Keeping weight off may be about more than just "calories in and calories out": Some diets may be better calorie-burners than others, a new study suggests.
The study, which involved people trying to maintain weight loss, found that participants burned more calories on a low-carb diet than a high-carb diet. Specifically, among participants with the same average body weight, those who ate a low-carb diet burned about 250 more calories a day than those on the high-carb diet, while engaging in similar levels of physical activity.
The findings, which are published today (Nov. 14) in the journal The BMJ, suggest that low-carb diets may help people keep weight off over the long term, a notoriously difficult feat.
"The type of calories you consume affect the number of calories you burn," David Ludwig, co-principal investigator of the study and co-director of the Boston Children's Hospital's New Balance Foundation Obesity Prevention Center, told Live Science. "These novel effects of food, beyond calorie content, may help make long-term weight control easier and more effective." [7 Tips for Moving Toward a More Plant-Based Diet]
However, some experts say it's too soon to recommend that the public switch to a low-carb diet like the one in the study for weight-loss maintenance, in part because the long-term health effects of such diets are unclear.
"It's too early to really say whether or not this type of low-carb diet is healthy in the long run," said Dana Hunnes, a senior dietitian at the Ronald Reagan UCLA Medical Center in Los Angeles, who was not involved in the study.
The "carbohydrate-insulin" model
The study aimed to test a hypothesis known as the "carbohydrate-insulin model of obesity." According to this idea, processed carbohydrates that have a high-glycemic index lead fat cells to store excess calories rather than burning them. (High-glycemic foods release sugar quickly into the bloodstream.)
However, some short-term studies (typically less than two weeks) have found no difference between high-carb and low-carb diets regarding the number of calories people burn. But the new study aimed to look at this question over a longer period, around five months.
The study involved 164 overweight adults who first underwent a weight-loss regimen in order to lose around 10 percent of their body weight. Then, they were randomly assigned to follow a low-, moderate- or high-carbohydrate diet — with 20, 40 or 60 percent of their calories coming from carbs, respectively — for 20 weeks. The low-carb group also consumed a higher amount of fat, but all groups consumed about the same amount of protein, sodium and added sugar. All of the participants were provided with fully prepared meals to ensure they were consuming these precise levels of nutrients.
The researchers also adjusted each participant's calorie intake so that they would maintain their current weight, and not gain or lose weight. The researchers then measured the participants' metabolic rate, or how many calories they were burning throughout the day. Participants also wore accelerometers to measure their level of physical activity.
When the researchers compared the calories burned by participants who weighed the same, they found that those on the low-carb diet burned 209 to 278 calories more per day than those on the high-carb diet.
If this difference persisted over time, it would translate to about a 20-pound weight loss over three years, without a change in calorie intake, the researchers said.
The effect was greatest among participants who naturally tended to secrete high levels of insulin after consuming glucose. (Insulin is a hormone that helps get sugar, or glucose, from the bloodstream into cells). Among these participants, those on the low-carb diet burned around 400 calories more per day than those on the high-carb diet.
This finding is consistent with the carbohydrate-insulin model, the researchers said. The model proposes that a lower- carb diet will lower insulin levels and "produce other beneficial hormone changes that lead fat cells to release their pent- up calories," Ludwig said. "With more calories in the blood — not trapped in fat cells — the brain and muscle have better access to the fuels they need."
Future studies
Still, the new study cannot prove that the low-carb, high-fat diet alone caused participants to burn more calories. Although the diets were as similar as possible between groups, other dietary factors, such as levels of certain nutrients not accounted for in the study, could have played a role in the effect, the researchers said. So more research is needed to understand why participants in the low-carb group burned more calories.
More studies are also needed to examine how the findings might be applied to weight-loss treatments to help people in a real-world setting.
For example, participants in the study had their meals prepared and precisely adjusted to maintain their weight. But in the real world "we're not adjusting our intake every week or days based on a scientific equation," Hunnes told Live Science, so it's unclear if the results would apply to people who were not following such a precise diet.
And because the study was 20 weeks, the long-term effects of the specific low-carb diet followed in the study — such as risks for heart disease or overall mortality — are not known, Hunnes added. (A recent study found that people who consumed both high- or low-carb diets were at greater risk for early death, compared with those who consumed a moderate-carbohydrate diet.)
Ludwig and colleagues have just started a trial that will look at the effects of three different diets on people's energy expenditure: a very-low carb diet; a high-carb/low-sugar diet; and a high-carb/high-sugar diet.
Originally published on Live Science.
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