Yes – a blood test called 25-hydroxyvitamin D, which, though expensive ($20 to $100 a pop), is covered by insurance. Your level should be at least 30 nanograms per milliliter of blood and not more than 100. If your level is way below that, say around 18, you need to take 1,200 international units or more a day of vitamin D supplements, either the type called D2 or D3, said Dr. Michael F. Holick, an endocrinologist and leading vitamin D researcher at the Boston University School of Medicine.
“Everyone should be tested,” said Holick, who consults for a lab that conducts the tests but does not get royalties. Many doctors hesitate to order the test because they still believe that rickets, a serious but now, extremely rare bone disease in children, is the only manifestation of vitamin D deficiency.
But even mild levels of vitamin D deficiency have been linked to cancers of the breast, prostate, and colon, as well as to multiple sclerosis, both type 1 and type 2 diabetes, heart disease, and stroke. A meta-analysis involving data on more than 57,000 people in 18 studies showed that those taking vitamin D supplements had a lower risk of dying for any reason during the study.
Vitamin D is a steroid-like hormone that the body makes after skin exposure to UV-B radiation from sunlight. After an inactive form of vitamin D is made in the liver, it is transformed in the kidney to an active form.
Many studies show that people, especially African-Americans and those in northern latitudes like New England, are at high risk of being deficient in vitamin D. A 2004 study of 307 healthy teenagers led by Dr. Catherine M. Gordon, an endocrinologist at Children’s Hospital in Boston, showed that 42 percent were vitamin D deficient.
Many doctors assume that because vitamin D is important for calcium metabolism, if blood levels of calcium are normal, vitamin D must be sufficient, too, said Holick. But this is not true.
Usually, Holick said, it’s only when doctors begin testing themselves for vitamin D deficiency that they “get religion” and begin taking vitamin D testing seriously. A growing number of researchers now recommend that everyone take at least 800 to 1,000 international units of vitamin D a day. It takes 10,000 IU’s a day for vitamin D to become toxic.
For what it’s worth, I take at least 1,400 IU’s a day.