Vitamin D
Vitamin D seems to have been keeping a low profile in the past few years. Recently, however, there has been a resurgence of interest in this fat soluble vitamin related to everything from the obvious, namely osteoporosis, to elusive killers such as high blood pressure, diabetes, and cancer. Along with these exciting possibilities comes new understanding about the exact role of vitamin D in the human body, and a bit of controversy on whether we're getting enough of the vitamin or maybe too much.
The discovery of vitamin D was not the kind that elicited shouts of "Eureka!" from the scientists studying it; it was more of a process, which began in 1919. That year, a researcher named Mellanby showed that cod liver oil and other foods could prevent and even cure rickets, a common disease afflicting many American children. Up to that point, scientists believed that vitamin A, which is also present in the oil, was responsible for this effect. A few years later, McCollum found that if he destroyed the vitamin A in the oil, it could still prevent and cure rickets. And it took twelve more years for scientists to prove that our bodies also produce vitamin D, and that it was chemically different from vitamin D from plant sources.
Some research purists decried the fact that, as they saw it, civilization changed vitamin D from a hormone into a vitamin. But in all fairness to civilization, the chemical compound is a sort of Jekyll and Hyde. If you use the basic definition of a hormone, that being a substance manufactured by one organ of the body affecting another organ or tissue, the vitamin does become a hormone. According to the definition of a vitamin (an essential organic compound required for growth and maintenance of life), vitamin D doesn't quite measure up, since the body can synthesize it. However, most people in the world wouldn't get enough because the body needs adequate exposure to sunlight in order to manufacture it, as illustrated by the American epidemic of rickets seen at the turn of the century before we fortified foods with vitamin D. These considerations generated some debate in scientific circles, but eventually the vitamin camp won out.
Vitamin D: What It Is and What It Does
Of the several forms of vitamin D, almost all begin with a precursor, or provitamin, which needs exposure to ultraviolet radiation (UVR) to eventually become an active form of the vitamin. Plants and bacteria make a precursor, ergosterol, which is converted to ergocalciferol, also called calciferol or Dz. In this form, we can only use a small amount for our vitamin D needs, but food manufacturers start with this source to make an active form of the vitamin and add it to foods to boost vitamin D content of commercial products. How do they do this? The same way other forms become active: exposure to UVR (the manufacturers irradiate ergosterol). After they've irradiated ergosterol and added it to their products,we eat the products and get active vitamin D.
In the human body, the precursor is a sterol compound related to a more infamous sterol, cholesterol. The liver makes the vitamin D precursor, 7-dehydrocholesterol, and releases it into the blood which carries it to the skin. When you're out on a sunny day, the sun's UVR converts the precursor to another form of the vitamin, cholecalciferol or vitamin D3. But vitamin D3 still needs a few more finishing touches to become truly active and do the jobs of vitamin D.
The next stop for vitamin D3 is the liver, where this important organ adds a small chemical group called a hydroxyl. And it takes one more step for the finished product to emerge, a trip to the kidney for the addition of yet another hydroxyl group. Those two hydroxylation reactions are crucial to making active vitamin D, so much so that a person with either liver or kidney disease may easily become deficient. The fancy name for the active vitamin reflects these two steps, with the numbers telling you where on the molecule the hydroxyl groups ended up:1,25-dihydroxycholecalciferoi. If that sounds complicated, it is: The entire process takes up to three days from when the precursor went out to catch some rays. You're not alone if you think it's complex-scientists have found thirty-three more active forms of the vitamin and still don't know what they do!
The main function of vitamin D is in building bone, and more specifically in getting minerals to bone, a process called bone mineralization. Vitamin D has a few tricks to accomplish this, several related to commandeering calcium and phosphorus and boosting their levels in the blood, making them available for bone work. These minerals, and a few others, are important in building bone because they are the key ingredients that make up bone tissue.
