In a study appearing in Tuesday's "Annals of Internal Medicine," Temple researchers found that people on low-carb regimens such as the Atkins diet spontaneously reduced their calories by 30 percent despite being offered unlimited helpings of protein-rich, high-fat foods, such as red meat.
On average, they cut out 1,000 calories a day when restricted to bird-like portions of carbs.
The study, funded by the American Diabetes Association and the National Institutes of Health, is the only trial of the Atkins diet ever done in a hospital setting where every calorie could be measured, Temple researchers said.
Appearing first in the 1860s, low-carb diets became hugely popular in recent years, spurring millions of people to try them.
Studies have shown they can work, at least short term, but the reason has been hotly debated.
"This study is very useful because it tells us it is about calories at the end of the day," said Gary Foster, a University of Pennsylvania obesity expert.
Stuart Trager, medical director for Atkins Nutritionals, said the research confirmed a low-carb diet fosters weight loss because it helps people consume fewer calories.
But other studies have suggested that metabolism plays a role, Trager said.
Unlike other research, where patients tracked their eating and exercise patterns, the Temple study measured every calorie that a group of 10 obese patients consumed and spent.
The patients, who also had Type 2 diabetes, ate their normal fare the first week, averaging 3,111 calories a day.
Their calories dropped to an average of 2,164 a day and they lost on average 4 pounds each over two weeks.
Hawk said he carried 315 pounds on his 6-feet, 1-inch frame when he entered the trial.