It's only a theory, and there are no reams of research or intensive clinical trials to back it up. But it makes sense to those in the know: Spring is a better time to lose weight and get in shape than the dead of winter.

It's only a theory, and there are no reams of research or intensive clinical trials to back it up. But it makes sense to those in the know: Spring is a better time to lose weight and get in shape than the dead of winter.

"It's absolutely true," says Jeff Wooten, a Raleigh personal trainer and owner of The Body Mechanic. "Just in my neighborhood, you see more and more people going outside. It's nice, you're feeling good. There's just more of an incentive to get out and do something."

That seems true now more than at the first of the year, when daylight is scarce and the temperatures more conducive to curling up with a tub of popcorn and a movie than lacing up the Nikes and going for a run. Well-intentioned though they may be, New Year's resolutions often are destined to fail. A study by Britain's University of Hertfordshire found that only 12 percent of 3,000 Brits and Americans surveyed succeeded with their resolutions.

For some, the spring is more conducive physiologically to getting fit. People who have seasonal affective disorder, or SAD, turn to carbohydrate-rich foods — starchy pasta and potatoes, and sweets — to stimulate the production of serotonin, a chemical in the brain that affects sleep, mood and appetite. Serotonin production is stimulated by sunlight.

"Carbs can be used for a quick lift," says Diana Koenning, a North Carolina nutritionist, "but you end up putting on all that weight."

For others, the motivation to get moving in spring is psychological.

Dr. Stuart Fischer was associate medical director for the Atkins Center, home of the low-carb Atkins diet, from 1988 to 1997. During his tenure, he noticed a curious trend.

"The phone would not ring for the first two months of the year," says Fischer. "Then in late March, early February people would start calling to make appointments.

"People suddenly started to realize that they were going to be seen more in public. When you start to take off some clothes, then you get down to business."

That revelation prompted him to develop the Park Avenue Diet, which, in addition to nutrition and exercise, encourages people to consider their overall appearance — including their hair, skin, clothes.

"The incentive," says Fischer, "is image."

In addition to longer, warmer days, there's another thing feeding our appetite for healthier living this time of year: "We're getting into the season when more wonderful fruits and vegetables are available," says nutritionist Koenning.

But there's a caveat or two about the spring-is-better theory:

While a spring-activated lifestyle does require more calories, Koenning says, "sometimes people mess with that, giving themselves permission to eat extra, beyond covering for the extra fuel they need."

Adds Keri Glassman, a nutritionist and contributor to Women's Health magazine, "Some research suggests ... warmer temperatures increase the need for temperature regulatory mechanisms to kick in, keeping the body cool, increasing the need for calories."