January is over, and the clock is ticking on perennial resolutions to shed weight. How long have you given yourself this time to lose those 10, 20, 30 or more pounds you packed on since adolescence?

And why does the weight almost always come back?

Unhealthy eating habits plus too little exercise, too much sitting around and an age-related loss of muscle mass are the main reasons, enhanced perhaps by genetics, environment, disability and other circumstances. Most of these can be overcome, dieticians say, by making small behavioral changes before setting weight loss goals.

Going on a "diet," they say, too often involves making radical, unsustainable eating changes in the hopes of losing weight quickly, only to mess up your metabolism and mind so much you could be setting yourself up for eventually weighing even more than when you started.

"Diet" is kind of a bad word to Jessica Cooper, a registered dietician in Salt Lake City who teaches people how to lose weight and counsels athletes on how to use nutrition to boost performance.

One of the first bits of advice she gives her clients is to focus less on outcomes and more on behaviors. "I talk about habits they need to change," she says. "If they start eating better, they start feeling better."

That is the gist of the second annual U.S. News and World Report "Best Diets" feature released in January, an analysis of 25 commercial and academic eating plans for their health and weight-loss potential.

Katherine Beals, a University of Utah associate professor of nutrition, was one of 22 experts who helped evaluate the eating plans, eventually breaking the rankings into bests for weight loss, diabetes, heart health, and healthy eating. The rankings also included the best commercial diet plans, easiest diets to follow and best overall.

"Many of the, quote unquote, diets we evaluated were not really for weight loss," she says.

Nor had the claims of some of them been subject to scientific analysis, something she and the other team members do regularly.

"If you’re a consumer, it’s very difficult to wade through the hundreds [of diet plans] that are available," she says. "But what I think is simple: Forget what you weigh. What do you need to be healthy?"

One answer: exercise.

"People tell me they don’t have time to exercise," Beals says. "Really? How much time do you spend watching TV? You need to be physically active every day....If you do those two things, your weight will settle at what is healthy for you."

Craving what you need • Cooper tells clients that if they are desperate for a certain unhealthy food, they are really craving nutrients. Trouble is, "it takes a lot of cheeseburgers and fries to get those vitamins and minerals," she says, "and you also get a lot of calories."

The diets that got top billing in the U.S. News report share a common denominator: lots of fruit and vegetables. They included the DASH, TLC and Volumetrics eating plans — which were developed to fight chronic ailments such as high blood pressure, diabetes and cardiovascular disease. Weight Watchers and Jenny Craig were the top two commercial diet plans.

None of them restrict entire classes of food, a big plus in the analyses.