Vitamin D and type 1 diabetes

Institute of Community Health Studies
New England Research Institutes
9 Galen Street
Watertown, MA 02472
E-mail: sharris{at}neri.org

Dear Sir:

In their recent case-control study, Stene et al (1) reported that the use of cod liver oil in the first year of life was associated with a reduced risk of type 1 diabetes, but that the use of other vitamin D supplements during the same period was not. This finding is in contrast with that of Hyppönen et al (2), who, in a prospective cohort study of the incidence of type 1 diabetes, found a strong, dose-dependent, protective effect of vitamin D supplements. Why this difference? One possibility is that vitamin D is protective only at relatively high doses. In the study by Hyppönen et al, few infants were given cod liver oil but >99% received some amount of vitamin D supplementation. For this reason, the reference group in dose-related analyses included infants receiving up to 50 µg/d, a dose 10 times that of the US recommendation (5 µg/d) (3). The risk of type 1 diabetes was sharply reduced at doses >50 µg/d. Stene et al did not collect information on individual doses of cod liver oil or other vitamin D supplements but did report that the manufacturers' recommended doses for both products was 10 µg/d. Thus, their data do not rule out a protective effect of vitamin D at doses >10 µg/d. The fact that they observed a protective effect of cod liver oil at the same dose is interesting, and, as they suggest, it may reflect an antiinflammatory effect of long-chain n-3 fatty acids. Certainly, the ability of both cod liver oil and other forms of supplemental vitamin D to prevent type 1 diabetes bears further investigation.

REFERENCES

1. Stene LC, Joner G, and the Norwegian Childhood Diabetes Study Group. Use of cod liver oil during the first year of life is associated with lower risk of childhood-onset type 1 diabetes: a large, population-based, case-control study. Am J Clin Nutr 2003;78:1128–34.
2. Hyppönen E, Läärä E, Reunanen A, Järvelin MR, Virtanen SM. Intake of vitamin D and risk of type 1 diabetes: a birth-cohort study. Lancet 2001;358:1500–3.
3. Standing Committee on the Scientific Evaluation of Dietary Reference Intakes. Dietary reference intakes for calcium, phosphorus, magnesium, vitamin D, and fluoride. Washington, DC: National Academy Press, 1997.