Vitamins and Minerals: What you need to know
about vitamin D
Understanding how vitamins and minerals interact in our body gives us a valuable means of taking our
health into our own hands. Research into the effects of these micronutrients has been enlightening. For
example, we now know that large fractions of our population are not getting enough vitamin D, and the
range of consequences may be far greater than we thought. Recent studies suggest that vitamin D does
more than help build strong bones; it may help to prevent hypertension, certain types of cancer, and
some autoimmune diseases. We also believe, with an increasing degree of certainty, that getting
enough folic acid can counteract some of the adverse effects of consuming alcohol.
Researchers are constantly making such discoveries, and often their findings have far-reaching effects.
But it’s worth keeping in mind that while nutrition research can yield powerful answers, it also raises
new questions. Nutrition is endlessly complex, and the research is invariably in a state of flux. Not only
do nutrients interact with one another (making it difficult to isolate their effects) but they also interact
with genes, drugs, and even lifestyle factors. For some nutrients there’s a narrow range between too
little and too much, and that amount varies from person to person.
What’s special about vitamin D? This fat-soluble vitamin is exceptional among vitamins in three ways.
First, it has a unique mechanism of action in the body. Second, you can’t get very much of it naturally
through your diet. And third, many of us are deficient in this vital nutrient. That final point is keenly
important, in light of burgeoning evidence that vitamin D’s health benefits extend far beyond its
reputation for building healthy bones. Over the past decade, studies suggest that adequate amounts of
vitamin D may lessen the risk of several types of cancer (including cancers of the colon, prostate, and
breast) and may also play a role in preventing high blood pressure, multiple sclerosis, and even
schizophrenia. And a 2007 meta-analysis of 18 randomized controlled trials showed that vitamin D
supplementation may even help people to live longer.
How D is different
Unlike other vitamins, which are antioxidants or key players in enzyme reactions in the body, vitamin D
functions in the body as a hormone—that is, a substance made by one organ that sends a chemical
message to another organ to elicit a specific response. One of vitamin D’s best-known and important
roles is to signal the intestines to absorb calcium into the bloodstream. Without sufficient vitamin D,
your body will break down bone to get the calcium it needs—no matter how much calcium you consume
through food and supplements. Vitamin D is also unusual be- cause it’s relatively scarce in normal diets.
To get just 400 international units (IU) of vitamin D (the current recommended daily amount for people
ages 50–71), you’d need to eat about 5 ounces of salmon, 7 ounces of halibut, 30 ounces of cod, or a 6-
ounce can of tuna, or drink four cups of milk. Few people include that much fish—or milk, for that
matter— in their daily fare. Milk actually doesn’t normally contain vitamin D, but nearly all the milk sold
in the market today is vitamin- D fortified. This practice began in the 1930s to combat rickets, a disease
that leads to soft, weak bones caused by vitamin D deficiency. It’s worth noting that dairy products
made from milk, such as cheese, yogurt, and ice cream, aren’t typically fortified with vitamin D and
contain only small amounts.
In fact, casual exposure to sunlight provides people with most of their vitamin D requirement. Sunlight
contains two forms of radiant energy, ultraviolet A (UVA) and ultraviolet B (UVB). UVB provides the
energy your body needs to generate vitamin D, which is why it’s known as the “sunshine vitamin.” If you
sit outside on a sunny day in the middle of summer wearing only a swimsuit, your body creates a
whopping 20,000 IU of vitamin D per hour. But even people who don’t sunbathe or spend a great deal of
time outside probably generate a fair amount just walking outside for short periods throughout the day,
depending on the latitude and the amount of skin that is exposed. However, a number of factors—
including the season, time of day, and where you live—can affect how much UVB reaches your skin.
What’s more, your age, skin color, and sunscreen use also influence your skin’s production of vitamin D.
A confluence of these factors can conspire to limit vitamin D levels, which is why a surprisingly large
number of our population—more than 50% among certain age groups—is deficient in vitamin D.