Evidence Used for Estimating Recommended Vitamin D Intake

Lack of accuracy in estimating dietary intake and skin synthesis

The unique problem of estimating total intake of a substance that can be provided in the diet or made in the skin by exposure to sunlight makes it difficult to estimate adequate total intakes of vitamin D for the general population. Accurate food composition data are not available for vitamin D, accentuating the difficulty for estimating dietary intakes. Whereas this has led two recent US national surveys to avoid attempting this task, the second National Health and Nutrition Examination Survey (NHANES II) estimated vitamin D intakes to be 2.9 μg/day and 2.3 μg/day for younger and older women, respectively. A recent study of elderly women by Kinyamu et al. concurred with this assessment, finding an intake of 3.53 μ g/day.

Skin synthesis is equally difficult to estimate, being affected by such imponderables as age, season, latitude, time of day, skin exposure, sun screen use, etc. In vitamin D – replete individuals, estimates of skin synthesis are put at around 10 μg /day, with total intakes estimated at 15 μg/day.

Because study after study had been recommending increases in vitamin D intakes for the elderly, it might have been expected that the proposed increases in suggested intakes from 5 μg/day (RDAs in the United States, RNIs in Canada) to 10 μg/day or 15 μg/day (AI) would be welcomed. However, a recent editorial in a prominent medical journal attacked the recommendations as being too conservative. This came on the heels of an article in the same journal reporting the level of hypovitaminosis D to be as high as 57 percent in a population of ageing (mean 62 years) medical in-patients in the Boston area.

Of course, such in-patients are by definition sick and should not be used to calculate normal intakes. Indeed, the new NHANES III study of 18 323 normal individuals from all regions of the United States suggests that approximately 5 percent had values of 25-OH-D below 27 nmol/l. Although the data are skewed by sampling biases that favour sample collection in the southern states in winter months and northern states in the summer months, even subsets of data collected in northern states in September give the incidence of low 25-OH-D in the elderly in the 6–18 percent range as compared with 57 percent in the institutionalized in-patient population. 

Ideally, such measurements of the normal population should be made at the end of the winter months and before UV irradiation has reached a strength sufficient to allow skin synthesis of vitamin D. Thus, the NHANES III study may still underestimate the incidence of hypovitaminosis D in a northern elderly population in winter. Nevertheless, in lieu of additional studies of selected human populations, it would seem that the recommendations of the Food and Nutrition Board are reasonable guidelines for vitamin D intakes, at least for the near future. This considered approach allows for a period of time to monitor the potential shortfalls of the new recommendations as well as to assess whether the suggested guidelines can be attained, a point that was repeatedly stated about the RDAs.

References:

Holick, M.F. 1994. McCollum award lecture, 1994: Vitamin D-new horizons for the 21st century. Am. J. Clin. Nutr., 60: 619-630.

National Academy of Sciences. 1997. Report on Dietary Reference Intakes for Calcium, Phosphorus, Magnesium and Vitamin D, Food & Nutrition Board, Institute of Medicine, US National Academy of Sciences. P. 7.1-7.30. . National Academy Press.

Kinyamu, H.K., Gallagher, J.C., Rafferty, K.A. & Balhorn, K.E. 1998. Dietary calcium and vitamin D intake in elderly women: effect on serum parathyroid hormone and vitamin D metabolites. Am. J. Clin. Nutr., 67: 342-348.

Centers for Disease Control and Prevention. 1998. National Health and Nutrition Examination Survey III, 1988-1994. CD-ROM Series 11, No. 2A. Hyatsville, MD.

National Research Council. 1989. Recommended Dietary Allowances 10th Edition. Report of the Subcommittee on the Tenth Edition of the RDA, Food and Nutrition Board and the Commission on Life Sciences . Washington DC: National Academy Press .

Health and Welfare Canada. 1990. Nutrition Recommendations. P. 90-93. Ottawa, ON Canada. Published by Health and Welfare Canada.

Utiger, R.D. 1998. The need for more vitamin D. N. Engl. J. Med., 338: 828-829.

Thomas, M.K., Lloyd-Jones, D.M., Thadhani, R.I., Shaw, A.C., Deraska, D.J., Kitch, B.T., Vamvakas, E.C. Dick, I.M., Prince, R.L. & Finkelstein, J.S. 1998. Hypovitaminosis D in medical inpatients. N. Engl. J. Med., 338: 777-783.

Looker, A.C. & Gunter, E.W. 1998. Hypovitaminosis D in medical inpatients. A letter to N. Engl. J. Med., 339: 344-345.

Fraser, D.R. 1983. The physiological economy of vitamin D. Lancet, I: 969-972.