Everyone knows that guy -- the one who can chug more than his fair share of beer, scarf down an entire pepperoni pizza, grab an ice cream, and make a donut run the next morning, all without gaining a pound.?

A new study suggests that people's bodies respond to the same food in very different ways.

"If you have two people who are given the same exact diet, they won't necessarily weigh the same." says Dr. Judith Korner, director at the Weight Control Center at Columbia University Medical Center.

Not happy with the way your body looks when you stray from your otherwise healthy diet? Blame mom and dad. Dr. Korner says genetics program your "set point," the weight your body strives to maintain. Some people may have a set point at a BMI of 22, while other bodies have a tendency to maintain a BMI of 28 or 30, depending on their genes and hormones."If you try and alter that set point, your body definitely fights against it," Dr. Korner continues. "There have been studies where you take people of any size, thin or obese, and if you underfeed them, there are changes in the body that try to get them back to their original weight. It also works the other way too -- the body will try to burn up that extra food to get them back to the set point."

The formula for weight gain is pretty simple: consume more calories than you burn, and vice versa. But how many calories you burn daily is much more dependent on your metabolism than hours spent at the gym. In fact, basic functions, like breathing and circulation, account for nearly three-quarters of the calories you burn daily, and your genetics determine this basal metabolic rate."[If your friend] is really having many more calories than you are, then his metabolism is probably faster and his body uses up calories much, much more quickly. That's probably how he was born," says Dr. Korner. " (It's like how) some people are born with blue eyes and some with brown eyes."

A pound of lettuce may weigh the same as a pound of fried chicken, but they certainly don't have the same effect on the body. That chronically thin friend who's constantly eating might actually be consuming foods that are less caloric than you think, Dr. Korner points out."It can look like someone's eating a lot, maybe they'll have this giant bowl in front of them, but the bowl may contain a salad of beans, chicken, and no dressing," she says. "It looks like they're just chewing and chewing and chewing, but their foods are less energy dense."There are people who say they eat whatever they want without gaining weight -- and they're not lying. While that might conjure up feasts of fried chicken, mac & cheese, and cream puffs in your mind, their palates may prefer less-caloric foods, allowing them to eat a much larger quantity.

Reality is not always as it seems. People tend to eat more and indulge in their favorite foods when they're out with friends, calorie counts be damned. Even if you saw your friend consume more chicken nuggets last night than you eat in six months, their regular diet is likely more modest."I think that it's an exaggeration. They may be eating what they want, but probably the number of calories they're consuming is not huge. If they start eating thousands of calories a day, I think that they would gain weight," says Dr. Korner.The opposite is true too, says Dr. Korner, who has seen patients that claim they've cut calories but still haven't seen the scale budge. Their perceptions of their diets are probably off."If you look at their food diary, you'll see a granola bar, and the patient thought that it was healthy but didn't realize the number of calories in this supposedly healthy granola bar was pretty significant. It's thermodynamically impossible to take in less [than you burn] and not lose weight." The best diet is always the one that works for you, not someone else's plan.