Okay, now for a bit more detail about Vitamin D. In this post we’ll look at where it comes from, and how we get it to where it’s needed in the body.
1) Vitamin D metabolism
There are actually two forms of vitamin D: vitamin D2, which is synthesized by plants, and vitamin D3 (cholecalciferol), which is synthesized by mammals, including us. This review will focus on D3, since that’s the most important form for humans.
There are two ways that humans get their daily vitamin D: sunshine (the most effective way) and diet.
First let’s clear up a common vitamin D misconception. We do not get vitamin D directly from the sun–that is, there are no tiny “Ds” raining down on us while we sunbathe by the pool. Rather, sunlight acts on a cholesterol-based vitamin D precursor our bodies make and converts that to previtamin D in the skin. Previtamin D is then converted to vitamin D3 by the heat of the skin.
That’s just the beginning…
Once it leaves the skin, vitamin D3 jumps on board a specialized protein that transports it to the liver, where it’s converted to 25-hydroxyvitamin D (25-OH-D). From there 25-OH-D surfs to the kidneys on another specialized protein where, through the mysteries of biochemistry, it transforms itself into 1,25-dihydroxyvitamin D (1,25-OH2-D), the most biologically active form of vitamin D. Then it’s off to the tissues of the body to work some magic.*
We can also get vitamin D from what we eat, though D isn’t distributed very well in natural foods. Vitamin D-rich foods include oily fish like salmon and swordfish, and egg yolks, butter and liver (Liver!! My Gram Sloan, a big chicken liver fan, was right…). Vitamin D is added to milk nowadays, of course, but even so, unless you’re a traditional Eskimo–eating a lot of fish, seal and whale blubber–it’s unlikely that your daily diet provides more than half of your vitamin D needs.
2) Sunshine: How much is enough?
Many factors go into determining how much sun exposure we require to ensure a healthy level of vitamin D in the bloodstream. Time of year, latitude, skin pigmentation, weather, air pollution, clothing and sunscreen all play a role. Basically, a fair-skinned, sunscreen-free person getting 10-15 minutes of total body exposure during the summer months will produce about 10,000-25,000 international units (IU) of vitamin D in 24 hours. So a Celtic-skinned person (such as myself) living in a nudist colony on the equator (not gonna happen) would do just fine, vitamin D-wise, from sunshine alone…until the skin cancer gets him, that is.
But all those other factors, from clothing to clouds, literally get in the way. Take the simple factor of skin pigmentation: it can take 5 to 10 times more sunlight for a dark-skinned individual than a light-skinned one to produce the same amount of vitamin D. Consider the plight of, say, a Nigerian-born child living in
Canada. Between the short northern winter days, cold-weather clothes and dark skin, the chances of getting enough vitamin D from sunshine are pretty remote.
And that’s true of just about everyone in the United States today, too, regardless of skin pigmentation. In a 1989 study on indoor air quality, the Environmental Protection Agency estimated that the average American spends 93% of his or her time indoors. Today, with computers, electronic games, and 600 TV channels to choose from, that sun-less percentage has likely increased, to the detriment of our vitamin D stores.
So, most of us are not getting nearly enough vitamin D from sunshine or diet. What potential impact does that have on our health?
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*(Vitamin D metabolism, which involves skin, liver, kidneys, bowel, bones, sunlight, diet, heat, several specialized proteins and dozens of specific chemical reactions–and which can be disrupted by things like rainy weather, or how often you wear a hat outdoors–is not a good argument for intelligent design.)