this, in turn, causes persons living at higher latitudes in North America to experience little or no UVB exposure, making them at risk for vitamin D deficiency. This assumption may not be entirely accurate. However, the question of latitude may work in tandem with other factors, discussed below, such as limited sun exposure overall or cultural and dietary practices. This section focuses only on the issue of latitude per se.
The relationship between UVB penetration and latitude is complex and not merely a function of distance from the equator. Other factors that come into play include the reduced atmosphere at the poles (about 50 percent less than at the equator), more cloud cover at the equator than at the poles, differences in ozone cover, and the duration of sunlight in summer versus winter. Geophysical surveys have indicated that UVB penetration over 24 hours during the summer months at Canadian north latitudes equals or exceeds UVB penetration at the equator (Lubin et al., 1998), suggesting that persons living in the northern latitudes are not necessarily receiving notably less total sunlight during the year. Rather, it suggests that there may be considerable opportunity during the spring, summer, and fall months in the far north for humans to form vitamin D and store it in liver and fat. Likewise, animals living in the same region that are consumed as part of the traditional diet are also rich sources of vitamin D (Keiver et al., 1988; Kenny et al., 2004; Brunborg et al., 2006; Kuhnlein et al., 2006).
These factors help to explain why latitude alone does not appear to predict serum 25OHD concentrations in humans. In a Finnish study, healthy subjects living above the Arctic Circle (latitude 66°N) did not have lower serum 25OHD levels than subjects living in southern Finland; in fact, the group living above the Arctic Circle had higher levels. Both groups achieved mean serum 25OHD levels above 90 nmol/L during the summer, whereas the mean serum 25OHD level at the winter nadir was 56 nmol/L in the south and 68 nmol/L in those living above the Arctic Circle (Lamberg-Allardt et al., 1983).
The DRIs for vitamin D established in this report are based on the assumption of minimal sun exposure. Therefore, they are regarded as adequate for persons who may be experiencing a reduced synthesis of vitamin D from sun exposure. Assuming that some population groups may be consuming less than the current DRI values for vitamin D, the question is to what extent are these persons at risk for vitamin D deficiency, or, conversely, to what extent can inadvertent sun exposure be expected to compensate for lower intakes for these persons?