That was the first question tackled by the Nutrition Science Intitiative’s Energy Balance Consortium of 10 obesity experts. (NuSI was largely financed by billionaire former hedge fund manager John Arnold and his wife, Laura.)
Kevin Hall, senior investigator at the National Institute of Diabetes and Digestive and Kidney Diseases, led two studies designed to answer the question: Do carbs drive you to gain more body fat because they boost levels of the hormone insulin?
“The argument is that people are consuming too many carbohydrates, which drive up insulin levels in the blood,” explains Hall.
“Insulin causes the body’s fat cells to suck in too many calories, and because calories are trapped in the fat cells, the rest of the body is starving. That makes you hungrier, so you eat more calories.”
And because the body is starving, adds Hall, “it slows down its metabolic rate, so it burns fewer calories.” So cutting carbs should boost calorie burning and shrink body fat.
At least that’s the theory.
Hall’s first study housed 19 people in a lab where they ate only the food the researchers provided. Those diets cut 800 calories either from carbs (about half of the cuts came from sugar) or from fat for one week each.1
“When we cut carbs, daily insulin secretion went down,” says Hall. If the carbohydrate-insulin theory were correct, “that should have released fat from their fat cells, boosting fat loss while relieving the internal starvation and therefore causing calorie burning to go up.”
It didn’t. “The number of calories they were burning went down,” says Hall. “So we found the opposite result.” Instead of speeding up fat loss, the low-carb diet actually slowed it down.
But that study didn’t cut carbs enough or last long enough, argued some critics. So Hall did a longer study using a very-low-carb diet.2
“After one month of eating a high-sugar, high-carbohydrate diet, we cut the carbs down to 5 percent, cranked the fat up to 80 percent, and kept protein and calories constant,” Hall explains.
The result: “The rate of fat loss actually slowed down for the first two weeks, and then picked back up to the normal rate again for the last two weeks,” says Hall. So the low-carb diet didn’t speed fat loss.
“We did see a very slight increase in the number of calories that were being burned—57 more a day—on the very-low-carb diet,” adds Hall. But NuSI’s Energy Balance Consortium had agreed beforehand that only an increase of at least 150 calories a day would be meaningful.
“Our results add to the evidence from many other controlled feeding studies on more than 500 people,” says Hall. Those studies failed to show that cutting carbs boosts calorie burning or fat loss more than cutting fat.3
“If anything,” says Hall, “there is a statistically significant greater fat loss and calorie burning on a low-fat diet. But the effects are so small that they’re physiologically meaningless.”
It’s still possible that a very-low-carb diet curbs dieters’ appetites. “We didn’t test that,” says Hall.
If so, that might explain why some studies report that people tend to lose more weight over the first few months when they are prescribed a low-carb diet.
“But over the long term that doesn’t seem to persist either,” says Hall. In studies that last a year or more, the difference in weight loss is negligible.4
“Sometimes you can’t see any significant difference, and sometimes you can see a few pounds difference that is clinically meaningless,” notes Hall.
The DietFits Study
“This study should be able to document, for the first time ever, what happens when free-living participants maintain compliance with a very-low-fat diet and a very-low-carbohydrate diet for an entire year,” says the NuSI website.
Christopher Gardner, professor of medicine at Stanford University, led that trial—called DietFits—which randomly assigned 609 overweight or obese people to either a healthy low-fat diet or a healthy low-carb diet.
“We told everyone in both groups to eat as little white flour and sugar and as many higher-fiber vegetables as possible,” Gardner explains.
But the participants weren’t told to cut calories. “If you prescribe calorie restriction, people feel deprived,” says Gardner. “So we just said, ‘Eat as low as you can on fat or carbs and don’t be hungry.’” And, whether they cut fat or carbs, “each group reported a 500-calorie reduction.”
After a year, each group had lost an average of about 13 pounds.5 And, as in earlier studies, the results varied dramatically. “Someone lost 60 pounds, someone gained 20 pounds, and we saw everything in between,” notes Gardner. “The range, which was similar in both diet groups, was stunning.”
It didn’t matter if people were resistant to their body’s insulin when they entered the DietFits study.
“We assumed that insulin-resistant people would do better on a low-carb diet—as they did in some earlier studies—but they didn’t,” says Gardner.
Maybe that’s because both groups were told to eat healthy foods, he suggests. “In some older studies, when researchers told people to eat less fat, they weren’t particular about which low-fat foods. Coke and white flour and sugar are low-fat.”
The full study hasn’t been published yet, Gardner’s team hasn’t yet analyzed data looking at the participants’ gut microbes, and two more NuSI studies are still in progress.
But the Arnolds are not funding new NuSI studies. If NuSI was looking for clear-cut answers, it didn’t find them.
1 Cell Metab. 22: 427, 2015.
2 Am. J .Clin. Nutr. 104: 324, 2016.
3 Gastroenterology 2017. doi:10.1053/j.gastro.2017.01.052.
4 JAMA 312: 923, 2014.