One way the vitamin makes more calcium and phosphorus available is by nudging your small intestine to absorb more of these minerals from the foods you eat. And in the best tradition of strategic thinking, vitamin D also attacks the other end: it elbows the kidney to hang on to more minerals instead of dumping them in the urine. So if you absorb more minerals from the foods you eat and you waste less in your urine, blood levels go up and everyone is happy, especially your bones.
But the third of vitamin D's tricks to bump up blood levels seems counterproductive; it increases bone demineralization, or loss of minerals from bone. This paradox becomes clear when you consider how new bone forms. Bones aren't the dead or inflexible structures they appear to be; they are dynamic and constantly changing. In order to make new bone cells, other cells have to be taken apart, a process called bone remodeling (just like the remodeling you might do to your house). So even this seeming loss eventually aids in building bone. And finally, other body parts need calcium, too. This mineral is important in nerve transmission and muscle function; in fact, it's so important that the body can't tolerate even seemingly slight dips in blood levels or these functions are compromised.
Vitamin D Deficiency: What Happens and Who's at Risk?
When you don't have enough vitamin D, the bones lose minerals, a process called demineralization, and become soft and pliable. This softening is called rickets in children, and it causes the legs to bend when carrying the child's weight, leading to permanently bowed legs. In adults, the same condition is called osteomalacia and is most common in women who don't get enough vitamin D or calcium. Osteomalacia can affect bone tissue in the limbs, chest, spine, and pelvis. Other problems, involuntary twitching and muscle spasms, arise as a result of low levels of calcium in the blood and reflect that mineral's role in muscle and nerve function.
The other bone disease is the one that you're probably more familiar with, osteoporosis. In this disease, the person loses bone tissue because of demineralization which makes the bones brittle or porous, leading to fractures. Osteoporosis is responsible for significant disability and chronic pain for millions of Americans: over 28 million Americans either have osteoporosis or are at high risk for its development. The National Osteoporosis Foundation estimates that one in two American women and one in eight men over the age of fifty will suffer an osteoporosis-related fracture in their lifetimes. For reasons yet unclear, 20 percent of the elderly who suffer a hip fracture die within months of the injury. And the monetary cost of osteoporosis is staggering, with estimated costs of nearly $14 billion every year in treatment of osteoporosis-related fractures. With these kinds of statistics, it's not surprising that scientists have turned over every conceivable stone to find the causes of the disease and better treatments.
The questions researchers asked are these: does low vitamin D cause this disease, and can supplements help treat it? Although vitamin D is a key player in bone mineralization by regulating calcium balance, researchers can't agree on the answer to either question. Some studies had shown that osteoporosis patients have lower blood levels of vitamin D compared to healthy people, which suggests a causative association. But other studies have reported no differences in vitamin D blood levels between patients with osteoporosis and controls. On the treatment front, some researchers thought that vitamin D might improve osteoporosis because it increases calcium absorption in the intestine. Again, the studies reported conflicting data, with one study actually demonstrating increased bone loss. Recently there has been some success with combination treatments that include vitamin D and calcium.
If you had asked a nutritionist just a few years ago whether most people get enough vitamin D, he or she probably would have said yes. The nutritionist might have gone on to describe the rickets epidemic at the turn of the century, and how the addition of vitamin D to milk stopped the deficiency in its tracks. He or she might also have pointed to a few groups who continue to be at risk, mostly people living in the northern regions who either don't get enough sunlight or those who have kidney and liver problems. While these risk factors still hold true, a new study suggests that many more Americans may be at risk for vitamin D deficiency.
Several studies within the past decade have shown that elderly people are at risk for several reasons. The most common reason is that the elderly, especially those who are in nursing homes, don't tend to get enough sunlight to convert the precursor in skin to the intermediate compounds needed for active vitamin D. In addition, as we get older, our bodies are less efficient at carrying out the conversion.
Food intake tends to be a problem as well. A person's ability to tolerate the lactose sugar in milk declines with aging, so many elderly people avoid milk, one of the best sources of the vitamin.
