I have been interested in disease incidence by latitude ever since I read that such a thing existed. Kidney stones, various cancers, irritable bowel disease, atherosclerosis, heart disease, depression, autism, multiple sclerosis, and diabetes are all more prevalent at higher latitudes. This has long been thought to be due to genetic differences of those who just happen to live in the north. Research has repeatedly shown this to be false. Autism rates among children of those who have immigrated from Uganda to Sweden are around 15%, which is 200 times that of the general population. (Gillberg, C. Et al. Autism in immigrants: children born in Sweden to mothers born in Uganda. J Intellect Disabil Res. 1995;39:141-4.)
The National Institutes of Health Office of Dietary Supplements recommendation for Vitamin D is 200IU, which is the minimum amount required to prevent rickets and maintain bone health. They recommend more for those over 50.
The Food and Nutrition Board has convened a panel to reconsider the issue and come up with a new Recommended Daily Allowance.
This is a complicated issue however, because Vitamin D is a fat soluble vitamin, and also because as most medical students know, Vitamin D is a poison. That, of course, depends on the amount taken. A popular mouse poison, D-Con works by thinning the blood and causing hemorrhaging. It is widely believed by Doctors that the tolerable upper limit is around 2000IU or even less. How can this be if this is the same amount recommended for Canadian mothers?
The popular sentiment has been wrong. Vitamin D deficiency is treated with doses up to 50,000IU. These doses are only administered every few weeks, and daily doses at 1/5th this level have been shown to cause toxicity in some individuals. The upper and lower limits still need to be discovered and set, but these limits vary from person to person. Let's hope the Food and Nutrition Board gets it right.
February was Vitamin D deficiency month in Canada, so there is at least official recognition of the problem of Vitamin D deficiency in some countries.
Supplementing with Vitamin D is hit and miss, and can only be done effectively and safely by monitoring blood levels. Supplementation isn't the only way of getting adequate amounts of Vitamin D. It is also created in your skin, through exposure to sunlight. Spending 15 minutes in the sun at noon can generate an astounding 20,000IU of Vitamin D. This can explain why disease incidence varies with latitude, and why those moving from countries near the equator to those at higher latitudes would experience more diseases.
There is also a racial factor. Those with darker skin generate less Vitamin D, and disproportionally suffer from the listed diseases when they don't get enough through sun exposure, diet, or supplementation.
How can you be sure you're not getting too little, or too much Vitamin D? Visit http://www.grassrootshealth.net/ where you can get a Vitamin D home test kit for $40. GrassrootsHealth is a consortium of scientists, institutions and individuals committed to solving the epidemic Vitamin D deficiency. Through their D*Action program they are hoping to gather data on Vitamin D blood levels, supplementation, and deficiency symptoms. You can find a better description of this program on their web site.
The Vitamin D Council is another good place for more information on this subject. They track news, research, and other publications about Vitamin D.