Why a kilogram of body weight does not have one energy cost

Body mass and tissue composition

Dividing daily energy expenditure by body weight produces an easy-to-read number. It does not make all kilograms physiologically equivalent. A comparison between a child and an adult therefore needs more than a shared unit on the vertical axis.

Consistent Eating highlights the changing proportions of organs, muscle and fat during development. A human study by Hsu and colleagues tested part of this explanation using organ measurements and indirect calorimetry in 15 children and 13 young adults. Children's relatively larger brain and liver contributions improved the expenditure model, but did not remove the effect of age. Published organ metabolic coefficients underestimated measured expenditure in the children. The authors concluded that both tissue proportions and changes in tissue metabolic rates probably contribute. [1]

Muscle matters, but its resting cost is modest

Reference estimates evaluated by Wang and colleagues put resting skeletal-muscle expenditure at about 13 kcal/kg of muscle/day, compared with 4.5 for adipose tissue. Major organs have much higher rates. These are tissue-specific estimates, not intake targets or evidence for a whole-body minimum. [3]

At those reference rates, adding 5 kg of muscle would contribute roughly 65 kcal/day at rest; replacing 5 kg of fat with muscle would change the estimate by about 42.5 kcal/day. These are illustrative calculations, not predictions of the total effect of training. Strength, mobility and exercise capacity remain important outcomes beyond this modest resting cost.

For EPK, preserving muscle is relevant to functional capacity, while differences in organ and tissue proportions complicate interpretation of the ratio. A declining RMR/kg value should not be attributed to muscle loss without measuring body composition and considering other explanations.

Interpret the measurement

This gives EPK research a more specific task. A high expenditure-per-kilogram value might reflect body composition, developmental physiology or other factors. It cannot identify which explanation applies without additional measurements. It also cannot, by itself, establish whether food intake is adequate.

The quantity being divided matters just as much. Basal expenditure is different from total dietary requirements. SACN's infant reference table includes energy for tissue deposition as well as total expenditure; its adult basal examples serve a different purpose. Combining these into a single resting-metabolism curve would obscure that distinction. [2]

A useful lifespan comparison

A useful lifespan analysis would report measured resting expenditure and total requirements separately, state whether the denominator is total body mass or fat-free mass, and compare age-appropriate models. Researchers could then ask whether EPK adds information about growth or function beyond those measurements. That approach keeps the books' central interest in energy and development while giving the hypothesis a fair test.

Related working papers

Read the relevant PDFs: EPK–001 to 007 are the revised version 1.2 papers; EPK–008 to 010 are version 1.0 supplements. Earlier PDFs and DOI records remain on the paper archive.