The UVB rays necessary for vitamin D synthesis can only penetrate the atmosphere when the sun is at 50° or more above the horizon. I knew that our vitamin D season was short, but when I did the calculations (using a US Navy website) I was shocked. The last day we could make vitamin D was August 21 and the next day will be April 18. On those two days there is about a half hour “window” centered on 1:15 pm (which is the sun-noon for Rosetown’s longitude). For most of June the “window” extends from 11:00 am to 3:30 pm. On sunny days outside these time “windows”, and on cloudy days, you can still get a sunburn and increase your risk of skin cancer from UVA, but you can’t make any vitamin D.
A recent study from New Zealand published in JAMA in October found a small but statistically insignificant reduction in upper respiratory tract infections (colds & “flu”) in healthy adults with supplementation of 100,000iu D3 monthly (equivalent to 3,330 daily). The average vitamin D level of the test population was low but not severely deficient (likely due to sun exposure), which may explain the small difference. For example, two previous studies with children in Mongolia and in Japan who had severe D deficiencies, showed much more significant improvement with D supplementation. Other studies have found that people with lower levels of D had more frequent colds or missed more work due to colds, and that newborns with a D deficiency were more susceptible to bronchitis and pneumonia. In any case there are many other good reasons to keep up your D levels – this is my 23rd column on this important vitamin.
To give credit where due, the New Zealand study appears to me to be well done. It was a fairly large controlled study with over 300 people. The dose was adequate - much closer to what some researchers are calling for. Having the subjects take their vitamin D dose monthly rather than daily seems unusual but since D is a fat soluble vitamin it's not unheard of (I recently heard a Naturopathic Doctor tell us that he takes his D once a month). It may not be the best way to keep one's level consistently up, though. And their conclusion, although disappointing, was not overstated. I quote:
In conclusion, we report that monthly administration of 100 000-IU doses of vitamin D3 did not reduce the incidence or severity of URTIs in healthy, predominantly European adults with near-normal vitamin D levels. Further research is required to clarify whether there is benefit from supplementation in other populations and with other dosing regimens.Similarly, after my previous rants about reporters exaggerating the dangers of vitamins in their news reports, I was pleased to see the Globe & Mail article actually tell people not to quit taking D:
That doesn’t mean you should stop supplementing with vitamin D. Mounting evidence suggests vitamin D deficiency may be linked to a number of diseases, most notably cancer, and that supplementation could be an effective way to reduce the risk. Although humans produce vitamin D through exposure to the sun, countries like Canada, that have lengthy winters and limited sunshine during those months, are increasingly advised to take vitamin D supplements.For more information on this or other natural health topics, stop in and talk to Stan; for medical advice consult your licensed health practitioner.