In an attempt to quantify the optimal level of 25-hydroxyvitamin D [25-(OH)D] needed to enhance health, a rigorous literature review was performed. The common definition of an optimal 25(OH)D serum concentration is generally regarded as that which maximally suppresses parathyroid hormone (PTH, which promotes bone loss), but estimates in this regard vary widely as PTH levels fluctuate with time of day, diet, physical activity, and renal function. In this paper, results from randomized controlled studies, dose-response trials, and prospective and epidemiologic data were summarized for the effect of a given level of 25(OH)D on multiple health endpoints other than PTH suppression or optimal calcium absorption, including bone mineral density, anti-fracture efficacy, lower extremity function, falls, dental health, and prevention of colorectal cancer. For all examined endpoints, optimal 25(OH)D serum concentrations started at 75 nmol/L (30 ng/mL), but the best outcomes were found with serum levels that ranged from 90 to 100 nmol/L (36-40 ng/mL). The current recommended daily intake of vitamin D depends upon age and ranges from 200 to 600 IU/d; unfortunately, optimal 25(OH)D levels cannot be attained in most people with the present suggested levels of intake. Thus, an increase in the currently recommended daily intake of vitamin D to more than 1,000 IU (40 μg) seems warranted for all ethnic and racial groups.