In a new study just published in the American Journal of Clinical Nutrition, Jackson, Leboff and other authors went back and looked at the data from the Women's health Initiative Calcium plus Vitamin D Clinical Trial. This trial looked at over 3000 postmenopausal women age 50-70.
They were looking to see if there was a correlation as would be expected between postmenopausal women's vitamin D levels, and the latitude where they live, as well as the amount of sun in their area of the country as well as their oral intake of vitamin D.
They found that although it would be presumed that these factors would be reflected in the blood level of a woman, it was not the case. There was a rough correlation but it was not the strict correlation that was expected.
They found that their model could explain only 21% of the variation in vitamin D levels after they adjusted for many other factors including food, outside activity, race, age, and the amount of sun their region gets per year.
The authors theorize that because so many people are aware that the sun can cause skin cancer, most people use sunblock or stay covered when they go out in the sun which was not the case many years ago before the relation between skin cancer and sun was recognized. They theorize that this may be why women in more southern climates did not necessarily have higher levels of vitamin D in their blood, as would have been expected.
The authors appropriately note also that the women had only one vitamin D blood test and because the study was done over several years that one blood test may not have been representative of what happened over the years.
The absolute numbers in the study were that over 50% of the sample of US postmenopausal women had blood levels less than 20 ng/ml. In addition, 13% of the women had blood levels less than 10 ng/ml. Note that if they were using the newly recognized minimal normal level of vitamin D i.e. greater than 30 ng/ML that these percentages would hvae been much higher. The 20 ng/milliliter level is only enough to prevent rickets.
The authors' summarize that models that predict blood levels of vitamin D by looking at oral intake and/or vitamin D made from sun exposure to the skin should now "be used with caution in making assumptions about the relation between vitamin D status and health outcomes."
This corroborates what I have said in my book and in my blog many times. Everybody should be getting a vitamin D blood test, at least once a year, just like they get a cholesterol or diabetes test once a year.
It is also why I chose to make available in my vitamin D store, an at-home blood test. In this way people who do not go to the doctor often or who do not have insurance, can find out their own blood level which is the only really accurate way to know where you stand in regards to vitamin D adequacy.