A diet with insufficient vitamin D in conjunction with inadequate sun exposure causes vitamin D deficiency, which is defined as a blood 25(OH)D level below 12ng/mL (30nmol/liter), whereas vitamin D insufficiency is a blood 25(OH)D level of 12–20ng/mL (30–50nmol/liter). An estimated one billion adults worldwide are either vitamin D insufficient or deficient, including in developed countries in Europe. Severe vitamin D deficiency in children, a rare disease in the developed world, causes a softening and weakening of growing bones, and a condition called rickets.
Vitamin D deficiency is found worldwide in the elderly and remains common in children and adults. Deficiency results in impaired bone mineralization and bone damage which leads to bone-softening diseases, including rickets in children and osteomalacia in adults. Low blood calcifediol (25-hydroxy-vitamin D) can result from avoiding the sun. Being deficient in Vitamin D can cause the absorption of dietary calcium to fall from the normal fraction (between 60 and 80 percent) to as little as 15 percent.
Dark-skinned people living in temperate climates have been shown to have low vitamin D levels. Dark-skinned people are less efficient at making vitamin D because melanin in the skin hinders vitamin D synthesis. Vitamin D deficiency is common in Hispanic and African-Americans in the United States, with levels dropping significantly in the winter. This is due to the levels of melanin in the skin, as it acts as a natural protectant from sun exposure.