Here’s a bunch of studies about cutaneous vitamin D synthesis (making vitamin D in your skin) :
Note: This is an extremely technical and detailed article explaining the process by which your body produces cutaneous vitamin D from sun exposure.
Traditionally Living Populations in East Africa Have a Mean Serum25-Hydroxyvitamin D Concentration of 115 nmol/l (2011)
Conclusion: In this study, researches traveled to Tanzania, East Africa to measure the population vitamin D levels of the Maasai and Hadzabe tribes. Despite wearing minimal clothing , spending the majority of every day outdoors, and living right at the equator, they actually had a mediocre average vitamin D level of 45 ng/ml. Most vitamin D experts recommend a consistent level of at least 50 ng/ml. So why are these half-naked, equatorial people who are in the sun all day long failing to achieve optimal vitamin D levels? 1) They have very dark skin (type 6) which blocks the UVB radiation necessary for vitamin D production and 2) they avoid direct sun exposure and seek out shade as much as they can. So if half-naked people living outdoors at the equator in Africa are failing to achieve optimal vitamin D levels, what is happening to people in Canada, New York, and the UK?…
Vitamin D Status and Its Adequacy in Healthy Danish Perimenopausal Women: Relationships to Dietary Intake, Sun Exposure and Serum Parathyroid Hormone (2001)
Conclusion: 2.5 year-long study of 2,016 healthy, white menopausal women in Denmark concludes that menopausal women who avoid summer sun exposure and do not supplement with vitamin D are prone to vitamin D insufficiency. Those who intentionally exposed themselves to regular sun exposure had the highest levels, those who reported occasional sun exposure had lower levels, and those reported actively avoiding sun exposure had the lowest levels. Active sunbathing was correlated more highly with vitamin D status than supplementation.The study also found that their vitamin D levels spiked significantly in the summer.
Conclusion: This study involving 10 subjects compared the plasma vitamin D increasing effects of UV exposure and oral vitamin D2 supplementation. Although an oral dose of 50,000 IU of vitamin D2 resulted in a much faster plasma vitamin D increase starting at just 4 hours after the dose, this increase began to drop rapidly and the vitamin D levels of the subjects returned to baseline after just two days. Plasma vitamin D level increases from UV exposure, on the other hand, took 10 hours to start, peaked at 24 hours, only began dropping at three days, and did not return to baseline until after seven days. This study proves that for a longer-lasting vitamin D boost, sunshine is far superior to oral vitamin D2. This study did not, however, test the effectiveness of vitamin D3, which has been found in many studies to be more effective than D2 for increasing plasma vitamin D levels.