According to a study published in the journal Archives of Internal Medicine by researchers from Harvard University and the University of Colorado, 70% of whites and 97% of blacks in the USA have insufficient blood levels of vitamin D.

Using data gathered by the U. S. National Centre for Health Statistics, researchers examined the vitamin D blood levels of 18,863 U. S. residents between 2001 and 2004. Their findings showed approximately
75% of teenagers and adults had levels below 30-nanograms per millilitre, which is on the low end of what is currently considered adequate for good health. Broken down ethnicity wise, it means 70% of whites, 90% of Hispanics and 97% of blacks are deficient in Vitamin D.

Based on guidelines adopted at the 13th Workshop Consensus for Vitamin D Nutritional Guidelines in 2007, deficiency is defined as any level below 20-nanograms per millilitre. Value upgraded based on more recent research, previously deficiency was defined as less than
11-nanograms per millilitre.

For long now, scientists have been aware that insufficient levels of vitamin D lead to the bone-softening disease of rickets in children, and also increase the risk of fractures and osteoporosis in adults. But, more recent research strongly suggests, even levels considered adequate for bone health are still too low for protecting against heart disease, cancer and auto-immune disorders.

The proportion of people with adequate vitamin D blood levels has dropped 50% between 1988 and 1994, which researchers attribute to people spending more time indoors than before, and when they do go out they are protected by high-SPF sunscreen.

Since, exposure to ultraviolet radiation from sunlight is required for the body to synthesize vitamin D, excessive use of sunscreen leads to deficiency. While, lighter-skinned people are able to produce all the vitamin D they need from exposing their face and hands to the sun for just 15-minutes per day, darker skinned people (depending on skin tone) need up to five times as much sun exposure.