Vitamin D is an essential nutrient that helps your body absorb and retain calcium, which is necessary for healthy bones. Vitamin D also affects your muscles, nerves and immune system. You get vitamin D naturally from your diet and from exposure to sunlight. You can develop a deficiency in vitamin D if your diet lacks sufficient amounts of it, or if your exposure to sunlight is limited. Certain health conditions can also result in a lack of vitamin D.
Diet
A lack of vitamin D can result because you don't consume enough of it in your diet. Most foods aren't naturally rich in vitamin D. Those that are rich include fatty fish, such as tuna and salmon, egg yolks and liver. Some foods, such as breakfast cereals and milk, are fortified with extra vitamin D. If you do not get enough vitamin D in your diet, you may need to take a vitamin D supplement.
Sunlight
Your body actually manufactures some of the vitamin D that it needs to function properly. Exposure of your skin to sunlight causes the production of the vitamin. Your body may not make enough vitamin D if your exposure to sunlight is limited, or if you cover your skin while in the sun. Protective sunscreen can also prevent the absorption of rays that produce vitamin D. According to the Harvard School of Public Health, sunscreen, when applied correctly, "reduces our ability to absorb vitamin D by more than 90 percent." Failure to make enough vitamin D from the sun also occurs more often in certain groups of people. Among them are people who are overweight or have had gastric bypass surgery, older adults and those who have darker skin tones.
Health Conditions
Certain health conditions may cause malfunctioning of vital organs, which can result in a lack of vitamin D. For example, the kidneys may not convert vitamin D to its normal active form, or insufficient amounts of vitamin D may be absorbed from the digestive tract. Conditions that are sometimes associated with these factors include liver diseases, Crohn's disease, cystic fibrosis, lactose intolerance and milk allergy.
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