Sugar lows: Participants on a low-carb diet showed greater total energy expenditure than those on the high-carb diet, according to research conducted by Dr David Ludwig, a nutrition professor at Harvard. Picture: TREVOR SAMSON
Sugar lows: Participants on a low-carb diet showed greater total energy expenditure than those on the high-carb diet, according to research conducted by Dr David Ludwig, a nutrition professor at Harvard. Picture: TREVOR SAMSON

A major new Harvard study shows that reducing carbohydrates and replacing them with fats speeds up metabolism. The research helps to overcome one of the biggest hurdles in conventional weight-loss diets.

The “plateau” — a metabolism slowdown — prevents further weight loss for people on conventional diets.

Lead author of the study published in the BMJ (British Medical Journal) is Dr David Ludwig, a nutrition professor at Harvard’s TH Chan School of Public Health. He says that the new study shows that “the case against carbohydrates is growing”.

Experts describe the randomised study as well-designed. Randomised controlled trials enjoy a reputation as the gold standard of scientific research. Research by US-based Virta Health, published in Diabetes Review in February, and experts say the Harvard study dishes up a recipe for fast, effective, safe and sustainable weight loss.

The Harvard scientists say their study is one of the “largest feeding studies ever conducted”. They avoid dealing with the controversy about the health benefits of animal- versus plantfood diets.

The study balances recent Harvard research in the Lancet journal, suggesting that low-carb diets will shorten lifespans — more so if diets contain animal foods. The Lancet study made headlines across the globe despite flaws. It was also epidemiological — observational or associational — so, unlike randomised controlled trials, could not imply causation.

In a New York Times report, Dr Dariush Mozaffarian, dean of the Friedman School of Nutrition Science and Policy at Tufts University, called Ludwig's study “profound”. It is “one more nail in the coffin of our global calorie-counting, low-fat fetish”, said Mozaffarian, who was not involved in the research.

He called for radical revision of official dietary guidelines: “It’s time to shift guidelines, government policy and industry priorities away from calories and low-fat and toward better diet quality.”

UK professor of genetic epidemiology Tim Spector, says Ludwig’s study effectively “kills the myth that all calories are equal”. It shows that “carbs and fats have different effects on our metabolic rate”, adds Spector, a specialist in genetics, epigenetics, microbiome and diet at King’s College, London.

Experts say the Harvard study further destabilises the foundation on which conventional low-fat weight-loss diets are built: the calories-in, calories-out obesity model. This holds that “a calorie is a calorie” and all people have to do to lose weight is eat less and move more.

Ludwig explained the significance of the new study in a Los Angeles Times editorial. “People have a hard time believing that weight control isn’t just a matter of calories eaten and calories burned,” he wrote. “But it’s not just how much but also what you eat that significantly affects your metabolism.” In turn, this affects how much weight will be gained or lost.”

Ludwig dismisses calories-in, calories-out diets as offering “no compelling biological explanation for the obesity epidemic”. Instead, he and his co-authors studied an alternate hypothesis about obesity, the carbohydrate-insulin model.

The model holds that overeating is not the underlying cause of long-term weight gain. Rather, the cause is the “biological process of gaining weight that causes us to overeat”. Ludwig explained a mechanism of action by which carbs lead to obesity.

“When we eat processed carbohydrates (particularly refined grains, potato products and sugars), our bodies produce more insulin. Too much insulin, one of the most powerful hormones, forces our fat cells into calorie-storage overdrive,” Ludwig said.

These rapidly growing fat cells then hoard too many calories, leaving too few for the rest of the body. This makes people hungry and if they persist in eating less, their metabolism slows down, he wrote.

In a BMJ press release, the Harvard scientists clarified their aim: “To better understand the role of dietary composition on energy expenditure.”

They compared the effects of diets varying in carb-to-fat ratio on energy expenditure over a 20-week period. In the trial, 234 overweight adults aged 18 to 65 with a body mass index of 25 or higher participated in an initial weight-loss diet for about 10 weeks. Of these, 164 achieved the target weight loss of around 10% of body weight.

The researchers then randomly assigned participants to follow either a high (60%), moderate (40%) or low (20%) carbohydrate diet for 20 weeks.

After adjusting for potentially influential factors over the 20-week, they found that participants on the low-carb diet showed greater total energy expenditure than those on the high-carb diet. They burned 209 to 278 kilocalories a day more. That’s about 50 to 70 kilocalories a day increase for every 10% decrease in the contribution of carbohydrate to total energy intake, the authors said.

The participants with the highest insulin secretion at the start of the study showed even greater difference in total energy expenditure between low- and high-carb diets: up to 478 kilocalories a day. That is consistent with the carbohydrate-insulin model, the authors wrote. If this effect persisted, “it would translate into an estimated 10kg weight loss after three years, assuming no change in calorie intake”.

Hormones involved in energy balance (ghrelin and leptin) also showed “potentially advantageous” changes in participants on low-carb diets.

The new study suggests a better strategy for weight loss: “Not counting calories but focusing on reducing carbs,” Ludwig wrote.

The study does not prove validity of the carbohydrate-insulin model and needs to be replicated, he said. However, it “credibly [makes] the case that all calories are not alike to the body”.

And novel effects of specific foods “might make a big difference in our ability to lose weight and keep it off”, Ludwig said.