Two hundred healthy women between ages 20 and 25 will be recruited from the first year classes in the Creighton University Schools of Medicine, Dentistry and Law. Each will be observed for a period of four years in order to test the hypothesis that calcium intake measurably influences the peak adult bone mass achieved after cessation of growth. The effect of other nutrients such as protein, phosphorus, alcohol and caffeine on this consolidation of bone mass will also be tested along with the effect of various self-chosen levels of physical activity. Dietary evaluations will be made at three-month intervals throughout the study and bone mass measurements by radio-grammetry, single photon absorptiometry and dual photon absorptiometry will be made at six-month intervals. Physical activity will be measured at six-month intervals using an activity monitor worn on the wrist for a seven-day period. We hope to confirm the findings of other studies of increased bone mass occurring after cessation of linear growth and to define the determinants of this increased mass. Thus, we hope to be able to show that peak skeletal mass is influenced by non-genetic factors, amenable to intervention early in life in a manner that can result in higher skeletal mass prior at the time when age-related bone loss begins at about age 40. This could result in greater skeletal mass remaining in old age and could offer protection from the crippling fractures of postmenopausal osteoporosis which afflicts nearly one half of our Caucasian women in their later years.