BETHESDA, MD. — Restricting dietary fat proved much more effective at lowering body fat than restricting carbohydrates in a well-controlled study conducted by researchers at the National Institutes of Health.
Studying 19 obese men and women over two extended visits, the researchers found a reduced fat diet led to body fat loss at a rate 68% greater than a diet cutting a similar number of calories through carbohydrate restriction.
The research findings were published in an article, “Calorie for calorie, dietary fat restriction results in more body fat loss than carbohydrate restriction in people with obesity.” The article appears in the Sept. 1 issue of Cell Metabolism. The paper was written by a team of authors led by Kevin Hall, a researcher with the N.I.H.
In the study, subjects stayed in the Metabolic Clinical Research Unit at the N.I.H. Clinical Center in Bethesda. Participants stayed in the unit for two extended visits, eating the same food and doing the same activities.
“For the first five days of each visit they ate a baseline balanced diet,” the N.I.H. said. “Then for six days, they were fed diets containing 30% fewer calories, achieved by cutting either only total carbs or total fat from the baseline diet, while eating the same amount of protein. They switched diets during the second visit.”
During the study period, subjects were under 24-hour supervision, giving researchers full control over what participants ate. The low-fat dieters lost an average of 3.1 ounces per day over six days. Low-carbohydrates dieters lost 1.9 ounces per day.
The authors explained that numerous research studies have identified fat loss as a more important goal than weight loss in the treatment of obesity.
“Often, the greater weight losses with low-carbohydrate diets were attributed to sodium and water imbalances,” the researchers said.
Rather than an attempt to promote low-fat dieting, the study was aimed at evaluating bold claims by advocates of low-carbohydrate diets. The advocates cited in the study included David Ludwig, a researcher at Harvard Medical School, and Gary Taubes, an influential writer on low-carbohydrate dieting.
The research was aimed at looking into a claim by Dr. Ludwig that cutting carbohydrates creates a metabolically favorable environment for burning off fat.
A 2002 feature article by Mr. Taubes in the New York Times, defending the Atkins diet, has been credited with triggering an explosion for a couple years of low-carbohydrate dieting in the early 2000s.
The N.I.H. authors sought to test a more recent conclusion by Mr. Taubes that “any diet that succeeds does so because the dieter restricts fattening carbohydrates.” Mr. Taubes in 2011 published, “Why we get fat and what to do about it.” The book contends that certain kinds of carbohydrates rather than fats or simply excess calories are to blame for rising obesity rates.
“In other words, body fat loss requires reduction of insulinogenic carbohydrates,” the researchers said. “This extraordinary claim was based on the observation that even diets targeting fat reduction typically also reduce refined carbohydrates,” the N.I.H. researcher said. “Since the primary regulator of adipose tissue fat storage is insulin, and a reduction in refined carbohydrates reduces insulin, carbohydrate reduction alone may have been responsible for the loss of body fat—even with a low-fat diet.”
The study found that, as Dr. Ludwig and Mr. Taubes suggested, a reduced-carbohydrate diet is indeed effective at lowering insulin secretion. What happened next did not fit into the Ludwig/Taubes model.
“Interestingly, study participants lost even more body fat during the fat-restricted diet, as it resulted in a greater imbalance between the fat eaten and fat burned,” Dr. Hall said. “These findings counter the theory that body fat loss necessarily requires decreasing insulin, thereby increasing the release of stored fat from fat tissue and increasing the amount of fat burned by the body.”
The researchers said their findings were strong.
“Calorie for calorie, restriction of dietary fat led to greater body fat loss than restriction of dietary carbohydrate in adults with obesity,” Dr. Hall said. “This occurred despite the fact that only the carbohydrate-restricted diet led to decreased insulin secretion and a substantial sustained increase in net fat oxidation compared to the baseline energy-balanced diet.”
Dr. Hall said the study undercuts fundamental assertions of Dr. Ludwig.
“In contrast to previous claims about a metabolic advantage of carbohydrate restriction for enhancing body fat loss, our data and model simulations support the opposite conclusion when comparing the restricted fat and restricted carbohydrate diets,” he said.
Even stronger was his language regarding Mr. Taubes assertions that cutting carbohydrates represents the only path to weight loss.
“We can definitively reject the claim that carbohydrate restriction is required for body fat loss,” he said.
While challenging the idea that carbohydrates were uniquely responsible for obesity, the researchers expressed the opinion that a reduced carbohydrate diet, if maintained over time, would result in weight loss comparable to low-fat diets. The results of their study matched a math model of human metabolism. When simulating what might happen over longer periods, the model predicted relatively small differences in body fat loss with widely varying ratios of carbs to fat.
“Indeed, the model simulations suggest that isocaloric reduced-energy diets over a wide range of carbohydrate and fat content would lead to only small differences in body fat and energy expenditure over extended durations,” Dr. Hall said. “In other words, while the present study demonstrated the theoretical possibility that isocaloric diets differing in carbohydrate and fat can result in differing body fat losses, the body acts to minimize such differences. The endocrine and metabolic adaptations that allow for the relative insensitivity of body fat to dietary macronutrient composition may themselves have effects on health over the long term, but this was not investigated in the present study.”
