"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. "There is a genetic basis for how much they can consume."
Here's what's going with those friends who eat whatever the hell they want, without seeing the slightest change in their waistlines.
They've got good genes
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."