Putting the
'D' in Diabetes
Evan David Rosen, M.D., Ph.D.
Assistant Professor of Medicine,
Harvard Medical School
Low
levels of vitamin D have long been known to predispose to osteoporosis
and rickets in children. It's for this reason that mothers used
to spoon feed cod liver oil, a rich source of vitamin D, to
their unwilling children. Nowadays we supplement milk products
with extra vitamin D, sparing legions of kids from the horrors
of cod liver oil, which tastes about as bad as it sounds. Now,
evidence is emerging that vitamin D may help prevent the onset
of type 1 diabetes as well.
This finding has actually been
anticipated for some time. Giving mice a drug called streptozotocin
can cause type 1 diabetes, but pretreatment with vitamin D can
forestall the appearance of the disease. A few small studies
have also shown that kids who took extra vitamin D or who ate
cod liver oil had lower rates of type 1 diabetes. A recent issue
of The Lancet reports a large study performed in Finland that
confirms these results. Researchers asked women who had children
in the 1960's whether they gave their kids extra vitamin D or
not. Then they looked at whether these kids developed type 1
diabetes over the next twenty years or so. The results showed
that kids who got extra vitamin D were about five times less
likely to develop diabetes, while kids with very low levels
of vitamin D (as seen by the fact that they actually developed
rickets, a bone disorder that occurs when vitamin D levels are
too low) got diabetes at three times the normal rate.
Now, a few caveats are in order.
First of all, this was a retrospective analysis that required
women to remember whether they gave vitamins to their kids twenty
years earlier. Also, the study took place in Finland, where
there are only two hours of daylight in the winter months. Low
levels of sunlight are associated with low levels of vitamin
D. Therefore, it's very unclear whether these results will apply
to folks who live in sunnier climates. Finally, this was not
a treatment-based study. What we really need is for someone
to take a couple of thousand babies, give half of them extra
vitamin D, and show that they get less type 1 diabetes. It's
unclear if such an experiment will ever be done, but even if
it were started yesterday, it would still take about 20 years
to get the answer.
So what do we do in the meantime?
Should we be giving kids extra vitamin D? Well first of all
let's make it clear that there aren't clear Recommended Daily
Allowances for vitamin D in kids or adults, at least in this
country. In other countries (such as Finland), the RDA for vitamin
D in kids used to be about 4,000 IU (International Units). Over
the years it dropped to 2,000 IU (which is what it was in the
1960's, when it would matter for the study we talked about above),
then 1,000 IU, then down to 400 IU. In fact 400 IU is still
the standard in most places, and is the amount of vitamin D
found in one adult multivitamin tablet. Clearly, everyone should
get at least that amount, and probably more if they get less
sunlight than usual, either because they're elderly, or they
live in northern latitudes, or because they like to watch cartoons
instead of playing outside.
Don't overdo the vitamin D, though,
as too much can cause problems ranging from confusion to constipation,
vomiting, and weakness.Stick to one or two multivitamins worth,
and you (and your kids) will be fine.
The final question concerns the
way vitamin D might have an effect on diabetes. At this point,
we really don't know how this works. Vitamin D raises calcium
levels in the body, but researchers have found that this has
nothing to do with the effect on diabetes. Interestingly, we
know that type 1 diabetes occurs when the immune system attacks
the insulin-producing beta cells of the pancreas, and vitamin
D has been shown to tamp down the immune response in several
important ways. Given that most of the other immunomodulatory
drugs under investigation can be quite toxic, vitamin D looks
like a very attractive treatment if it turns out to work.
References:
1. Hypponen E, Laara E, Reunanen
A, Jarvelin MR, Virtanen SM. Intake of vitamin D and risk of
type 1 diabetes: a birth-cohort study. Lancet. 2001; 358: 1500-3.
2. Riachy R, Vandewalle B, Belaich
S, Kerr-Conte J, Gmyr V, Zerimech F, d'Herbomez M, Lefebvre
J, Pattou F. Beneficial effect of 1,25 dihydroxyvitamin D3 on
cytokine-treated human pancreatic islets. Journal of Endocrinology.
2001 169: 161-8.
3. Stene LC, Ulriksen J, Magnus
P, Joner G. Use of cod liver oil during pregnancy associated
with lower risk of Type I diabetes in the offspring. Diabetologia.
2000; 43: 1093-8.
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