CELESTE HEADLEE, HOST:
I'm Celeste Headlee and this is TELL ME MORE from NPR News. Michel Martin is away. Some surprising news now about vitamin D. According to two major reviews in the British Medical Journal published last week, people with low levels of vitamin D could be more likely to die from cancer, heart disease and a number of other illnesses.
Now, you may think of vitamin D as the sunshine nutrient, and we do produce it when we're exposed to the sun. But getting a healthy level of vitamin D is actually a lot more complicated than just going outside or swallowing a few supplements.
Helping us to sort through some of this, we have Marion Nestle. She's a professor of nutrition at New York University, and the author of several books on food and nutrition. Marion, welcome.
MARION NESTLE: Oh, glad to be here.
HEADLEE: Can - maybe you can help me sort through what, exactly, we know and we don't because there appear to be certain things that we don't understand yet, including whether vitamin D is causing these diseases, or it's just that vitamin D happens to be present in a lot of people who have them - I mean, not present.
NESTLE: Not present, right.
HEADLEE: Right.
NESTLE: Well, first of all, it is not a vitamin. It's a hormone, and you have to think of it as a hormone. It's a hormone that's made in the skin by the action of sunlight. And people who live in temperate latitudes and are out in the sun for really just a few minutes a day are going to be making enough vitamin D to meet their needs for all of the processes that vitamin D is responsible for.
It has a lot to do with bone health and other kinds of processes, like all hormones. Its full range of activities isn't very well-understood, and there are lots of uncertainties and need for more research about it. And in part, because of the uncertainties, people like to attribute a lot of conditions to lack of vitamin D. And also, the other problem about it is that it's very difficult to measure the active hormone.
So instead, doctors measure an intermediate and synthesis of the hormone, which may or may not be related to actual hormone activity. So it's a mess, and it's complicated. And it's not surprising that studies come out with lots and lots of different results.
HEADLEE: Right.
NESTLE: Some studies show that if you don't have enough vitamin D, you're going to have more of a higher risk for cancer and heart disease - as these studies did...
HEADLEE: Right.
NESTLE: ...Or one of these studies did. And some of them show that it makes absolutely no difference at all, and it could potentially be harmful, particularly if you use supplements.
HEADLEE: Right. Well let's - we only have about a minute left. But I wanted to say for people who are listening, taking a supplement isn't necessarily going to fix your vitamin D levels, right?
NESTLE: Oh, no. It's much better to go out in the sun because if you're out in the sun, you're creating the hormone in the way that it's supposed to be created. And you have to think of this as hormone replacement therapy, with all of the confusing and unknown and risks associated with hormone replacement therapy.
This isn't something that you want to do lightly. It's not really a vitamin in the sense that we get vitamins from food. It's much more complicated than that. So I would say get outside, be active, eat a healthy diet.
HEADLEE: That's Marion Nestle, professor of nutrition at New York University. I'm sure eating a healthy diet is probably something you add onto everything, every piece of advice that you give.
NESTLE: It absolutely is.
HEADLEE: She joined us by phone from her home office. Marion, thank you so much.
NESTLE: My pleasure. Transcript provided by NPR, Copyright NPR.