The old joke has a man going to the doctor and saying, "It hurts when I do this. What should I do to make it go away?" 

"Stop doing it," says the doctor.

The diabetes variation on the joke has a person with type 2 going to an endocrinologist and saying, "No matter what diet I'm on-low-carb, low-fat, high-fat, high-protein-I can't lose weight. What should I do to make the weight go away?"

"Don't eat so much," says the endocrinologist.

So, the obvious once again makes itself known: A two-year study of four diets with varying combinations of protein, fats, and carbohydrates has concluded that the combinations don't matter as much as the calories consumed.

In other words, eat less.

The study, by the Harvard School of Public Health and the Pennington Biomedical Research Center at Louisiana State University, tracked 811 men and women of different ages, incomes, and geographic locations. They were split into four groups, each of which ate one of the following diets:

• Low-fat, average protein: 20 percent of calories from fat, 15 percent of calories from protein, 65 percent of calories from carbohydrate 
• Low-fat, high-protein: 20 percent fat, 25 percent protein, 55 percent carbohydrate 
• High-fat, average protein: 40 percent fat, 15 percent protein, 45 percent carbohydrate 
• High-fat, high-protein: 40 percent fat, 25 percent protein, 35 percent carbohydrate

All the diets were "heart healthy," replacing saturated with unsaturated fats and emphasizing large daily helpings of fruits, vegetables, and whole grains.  

Participants were told to shave 750 calories from their typical daily food intakes, but no participant had a daily intake below 1,200 calories. Participants were also asked to exercise moderately for a total of 90 minutes each week.

Participants kept a daily food and drink diary and posted to a Web-based program that let them know how closely they were hewing to their goals. They also received one-on-one counseling every eight weeks over the two-year period and met in group sessions at least twice monthly over the course of the study.

The results for each diet group were remarkably similar: All participants had lost an average of 13 pounds by the six-month mark (which they slowly regained-a common occurrence in weight-loss programs) and finished the two years with an average nine-pound weight loss. Additionally, the extent of their weight gain was a fraction-20 percent-of the weight gain experienced by participants in prior studies. 

The researchers concluded that as long as diets were heart healthy, the various combinations of nutrients were not as important to weight loss as simply eating fewer calories. 

They also found that the participants who regularly attended counseling sessions, whether one-on-one or in groups, lost significantly more weight (an average of 22 pounds) than participants who didn't (nine pounds). There is apparently a social aspect to weight loss, a sort of "misery loves company" factor, that bolsters people's resolution to stick to a reduced-calorie diet.   

The National Heart, Lung, and Blood Institute of the National Institutes of Health funded the study, which appears in the February 26, 2009, issue of The New England Journal of Medicine.