Vitamin D deficiency is widely overestimated, doctors warn

Misunderstandings about the recommended amount of vitamin D have led to misinterpretation of blood tests and many people thinking they need more than they really do, some experts who helped set the levels write in Thursday's New England Journal of Medicine .

Too much vitamin D can lead to high levels of calcium in the blood, which can cause nausea, constipation, kidney stones, an abnormal heart rhythm and other problems.

"If you're chasing a lab number, that will lead to many people getting higher amounts of vitamin D than they need," and labs vary a lot in the quality of testing, Manson said.

A big study she is helping lead is testing whether higher levels lower the risk of cancer, heart disease, stroke, memory loss, depression, diabetes, bone loss or other problems.

Nearly 26,000 people have been taking 2,000 units of D-3 (the most active form of vitamin D, also known as cholecalciferol) or dummy pills every day for five years.