A recent survey of American adolescents revealed that fully 70% are deficient in vitamin D.  Vitamin D is critical for bone growth and proper immune function.  Low levels of vitamin D can cause the bone disease Ricketts in children, and is associated with increased incidence of cancer and heart disease in adults. 

The current study also finds strong links in children between low vitamin D levels and both high blood pressure and diabetes. 

Your body can make vitamin D when your skin is exposed to sunlight.  Fifteen - twenty minutes of sun exposure per day in light clothing is adequate, and puts you at little risk for sun damage.  You can also get vitamin D from oily fish like sardines, mackerel and herring, and from liver.  Mushrooms are a good vegetarian source which ave a multitude of other health-enhancing benefits.  (Mushrooms are nowhere near as rich a source of vitamin D as oily fish and liver, however.)

We routinely recommend cod liver oil for adults and children.  Cod liver oil from a quality manufacturer like Carlson's or Nordic Naturals is a great source f vitamin D andomega-3 essential fatty acids.  It should e viewed as a regular component of a healthy diet.  In adults we recommend 2,000 - 4,000 mg per day with food, and in children 1,000 - 4,000 mg per day depending on individual circumstances.