How many calories do you need a day?

US dietary guidelines give wider ranges, from about 1,600 to 2,400 calories a day for adult women and 2,000 to 3,000 for adult men. Around 60 to 70 percent of the energy you use goes on your basal metabolic rate, the energy needed simply to keep your body alive at rest. The brain alone uses about 20 percent of your energy, despite being only about 2 percent of your body weight. Very active people need far more: Tour de France riders can burn 5,000 to 6,000 calories or more on a hard mountain stage. A food calorie is really a kilocalorie, the energy needed to warm one kilogram of water by one degree Celsius.