Growth and host-pathogen interactions.

Differing trajectories of infant and child growth are associated with different patterns of disease and mortality in adulthood. Since postnatal growth patterns are partially modifiable by diet, these associations raise fresh questions about what constitutes an optimal growth rate. We use data from contemporary societies that still suffer poor nutrition and high burdens of infectious disease to illustrate early growth patterns that have likely been typical of our evolutionary past. Pathogenic assault is a major suppressor of growth; populations frequently average -1.0 to -1.5 z scores (standard deviations relative to standard growth curves) for height, and -2.0 to -2.5 z scores for weight, body mass index and head circumference. Many infections are symptomatic (e.g. diarrhea, malaria, pneumonia, HIV), but others are subclinical (e.g. hepatitis B, cytomegalovirus, Epstein-Barr virus, herpes, Helicobacter pylori). The great majority of young children become infected by multiple pathogens which initiate a downward cycle of infection --> suppressed appetite and malabsorption --> reduced growth --> lowered immunity --> repeated infection. Examination of the evolutionary 'norm' for early growth, and the external environmental factors that influenced it, may provide clues towards identifying the current day optimum for growth.