Many mammals do not synthesize Vitamin D in significant amounts from sunshine. It is not really about the hair directly so much as the lower activity of relevant enzymes. Dogs and cats do not need sunshine to make Vit D, and in one study cats were shaved and placed outdoors without any appreciable increase in their Vitamin D levels. (Dietary Vitamin D Dependence of Cat and Dog Due to Inadequate Cutaneous Synthesis of Vitamin D
K. L. How, H. A. W. Hazewinkel and J. A. Mol)
The greater the reliance on live prey among mammals, the more they seem to rely on dietary vitamin D rather than production driven by UV. But this correlation isn’t perfect, and it varies with natural history as well.
Overall, there’s a lot of variation. Many reptiles (lizards, tortoises) require UV to make Vit D, but most snakes do not. Most primates do appear to need sunlight to make Vitamin D regardless of hair density (e.g. Relationship between sunlight exposure, housing condition, and serum vitamin D and related physiologic biomarker levels in captive chimpanzees (Pan troglodytes). Videan EN, Heward CB, Fritz J, Murphy J, Cortez C, Su Y.) I’m not sure about nocturnal animals generally, but I found one reference which suggests that some which do not have a dietary vitamin D source may have very little Vit D, and yet their calcium and phosphorus balance is perfectly normal, suggesting an entirely different way of dealing with the physiologic features Vitamin D regulates in other mammals (Vitamin D metabolism in a frugivorous nocturnal mammal, the Egyptian fruit bat (Rousettus aegyptiacus). Cavaleros M, Buffenstein R, Ross FP, Pettifor JM.).
All of the individual species variation and caveats here illustrate why evolutionary rationales for nutritional or other health maintenance practices can be dicey. A perfectly sound chain of reasoning can turn out to be false due to one little unknown or overlooked fact.
Very interesting article on this topic in the July 26 NYTimes. Read it HERE
What Do You Lack? Probably Vitamin D
By JANE E. BRODY
Published: July 26, 2010
Vitamin D promises to be the most talked-about and written-about supplement of the decade. While studies continue to refine optimal blood levels and recommended dietary amounts, the fact remains that a huge part of the population — from robust newborns to the frail elderly, and many others in between — are deficient in this essential nutrient.
... both Dr. Giovannucci [Harvard School of Public Health] and Dr. Holick [Boston University] say it is very hard to reach ... toxic levels. Healthy adults have taken 10,000 I.U. a day for six months or longer with no adverse effects. People with a serious vitamin D deficiency are often prescribed weekly doses of 50,000 units until the problem is corrected. To minimize the risk of any long-term toxicity, these experts recommend that adults take a daily supplement of 1,000 to 2,000 units.
Most meds are manufactured in dosing units. One of the things drilled into me as a new nurse is that if you are giving more than one pill, more than a one milliliter injection, or one vial of a medication, think twice and go check your order, then your references…
FYI the most recent issue of the Berkeley Wellness Letter (published by the UC Berkeley School of Public Health) has a short article about Vitamin D. (Vol. 26, Issue 11, Aug. 2010):
People who are obese or have osteoporosis, limited sun exposure, or absorption problems may need as much as 2,000 IU a day to maintain adequate blood levels of D. The Wellness Letter advises 800 to 1,000 IU a day for most people. The government recommends 200 to 600 IU a day, depending on age, but is expected to soon advise higher amounts. Vitamin D is found in few foods, so most people need to take supplements to reach these levels.
Then the drug companies need to reconfigure their dosages. I just hope it will not be a repeat of the vitamin E errors.
What were the problems with E?
You can get tablets or capsules of 1,000 or 2,000 IU in drugstores now; I’ve seen higher dosages as well, though harder to find.
Initial studies showed vitamin E protected women from breast cancer, and they were recommending higher doses for all women. Closer studies showed the higher doses actually increased the likelihood of developing breast cancers, so they had to quickly back pedal on their recommendations.
PARIS (AFP) – Ordinary calcium supplements taken by the elderly to strengthen bones may boost the risk of heart attacks, according to a study released Friday.
The findings, published in the British Medical Journal, suggest that the role of calcium in the treatment of osteoporosis should be reconsidered, the researchers said.
Calcium tablets are commonly prescribed to boost skeletal health, but a recent clinical trial suggested they might increase the number of heart attacks and other cardiovascular problems in healthy older women.
NB: they note that the chances of harm rise if you take more than 4,000 IU/day, and particularly 10,000 and over.
There is a deeply misleading, indeed borderline irresponsible, article on this in the NYTimes today by Gina Kolata, HERE. Kolata swallows the ledes, which are the huge increases in RDA and safe upper bound. Indeed, the article doesn’t even mention them!
Kolata makes all her hay from the report’s claim that most Americans don’t need to take supplements, and that some people are dosing at levels that may be dangerous. (The important area is in blood levels of Vitamin D, which the report says should be above 20 ng/ml. If indeed most Americans are well above that level, then it’s true that there isn’t much concern).
However it’s far from clear how someone who works inside during daylight hours, doesn’t drink more than a cup of milk a day (and probably significantly less) and doesn’t eat salmon or mackerel regularly is going to get anywhere near 600-800 IU/day of Vitamin D without supplementation. FYI: fortified milk has 400 IU/quart or 100 IU/cup. That’s only somewhere between 12-17% of your RDA of Vitamin D (one sixth to one eighth of what you require to remain healthy). Since Vitamin D is available in virtually no other food sources, where are we supposed to be getting it?
The lack will be particularly acute for those with dark skin or those who live at northern latitudes, and particularly during winter months when the sun is low in the sky. ... and again, for those who don’t get sun exposure at all because they work indoors during daylight hours.
While Kolata (and to some extent the authors of the study) are right to point out the dangers of overdosage, certainly for many if not most Americans the numbers simply don’t add up unless one takes a small supplement. And since there isn’t any recognized danger under 4,000 IU/day, I continue being happy taking my 1,000-2,000 IU/day supplements.