Humans can make vitamin D just by exposing their skin to sunlight. The first humans, living in sunny Africa, had no problems with this. Their skins became as dark as possible to prevent the overabundance of sun from doing them harm. But what happened when humans started living in the cold north, and needed to cover their skin much of the year?
One thing they did was evolve lighter colored skins that allowed the UV rays of the sun to more easily penetrate. But there was more going on that led to the spread of this mutation. A post on ScienceBlogs.com talks about the science:
Frank W. Sweet published an essay in 2002* which offered that the feasibility of a farming lifestyle at very high latitudes in Europe due to peculiar climatological conditions served to drive Europeans to develop light skins over the last 10,000 years. In short, Sweet argues that the diets of pre-farming peoples were richer in meats and fish which provided sufficient Vitamin D so that skin color was likely light brown as opposed to pink. But with the spread of agriculture Vitamin D disappeared from the diets of northern European peoples and so only by reducing their melanin levels could they produce sufficient amounts of this nutrient to keep at bay the deleterious consequences of deficiencies.
In part 2 of his post, the pseudonymous Razib makes the lactose connection clear.
Adult lactose digestion, blonde hair, very light skin and blue eyes are all extant at very high frequencies around the Baltic sea. The last three characters are diagnostic for northern European populations.1 We know that lactose tolerance is a recent adaptation, within the last 10,000 years. We know that one of the major skin color genes in humans, SLC24A5, has swept to near fixation in Europeans relatively recently, on the order of 10,000 years. We also know that OCA2 has been subject to a recent bout of selection. Agriculture arose in the Middle East about 10,000 years ago and finally "conquered" southern Scandinavia as late as 5,000 years ago. I am beginning to suspect that the cultural revolution prompted by the transition from hunting & gathering to farming was followed by a genetic revolution due to co-evolutionary dynamics. In exchange for scalable caloric resources in the form of grains human populations were faced with a narrowed diet so that their nutrient streams became depauperate. Where it was feasible these agricultural populations naturally turned to animal husbandry to supplement their diet and round out their nutritional balance. Like grain farming dairying is a relatively scalable enterprise, especially if the cattle are fed upon unpalatable leavings or fallow land which would otherwise be underutilized. In the specific context of Vitamin D, though pre-fortified cow milk is a poor source of this nutrient in comparison with fish, it would have provided a non-trivial supplement (also, there is some reason to assume that modern cattle might produce somewhat deficient milk in terms of Vitamin D load because of factors of diet and sun exposure).
It therefore shouldn't come as a shock to anyone that those living in colder climates of the United States are also at risk for Vitamin D deficiency.
A press release on Eurekalert.com highlighted the findings of a new study just published in the American Journal of Clinical Nutrition, Vol. 86, No. 1, 150-158, July 2007. "Risk factors for low serum 25-hydroxyvitamin D concentrations in otherwise healthy children and adolescents," by Francis L Weng, Justine Shults, Mary B Leonard, Virginia A Stallings and Babette S Zemel.
Many otherwise healthy children and adolescents have low vitamin D levels, which may put them at risk for bone diseases such as rickets. African American children, children above age nine and with low dietary vitamin D intake were the most likely to have low levels of vitamin D in their blood, according to researchers from The Children’s Hospital of Philadelphia.
A study in the current issue of the American Journal of Clinical Nutrition measured blood levels of vitamin D in 382 healthy children between six years and 21 years of age living in the northeastern U.S. Researchers assessed dietary and supplemental vitamin D intake, as well as body mass, and found that more than half of the children had low blood levels of vitamin D. Of the subjects, 55 percent of the children had inadequate vitamin D blood levels and 68 percent overall had low blood levels of the vitamin in the wintertime.
Vitamin D is crucial for musculoskeletal health. The primary dietary source of the vitamin is fortified milk, but the best way to increase vitamin D levels is from exposure to sunshine. Severe deficits in vitamin D may lead to muscle weakness, defective bone mineralization and rickets. In addition to musculoskeletal effects, vitamin D is important for immune function, and low blood levels of the vitamin may contribute to diseases such as hypertension, cancer, multiple sclerosis and type 1 diabetes. Decreased blood levels of vitamin D have also been linked to obesity.
Milk needs to be fortified with Vitamin D because it does not have any naturally. But milk is a good source of calcium, and the new agricultural diet of the northern Europeans lacked sufficient calcium. That's one of the reasons that the lactose tolerance mutation and the white skin mutation appear to have evolved in close harmony. Milk still has an important role to play for those who want it.
An article on the study by Joi Preciphs of Bloomberg News makes the point forcefully:
Vitamin D-deficient diets are also associated with milk allergy; intolerance to lactose, a sugar found in dairy products; and strict vegetarianism.
You can get Vitamin D from other sources than milk, and calcium pills with Vitamin D are widely available. But milk-free diets all too easily can lead to this problem for those who are not aware.
Dairy-free is a choice, and can be a perfectly healthy one. But you have to understand all of its ramifications and make the adjustments needed to stay healthy when you remove a good source of many bioavailable nutrients. Eat smart.
*Essays on the Color Line and the One-Drop Rule.