”

In other words, a calorie is a calorie, and the type of calorie makes little if any difference. This appears to be particularly true over the long periods of time—years and decades—during which most adults gain weight. Or, less frequently, lose it.

Several years ago, Hall oversaw a multi-center trial, funded by Taubes’ research group, to test the carbohydrate-insulin-fat gain hypothesis. The results of that trial are being prepared for publication. It will be interesting to see how they compare to the new NIH study results.

Hall has been featured by Runner’s World Newswire recently for his research showing that it takes more than 3,500 calories to lose or gain a pound of body weight.

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