Latitude and vitamin D production in the skin
Except during the summer months, the skin makes little if any vitamin D from the sun at latitudes above 37 degrees north (in the United States, the shaded region in the map) or below 37 degrees south of the equator. People who live in these areas are at relatively greater risk for vitamin D deficiency.
In search of vitamin D
Under the right circumstances, 10 to 15 minutes of sun on the arms and legs a few times a week can generate nearly all the vitamin D we need. Unfortunately, the “right circumstances” are elusive: the season, the time of day, where you live, cloud cover, and even pollution affect the amount of UVB that reaches your skin. What’s more, your skin’s production of vitamin D is influenced by age (people ages 65 and over generate only one-fourth as much as people in their 20s do), skin color (African Americans have, on average, about half as much vitamin D in their blood as white Americans), and sunscreen use (though experts don’t all agree on the extent to which sunscreen interferes with sun-related vitamin D production).
Lack of sun exposure would be less of a problem if diet provided adequate vitamin D. But there aren’t many vitamin D–rich foods (see chart, below), and you need to eat a lot of them to get 800 to 1,000 IU per day. People who have trouble absorbing dietary fat — such as those with Crohn’s disease or celiac disease — can’t get enough vitamin D from diet no matter how much they eat (vitamin D requires some dietary fat in the gut for absorption). And people with liver and kidney disease are often deficient in vitamin D, because these organs are required to make the active form of the vitamin, whether it comes from the sun or from food.
Selected food sources of vitamin D
Food Vitamin D (IU*) Salmon, 3.5 ounces 360 Mackerel, 3.5 ounces 345 Tuna, canned, 3.5 ounces 200 Orange juice, fortified, 8 ounces 100 Milk, fortified, 8 ounces 98 Breakfast cereals, fortified, 1 serving 40–100 *IU = international units
Source: Office of Dietary Supplements, National Institutes of HealthFor these and other reasons, a surprising number of Americans — more than 50% of women and men ages 65 and older in North America — are vitamin D–deficient, according to a consensus workshop held in 2006.
Canadians are really at risk of not having enough Vitamin D. If you are old like me - even worse. As you can see - food will not make up the difference.
Sun block is also a caution - it screens out UVB - which is what makes Vitamin B. Better to wear clothes and take them off for a bit that screen out the spectrum that we need for our health. More on Sun block later.
