SAN DIEGO – In adults with obesity, lowering intake of dietary fat may lead to greater body fat loss than lowering intake of dietary carbohydrates, results from a small federally funded study showed.

“A lot of focus has been placed on the idea of the competing importance of dietary carbohydrate versus dietary fat in the treatment and prevention of obesity,” lead study author Kevin D. Hall, Ph.D., said during a press briefing at the meeting of the Endocrine Society. “The fashion of which is the evil macronutrient of choice at one time or another is vacillating back and forth. More recently I would say that dietary carbohydrate has been the one demonized, with sugars and refined carbohydrates being the main culprit.”

Dr. Hall, a senior investigator at the National Institute of Diabetes and Digestive and Kidney Diseases, went on to note that at the level of the whole body, “you can think of fat balance as a balance between the intake of fat and how much fat is being burned. So if you’re eating a certain amount of fat and burning that same amount of fat, your body fat will be stable. The only way to perturb that fat balance at the whole-body level is to induce an imbalance between intake of fat and the amount of fat that is being burned.” The low carbohydrate advocates will suggest that an obese adult should decrease carbohydrate intake in the diet, thereby lowering insulin. This in turn has the effect of releasing fat from the fat cells, thereby leading to an imbalance that will eventually rebalance at a lower level of body fat. Alternatively, another way an obese adult could potentially achieve the same results is to cut dietary fat. The resulting state of fat imbalance would promote loss of body fat, according to Dr. Hall.

In an effort to investigate the whole-body energy expenditure and metabolic fat balance from selective restriction of dietary carbohydrate versus dietary fat, the researchers enrolled 19 nondiabetic volunteers with a mean age of 34 years and a mean body mass index of 36 mg/m². Of the 19 volunteers, 10 were men. The volunteers were housed in a metabolic ward at the National Institutes of Health for a pair of 2-week inpatient visits. For 5 days, everyone was fed a eucaloric baseline diet of 50% carbohydrate, 35% fat, and 15% protein that was designed to provide them with the number of calories they needed to maintain their body weight. Next, the volunteers were randomized to one of two groups where they received a 30% reduced energy diet by restriction of their intake of either fat or carbohydrate. “We provided all of their food; they had no choice in what they ate,” Dr. Hall said.

After a 2-4-week washout period the volunteers returned to the ward and received the alternative diet after the same 5-day baseline run-in phase. “The idea here was [that] we knew exactly how much they ate, and during the time they spent in metabolic chambers we could measure how much fat they burned,” he explained. “So we could calculate how fat oxidation changed as well as the fat imbalance in the body. The low-carb diet selectively reduced carbohydrates, keeping fat and protein at the constant levels, whereas the low-fat diet selectively reduced fat, keeping carbohydrate and protein at baseline levels.”

During the 6-day diet phase the researchers cut 30% of participants’ calories, selectively from either carbohydrates or fat. This translated into about 800 calories per day being cut from both diets.

Dr. Hall reported that reduction of dietary carbohydrates led to a significant increase in whole-body 24-hour fat oxidation (P< .0001), while isocaloric reduction of dietary fat in the same subjects had no significant effect on 24-hour fat oxidation (P = .15). However, the cumulative metabolic fat balance (adding up the daily imbalance between intake and expenditure of fat) indicated that the reduced fat diet resulted in about 80% greater body fat loss, compared with the reduced carbohydrate diet (P = .0003).

Dr. Hall acknowledged certain limitations of the study, including the fact that it did not address the long-term efficacy of low-carbohydrate or low-fat diets. One remaining key unanswered question is whether “the low fat diet continue to outpace the low carbohydrate diet with respect to body fat?” he asked. “This is a short-term study, a small number of people with obesity under metabolic ward conditions. We can’t extrapolate about what might happen to their health with respect to these diets, if they were able to adhere to them for long periods of time.”

The study was funded by the Intramural Research Program of the NIDDK. Dr. Hall reported having no relevant financial conflicts.

dbrunk@frontlinemedcom.com

On Twitter @dougbrunk

Lowering dietary fat, not carbs, led to greater body fat loss