We make vitamin D in our skin using a type of solar radiation called ultraviolet B (UVB). Unfortunately, UVB is filtered out by the atmosphere. As the Earth tilts on its axis in winter, the sun’s rays travel through more atmosphere to get to us, and more and more UVB is filtered out. Some estimates have it that, in our part of the country, we simply do not make vitamin D during 4-5 months of every year. Even in summer, morning and afternoon sun has to angle through more atmosphere, filtering out most if not all UVB.
Dark-skinned people have an especially difficult time using UVB. Which isn’t surprising when you think about it: after all, dark skin is built-in sunblock! In the Boston study, for example, black women averaged half the blood D levels white women did. Obviously, supplements are especially important here.
So how much vitamin D can we make, anyway? Well, that depends on how much sun we get, how pale we are (taking into account our current tan), and how much of our body is exposed (and how big that body is in the first place!) Scientists talk about a “minimal erythemal dose” (or MED) of UVB, which is a fancy way of saying “just enough sunlight so you just start to turn pink.” A full-body MED equals 10,000-25,000 i.u. of oral vitamin D. But when I say “full-body,” bear in mind, I mean “completely naked.” Let’s say instead that you get an MED in jeans and a t-shirt: you lose 85% of that10-25,000 i.u. – and that still assumes that you tan underneath your chin, etc. Wearing a coat and gloves slashes 95% from the original totals. And this, of course, is only valid if we’re getting good UVB, which we don’t in winter.
One final note: D is converted to 25-hydroxyvitamin D in the liver, before it becomes active 1,25-dihydroxyvitamin D in the kidneys or tissues. So if your kidneys or liver are impaired, you’ll have a harder time utilizing your vitamin D. Also know that you need the mineral boron to activate D. Unfortunately, many multivitamins leave out this crucial mineral because the U.S. government has not yet established a recommended daily allowance for it. A 3-6 mg dose is probably fine.
So let’s move on to the health benefits, starting with osteoporosis, where there’s good reason to believe that supplementing with D is every bit as important as calcium, if not more so. Bear in mind, most of us have enough calcium; we just don’t use it properly. D promotes the absorption of calcium into the blood-stream, and once there, stimulates it being laid down as bone. It also helps keep our bone-building cells alive.
To be fair, clinical research on D and osteoporosis is inconsistent. Some studies show benefit; some don’t. But when you look specifically at trials that use at least 800 i.u. (instead of the standard 400), and that use D3, it consistently does show benefit. In the four long-term, double-blind trials that used this dose, there was a roughly 30% reduction in hip fracture. As far as I’m aware, this dose has not yet been studied in relation to the spine.
The neat thing about vitamin D and osteoporosis is that it protects our bones in a way that doesn’t have anything to do with strengthening the bones themselves. It turns out that higher blood vitamin D levels are linked to greater muscle strength and coordination in the elderly. So, fewer falls.
And then there’s cancer. Much of the research that connects vitamin D with cancer uses UVB exposure as an indicator of vitamin D levels. While this should be fairly accurate, it can’t account for all possible confounding factors. (I mean, who knows – maybe less UVB [meaning it’s colder] means people spend more time indoors with the windows closed, and it’s actually a build-up of chemical fumes inside the house that increases cancer risk, not a vitamin D deficiency…) Still, some of the research is highly compelling, especially when you add some of the epidemiological data to some of the biochemistry we know, showing how D could be involved. In fact, test tube research has identified over half-a-dozen mechanisms that D could use to prevent cancer. Many cancers are linked to a specific vitamin D receptor subtype.
By far the most compelling work connecting D to cancer prevention is a 2002 paper published in the journal Cancer (94: 1867-1875), in which the author, William Grant, uses methodology so simple, so elegant in concept as to be brilliant. Grant took July UVB levels for the entire country from a NASA database, and mapped it against the National Institutes of Health’s database of regional cancer mortality over 44 years. The results linked vitamin D to prevention of Non-Hodgkin’s Lymphoma, and cancers of the bladder, breast, rectum, colon, esophagus, stomach, ovaries, prostate, kidneys, and uterus.
But remember, this isn’t necessarily due to vitamin D. Increased UVB, which allows us to make that vitamin D, might be correlated to the decrease of another risk factor entirely. Well, two years after the original publication, Grant and his colleagues re-examined their data with this in mind, factoring in other risk factors like smoking, urbanization, and poverty. And when they did so, the results did in fact shift… to include prevention of Hodgkin’s Lymphoma, as well as cancers of the cervix, gall bladder, larynx, mouth, and pancreas. Grant writes: “In most cases the association [of cancer risk] with UVB radiation for July is stronger than that for any other factor.”
And what about skin cancer – doesn’t the sun cause that? Well, first of all an MED is different from a sunburn. Secondly, you can protect yourself with green tea and fish oil, which have been shown to decrease the pre-pre-cancerous changes in skin following sunburn. And finally, even in a worst case scenario where the sun is every bit as bad for your skin as it’s made out to be, you’re still trading seventeen cancers for one.
And it’s not just cancer and osteoporosis. Numerous studies in countries all over the world connect UVB exposure at all ages with lower incidences of multiple sclerosis. Similar studies connect UVB with prevention of type I diabetes. For example, one study found that maternal UVB exposure during the second trimester (when the pancreas develops) lowers risk of the disease. Another study (published in the Lancet) found that infants with the highest vitamin D intake had an 80% reduced risk of developing the disease.
What MS and type I diabetes have in common is that they’re both autoimmune disorders, where our immune systems get confused and attack parts of us. The research on D and autoimmunity is still in its infancy, but it’s interesting to see some of the test tube evidence pointing to a very strong role for vitamin D in retraining our immune systems to be more tolerant. We may see some major developments here in the next few years. (And in the meantime, why not just take some D?) (By the way, at least one pharmaceutical company is actually working on drugs to stimulate vitamin D receptors for this purpose).
What else? Well, one study found that elderly people with chronic, unexplained muscle-and-bone pain improved when given D. Two small studies found that vitamin D, between 400-800 i.u. a day, improved mood. In one of these, involving 44 people with seasonal affective disorder, mood improved within 5 days.
And speaking of mood and behavior. One study found that babies born in the spring (meaning their brains developed in winter, with low UVB) were more likely to develop anxiety neurosis. Another study found the same thing for bipolar disorder. Does this implicate maternal vitamin D deficiency in how our personalities develop? Perhaps it just proves that whole zodiac/astrology thing… Maybe all that star sign jazz is true, and it’s all because of vitamin D! On this one, I think, it’s way too early to jump to conclusions…
 It’s no coincidence that we see lighter-skinned indigenous people the further we get from the equator – with one major exception: Eskimoes are remarkably dark-skinned for their latitude, probably reflecting their high D intake.
 Then again, the same can be said about magnesium, fish oils, strontium, and vitamin K2.
 But it’s a lot cheaper and faster than giving a thousand people vitamin D every day for 50 years… Besides, most research on cancer prevention is based on epidemiological, not intervention trials. It’s no different than the research on smoking.