The FDA generally bases nutritional labels off a 2,000 calorie a day intake.
Doing the conversion from calories to watt hours I get: 2.326 Wh or 8373.6 joules (watt seconds)
If you were to compare that to a cell phone battery (3.3v) the battery would have a storage size of 0.705 Ah or 705 mAh. Most smart phone batteries are larger. If you were to divide that power over 24 hours thats an average usage of 29 mA.
Given such a small amount of power it hardly seems possible the human body needs so little energy. Consider a flight of stairs offers a height difference of 3 meters (approx 9ft). Google says the average American weighs around 81.9kg (180lbs). Gravity is 9.78m/s^2.
http://www.arachnoid.com/gravitation_equations/power_energy_gravity.html
81.9kg * 9.78m/s^2 * 3m = 2402.946 joules of energy to go up the stairs.
8373.6 joules / 2402.946 joules = 3.84
Four trips up the stairs would be enough to consume 2000 calories.
We haven't factored in walking around, and the energy required to keep your body temperature up over the lower ambient temperature, or anything else.
Are my calculations off?