In adults, vitamin D deficiency may result in a softening of the bones known as osteomalacia. This condition is treated with vitamin D, sometimes in combination with calcium supplements. Osteomalacia should be diagnosed, and its treatment monitored, by a doctor. In people of any age, vitamin D deficiency causes abnormal bone formation. In addition, vitamin D deficiency can cause muscle weakness, which improves with vitamin D supplementation.⁷ Vitamin D deficiency occurs more commonly following winter, owing to restricted sunlight exposure during that season. Living in an area with a lot of atmospheric pollution, which can block the sun's ultraviolet rays, also appears to increase the risk of vitamin D deficiency.⁸⁹

Vitamin D deficiency is more common in strict vegetarians (who avoid vitamin D-fortified dairy foods), dark-skinned people,¹⁰ alcoholics, and people with liver or kidney disease. People with liver and kidney disease can make vitamin D but cannot activate it.

Vitamin D deficiency is more common in people suffering from intestinal malabsorption, which may have occurred following previous intestinal surgeries, or from celiac disease.¹¹ People with insufficient pancreatic function (e.g., those with pancreatitis or cystic fibrosis) tend to be deficient in vitamin D. Vitamin D deficiency is also common in individuals with hyperthyroidism (Graves' disease), particularly women.¹²

In children, vitamin D deficiency is called rickets and causes a bowing of bones not seen in adults with vitamin D deficiency. Vitamin D deficiency is common among people with hyperparathyroidism, a condition in which the parathyroid gland is overactive. In a study of 124 people with mild hyperparathyroidism, vitamin D levels were below normal in 7% of them and suboptimal in 53% of them.¹³ Vitamin D deficiency is also common in men with advanced prostate cancer. In one study, 44% of 16 men with advanced prostate cancer had decreased blood levels of vitamin D.¹⁴

One in seven adults has been reported to be deficient in vitamin D.¹⁵ In one study, 42% of hospitalized patients under age 65 were reported to be vitamin D deficient.¹⁶ In this same study, 37% of the people were found to be deficient in vitamin D, despite the fact they were eating the currently recommended amount of this nutrient. Vitamin D deficiency is particularly common among the elderly. Age-related decline in vitamin D status may be due to reduced absorption, transport, or liver metabolism of vitamin D.¹⁷