1. Causes of Vitamin D Deficiency
Vitamin D deficiency can occur for a number of reasons:
You don't consume the recommended levels of the vitamin over time. This is likely if you follow a strict vegetarian diet, because most of the natural sources are animal-based, including fish and fish oils, egg yolks, cheese, fortified milk, and beef liver.
Your exposure to sunlight is limited. Because the body makes vitamin D when your skin is exposed to sunlight, you may be at risk of deficiency if you are homebound, live in northern latitudes, wear long robes or head coverings for religious reasons, or have an occupation that prevents sun exposure.
You have dark skin. The pigment melanin reduces the skin's ability to make vitamin D in response to sunlight exposure. Some studies show that older adults with darker skin are at high risk of vitamin D deficiency.
Your kidneys  cannot convert vitamin D to its active form. As people age their kidneys are less able to convert vitamin D to its active form, thus increasing their risk of vitamin D deficiency.
Your digestive tract cannot adequately absorb vitamin D. Certain medical problems, including Crohn's disease, cystic fibrosis, and celiac disease, can affect your intestine's ability to absorb vitamin D from the food you eat.
You are obese. Vitamin D is extracted from the blood by fat cells, altering its release into the circulation. People with a body mass index of 30 or greater often have low blood levels of vitamin D.

2. Treatment for vitamin D deficiency
Treatment for vitamin D deficiency involves getting more vitamin D -- through dietand supplements. Although there is no consensus on vitamin D levels required foroptimal health -- and it likely differs depending on age and health conditions -- aconcentration of less than 20 nanograms per milliliter is generally consideredinadequate, requiring treatment.
Guidelines from the Institute of Medicine increased the recommended dietaryallowance (RDA) of vitamin D to 600 international units (IU) for everyone aged 1-70,and raised it to 800 IU for adults older than 70 to optimize bone health. The safeupper limit was also raised to 4,000 IUs.
If you don't spend much time in the sun or always are careful to cover your skin(sunscreen inhibits vitamin D production), you should speak to your doctor abouttaking a vitamin D supplement, particularly if you have risk factors for vitamin Ddeficiency.

3. How Much Vitamin D Do You Need?
Bone health was the single focus of the Institute of Medicine's recommendations on how much vitamin D and calcium people should get.
The recommendations for adults up to age 69 rose to 600 IU/day, and to 800 IU/day for adults starting at age 70. Older adults need more vitamin D because as they age, their skin does not produce vitamin D efficiently, they spend less time outdoors, and they tend to not get enough vitamin D.
The committee did not consider the emerging research on any other conditions. Patsy Brannon, PhD, RD, a Cornell University professor of nutritional sciences and a member of the IOM committee, spoke about that at the American Dietetic Association's 2011 annual meeting in San Diego. "The committee of 14 scientists reviewed more than 1,000 publications and determined that the evidence was inconsistent and inconclusive to include any other health benefits in the new recommendations," Brannon said. "The committee is not dismissing the role of vitamin D in other areas, we need more clinical trials, consistent evidence, and evidence that supports causality."

4. How to Get Vitamin D
There are several ways to boost your daily vitamin D:
Sunshine Sunshine is the most natural way to get vitamin D. The vitamin D you get this way is easily used by your body, it is free, and you can combine it with exercise for a great bone building combo. For white women who live where there is strong sunlight, about 15 to 20 minutes three times a week will usually produce enough vitamin D to keep you from being deficient.
For some people, sunshine is difficult to come by. If you wear clothing that covers all of your skin, if you live in a northern or rainy climate, or if you are dark-skinned, you may have trouble getting enough sunshine to make adequate levels of vitamin D. 
Food There are several food sources for vitamin D. Fish such as salmon, tuna and mackerel are sources of this vitamin, as are egg yolks, cheese and beef liver. Vitamin D-fortified milk and other foods are also good sources.