A key factor in the determination of body composition over the lifecourse is fat accumulation during childhood. Periods of life associated with the greatest changes in organ development and growth, i.e. early childhood, have the most significant effect on body composition, energy balance, and metabolism. Early childhood (age 3 to 7 years) represents a critical transition for the basis of adaptability in body composition, due to the rapid growth and development that occurs. Plausibly the phenotype underlying obesity and related health risk may be determined by body composition during this critical period.