Environmental temperature and human growth in early life

A link between adult human body size and environmental temperature, evolved through adaptation to heat stress, was first recognized a century ago and is now well accepted in human biology. Increasing heat stress favours smaller body size and an increased ratio of surface area to mass. However, possible effects of temperature on growth in early life have not been considered. Many developing country populations inhabit relatively hot environments compared to industrialized populations, but growth faltering in developing countries is invariably attributed to the combination of poor nutrition and infection. The aim of this study was to model the theoretical effect of growth faltering on cooling capacity in early life. Data on weight, length and total energy expenditure were selected from the literature. Three types of growth (normal growth, stunting and wasting) were considered, and the relationship between body size and heat production investigated. Relative to normal growth, stunting causes little increase in the area-to-mass ratio until 2 years of age, and results in little increase in cooling capacity during infancy. Wasting increases the area-to-mass ratio by > 15% in early infancy, and by 10% thereafter, equivalent to a similar increase in cooling capacity. These findings demonstrate theoretically that growth faltering can relieve heat stress in both infancy and childhood. The hypothesis that heat stress plays a role in human growth faltering in hot environments therefore merits empirical investigation.