In the long run, it's the calories -- not the fat nor protein nor carbs -- that matter, according to a new study comparing diets.
Weight-conscious Americans snap up the latest diets, from the low-fat Dean Ornish approach, to the high-protein Atkins plan, to the compromise called South Beach. But scientific studies evaluating the diets' effectiveness have had mixed results, though, frustrating consumers who struggle to shed pounds.
A team led by Dr. Frank M. Sacks of the Harvard School of Public Health designed a clinical trial that randomly assigned 811 men and women to eat one of four reduced-calorie, heart-healthy diets with different levels of fat, protein, and carbohydrates for two years. The dieters were asked to exercise for a total of 90 minutes each week and they were invited to attend regular group sessions, in addition to receiving periodic individual counseling.
"What we found is that the most important thing for people to lose weight is to choose a heart-healthy diet and to keep the amounts down," Sacks, lead author of the article appearing in the New England Journal of Medicine,said in an interview. "It's not so important whether they eat higher carbohydrates or higher protein or lower carbohydrates or lower protein. What really matters is just plain, simple old quantity: how much people eat."
All the diets worked the same when measured by lost pounds and reduced waist circumference, regardless of the nutrients they emphasized. The 80 percent of participants who stuck with the diets until the end lost an average of 13 pounds in the first six months and kept about 9 pounds off after two years. Dieters who had the best record of attending counseling sessions lost 22 pounds. Waistlines shrank by about 2 inches throughout all groups.
In the end it was the calories they didn't eat that made the difference. All four diets cut out 750 calories a day, the researchers said, prescribing a minimum intake of 1,200 calories per day.
"It's just the calories that count," Sacks said. "It does really make sense [if] how you gain weight is by eating more calories than you burn, you lose weight by eating less."
Cholesterol, blood pressure, and triglyceride levels also improved modestly across the groups.
"These results show that, as long as people follow a heart-healthy, reduced-calories diet, there is more than one nutritional approach to achieving and maintaining a healthy weight," Dr. Elizabeth G. Nabel, director of the National Heart, Lung, and Blood Institute, said in a statement. The institute funded the study.
The participants, who were classified as overweight or obese based on body mass index scores from 25 to 49, agreed to eat diets high in whole grains, fruits, and vegetables. They recorded what they ate or drank in a web-based program that tracked their progress.
Most participants had trouble meeting their assigned targets for fat, protein, or carbohydrate, the study authors noted. That was especially true for the high-protein and low-fat groups, but the differences between what the four groups ate were bigger than in previous studies, so still large enough to draw conclusions, they said.
"The further diet is from a person's customary intake, the harder it is to stick with it in the long term," Sacks said, adding that the study's conclusion that calories matter most will make it easier to follow than more restrictive diets.
Susan B. Roberts, a professor at the Friedman School of Nutrition Science and Policy at Tufts University, was troubled by the limited success participants had in staying on the diets. If they ate similar amounts of protein, she said, it's not surprising that their weight loss was similar.
"I think these large studies that fail to cause much change in diet and produce minimal weight loss don't really help us move forward because they don't tell us what does work," she said in an e-mail interview. "There are four generally recognized ways to reduce hunger and increase satiety based on detailed clinical studies - high protein, high fiber, high volume (i.e., low energy density) and also low glycemic index carbs. What I would really like to see is more focus on these factors rather than just fat, carbs and protein."
An editorial also appearing in the Journal said focusing on diet components might be less important than the behavior of the dieters and the environment around them.
"Even these highly motivated, intelligent participants who were coached by expert professionals could not achieve weight losses needed to reverse the obesity epidemic," Martijn B. Katan of the Institute of Health Sciences at VU University in Amsterdam writes. "We do not need another diet trial; we need a change of paradigm."
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