Vitamin D insufficiency is highest among people who are elderly, institutionalized, or hospitalized. In the United States, 60% of nursing home residents  and 57% of hospitalized patients  were found to be vitamin D deficient.
However, vitamin D insufficiency is not restricted to the elderly and hospitalized population; several studies have found a high prevalence of vitamin D deficiency among healthy, young adults. A study determined that nearly two thirds of healthy, young adults in Boston were vitamin D insufficient at the end of winter. 
An analysis of data on 2877 US children and adolescents (age, 6-18 y) from the National Health and Nutrition Examination Survey (NHANES) 2003-2006 indicated that, based on current Institute of Medicine Committee guidelines, about 10.3% of this population (an estimated 5.5 million) had inadequate vitamin D (25(OH)D) levels (< 16 ng/mL), and 4.6% (an estimated 2.5 million) had levels placing them at risk of frank deficiency (< 12 ng/mL). [23, 24] Adolescents (age, 14-18 y) and obese children had the highest risk of 25(OH)D deficiency and inadequacy, and these risks were also higher among girls than boys (of any age and body mass index) and among nonwhite children. 
Vitamin D status may fluctuate throughout the year, with the highest serum 25(OH)D level occurring after the summer and the lowest serum 25(OH)D concentrations after winter. A study by Shoben at el demonstrated that mean serum 25(OH)D concentrations can vary as much as 9.5 ng/mL. Factors such as male sex, higher latitude, and greater physical activity levels were found to be associated with greater differences in serum 25(OH)D concentrations in winter and summer.