What is Vitamin D?
Vitamin D is a fat-soluble vitamin. Because the body can create its own vitamin D, it is technically not a vitamin, but a pro-hormone, a precursor to the hormones the body needs to function. Vitamin D has no active role in the body other than to be converted to usable hormones by the liver and kidneys.

This vitamin (we’ll stick with vitamin since that’s how it’s known commonly) is responsible for maintaining blood levels of calcium and phosphorus, growing bone, and shoring up the immune system. Not enough vitamin D -- a vitamin D deficiency -- can cause pain, hormone problems, muscle weakness, and more.

Symptoms and Health Risks of Vitamin D Deficiency
Symptoms of bone pain and muscle weakness can mean you have a vitamin D deficiency. However, for many people, the symptoms are subtle. Yet even without symptoms, too little vitamin D can pose health risks. Low blood levels of the vitamin have been associated with the following:

• Increased risk of death from cardiovascular disease
• Cognitive impairment in older adults
• Severe asthma in children
• Cancer

Research suggests that vitamin D could play a role in the prevention and treatment of a number of different conditions, including type1 and type 2 diabetes, hypertension, glucose intolerance, and multiple sclerosis.

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, 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.


Where Can I Find Vitamin D?
For humans, there are two ways of getting vitamin D. The body can create vitamin D in the skin when it is hit with UVB radiation. Or the body can get vitamin D from dietary sources. Ultimately however, all vitamin D in the food-chain begins with some action of an organism with sunlight. So obviously, the first place you can turn to get some vitamin D action going on is the source of the UVB rays that help the plant and animal kingdoms make the vitamin in the first place. A little sunlight isn’t going to cause skin cancer. Vitamin D deficiency actually predisposes one to skin cancer. Of course, that’s not freedom to go sunbathe for hours on end.
So if you are either scared of the sun or live much above the sub-tropical zones, it’s going to be hard to get enough vitamin D from the sun. Luckily, there are some rich sources of the vitamin that fit perfectly into a lifestyle of eating real foods.

• Cod liver oil – 1tsp, 450IU
• Salmon, cooked, 3 1/2 oz: 360 IU
• Mackerel, cooked, 3 1/2 oz: 345 IU
• Sardines, canned in oil, drained, 3 1/2 oz: 270 IU
• Pork lard, 1 tbsp – 140IU
• Beef Liver, cooked, 3.5oz – 30IU
• Whole Egg – 25IU

Source: FitnessSpotlight.com and WebMD.com