Adequate vitamin D is necessary for muscle and nerve function, a healthy immune system and calcium absorption. This makes deficiency a serious concern. Your body makes the active form of vitamin D through a three-step process. First, your skin converts sunlight into cholecalciferol or D3. You can also get D3 from limited dietary sources and supplements. Next, your liver converts D3 into calcidiol or 25-hydroxyvitamin D, the form in which your body stores vitamin D. Finally, your kidneys convert calcidiol into the active form called calcitriol or 1,25-dihydroxyvitamin D.
Vitamin D Tests
A 25-hydroxyvitamin D blood test measures vitamin D. Although reference ranges vary among labs, a range of approximately 30 nmol/L to 125 nmo/L is considered normal, with a level equal to or above 50 nmol/L being sufficient for health, according to the National Institutes of Health. Your doctor may order a test to check 1,25-dihydroxyvitamin D to see if your kidneys are handling the final vitamin D processing step. A test for 1,25-dihydroxyvitamin D should not be used to determine vitamin D deficiency, however.
Sun Exposure
If you are deficient, it may be due to lack of sun exposure. The best time to get the UVB rays necessary for vitamin D synthesis is between 10 a.m. and 2 p.m. The latitude at which you live impacts the amount of exposure needed. The Vitamin D Council says if you have fair skin and live in Miami, you need only six minutes of sun in summer and 15 in winter, but if you live in Boston, you might need as much as one hour in summer. In a northern area such as Boston, you cannot get enough exposure in winter to trigger vitamin D synthesis. The use of sunscreen also reduces the amount of UVB rays that reach your skin.
Age and Skin Pigmentation
The skin of people older than 60 does not synthesize vitamin D as efficiently as younger skin. Older people may need up to four times as much sun as someone younger, according to the Vitamin D Council. Skin pigmentation is also a factor. Vitamin D synthesis is triggered in fair skin in just six to 15 minutes depending on latitude, but if you have dark skin, it may take up to six times as much exposure. The melanin in skin in nature's way of protecting against skin damage from the sun. This is an advantage if you have dark skin and live in a tropical area, but if you have dark skin and live in the North, you can easily become vitamin-D deficient.
Diet and Malabsorption Disorders
The best food sources of vitamin D are fatty fish, including salmon, mackerel and tuna. A small amount is present in egg yolks, cheese, beef liver and mushrooms. In the United States, milk and many cereals are fortified with D. If your diet does not regularly include these foods and you do not get enough sun exposure, you may need supplements. Because vitamin D requires fat in order to be absorbed, you could be deficient if you have Crohn's disease, celiac disease or another disorder that prevents your body from absorbing fat.
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