The first week in January is starting time for a lot of new dieters. So wouldn't you like to know which diet works best? Doctors this week came up with an answer: all of them—if you stick with them. People who swear that low-carb Atkins is better than small-portion Weight Watchers, or that the nutrient-focused Zone is better than both, may have to change their tunes. Arguments over which diets melt away pounds the best may not be as important as arguing over which one is easiest to adhere to. The diets also proved equally adept at reducing heart-disease risk factors.

What the researchers wanted to know: Was any one of several popular diets better than the others at reducing weight or reducing the risk of heart disease?

What they did: A total of 160 overweight or obese men and women were randomly assigned to try a diet for one year. Forty each went on the Atkins diet (low carbs and no fat restrictions), the Zone (balance macronutrients and intake of sugars), Weight Watchers (restrict portion sizes and overall calories), and the Ornish "Eat More, Weigh Less" diet (low fat). People had an average body mass index of 35; 25 is the beginning of the overweight category. They also had high cholesterol levels, high blood pressure, and high blood sugar. People were asked to strictly follow their diet for two months, monitored by the medical team, and then follow it as best they could for the remaining 10 months.

What they found: In each diet group, after one year, 25 percent of the participants lost about 5 percent of their body weight. Another 10 percent of people in each group lost more than 10 percent of their body weight. There wasn't any meaningful difference between the groups, the researchers say. Not surprisingly, those who finished the year lost the most weight. But in each group there was a high percentage of dropouts before the year was up: 47 percent in Atkins, 35 percent for the Zone, 35 percent for Weight Watchers, and 50 percent for Ornish. The diet completers also had similar reductions in their cholesterol levels, blood pressure, C-reactive protein (another heart risk factor), and blood sugar.

What the study means to you: "Our findings challenge the concept that one type of diet is best for everybody," the researchers write. Atkins and Ornish might be too extreme for many people, as indicated by the high dropout rate, but they worked—modestly—for some people. The best thing to do, the doctors suggest, is to match food preferences, lifestyles, and heart risk profiles of each patient to a diet, and don't think that one size fits all. People in this study were randomly assigned to a diet; the researchers suspect dropout rates would have been lower if people were allowed to pick their own.

Caveats: The dropouts in this study were not followed, and the researchers assumed their weight stayed the same. If some dropouts kept losing weight—unlikely as that may seem—the conclusions of the study might have been different.

Find out more: Diet news can be tricky to digest. The Harvard School of Public Health has some tips for making sense of it.

Read the article: Dansinger, M.L. et al. "Comparison of the Atkins, Ornish, Weight Watchers, and Zone Diets for Weight Loss and Heart Disease Risk Reduction." Journal of the American Medical Association, Jan. 5, 2005, Vol. 293, No. 1, pp. 43–53.

Abstract online: http://jama.ama-assn.org