Most of the energy you burn each day goes on simply staying alive
Lying still, awake and unfed, the human body still spends roughly seven tenths of its daily calories just running itself: breathing, pumping blood, holding its temperature, firing nerves. That idling cost is the basal metabolic rate, and a study in Scotland found it can more than double from one adult to the next.
Measuring it properly is fussy. The subject must be calm, physically rested, in a room neither warm nor cool enough to trigger any response, and not digesting a meal. Laboratories either capture body heat directly or analyse the oxygen and carbon dioxide in each breath; a looser version taken under easier conditions is called resting metabolic rate. For cold-blooded animals such as fish and reptiles, biologists use a related figure, standard metabolic rate, which records the temperature too.
Why does it vary so much? A 2005 study of 150 Scottish adults found daily values ranging from 1,027 to 2,499 kilocalories, averaging 1,500. Differences in fat-free mass, meaning muscle, organs and bone, explained 62 percent of that spread, fat mass another 7 percent, and age only 2 percent. More than a quarter remained a mystery, unexplained even by sex, hormones or the size of energy-hungry organs such as the brain. Once expressed per kilogram of lean tissue, adult men and women come out essentially the same. The figure typically slips 1 to 2 percent per decade after age twenty, mainly as lean mass shrinks, and it rises during the second half of the menstrual cycle and with fever or injury.
In 1919 J. Arthur Harris and Francis Benedict published a formula to estimate it from weight, height, age and sex, since nobody could then measure lean tissue directly; it was revised in 1984. Oddly, the average person's basal rate appears to have fallen over recent decades even after allowing for body composition.
Animals tune theirs seasonally. Birds and rodents raise it in the cold, and the red knot, a shorebird, boosts it by about 40 percent before flying north, largely because its flight organs grow. The regulating centre is the hypothalamus, which also manages hunger through the hormones leptin and ghrelin.
Source: Basal metabolic rate
