The largest organs are the quietest. For a typical U.S. adult, muscle accounts for 42 percent of body weight but only 16 percent of BMR, about 280 kcal per day (about 6 kcal/day per pound). Your skin weighs 11 pounds but burns only 30 kcal per day; your skeleton weighs a bit more but burns even less. Fat cells are more active than you might think. They make hormones and traffic in glucose and lipids to maintain energy supply to the body. Still, each pound of fat burns only about 2 kcal per day, for a total of about 85 kcal per day for a typical 150-pound adult with 30 percent body fat.