Studies have shown that African American children living in urban areas are also at risk for vitamin D deficiency for several reasons. Skin pigmentation affects the conversion reaction in this way: darker skin makes less vitamin D compared to lighter skin given the same exposure to sunlight. Within three hours, the amount of vitamin D available is the same in the darker-skinned person as the lighter-skinned person made in thirty minutes. So it takes more exposure for the darker-skinned person, and in northern regions this may be a problem. The urban setting may mean that parents keep children indoors because of safety concerns. To make matters worse, smog in urban areas filters out the sun's UVR so that less conversion takes place.
Nutritionists have known for several years about those high-risk groups, but a 1998 study shows that many Americans may be at risk. Researchers at a Boston hospital reported that among almost 300 hospitalized patients, more than half could be considered vitamin D deficient based on blood tests measuring vitamin D. When they asked patients about their diets, the researchers found that the majority didn't get the recommended amount of the vitamin. Of more concern was the finding that 37 percent of patients who did meet the recommended amount were also deficient. They concluded that not only are most Americans not getting enough vitamin D, the recommendation may not be high enough. Of course, you need to remember that the researchers were studying sick older people, and it's reasonable to assume that illness can affect vitamin D status.
How Much Is Enough and How to Get It
Only animal foods contain active vitamin D, and good sources include liver, butter, fatty fish, and egg yolks. For most Americans, the best sources of vitamin Dare foods that are fortified with it. Milk contains 100 IV per cup and constitutes the major source for many segments of the population. Vitamin D was one of the bone health nutrients which received new recommendations last year in Dietary Reference Intakes (DR!). The recommended amount stayed the same for adults, 200 IV, but the committee established new age categories and raised the level for older people.
The DRI for vitamin D, like the RDA before it, was difficult to establish because of variability in people's exposure to sunlight. In fact, people in warm, sunny climes regularly exposed to sunlight don't require a dietary source of the vitamin. The use of sunscreens may limit vitamin D conversion from sunlight, but most experts agree that the benefit of preventing skin cancer outweighs this factor. Considering most Americans spend a fair number of hours indoors, especially those who live in northern climates or institutions, a dietary source of vitamin D is essential. If you are seventyone years old, it might be very difficult to eat enough foods high enough in vitamin D to get 600 IV, which is the equivalent of six cups of milk, or one pound of shrimp, or twentyone eggs per day! A supplement would seem to be the only way to achieve the recommendation.
In an editorial in the New England Journal of Medicine, accompanying the Boston hospital study, the author points out the difficulty for elderly people in meeting the DRI through diet alone. He also points out that the study showed 37 percent of patients who did meet the recommended level of intake still had low blood levels of vitamin D, suggesting that the new DRI is too low. Only further studies will confirm if he's right, and there is concern that people might get too much vitamin D.
As with most fat soluble vitamins, vitamin D can be toxic at high levels of intake. The side effects include excess calcium pulled from bone, and high levels of calcium in the blood and urine. The high enough blood level of calcium, hypercalciuria, can cause calcium to be deposited in soft tissue such as the heart and blood vessels. This can result in irreversible damage to the heart and kidneys, leading to death. You don't have to worry about too much sunlight and toxicity because the UVR itself breaks down excess amounts of the vitamin in skin.
The new DRI added a category called tolerable upper intake level (VL) to give consumers an estimate on the highest amount of nutrient intake that would still be safe. For adults of all ages, the VL for vitamin D is 2,000 IV, which is only about three times the DR! for people over age seventy. In a tragic event that demonstrated the vitamin's toxic effects, several people became ill and two died after drinking milk from a dairy that had mistakenly added 500 times the standard amount of vitamin D.
Physicians in Los Angeles recently reported on four patients who were suffering from bone density loss, possibly osteoporosis, and high calcium blood levels. They also had high blood levels of vitamin D, and all four patients had been taking several multivitamin and other dietary supplements, leading the physicians to suspect vitamin D overdose.