Low-carb diets make us burn more calories, finds largest, longest feeding trial to date

Low carb diet helps us burn calories faster, supporting the Carbohydrate-Insulin Model of obesity
Supporting the Carbohydrate-Insulin Model of obesity, a new study finds that low-carb diets increase our energy expenditure.

Most people who diet to lose weight regain the pounds within a year or two, in part because the body adapts by slowing down metabolism and burning fewer calories. A new study known as the Framingham State Food Study, or (FS)2, suggests that low-carb diets can help people keep the weight off, showing that eating fewer carbohydrates increases the number of calories burned. The findings, published today in the BMJ, could help make obesity treatment more effective.

“This is the largest and longest feeding study to test the ‘Carbohydrate-Insulin Model,’ which provides a new way to think about and treat obesity,” says David Ludwig, MD, PhD, who with Cara Ebbeling, PhD, co-directs the New Balance Foundation Obesity Prevention Center at Boston Children’s Hospital. “According to this model, the processed carbohydrates that flooded our diets during the low-fat era have raised insulin levels, driving fat cells to store excessive calories. With fewer calories available to the rest of the body, hunger increases and metabolism slows — a recipe for weight gain.”

The Carbohydrate-Insulin Model and the idea that not all calories are alike have been the subject of much recent debate. Ludwig contends that prior studies haven’t been rigorous enough or long-lasting enough to provide a proper test. The new study, led by Boston Children’s in partnership with Framingham State University, tightly controlled what people ate for 30 weeks, providing them with fully prepared food-service meals. Participants’ weight, insulin secretion, metabolic hormones and total energy expenditure (calories burned) were carefully tracked.

Comparing carb levels head to head

After careful telephone screening of 1,685 potential participants, Ebbeling, Ludwig and colleagues enrolled 234 overweight people age 18 to 65 (body mass index, 25 or higher) to an initial weight-loss diet for about 10 weeks. Of these, 164 successfully lost 10 to 14 percent of their body weight and continued to the maintenance phase. For this, they were randomized to follow high-, moderate- or low-carbohydrate diets for an additional 20 weeks — with carbs comprising 60, 40 and 20 percent of total calories, respectively.

Our observations challenge the belief that all calories are the same to the body.

In all three groups, the carbs were of high quality, conforming to guidelines for minimizing sugar and using whole rather than highly processed grains. Total calorie intake was adjusted to maintain weight loss, so weight did not change notably in any group. Instead, the goal during this phase was to compare energy expenditure — how the different groups burned calories at the same weight.

Over the 20 weeks, total energy expenditure (measured with the doubly labeled water method) was significantly greater on the low-carb versus the high-carb diet. At the same average body weight, participants who consumed the low-carb diet burned about 250 kilocalories a day more than those on the high-carb diet.

“If this difference persists — and we saw no drop-off during the 20 weeks of our study — the effect would translate into about a 20-pound weight loss after three years, with no change in calorie intake,” says Ebbeling.

In people with the highest insulin secretion at baseline, the difference in calorie expenditure between the low- and high-carb diets was even greater, about 400 kilocalories per day, consistent with predictions of the Carbohydrate-Insulin Model. Ghrelin, a hormone thought to reduce calorie burning, was significantly lower on the low- versus high-carb diet.

“Our observations challenge the belief that all calories are the same to the body,” says Ebbeling. “Our study did not measure hunger and satiety, but other studies suggest that low-carb diets also decrease hunger, which could help with weight loss in the long term.”

Next up: Sugar

Ludwig and Ebbeling recently launched another clinical trial called FB4, in which 125 adults with obesity are asked to live in a residential center for 13 weeks. They’re being randomized to one of three diets: very-low-carb, high carb/low sugar or high carb/high sugar diets, with calorie intakes individually matched to their energy expenditure. Results are expected in 2021.

The current study was funded by the Nutrition Science Initiative (via grants from the Laura and John Arnold Foundation and the Robert Lloyd Corkin Charitable Foundation), the New Balance Foundation, Many Voices Foundation, Blue Cross/Blue Shield and the National Institute of Diabetes and Digestive and Kidney Diseases. See the paper (DOI: 10.1136/bmj.k4583) for a full list of authors and affiliations.

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