Available data on metabolic utilization of vitamin D3 indicate a total daily requirement of approximately 4000 international units (iu) (100 microg) or twice the current tolerable upper intake level (UL). In young individuals, most of this comes from the skin. However, cutaneous vitamin D3 synthesis declines with age, creating a need for increasing oral intake to maintain optimal serum 25-hydroxyvitamin D [25(OH)D] concentrations. Estimates of the population distribution of serum 25(OH)D values, coupled with available dose-response data, indicate that it would require input of an additional 2600 iu/d (65 microg/d) of oral vitamin D3 to ensure that 97.5% of older women have 25(OH)D values at or above desirable levels. The age-related decline in cutaneous input, taken together with the UL, creates a substantial barrier to the deployment of public health strategies to optimize vitamin D status in the elderly.