The primary role of sufﬁcient vitamin D during youth and adolescence is optimization of BMD. For example, serum 25(OH)D levels were found to be strongly correlated with BMD for peripubertal Finnish girls53 and young Finnish men.54 A study in Boston reported that 24 percent of 307 adolescents recruited during an annual physical examination were vitamin D deficient (serum 25(OH)D ≤ 15 ng/mL), with 14 percent severely vitamin D deficient (25(OH)D ≤ 8 ng/mL);55 the deficiencies were highest among African Americans. A study based on the National Health and Nutrition Examination Survey (NHANES) III found adolescents were more likely to be vitamin D insufficient, rather than deficient, in low-latitude winter and high-latitude summer populations.56 There are 4-5 months of the year when vitamin D cannot be produced from solar UVB irradiation in Boston at 42.5° N latitude.57 Another important role of vitamin D during youth appears to be in reducing the risk of MS. A study in Tasmania found that children ages 615 years reporting the highest amount of sun exposure, especially in winter, had an odds ratio of 0.31 (95% conﬁdence interval (CI): 0.16-0.59) of developing MS compared with those experiencing less than one hour of sun exposure dai-ly.58 It is well known that the risk of MS increases rapidly with increasing latitude. This ﬁnding has been demonstrated in Australia,59 Europe,60 and the United States.61,62Figure 1 shows the latitudinal dependence for U.S. veterans at the time of entry into World War II and the Korean Conﬂict.62 Wintertime serum 25(OH)D values are much more likely to follow a simple latitudinal dependence due to the reduced number of days during which vitamin D can be produced from solar UVB at the higher latitudes.57 In the winter, little if any vitamin D can be made in the skin above 37° N latitude,and serum 25(OH)D levels reach their nadir in February or March in the northern hemisphere.57,63 In summer, the level of serum 25(OH)D is generally adequate. Summertime UVB irradiation does not follow a simple latitudinal dependence, due to the higher surface elevation and lower tratospheric ozone layer for states west of and including the Rocky Mountains.64 The best explanation for this latitudinal variation is strengthening of the immune system, specially in winter, which can then help prevent viral infections from giving rise to MS.11,19,45-47,65-67 For example, vitamin D regulates T-helper 1 (Th1) and dendritic cell function.