Nutritional Factors Affecting Calcium Requirements - Vitamin D

FAO/WHO and other Nutritionists...verbatim.

Vitamin D

Jones, G., Strugnell, S.A. & DeLuca, H.F. 1998. Current understanding of the molecular actions of vitamin D. Physiol. Revs., 78: 1193-1231.

Mellanby, E. 1918. The part played by an "accessory factor" in the production of experimental rickets. A further demonstration of the part played by accessory food factors in the aetiology of rickets. J. Physiol., 52: 11-53.


Telfer, S.V. 1926. Studies in calcium and phosphorus metabolism. Q. J. Med., 20:1-6.

One of the first observations made on vitamin D after it had been identified in 1918 was that it promoted calcium absorption. It is now well established that vitamin D (synthesised in the skin under the influence of sunlight) is converted to 25OHD in the liver and then to 1,25(OH)2D in the kidneys and that the latter metabolite controls calcium absorption.

Fairweather-Tait, S., Prentice, A., Heumann, K.G. et al. 1995. Effect of calcium supplements and stage of lactation on the calcium absorption efficiency of lactating women accustomed to low calcium intakes. Am. J. Clin. Nutr., 62:1188-1192.

Prentice, A., Jarjou, L.M.A., Stirling, D.M., Buffenstein, R. & Fairweather-Tait, S. 1998. Biochemical markers of calcium and bone metabolism during 18 months of lactation in Gambian women accustomed to a low calcium intake and in those consuming a calcium supplement. J. Clin. Endocrinol. Metab., 83: 1059-1066.

However, plasma 25OHD closely reflects vitamin D nutritional status and because it is the substrate for the renal enzyme which produces 1,25(OH)2D, it could have an indirect effect on calcium absorption. The plasma level of 1,25-(OH)2D is principally regulated is through increased gene expression of the 1-α-hydroxylase(CYP1α) and not by increased 25OHD levels. This has been seen consistently in animal studies, and the high calcium absorption and high plasma 1,25-(OH) 2D observed in Gambian mothers is consistent with this type of adaptation.

However, increasing latitude may compromise vitamin D synthesis to the degree that 25OHD levels are no longer sufficient to sustain adequate 1,25-(OH)2D levels and efficient intestinal calcium absorption,although this theory remains unproved.

Regardless of the mechanism of compromised vitamin D homeostasis, the differences in calcium absorption efficiency have a major effect on theoretical calcium requirement, as illustrated in Figure 18, which shows that an increase in calcium absorption of as little as 10 percent reduces the intercept of excreted and absorbed calcium (and therefore calcium requirement) from 840 to 680 mg. (The figure also shows the great increase in calcium requirement that must result from any impairment of calcium absorption.)