"The ability of vitamin D to prevent postmenopausal breast cancer and colorectal cancer was discovered by examining a map in 1974, resulting in a publication in 1980 by Garland and Garland of The Johns Hopkins University. There were then a dozen confirmatory observational studies, including research led by Edward D. Gorham, PhD. The effect of vitamin D3 on risk of invasive cancers was further confirmed by an RCT performed by Joan Lappe, Robert Heaney and their colleagues at Creighton University of Omaha in 2007. In the meantime, the number of cancers associated with low UVB levels (and most likely low 25(OH)D) levels, has been substantially expanded by William B. Grant, Ph.D., of the Sunlight and Nutrition Research Center in  San Francisco CA; and Sharif B, Mohr, Ph.D., Rafael Cuomo, MPH and June Kim of UC San Diego. I think that Drs. Lappe and Heaney should be given a Nobel Prize for their experiment in 2007, and am hoping for this.  Due to these studies, there is now no question that vitamin D3 prevents postmenopausal breast cancer and colorectal cancer, along with both types of diabetes, multiple sclerosis and other diseases. The safe upper limit dosages of the National Academy of Sciences (2001) for everyone aged 9 years and older is 4,000 IU/day. It should also be noted that the NOAEL (no adverse effect level) is 10,000 IU/day. 

A reasonable serum 25(OH)D target is now 50-70 ng/ml.  There may be some minor risks of aiming above 40-60 ng/ml, but the benefits are so great and the risk is so low that this target is becoming a reasonable choice. Always tell your doctor what you have in mind, and seek his or her advice. He or she can take your personal medical history into account."

Cedric F. Garland, Dr PH FACE

UC San Diego School of Medicine

"The effect of solar UVB exposure on cancer mortality rates is clearly evident in the maps of cancer mortality rates in the United States. Rates for 15 types of cancer are lowest in the southwest, highest in the northeast. The inverse correlation between solar UVB doses in summer and cancer mortality rates is very strong for 15 types of cancer even after adjustment for other cancer risk-modifying factors such as alcohol consumption, diet, and smoking [Grant, 2006].

Observational studies also provide strong support for the UVB-vitamin D-cancer hypothesis. Even the International Agency for Cancer Research agrees that vitamin D reduces risk of colorectal cancer [IARC, 2008]. There is a very strong inverse correlation between vitamin D level and incidence of breast cancer based on 11 case-control studies from 7 countries [Grant, 2015].

Since many adverse health outcomes have been linked to low vitamin D levels, the prudent thing to do is maintain levels above 40 ng/mL (100 nmol/L). There are few if any adverse health effects associated with vitamin D levels up to 100 ng/mL [Grant, 2016]."