More commonly, studies have found that overweight individuals tend to underestimate how many calories they consume, and overestimate their physical activity level. There may be certain genetic dispositions or stress factors that lead to excess calorie intake, but this is unrelated to metabolism -- in fact, obese individuals actually have higher resting metabolic rates than their skinny counterparts. Ever notice how people on The Biggest Loser can drop 10 pounds in a week while you fight to lose one? Larger bodies require greater energy (calories), and a stronger engine (high metabolic rate) to process them. When that energy is reduced, so is the body’s energy storage (fat), while the high metabolic rate sticks around for a while.
Metabolism slows down when you get older
You probably remember the glory days, when you could eat whatever the hell you wanted without gaining an ounce. Now it seems like you can’t even touch a potato chip without packing on the pounds. It’s natural (aka easy) to blame this on age.
As in the case of genetics, this is partially true. Until you reach adulthood, your body is under construction, and excess energy typically gets put to work, so your waist doesn’t suffer the consequences. Once you’re in your 20s, the construction site shuts down, and your energy needs decline.