Summary
Metabolism is not just about energy—how the body handles nutrient fuel and converts it to useable energetic currency. Metabolism also encompasses synthesis, modification, and exchange of the building blocks for all aspects of cellular function and acts as a sensor and regulator of cellular activities, in which individual moieties within metabolic pathways influence cellular responses. A substantial amount of the energy taken in each day is required to simply sustain life; the energetic demands of physical activity are superimposed on a vastly integrated machinery. Metabolic status has been linked to innumerable diseases and disorders, including those most prevalent with age (1–3). On page 808 of this issue, Pontzer et al. (4) analyze energy expenditure in more than 6400 males and females from 29 countries across the globe, aged between 8 days and 95 years, and show distinct metabolic phases during development and aging.
