Vitamin D is considered nature’s sunshine vitamin, and it's a vitamin we can make in our bodies… under proper conditions. Large-scale studies have found that deficiency is widespread in not just adults but in children as well. Vitamin D promotes healthy growth and development; supports teeth, bone, and muscle health; assists with a healthy immune and cardiovascular system; and aids in a healthy mood.
 
Many children, like adults, do not get enough time in the sun during certain crucial times of the year to make and maintain adequate 25(OH)D levels, which is the form made after D3 is converted in the liver. In addition to lack of time in the sun, it's also difficult to synthesize adequate amounts of vitamin D from sunlight due to sunscreen, latitude, color of skin, and various other factors. There are also not many naturally-occurring dietary sources of vitamin D. 
 
It can be found in the flesh of fatty fish like salmon (approximately 500 IU (12.5 mcg) in 3 ounces), mackerel, and tuna. It's also in fish liver oils, but not many people consume these on a weekly basis. Most of the dietary vitamin D comes from fortified foods; therefore, many should look to supplemental vitamin D3 to maintain adequate levels throughout the year. Research has found that vitamin D3 is the preferred form to supplement with to increase levels effectively.
 
Various institutions have stated their reference range recommendations for vitamin D, and they vary widely. The Institute of Medicine Food and Nutrition Board sets the standard and states adults should have a vitamin D level of 20+ng/mL. Many disagree, and feel it's too low. The Endocrine Society states sufficiency as 30-100 ng/mL, and the Vitamin D Council states adequacy as 40-80 ng/mL. The optimal vitamin D level range is not yet established, but somewhere between 30-70 ng/mL is desirable, based on research.
 
The amount necessary to increase and maintain D levels is different for everyone and varies throughout the season. If your level is very low, a practitioner may recommended you take anywhere from 4-10,000 IU (10-250 mcg)/day for several months until levels increase into the adequate range.
 
Make sure to have your levels tested regularly. It takes at least 3 months to increase levels, and you can do a re-test to see if you have adequately increased your levels within 3-6 months. If your level is within the desirable range, then it's recommended to retest in one year.
 
Be sure to stay on top of your D levels all year long, as it's so crucial for the entire body.