Re: influenza

The amount of vitamin D that can be obtained by a fair skinned, red haired person in 15 minutes will take 8 hours for someone dark skinned living near the equator.

Most people end up working in doors, so their D levels go down.

Vitamin D is a hormone and needs to be converted by an enzyme in the body.

If there are any problems with the D receptors, liver, kidneys, enzymes, the D may not work.

"Vitamin D from the diet or dermal synthesis is biologically inactive and requires enzymatic conversion to active metabolites. Vitamin D is converted to 25-hydroxyvitamin D, the major circulating form of vitamin D, and then to 1,25-dihydroxyvitamin D, the active form of vitamin D, by enzymes in the liver and kidney. 1,25-dihydroxyvitamin D binds to the intracellular vitamin D receptor to activate vitamin D response elements within target genes. The half-life of 1,25-dihydroxyvitamin D is four to six hours, compared with two to three weeks for 25-hydroxyvitamin D and 24 hours for parent vitamin D."

http://www.uptodate.com/contents/metabolism-of-vitamin-d