According to a study made by a team of researchers at Children's Hospital Boston, many young children do not get enough vitamin D. This deficiency may not be very visible, but it does make itself noticed in case of broken bones. It also weakens the immune system. 

The study involved 380 patients and its findings were published in the June 2008 issue of Archives of Pediatrics & Adolescent Medicine.

It showed that two out of five U.S. children with ages between 8 months and 2 years had less-than-optimal blood levels of vitamin D. The outcome of the research also showed that one-third of children who were deficient had changes in bone density seen on X-rays of the wrist and knee. A few even had signs of rickets, or softening of the bones caused by severe deficiency of vitamin D.

Vitamin D is of extreme importance for bone growth and calcium absorption in the intestine. It also plays a role in strengthening the immune system. The sun is the best source of vitamin D. Ultraviolet rays determine its production in the skin.

Parents usually cover young children with sunscreen to protect them from harmful UV rays, so children may not have enough sun exposure to produce sufficient levels of vitamin D.

Diet is another source of the vitamin. Milk products, bread and cereals are often fortified with it.

Babies fed with enriched formulas and toddlers who drink milk are more protected from vitamin D deficiency, and breast-fed infants who did not receive vitamin D supplements are at a higher risk.