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New Theory on Why Low-Carb Diets Work
When researchers restricted carbs, dieters simply ate less

(HealthDayNews) -- A new study involving obese individuals suggests the reason the Atkins, Zone and other low-carb regimens help people lose weight is that dieters don't substitute fatty or sugary foods for the carbohydrates they lack.

Instead, they simply eat less food.

"Take the carbohydrates away, and I expected the participants would just eat more of the other stuff," said researcher Dr. Guenther Boden, a professor of biochemistry at Temple University, in Philadelphia.

"But they didn't. In fact, it turned out they ate 1,000 calories less every day," he said.

The findings, published in the March 15 issue of the Annals of Internal Medicine, may help allay concerns these diets raise heart risks linked to increased fat consumption.

According to Boden, numerous theories have been floated as to how low-carbohydrate diets trigger weight loss.

"The possibilities were: People eat less, they expend more calories, they don't really lose body mass but instead they lose water, and a fourth possibility -- very popularly expressed -- that carbohydrate calories are somehow different from other calories," he said.

To help determine the correct answer, his team sequestered 10 obese patients, all diagnosed with type 2 diabetes, in a controlled, clinical environment where diets were strictly monitored for three weeks. Boden's team also used the very latest technology to assess weight-related outcomes such as body mass loss, water loss, and total calories expended.

For the first week, participants ate their usual mixed diet. But during the last two weeks the researchers restricted their intake of carbohydrates from an average of 300 grams per day to just 20 grams a day.

At the same time, a tempting array of fatty, sugary and other foods was readily available to all.

"We told them 'Look, you can eat as much of anything else as you want, whenever you want,' " Boden said.

The result: By the end of the two-week low-carb regimen, patients lost an average of 1.65 kilograms (3.6 pounds) and reduced their daily caloric intake by nearly 1,000 calories -- from an average of 3,111 calories before they began the diet, to just 2,164 calories while on the low-carb regimen.

"In other words, they self-corrected their previously excessive appetites down to normal," Boden said.

And that magic number of around 2,100 calories per day "turned out to be exactly the amount of energy they should've been consuming to start with" to avoid weight gain, he added.

As happens naturally with weight loss, diabetes risk factors such as insulin and blood-glucose levels began to noticeably improve. So did levels of unhealthy blood fats called triglycerides -- a finding noted in previous studies that looked at the effects of low-carb diets on cardiovascular health.

The study, which was funded by grants from the National Institutes of Health and the American Diabetes Association, still leaves unanswered the question of why carb-deprived individuals don't reach for sugars, proteins or fats.

"The only thing that makes sense to me is a drop in insulin," Boden said. "I've been treating diabetics for decades, and every time I start them on insulin they gain weight. So I am sure insulin has something to do with appetite."

"I can't prove it, of course," he added, "Because we still know so very little about appetite. Everyone's on thin ice there."

Dietitian Cathy Nonas is director of the obesity and diabetes program at North General Hospital, in New York City, and a spokeswoman for the American Dietetic Association. She said the study doesn't tell scientists much they didn't already know about low-carb diets.

"If you look at all of the Atkins data that's ever been done, including USDA White Papers and so forth, people lose weight on the Atkins diet because they eat lower amounts of calories," she said. "And that's true of most diets."

And Nonas said previous studies have suggested that cutting back on one type of food doesn't necessarily mean people are going to gorge on another.

She's also concerned that the Temple study didn't include a control group -- participants the researchers could have used for comparison purposes.

"The problem here," she said, "is that we don't have a study where you also looked at taking away meat, for example -- would we have seen similar, greater, or less change in weight?"

But another expert believes the new study "adds to the literature suggesting that low-carbohydrate diets may have a place in the treatment of obesity."

In his editorial comment, Dr. George A. Bray, of the Pennington Biomedical Research Center, in Baton Rouge, La., called the Temple research "nicely done."

Long-term studies focused on the Atkins diet have found short-term weight loss that often exceeds that seen in other diets, Bray writes, "but the differences vanished after 12 months." He believes low-carb regimens should be simply added to a growing list of relatively safe weight-loss options for America's overweight and obese.

Nonas remains skeptical that any diet that excludes a whole food group can be healthy -- or sustainable -- over the very long term, however. And she believes Americans need only look abroad to find an ideal dietary model for life.

"All of the societies with low levels of the kinds of diseases [that plague us] have diets with lots of vegetables and fruits, a small amount of whole grains, portion-control, and a much higher fiber intake," she said. "And fiber isn't something that's been high on the list in any of these studies."

More information

For lots more on diet and nutrition, head to the American Dietetic Association.

By E.J. Mundell

SOURCES: Guenther Boden, M.D., program director, General Clinical Research Center, Temple University School of Medicine, Philadelphia; Cathy Nonas, M.S., R.D., C.D.E., director, obesity and diabetes program, North General Hospital, New York City, and spokeswoman, American Dietetic Association; March 15, 2005, Annals of Internal Medicine
Last Updated: January 2006
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