Sunlight is better than milk as a source of 'not a vitamin' D

Editor's note: Nationally recognized nutrition expert Marion Nestle answers readers' questions in this column written exclusively for The Chronicle. E-mail your questions to food@sfchronicle.com, with "Marion Nestle" in the subject line.

Q:I recently learned that the milk we give our toddler is not fortified with vitamin D. The dairy wants its milk to be natural. But kids don't get enough vitamin D and milk is one of its few sources. Shouldn't parents be warned against buying this milk?

A: Ordinarily, I don't talk about single nutrients. People eat food, not nutrients. But vitamin D is the latest "hot" nutrient, the one advertised on banners across the fronts of cereal boxes. And it has its own advocacy groups who argue that vitamin D deficiency is epidemic in America and everyone should be taking supplements. The pushing of any one nutrient always makes me uneasy, mainly because we need all of them all the time.

But Vitamin D is so weird, poses such difficult dilemmas, and it's so scientifically uncertain and controversial that it's worth a discussion.

Vitamin D is weird because it is not a vitamin. Vitamins are chemicals found in foods that our bodies cannot make. But hardly any natural foods contain this particular chemical. Plants have none. Meat has little. Wild fatty fish like salmon have reasonable amounts, but farmed fish have much less. Fish liver has enormous amounts, but nobody eats it. We call this chemical vitamin D only because it was discovered at the same time as other vitamins.

Not-a-vitamin D is actually a hormone produced by the action of sunlight on skin. Like other hormones, it has many roles in the body, not all of them well understood. Its best known function is to regulate bone mineralization. Without enough of this hormone, bones do not harden and children develop the bowed legs characteristic of rickets (in adults, this is called osteomalacia). Because receptors for this hormone are found in many body tissues, scientists suspect it has other physiological functions and are actively investigating these possibilities.

Producing the hormone takes many separate steps. First, sunlight converts a cholesterol derivative in skin into an intermediary. Next, the intermediary is converted to the chemical we call vitamin D, but this is biologically inactive. To become the active hormone, vitamin D goes to the liver where it is converted to a precursor hormone that circulates in blood. Finally, the precursor travels to the kidney, which converts it to the functioning hormone.

So fortifying milk with vitamin D is really doing a form of hormone replacement therapy.

The Food and Drug Administration does not require milk to be fortified with vitamin D. If milk is fortified, it must contain 400 international units (IU) per quart. This amount meets recommended levels for children and younger adults. For older adults, who make less vitamin D in response to sunlight, the recommendation is 600 IU per day. These are tiny amounts.

If you live in a sunny climate - from the equator to the latitude of Northern California or Connecticut - exposing your skin to full sun for around 15 minutes a day will produce 10,000 to 20,000 IU of vitamin D. People with darker skins need more sunlight exposure, and so do the elderly. And sun-induced vitamin D is stored to be used as needed.

If you live further away from the equator, you will not be able to make much, if any, vitamin D in winter months and must draw on stored amounts. And you won't make any at all if you don't go outdoors. Hence, fortification.

Although fortification of milk once ended rickets in the United States, new cases are now turning up among breastfed babies born to mothers who do not go outside much during pregnancy.

Here is the first dilemma: You need sunlight to make vitamin D. But sun exposure leads to wrinkles and skin cancers. If you use sunscreen to avoid these problems, you make much less vitamin D.

Another dilemma: Vitamin D created by the action of sunlight on skin is regulated by the body and does not cause harm. But excessive supplementation with synthetic vitamin D is toxic; it causes too much calcium to accumulate in the blood.

How much is safe? The recommended limit is 2,000 IU per day, but vitamin D advocates say much higher doses are required for optimal health. Other scientists disagree; they say 2,000 IU is already too high to be safe.

Scientists cannot resolve this disagreement because they do not yet have a good assay for vitamin D status. The standard method measures levels of the precursor hormone in blood. This technique is notoriously unreliable, not least because there is no standard reference material against which to compare results from different laboratories. The recent recall of vitamin D tests by Quest Diagnostics is one consequence of this problem.

Even with a better method, it is not clear how to interpret low levels of a precursor that is an intermediary between vitamin D and the active hormone. A recent scientific review noted an urgent need for further research to answer the most fundamental questions about the risks and benefits of sun exposure, fortification and supplements; the optimal amounts; and the role of the hormone in body functions beyond bone mineralization.

The Institute of Medicine has just appointed a committee to address such questions. Let's hope its report comes quickly. As for what to do in the meantime, I tend to err on the side of caution. We know that vitamins work best and are safest in small doses. Perhaps this is also true of this particular hormone.

As I read the evidence, those of us in reasonably sunny climates can resolve the vitamin D dilemma by getting outside in the sun - unprotected by sunscreen - for a few minutes a day. Play with your toddler outside for 15 minutes, and then lather on the sunscreen. Do this, and it won't matter if your favorite dairy refuses to fortify.

If you think that you and your child are not getting enough sunshine, you can always take a vitamin D supplement or cod-liver oil, which is rich in vitamin D. But until further research confirms the safety and benefits of higher amounts, I'd keep that supplement to what pediatricians now recommend - 400 IU per day.