We investigated the effects of dietary protein on plasma IGF-I levels and muscle fiber cross-sectional area (CSA). Twelve healthy elderly women were randomly assigned to a weight-maintaining diet containing either 1.47 (marginal) or 2.94 (adequate) g protein/kg body cell mass (BCM)/d, (0.45 and 0.92 g/kg body weight/d, respectively) for 10wks. Plasma IGF-I levels and muscle fiber areas and distributions were evaluated at baseline and 10wks. After 10wks, both IGF-I and type I fiber CSA had declined significantly in subjects fed the marginal diet (30.1+/-2.1% and 32.7+/-7.9%, respectively) while they increased in those fed the adequate diet (19.5+/-7.0% and 22.3+/-7.5%, for IGF-I and type I CSA, respectively). The change in IGF-I was the only significantly associated with the change in type I fiber CSA (r2=0.70; p<0.03). These findings show that marginal dietary protein intakes will result in losses of muscle mass in the elderly and suggest a role for plasma IGF-I as a biochemical marker for the histological changes in skeletal muscle.