A new study suggests that cutting back on carbohydrates may help speed up metabolism and prevent weight gain. 

By Amanda MacMillan
November 15, 2018

Gaining back unwanted pounds after a period of weight loss is an all-too-common problem, and it’s not just about flagging willpower. Even when people follow their diet and exercise routine to a T, it’s not uncommon for their bodies to adapt to those missing pounds by slowing down their metabolism and burning fewer calories. This can lead to slowed progress, or even a reversal from weight loss to weight gain.

Now, a new study suggests that cutting back on carbs may boost metabolism and help people burn more calories, according to new research published yesterday in BMJ. The study authors say their findings challenge the belief that all calories work the same in the body—and suggest that the dreaded weight regain after dieting may be avoided by sticking to a low-carb eating plan.

The study included 164 overweight individuals who had just lost 10 to 14% of their body weight during an initial 10-week dieting period. Those people were split into groups and were assigned to either a low-, moderate-, or high-carbohydrate diet for an additional 20 weeks. Total calorie intake in all three groups was adjusted throughout the study so that none of the participants gained or lost significant amounts of weight.

Over those 20 weeks, the study authors kept track of participants’ energy expenditure, or the total number of calories they were burning. And they found that, at the same average body weight, those on the low-carb diet burned about 250 calories more per day than those on the high-carb diet.

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“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," said Cara Ebbeling, PhD, co-author of the study and co-director of the New Balance Foundation Obesity Prevention Center at Boston Children's Hospital, in a press release.

So why the big difference in results? David Ludwig, MD, Ebbeling’s co-author and co-director, ventured one possible explanation. Processed carbohydrates—which “flooded our diets during the low-fat era”—raise insulin levels, he said in the press release, which drives fat cells to store excess calories. This increases hunger and slows metabolism, which is “a recipe for weight gain.”

Cutting back on carbs, on the other hand, allows the body’s metabolism to speed back up to normal levels, the authors suggest. They also found that ghrelin, a hormone thought to reduce calorie burning, was significantly lower on the low- versus the high-carb diet.

This certainly sounds encouraging, especially for anyone who’s achieved a weight-loss goal only to see their hard-earned results fade away (and their waistline expand) over the following months. And this isn’t the first time that low-carb diets have gotten a thumbs-up for weight loss: Plenty of other research—and anecdotal evidence—suggests that these types of diets (like Atkins or the extremely popular ketogenic diet) can deliver real results.

But then again, we’ve also heard the opposite: That low-carb diets don’t work long-term, that they can affect mood and make people feel stressed, and that you can absolutely eat carbs (even pasta!) and still lose weight. So before you decide that giving up bread and loading up on meat is the answer to keeping off unwanted pounds, it’s important to consider all the facts.

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First, this study wasn’t looking at just any low-carb eating plan; it featured a very specific regimen of pre-formulated meals, with fat, protein, and carbohydrate content calculated to the exact gram. So it’s only natural that people attempting to follow a similar diet, without the help of scientists and ready-made dinners, might not have the same success rates in real life.

Second, the carbs provided to all three groups were all high quality, according to the study: They consisted of whole grains (rather than highly processed ones) and minimal sugars—so no candy or pastries, for example.

Cynthia Sass, MPH, RD, Health’s contributing nutrition editor, says it’s important to remember that low-carb doesn’t automatically mean healthy. “I think at this point we can all agree that low-fat diets aren’t optimal for health, particularly when the carb sources are processed and refined,” she says. However, she adds, “just as not all calories are created equal, not all low-carb diets are created equal.”

Even on low-carb diets, she says, there should still be room for healthy carbohydrates—like non-starchy veggies, berries and other fresh fruit, and small portions of whole grains, pulses, and starchy vegetables like sweet potatoes. “Think a half cup, about the size of half a tennis ball per meal, rather than none,” she says.

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Fitting in these healthy foods will ensure you’re getting protective antioxidants, phytonutrients, vitamins, minerals, and fiber, she says. Plus, “whole, plant-based foods support a healthy gut microbiome, which is vital for immunity, mood, and digestive health,” she adds.

The study authors are hopeful that their findings may have major implications for the treatment of obesity. But more research is needed, they say, to compare different types of low-carb diets—including extreme carbohydrate restriction such as in the keto plan. Even if the benefits suggested in this study are confirmed, they wrote in their paper, still more work would be necessary “for optimal translation to public health.”

For now, Sass says, the most important components of long-term weight loss—and long-term health—remain unchanged. “It’s important to find an approach that is realistic, sustainable, and allows you to feel well mentally and physically,” she says.

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