How much sun is optimal?

Here is a report of a paper about a finding that incidence of kidney cancer in a sample of 175 countries is positively correlated with increased distance from the equator. Here's the link to the story (not the paper), and an excerpt from the story:

The researchers created a graph with a vertical axis for renal cancer incidence rates, and a horizontal axis for latitude. The latitudes range from -90 for the southern hemisphere, to zero for the equator, to +90 for the northern hemisphere. They then plotted incidence rates for 175 countries according to latitude. The resulting chart was a parabolic curve that looks like a smile (see accompanying images).

"The plot points created a curve roughly resembling a smile, with countries with high incidence rates at the left and right, and those with low incidence rates in the center, just a few degrees from the equator," said Garland. "Countries with the highest cancer rates were places like New Zealand and Uruguay in the southern hemisphere and Iceland and the Czech Republic in the northern hemisphere. Clustered at the bottom of the curve with lowest incidence rates were Guam, Indonesia and other equatorial countries on most continents, including many varied equatorial cultures."

In addition to UVB, the researchers analyzed cloud cover and intake of calories from animal sources for their association to kidney cancer. The scientists were able to determine the contributions of each independently. After accounting for cloud cover and intake of animal protein, UVB exposure still showed a significant independent association with incidence rates.

"Because the distinctive "smiley" parabolic curve is present for both sexes, it is unlikely that the international differences are due to occupational exposures, which usually vary according to gender, " said co-author Sharif B. Mohr, M.P.H.

So, how much sun is optimal probably depends on your constitutive pigmentation, ability to tan, your diet, and types of exposure to the sun (long term, short term). That's all I can think of right now.