Q: How long does it take to gain a pound of fat?

   Activity level, age, and size determine how many calories you need in a day to maintain your weight. After you eat something, your body takes days to process the nutrients that are burned for fuel, and then stores excess calories as fat.

   It was thought that 3,500 extra calories equaled a pound, but new research found that the number varies. For example, over time, lean people gain less obese people  do after taking in the same amount of additional calories.