We are one of the few animals on earth without feathers, scales or fur. It turns out to be a bit risky not to have a covering on the skin, but there may be some advantages to being hairless. Though we don’t have fur to block the sun we are able to make a portable, temporary sunscreen that allows us to protect our skin from the sun when we need to. All animals on the planet, as far back as insects, fish and reptiles use their skin to make vitamin D, and so do we. We make vitamin D from the UVB rays of the sun as they hit our skin, but only outside and with direct sun exposure. UVB is the only wavelength of light that changes 25 dehydroxycholesterol in our skin to a “pre” vitamin D, the chemical that then becomes vitamin D25 OH. The clothing that we wear and the sunscreen that we put on both block the UVB rays. They also block the production of vitamin D on the skin! Because the D in most food supplements is vitamin D2, (D2 is what rats, nocturnal animals, use), the food supplementation that the US government has provided in milk does not replace the vitamin D3 made on the skin. (Most doctors do not know the difference, and the current Family Practice recommendations are still vitamin D2 50,000 IU once a week.)
How to use the melanin in your skin to your and your child’s advantage:
Human skin coloring is based on a chemical called melanin. Melanin is a chemical that is designed to absorb energy. In the skin it is used to absorb potentially damaging UVB light. High energy light rays of UVA and UVB frequency from the sun or from a tanning bed can damage the DNA in our skin cells. That damage, if not repaired, can lead to skin cancer. We hairless beings incorporated a wonderful, home-made sunscreen into the surface of our skin. We can make it on demand and adjust it in relation to our sun exposure. We also use the vitamin D on our skin to repair that same DNA damage. So… way, way before doctors or sunscreen ever existed, our bodies had this all worked out!
In summary: be smart, be careful, listen to your body. If you can make enough vitamin D in the summer to make you sleep all the way through the winter then you don’t need to supplement. The bad effects of vitamin D deficiency don’t happen in one season, they accumulate over years of sleeping badly and not repairing our body for years on end. In normal populations that lived away from the equator there was a normal yearly change in the vitamin D blood level. It went up in the Fall and down again in the Spring. Probably, if the lowest Winter D level was 40 ng/ml the intestinal bacteria changed enough for us to gain a little weight, then when the D went up again in summer the normal bacteria grew back and we would lose the extra pounds.