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Why Do Low-Carb Diets Cause Weight Loss?

Diets Work Because People Eat Less, New Study Shows

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March 14, 2005 - Low-carb weight loss dietsLow-carb weight loss diets do work, at least in the short term, but why?

A meticulously conducted short-term study may have the answer. Temple University researchers reported that participants in a new study lost weight when they restricted carbohydrates simply because -- drum roll, please -- they ate fewer calories.

The researchers found no evidence to support other popular theories for why low-carb diets work, such as the idea that calories from carbs are somehow burned less efficiently than calories from other sources.

"It had nothing to do with water being shed or with carbohydrates being somehow different in the way that they are metabolized by the body," lead researcher Guenther Boden, MD, tells WebMD.

"The people in our study ate less. It was a simple as that."

"Some very prominent researchers have bought into the idea that you can cut carbohydrates without cutting calories and lose weight. We found that people lost weight because they took in fewer calories."

Impact of Low-Carb Diets on Calories

A whole lot fewer calories. The 10 obese study participants, who all had type 2 diabetes, cut their calorie intake by a third while on the most carbohydrate-restrictive phase of the low-carb diet.

Because the subjects were housed in a hospital research center for the length of the 21-day study, investigators were able to track every calorie that was eaten and burned. They also measured fat- and water-derived weight loss, blood sugar control, and cholesterol levels.

The participants ate their regular diets for the first seven days. They then followed the low-carb diet for the next two weeks, limiting carbohydrates to just 20 grams a day but eating unlimited amounts of protein and fat.

Prior to starting the low-carb diet, the subjects ate an average of 3,100 calories a day. While on the low-carb diet, they ate about 2,100 calories, even though they were told to eat as much of the permitted foods as they wanted.

The participants lost an average of 3.6 pounds while on the low-carb diet, their blood sugar level normalized and their cholesterol levels also improved.

The study is published in the March 15 issue of Annals of Internal Medicine.

"In my opinion this study proves conclusively that the weight loss in the Atkins diet is due to reduced calorie intake, period," Boden says.


Can Carbohydrates Stimulate Appetite?

Boden says he was amazed to find that the participants did not compensate for eating fewer carbs by eating more of other foods. He added that they seemed happy with the low-carb diet and did not complain of hunger.

"They ate 1,000 fewer calories a day and did not miss them," he says. "That told me that it was the carbs that fueled their excessive appetites in the first place. In my opinion, carbohydrates do stimulate appetite."

But weight loss researcher George Bray, MD, tells WebMD that he believes the monotony of low-carb diet explains why people eat less. It is also why most people don't stay on any restrictive weight loss plan for very long.

"The strategy for every weight loss diet that I know if is to restrict choice," he says. "Monotony is essential to why people eat less."

He says there is no convincing evidence that low-carb diets are easier to stay on or help people lose more weight than other approaches to weight loss. He cites a recent year-long study comparing the Atkins low-carb diet to three other popular diets.

The people in the study lost about the same amount of weight in a year -- a modest 4 to 7 pounds -- whether they were on Atkins, Ornish, Weight Watchers, or the Zone. And compliance was a problem with all of the diets. Only about half of those on Atkins or Ornish stayed on the diets for a year, and about 65% of those on the Zone or Weight Watchers diets stuck with the plans.

"Some people on each of these diets did very well and others didn't," Bray says. "I think the message is that one approach to weight loss is not the answer for everyone."

In an editorial accompanying the study by Boden and colleagues, Bray suggests that dieters might lose more weight by switching weight loss strategies from time to time.

"Switching between different diets with different approaches to food restriction may be the best approach to the long-term management of obesity," he writes.

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SOURCES: Boden, G. Annals of Internal Medicine, March 15, 2005; vol 142: pp 403-411. Guenther Boden, MD, professor of medicine; chief, division of endocrinology/diabetes/metabolism, Temple University School of Medicine, Philadelphia. George A. Bray, MD, professor of medicine, Pennington Biomedical Research Center, Louisiana State University, Baton Rouge.
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