If I described a substance with the following properties, what would you guess it was?
-It's synthesized by the body from cholesterol
-It crosses cell membranes freely
-It has its own nuclear receptor
-It causes broad changes in gene transcription
-It acts in nearly every tissue
-It's essential for health
There's no way for you to know, because those statements all apply to activated vitamin D, estrogen, testosterone and a number of other hormones. Vitamin D, as opposed to all other vitamins, is a steroid hormone precursor (technically it's a secosteroid but it's close enough for our purposes). The main difference between vitamin D and other steroid hormones is that it requires a photon of UVB light for its synthesis in the skin. If it didn't require UVB, it would be called a hormone rather than a vitamin. Just like estrogen and testosterone, it's involved in many processes, and it's important to have the right amount.
The type of vitamin D that comes from sunlight and the diet is actually not a hormone itself, but a hormone precursor. Vitamin D is converted to 25(OH)D3 in the liver. This is the major storage form of vitamin D, and thus it best reflects vitamin D status. The kidney converts 25(OH)D3 to 1,25(OH)D3 as needed. This is the major hormone form of vitamin D. 1,25(OH)D3 has profound effects on a number of tissues.
Vitamin D was originally identified as necessary for proper mineral absorption and metabolism. Deficiency causes rickets, which results in the demineralization and weakening of bones and teeth. A modest intake of vitamin D is enough to prevent rickets. However, there is a mountain of data accumulating that shows that even a mild form of deficiency is problematic. Low vitamin D levels associate with nearly every common non-communicable disorder, including obesity, diabetes, cardiovascular disease, autoimmune disease, osteoporosis and cancer. Clinical trials using vitamin D supplements have shown beneficial and sometimes striking effects on cancer, hypertension, type 1 diabetes, bone fracture and athletic performance. Vitamin D is a fundamental building block of health.
It all makes sense if you think about how humans evolved: in a tropical environment with bright sun year-round. Even in many Northern climates, a loss of skin pigmentation and plenty of time outdoors allowed year-round vitamin D synthesis for most groups. Vitamin D synthesis becomes impossible during the winter above latitude 40 or so, due to a lack of UVB. Traditional cultures beyond this latitude, such as the Inuit, consumed large amounts of vitamin D from nutrient-rich animal foods like fatty fish.
The body has several mechanisms for regulating the amount of vitamin D produced from sunlight exposure, so overdose from this source is impossible. Sunlight is also the most effective natural way to obtain vitamin D. To determine the optimal blood level of vitamin D, it's instructive to look at the serum 25(OH)D3 levels of people who spend a lot of time outdoors. The body seems to stabilize between 55 and 65 ng/mL 25(OH)D3 under these conditions. This is probably near the optimum. 30 ng/mL is required to normalize parathyroid hormone levels, and 35 ng/mL is required to optimize calcium absorption.
Here's how to become vitamin D deficient: stay inside all day, wear sunscreen anytime you go out, and eat a low-fat diet. Make sure to avoid animal fats in particular. Rickets, once thought of as an antique disease, is making a comeback in developed countries despite fortification of milk (note- it doesn't need to be fortified with fat-soluble vitamins if you don't skim the fat off in the first place!). The resurgence of rickets is not surprising considering our current lifestyle and diet trends. In a recent study, 40% of infants and toddlers in Boston were vitamin D deficient using 30 ng/mL as the cutoff point. 7.5% of the total had rickets and 32.5% showed demineralization of bone tissue! Part of the problem is that mothers' milk is a poor source of vitamin D when the mother herself is deficient. Bring the mothers' vitamin D level up, and breast milk becomes an excellent source.
Here's how to optimize your vitamin D status: get plenty of sunlight without using sunscreen, and eat nutrient-rich animal foods, particularly in the winter. The richest food source of vitamin D is high-vitamin cod liver oil. Blood from pasture-raised pigs or cows slaughtered in summer or fall, and fatty fish such as herring and sardines are also good sources. Vitamin D is one of the few nutrients I can recommend in supplement form. Make sure it's D3 rather than D2; 3,000- 5,000 IU per day should be sufficient to maintain blood levels in wintertime unless you are obese (in which case you may need more and should be tested). I feel it's preferable to stay on the low end of this range. Vitamin D3 supplements are typically naturally sourced, coming from sheep lanolin or fish livers. A good regimen would be to supplement every day you get less than 10 minutes of sunlight.
People with dark skin and the elderly make less vitamin D upon sun exposure, so they should plan on getting more sunlight or consuming more vitamin D. Sunscreen essentially eliminates vitamin D synthesis, and glass blocks UVB so indoor sunlight is useless. Vitamin D toxicity from supplements is possible, but exceptionally rare. It only occurs in cases where people have accidentally taken grotesque doses of the vitamin. As Chris Masterjohn has pointed out, vitamin D toxicity is extremely similar to vitamin A deficiency. This is because vitamin A and D work together, and each protects against toxicity from the other. Excess vitamin D depletes vitamin A, thus vitamin D toxicity is probably a relative deficiency of vitamin A.
I know this won't be a problem for you because like all healthy traditional people, you are getting plenty of vitamin A from nutrient-dense animal foods like liver and butter. Vitamin K2 is the third, and most overlooked, leg of the stool. D, A and K2 form a trio that act together to optimize mineral absorption and use, aid in the development of a number of body structures, beneficially alter gene expression, and affect many aspects of health on a fundamental level.
Thanks to horizontal.integration for the CC photo.