Vitamin D requirements vary from one human population to another. If solar UV is too weak or the skin too dark, there will be insufficient synthesis of vitamin D in the skin. Consequently, natural selection will favor those individuals who use this vitamin more efficiently or have alternate metabolic pathways.

There has apparently been selection to reduce vitamin D requirements among the Inuit people of Alaska, northern Canada and Greenland (

Frost 2012

;

Frost 2018

). Above the Arctic Circle, solar UV is usually too weak to permit vitamin D synthesis in the skin. Some vitamin D is obtained from the diet, i.e., fatty fish and certain marine mammals, but such sources are insufficient. According to a study from Nunavut and the Northwest Territories, even Inuit on a traditional diet get less than the recommended minimum of 10 µg/day (

Kolahdooz et al. 2013

).

Inuit have adapted to this insufficiency through physiological changes: receptors that bind more tightly to the vitamin D molecule; a lower set-point for calcium-regulated release of parathyroid hormone; and conversion of vitamin D at a higher rate from its common form to its most active form. There may be other adaptations. For instance, Inuit breast milk might be richer in ß-casein, which seems to facilitate the body's use of vitamin D (

Frost 2018

).

Vitamin D levels are generally low in dark-skinned human populations, even those that still inhabit the tropical zone and receive intense sunlight. This was the conclusion of several studies:

• Vitamin D levels were below 75 nmol/L in 51% of young tanned Hawaiians with 22.4 hours per week of unprotected sun exposure (Binkley et al. 2007).

• Among south Indians, 44% of the men and 70% of the women had less than 50 nmol/L. They were "agricultural workers starting their day at 0800 and working outdoors until 1700 with their face, chest, back, legs, arms, and forearms exposed to sunlight" (
• Harinarayan et al. 2007).

• In Saudi Arabia, levels were below 25 nmol/L in respectively 35%, 45%, 53%, and 50% of normal male university students of Saudi, Jordanian, Egyptian, and other origins (
• Sedrani 1984
• ).

• In a sample of healthy Middle Eastern athletes, 91% had less than 50 nmol/L (
• Hamilton et al. 2010
• ).

• Variation is significant even among Europeans. Vitamin D levels are lower in central and southern Europeans than in lighter-skinned Swedes (
• Snellman et al. 2009
• ).

• A meta-analysis concluded that vitamin D levels are significantly lower in people of non-European origin than in people of European origin. Levels are consistently low in non-Europeans regardless of latitude (
• Hagenau et al. 2009
• ).

Vitamin D levels are apparently lower in dark-skinned human populations for genetic reasons. A study of African Americans found that serum 25(OH)D decreased linearly with increasing African ancestry, the decrease being 2.5 to 2.75 nmol/L per 10% increase in African ancestry. The study also found that sunlight and diet were 46% less effective in raising these levels among subjects with high African ancestry than among those with low/medium African ancestry (

Signorello et al. 2010

).

If vitamin D levels are naturally lower in darker-skinned populations, the optimal range of levels will likewise be lower, and toxic effects will develop at lower levels

^{[1][2][3][4][5][6][7][8][9][10]}

.