Figure 60-5 Body composition changes rapidly in premature and term infants during the first 12 months of life. Their high water content provides a large volume of distribution for water-soluble medications, whereas their low fat and muscle content provides a small reservoir for drugs that depend on redistribution into these tissues for termination of drug effects. Thus, body composition may significantly affect pharmacokinetics and pharmacodynamics. (Data from Friis-Hansen B: Body composition during growth. In vivo measurements and biochemical data correlated to differential anatomical growth. Pediatrics 47:169–181, 1971.)