It’s one of the most hotly contested areas of dieting: How much do carbohydrates matter when it comes to weight loss?

If you ask a number of celebrities and authors of diet books, it’s pasta, bread, and cookies that stand between you and a svelte physique.

These low-carb proselytizers make very specific claims about the effect cutting carbs has on the body, suggesting that it can speed up fat loss and increase calorie burn. Indeed, many dieters have found at least short-term success following low-carb schemes like the Atkins or Dukan diet.

A group of researchers published the best test of those claims to date in the journal American Journal of Clinical Nutrition.

They didn’t find evidence that carbs are the magic key to weight and fat loss. But the study demonstrates just how controversial and fraught the low-carb idea is, and how, despite all the magical claims, there’s a lot we still don’t understand about this diet.

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The carbohydrate-insulin hypothesis

The main scientific model behind the low-carb approach is the ”carbohydrate-insulin hypothesis,” which journalist Gary Taubes, Harvard professor David Ludwig, University of California San Francisco’s Robert Lustig, and others have extensively promoted, including in a New York Times piece by Taubes in July 2017. It suggests that a diet heavy in carbohydrates (especially refined grains and sugars) leads to weight gain because of a specific mechanism: Carbs drive up insulin in the body, causing the body to hold on to fat and suppress calorie burn.

According to this hypothesis, to lose weight you reduce the amount of carb calories you eat and replace them with fat calories. This is supposed to drive down insulin levels, boost calorie burn, and help fat melt away.

This method arose as an alternative to the classic approach to dieting, in which calories in general are restricted. So instead of just cutting calories, you’re supposed to change the kinds of calories in your diet to lose weight.

But what’s often lost in all the boosterism around the low-carb approach is that it is still an unproven hypothesis in science.

Most tests of low-carb diets have involved either measuring what people eat over long periods of time or assigning people to different diets, and then tracking their weight and health outcomes. But people can’t always stick to the diets they are assigned to for long periods. And when you measure what people are eating naturally, there’s always a chicken-and-egg problem, which is why many diet studies are marred by confounding factors and flaws. (We explained them here at Vox.)

A study tested the low-carb model — and found little success

The study, led by National Institutes of Health obesity researcher Kevin Hall, tried to address those limitations in an effort to see whether a very low-carb diet (and resulting drop in insulin) led to that often-touted increase in fat loss and calorie burn.

Hall and his colleagues confined 17 overweight and obese patients to the hospital for two months, where they measured their every movement and carefully controlled what they were eating. (Diet researchers call this the “gold standard,” since it was an extremely well-controlled experiment, with all food provided, and it used the best technologies for measuring energy expenditure and body composition.)

For the first month of the study, participants were put on a baseline diet, which was designed to be similar to what they reported they were eating outside the hospital, including lots of sugary carbohydrates. For the second month, the participants got the same amount of calories and protein as they did in the first month of the study, but this time they ramped up the amount of fat in their food and got far fewer carbs.

The researchers were then able to measure what happened to the participants’ insulin production, and related energy burn and fat loss, when they ate fewer carbs.

The results weren’t nearly as dramatic as low-carb boosters claim. “In this case,” Hall said, “we saw daily insulin secretion drop substantially within the first week and stay at a low level. But we only saw a small transient increase in energy expenditure during the first couple of weeks of the [low-carb] diet, and that essentially vanished by the end of the study.”

That short-lived increase in calorie burn amounted to about 100 extra calories per day — less than the 300 to 600 calories promised by low-carb gurus. And compared with the baseline diet, the low-carb diet did not cause subjects to experience an increase in fat loss. To be more specific, it took the full 28 days on the low-carb diet for the subjects to lose the same amount of fat as they did in the last 15 days on the baseline (higher-carb) diet that wasn’t even designed to get them to lose weight.

In other words, the researchers did not find evidence of any dramatic effects after switching to a low-carb diet.

”According to the insulin-carbohydrate model, we should have seen an acceleration in the rate of body fat loss when insulin secretion was cut by 50 percent,” Hall said. But they didn’t, which he thinks suggests that the regulation of fat tissue storage in the body has to do with more than just insulin levels and their relationship with the carbs we eat.

The results of the study also echoed a previous paper on the insulin-carbohydrate model, where Hall found that when people who cut fat in their diets, they had slightly greater body fat loss than when they cut the same number of calories from carbs. (Here’s Hall’s new review of the literature on the carb-insulin model of obesity.)

”These studies represent the first rigorous scientific tests of the carb-insulin model in humans,” Hall added. “The public needs to understand that this [insulin-carbohydrate] model now has pretty strong evidence against it.”

Can pasta and bread lovers now rejoice?

The study is a real blow to the low-carb camp, said Richard Bazinet, a professor of nutritional sciences at the University of Toronto. “For the [insulin-carbohydrate] hypothesis to be true, you’d expect they’d lose more weight, and have increased energy expenditure in the low-carb group. But the researchers just didn’t see that.”

But before we throw out the low-carb approach to weight loss and load up on a bowl of linguini, let’s be clear: This study had some important limitations, leading some researchers to react more cautiously. It lacked a control for comparison, and while the baseline diet was designed to keep participants at about the same levels of energy burn they experienced outside of the study, the participants started to lose weight on that diet too. So they were already slimming down by the time they started their low-carb month.

And while the study was designed to overcome some of the limitations of real-world diet studies, a highly controlled setting that amounts to confining people to a hospital and lab isn’t exactly representative of how people actually live and eat.

”These points, along with the small sample size and short-term follow-up, prevent the ability to draw conclusions about the effects of a very low-carbohydrate versus usual carbohydrate diet,” said Deirdre Tobias, an associate epidemiologist at Harvard Medical School and Brigham and Women’s Hospital.

What’s more, one of the promises of the low-carb, high-fat diet is that when people start eating this way, they naturally cut back on calories because they’re more satiated (from the protein and fat in their diet). This study didn’t measure that either, since the participants were forced to stick to strictly measured menus.

But as Bazinet points out, “The study ... doesn’t see any [relationship between a decrease in insulin and an increase in fat loss]. Show me a better study that supports this.”

There isn’t any, he added.

Other big studies comparing popular diets of different macronutrient compositions also suggest that the low-carb approach probably isn’t a sustainable solution for weight loss. While low-carb diets seem to outperform their higher-carb counterparts in the short term, that effect goes away after about one year.

A 2015 review of the research on different types of diets in the Lancet found that low-carb diets outperform low-fat diets. But as a related commentary (also authored by Hall) pointed out, the difference in weight loss among groups of dieters was tiny: “Participants prescribed low-carbohydrate diets lost only about 1 kg of additional weight after 1 year compared with those advised to consume low-fat diets.”

In a high-quality randomized control trial, published in JAMA in 2018, study participants following low-carb and low-fat diets once again lost about the same amount of weight after a year: 13 pounds in the low-carb group versus 12 pounds in the low-fat group. And when the participants’ individual weight loss was charted by the researchers, they found the exact same variation between the two groups, as this analysis from Examine.com shows:

Here's the individual weight loss/gain from the recent major LF vs LC study.



It's impressive how both amount and distribution is near-equal.



Full analysis: https://t.co/Ewl1DCXKle pic.twitter.com/Wo2nuDisc9 — Examine.com (@Examinecom) February 20, 2018

Tobias urged dieters not to lose sight of the bigger picture. “Low-carb versus low-fat should not be the focus for people selecting a weight loss diet.” The focus, she said, should be on improving the quality of food that people eat instead.