Deficient in Vitamin D (IMAGE) University of Delaware Caption As many as 25% of Americans are vitamin D deficient. Diet, the amount of fat we store in our bodies, sun exposure, skin pigmentation, and supplement usage all impact vitamin D levels in the blood. Credit Jeffrey Chase/University of Delaware Usage Restrictions None License Original content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.