Your body needs vitamin D to absorb calcium. Calcium is necessary to build strong bones. The three main sources of vitamin D are sunlight, food and supplements/medications.
Your skin makes vitamin D from the ultra-violet light (UVB rays) in sunlight. Your body is able to store the vitamin and use it later. Due to concerns about skin cancer, many people stay out of the sun, cover up with clothing and use either sunscreen or sun block to protect their skin. Because of this, many people need to get vitamin D from other sources.
Vitamin D is available in only a few foods, so it is difficult to get your daily requirements from food alone. Natural sources of vitamin D are found in fatty fish such as mackerel, salmon and tuna. It is also found in egg yolks and liver. Additional vitamin D is typically added to most brands of milk, as well as some brands of juice, soymilk and cereals. Currently vitamin D is not added to cheese, yogurt, butter or other dairy products.
There are many over the counter supplements with added vitamin D. Check your multi-vitamin and calcium supplements to see how much you are getting every day. There are also prescriptive forms of vitamin D available from your health care provider.
If you want to know if you are getting enough vitamin D there is a simple blood test that checks your level. Talk with your health care provider to see if you should have this test done. The amount of vitamin D you need will depend on your general state of health, medications you take and your age.