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Should You Cut Carbs, Not Calories?

A new study gives lots of weight to the idea that low-carb diets are key to keeping off pounds

spinner image an assortment of low carb food
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A major new study could help more weight-loss winners keep off the pounds they've shed — if they can pass on the bread basket.

The research, published in the journal BMJ, found that among study participants who had just successfully lost weight, those who followed a diet with the lowest percentage of carbohydrates not only maintained their new, lower weight but also had a higher resting metabolism. That is, they burned calories at a higher rate when, say, they were sitting around watching TV than those eating more carbs would. That last detail is what has some experts excited about the results — calling them “profound” and potentially guideline-shifting — because it indicates a possible way forward for obesity treatments.

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As the authors acknowledged in the study, such treatments have been difficult even for people working quite hard to follow them. “With weight loss, hunger increases and energy expenditure decreases — physiological adaptations that defend against long-term weight change.” Their work put participants into that challenging situation, since they’d lost 12 percent of their body weight in the 10 weeks prior to the start of the carb-measuring phase of the work.

In that phase, the 164 adult participants were given meals that featured either 20, 40 or 60 percent of their daily calories from carbs. (The percentage of protein was held constant for all.) From there, researchers measured total energy expenditure, resting energy expenditure, and related things like the levels of hormones such as leptin and ghrelin that aid in metabolism.

Compared with the group eating the highest amount of carbs, the group eating the lowest ended up burning roughly 250 more calories a day — an amount that one expert speaking to the New York Times equated to a 20-pound weight loss after three years on such a diet. (It's also worth noting that those in the middle group, consuming a moderate amount of carbs, also burned more calories daily than those eating the most carbs.) Experts pointed out that those on the low-carb diet had the lowest levels of the hormone ghrelin, which promotes the feeling of hunger and the storage of fat, and that this could be one key to the results. 

While the findings aren’t expected to be the last word on carbs and dieting, they could, the study's authors note, help answer what was one of the bigger questions on their minds in the design of their research: “Why the average person today, compared with 40 years ago, seems to be ‘defending’ a much higher body weight.”

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Part of the answer, they believe this study suggests, could be related to the effect of eating higher levels of carbs, with their higher glycemic load, on things like insulin levels. After a meal full of carbs, they wrote, higher levels of insulin seem to direct the metabolic process away from “oxidation” (broadly, burning stuff off) and closer to “storage in adipose tissue” (building up fat reserves). This carbohydrate-insulin model, they write, “offers a physiological mechanism for understanding why obesity rates have increased since the 1970s in the United States, as dietary fats were replaced with high glycemic load foods, including refined grains and added sugars.”

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