[1] Because adipose tissue does not use much energy to maintain, fat free mass is a better predictor of metabolic rate.
[2] At rest, the largest fractions of energy are burned by the skeletal muscles, brain, and liver; around 20 percent each.
[5] This excess reduction is termed adaptive thermogenesis and it is estimated that it might compose 50 to 100 kcal/day in people actively losing weight.
Two of the earliest weight loss drugs, 2,4-dinitrophenol and thyroid hormone, increase energy expenditure, but both were withdrawn from use due to risks.
[12] Formulas have been devised to estimate energy expenditure in humans, but they may not be accurate for people with certain illnesses[13][14][15] or the elderly.