How Does Energy Expenditure Change Over Time?

One of the often repeated wisdoms regarding changes in energy expenditure as we age, is that we lose about 10-15 Cal per year. According to this formula, someone going from age 20 to age 50 would lose about 300-450 Cal in energy requirements. 

But how accurate is this figure and does it really hold true?

This is the topic of perhaps the most comprehensive analysis of human energy expenditure over the lifespan ever to be conducted, by Herman Pontzer and colleagues, published in SCIENCE.

The researchers investigated the effects of age, body composition, and sex on total expenditure using a large (n= 6421 subjects; 64% female), diverse (n = 29 countries) database of doubly labeled water measurements for subjects aged 8 days to 95 years. In addition they looked at published measures of basal expenditure in neonates and doubly labeled water–measured total expenditure in pregnant and postpartum women. 

After adjusting for body size to isolate potential effects of age, sex, and other factors, they found four distinct phases of human energy expenditure. 

The first phase applied to neonates, up to 1 year of age. While during their first month neonates had a size-adjusted energy expenditures similar to that of adults, this increased rapidly in the first year so that between 9 and 15 months of age, adjusted total and basal expenditures were nearly ~50% elevated compared with that of adults.

The second phase applied to juveniles, 1 to 20 years of age. While total and basal expenditure continued to increase with age throughout childhood and adolescence along with fat-free mass, size-adjusted expenditures steadily declined at a rate of about 3% per year till about age 20, after which it plateaued at adult levels. In contrast to what one might expect, there was no indication of a pubertal increases in adjusted total or basal expenditure. Although men tended to have a higher energy expenditure, the rate of decline was the same for men and women. 

Over the third phase, from 20 to 60 years of age, total and basal expenditure and fat-free mass remained stable from ages 20 to 60 years in both sexes. During pregnancy, adjusted total and basal expenditures remained stable with the elevation in unadjusted expenditures matching those expected from the gain in mothers’ fat-free mass and fat mass.

Finally, during the fourth phase, starting at about age 60, total and basal expenditure declined at a rate that exceeded the loss attributable to the steady reduction in fat-free mass and fat mass. Thus, adjusted total expenditure declined by –0.7 per year, and adjusted basal expenditure fell at a similar rate. For subjects 90+ years of age, adjusted total expenditure was ~26% below that of middle-aged adults.

In additional analyses, the researchers also found that both physical activity and tissue-specific metabolism contribute to total expenditure and its components across the life span. Elevated tissue-specific metabolism in early life may be related to growth or development, whereas reduced expenditures in later life may reflect a decline in organ-level metabolism.

These observations have several important implications. Firstly, these data contradict the notion that energy requirements change in a continuous fashion over time; rather, each of these phases are distinct, with energy expenditure remaining rather stable over long periods of adult life, with significant changes occurring in childhood and old age. 

Secondly, the researchers note that there are considerable interindividual variations in expenditure even when controlling for fat-free mass, fat mass, sex, and age – meaning that some individuals require fewer calories that others. 

Clearly, understanding the complex biology that underlies these metabolic changes over the life course as well as the variation among individuals will likely help reveal the roles of metabolic variation in health and disease.

Berlin, D