Why you need at least 5,000 units of Vitamin D orally

http://blog.vitamindcouncil.org/2013/01/16/sun-exposure-is-just-a-little-bit-enough/

Dr Mercola recommends getting sun exposure to increase your Vitamin D levels. 

But John Cannel, MD claims it is hard to get Vitamin D levels to increase just by sun exposure without a swim suit.

About John Cannell, MD

Dr. John Cannell is founder of the Vitamin D Council. He has written many peer-reviewed papers on vitamin D and speaks frequently across the United States on the subject. Dr. Cannell holds an M.D. and has served the medical field as a general practitioner, itinerant emergency physician, and psychiatrist.

"Perhaps forearms, face and hands are just not enough skin surface to make meaningful amount of vitamin D.  Such brief sun exposure may not be long enough to make meaningful amount of vitamin D.

Our hunter-gatherer equatorial ancestors had very dark skin but wore no or little clothing and were outside most or all of the day. Recent studies indicate such people had vitamin D levels around 50 ng/ml. This indicates a vitamin D input of about 5,000 to 10,000 IU/day.
Vitamin D status in indigenous populations: Part 1. Posted on August 27, 2012 by John Cannell, MD
That’s why the Vitamin D Council recommends full body sunbathing, not incidental sun exposure. Make sure your shadow is shorter than you are so you know you are making vitamin D. Also, since most people can’t sunbath every day, and because the vitamin D winter is so severe, we recommend 5,000 IU/day on the days you don’t sunbathe./