1988 paper by Webb and Hollick postulated a limitation of vitamin D production with latitude and season
2009 paper appear to say that is only true for caucasians
"A major analysis published in 2009 (3) pooled together the results of 394 studies examining vitamin D levels in over 30,000 people all across the globe in order to investigate the effect of latitude on vitamin D status. The authors only included people who were native to the area in which they were living, and who were free-living. They concluded that there was only an effect of latitude in Caucasians. There was no effect of latitude in people with non-Caucasian ancestry."
"The reason this deals such a major blow to the latitude hypothesis is that it is precisely people with white skin who dwell outside the equatorial regions who are supposed to be among the most vulnerable, but Caucasians actually had 45% higher levels of vitamin D than non-Caucasians! "
One 2007 paper "They came to the "startling" conclusion that latitude was only related to vitamin D production during the coldest four months of the year."
"Another 2007 study conducted in Adenes, Norway (2) provided limited evidence suggesting that even at this far north latitude of 68 degrees vitamin D production begins in late February. The study was not anywhere near as rigorously controlled as Webb and Holick's test tube study, but it was conducted in live human beings."
Masterjohn also discussed use of coconut oil on the Pacific Islands to absord UV rays.
A quick search of the internet seems to find coconut oil but used to increase and decrease tanning - not conclusive