|
Vitamin D Photosynthesis and Latitude
An early study done in Boston and Canada is often cited to support the conviction that latitude dramatically influences the amount of solar radiation available to synthesize vitamin D3. [1] However, authorities who conducted recent studies refute this hypothesis. Blaney et al. [2] found that vitamin D deficiency was not as common as speculated in Canadian adults living in the Pacific Northwest and was not found in the majority of patients with diseases previously associated with vitamin D deficiency. Kimlin concluded, "It may no longer be correct to assume that vitamin D levels in populations follow latitude gradients." [3] And Lubin stated, "Geophysical surveys have shown that UV-B penetration over 24 hours, during the summer months at Canadian north latitudes when there are many hours of sunlight, equals or exceeds UV-B penetration at the equator." [4]
In addition, our bodies have mechanisms for preserving the vitamin D we acquire during the summer; which have evolved to stabilize and maintain serum levels of 25(OH)D in environments with variable vitamin D availability. The D binding protein (DBP) optimizes and stores 25(OH)D for later use; it also binds 1,25(OH)2D, as well as the parental vitamin D itself. DBP sequesters vitamin D sterols in the serum, prolongs their serum half-lives, and provides a circulating store of vitamin D to meet transient periods of deficiency. [5] A review by Ross et al. reports that ample opportunities exist to form vitamin D (and store it in the liver and fat) from exposure to sunlight during the spring, summer, and fall months even in the far north latitudes. [6] Cutaneous vitamin D synthesis seems to be a major contributor to vitamin D status, even at northern latitudes. [7]
References
1. Webb AR, Kline L, Holick MF. Influence of season and latitude on the cutaneous synthesis of vitamin D3: exposure to winter sunlight in Boston and Edmonton will not promote vitamin D3 synthesis in human skin. Clin Endocrinol Metab. Aug 1988;67(2):373-8.
2. Blaney GP, Albert PJ, Proal AD. Vitamin D metabolites as clinical markers in autoimmune and chronic disease. Ann N Y Acad Sci. Sep 209;1173:384-90.
3. Kimlin MG, Olds WJ, Moore MR. Location and vitamin D synthesis: is the hypothesis validated by geophysical data? J Photochem Photobiol B. Mar 2007;86(3):234-9.
4. Lubin D, Jensen EH, Gies HP. Global surface ultraviolet radiation climatology from TOMS and ERBE data. J Geo Res. 1998;103(D20):26061-91.
5. Berg J. Vitamin D-binding protein prevents vitamin D deficiency and presents vitamin D for its renal activation. Eur J Endocrinol. Oct 1999;1(4):321-2.
6. Ross AC, Taylor CL, Yaktine AL, Del Valle HB. Dietary Reference Intakes for Calcium and Vitamin D. Washington, D.C.: National Academy of Sciences; 2010. 0-309-16394-3.
7. Helund L, Brembeck P, Plausson H. Determinants of vitamin D status in fair-skinned women of childbearing age at northern latitudes. PLoS One. Apr 2009;8(4):e60864.
| 
