This is a highly thought provoking and unusually data rich and so exceptional paper looking at the issue of vitamin D.
The second link; a table looking at vitamin D falls in submariners is highly thought provoking, as are all of the tables.
A definite recommend for a quick scan or more if you have the time for the data it presents.
http://ajcn.nutrition.org/content/69/5/842.full
Vitamin D supplementation, 25-hydroxyvitamin D concentrations, and safety1,2
Reinhold Vieth
Many arguments favoring higher intakes of calcium and other nutrients have been based on evidence about the diets of prehistoric humans (1). Likewise, the circulating 25-hydroxyvitamin D [25(OH)D; calcidiol] concentrations of early humans were surely far higher than what is now regarded as normal. Humans evolved as naked apes in tropical Africa. The full body surface of our ancestors was exposed to the sun almost daily. In contrast, we modern humans usually cover all except about 5% of our skin surface and it is rare for us to spend time in unshielded sunlight. Our evolution has effectively designed us to live in the presence of far more vitamin D (calciferol) than what most of us get now, yet there is no consensus about what vitamin D intakes are optimal or safe.
See corresponding editorial on page 825.
Unlike anything else used in the fortification of foods, the purpose of vitamin D is to correct for what is an environmental deficit (less ultraviolet exposure) and not to correct for lack due to classical nutritional reasons. With a few exceptions reviewed by Takeuchi et al (2), there is little or no vitamin D in the kind of foods that humans normally eat. Therefore, conclusions about the efficacy and safety of vitamin D must be in the context of the role of environmental factors.
Before 1997, the recommended dietary allowance of vitamin D (RDA; 3) for infants and children was 10 μg (400 IU). In essence, the scientific basis for this dose was that it approximated what was in a teaspoon (5 mL) of cod-liver oil and had long been considered safe and effective in preventing rickets (4). The basis for adult vitamin D recommendations has been even more arbitrary. Thirty-six years ago, an expert committee on vitamin D could provide only anecdotal support for what it referred to as “the hypothesis of a small requirement” for vitamin D in adults and it recommended one-half the infant dose, just to ensure that adults obtain some from the diet (5). In England, an adult requirement of only 2.5 μg (100 IU)/d was substantiated on the basis of 7 adult women with severe nutritional osteomalacia whose bones showed a response when given this amount (6). The adult RDA of 5 μg (200 IU)/d was described as a “generous allowance” in the 1989 version of American recommended intakes (3)—but why was this “generous” and in relation to what? It is remarkable that despite the widespread intake of 5 μg (200 IU) vitamin D/d, there is still no published data showing that this dose has any effect on the serum 25(OH)D concentration in adults.
http://ajcn.nutrition.org/content/69...expansion.html
Decline in 25-hydroxyvitamin D [25(OH)D] concentrations under acutely sun-deprived living conditions