By Michael Gowan

You're trying hard to lose a few pounds through dietary diligence, eating healthy meals, and counting calories. And then you see a skinny person eating donuts and pizza, seemingly without a calorie care in the world.

It sure seems like some people can eat whatever they want and not gain much weight, while others can't waver from a strict calorie count without putting on pounds.

Myth or Fact?

So, can some people eat whatever they want and not gain weight?

Elisabetta Politi, nutrition manager of Duke Diet & Fitness Center, says this is likely true. "Some people tend to gain weight and some people tend to maintain weight," she says.

Research shows that people who are prone to maintaining their weight, when given more to eat than they need, tend not to gain much weight. In the same studies, people who tend to gain weight, when given more to eat than they need, gain more weight.

Politi attributes this discrepancy to a theory known as "set point." Your body has a resting metabolic rate -- the amount of calories you need in a day to meet basic functions like maintaining heart rate, breathing, and maintaining body temperature.

According to the set-point theory, our bodies raise our metabolism if we eat more calories than we need. If we eat fewer calories than we need, our body slows down our metabolism to maintain weight.

If you have a good set point, your resting metabolic rate goes up when you eat more calories than your body needs, and the extra calories burn off. You do not gain weight.

In the Genes

Your set point is likely determined by genetics. "Obesity is not just a problem of will power. Some people just have a strong genetic disposition to gain weight." Politi says anecdotal evidence she has seen at the Diet & Fitness Center supports the set point theory.

"Working in this environment, I see how some individuals are very susceptible to weight gain and have to be very vigilant to reach and maintain a healthy weight."

Of course, it's not all about genetics. Exercise and a proper calorie intake both play a key role in weight gain or loss. Regardless of your genes,  the more active you are, the more calories you need to maintain your weight.

And here's a reality check for those with favorable set points -- your set point can change as you age. That explains how, except for a very few individuals, almost everyone gains weight with age.