A recent New Zealand study suggests that supplementing intake of Vitamin D doesn't reduce your chances of catching a cold. Earlier studies have found a correlation between Vitamin D levels and catching a cold. The lesson? The research is interesting, but we can always use more of it.
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The new study, published in the Journal of the American Medical Association, tracked the frequency of colds in 322 New Zealanders over 18 months. Half the group received regular Vitamin D supplements; the other half received a placebo. There was no statistically significant difference in the number of colds (upper respiratory tract infections) in either group, the severity of those colds, or the number of days of work missed as a result.
Other research has pointed in different directions. One study of 19,000 by the University of Colorado found people with higher Vitamin D levels were less likely to have respiratory infections. That doesn't contradict the New Zealand study; it's possible, for instance, that people with higher Vitamin D levels had higher levels of sun exposure because they exercised more, which improved their overall health.
Another study of Mongolian children found their risk of respiratory infection was reduced by a daily dose of Vitamin D-enhanced milk. However, that test pool already had lower-than-usual Vitamin D levels, so we don't yet know if the same results would be evident in people with healthy levels of Vitamin D.
Principal investigator Dr David Murdoch highlighted that point in the release announcing the study:
VIDARIS is the first study to convincingly show that vitamin D does not prevent colds in healthy adults. However, it is important to note that very few people in our study had extremely low levels of vitamin D at the beginning. So, our findings may not apply to these people and to children who should now be the focus of further research.
The big lesson? Science is useful because it repeatedly tests the same propositions over time. A single study is a useful data point, but not "proof" of anything absolute. It's the combination of all the different results that gives us confidence in what we know.
In the meantime, taking extra vitamins may not do you any harm unless the doses are ridiculously high, but chances are a healthy diet will give you the same benefits for less expense.