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The story of vitamin D and its association with other diseases
Vitamin D is a fascinating molecule with a fascinating story.
Historically, “vitamins” were defined as chemicals that humans required from their environment that were “vital” to human health.  These chemicals were needed only in very small amounts to prevent disease; an absence of a particular vitamin in the diet led to a specificdeficiency disease: vitamin C, scurvy; thiamine, beri beri.  Other vitamin deficiencies were found to be a bit more complicated: vitamin B12 deficiency was found to cause a type of anemia, dementia, and spinal cord problems.
Because vitamins are required in such small amounts, and are often present in small amounts in foods, their discovery was an opportunity to prevent and cure several diseases. One of these diseases was rickets.
Rickets was common in America and Northern Europe when vitamin D was discovered early in the 2oth century.  A series of fascinating experiments found that a fat-soluble substance present in cod-liver oil had the ability to prevent and cure rickets.  Environmental observations also found that children in the tropics were less likely to develop rickets than thosein high northern latitudes, and sunlight seemed to prevent the disease.
It is nearly impossible to ingest sufficient vitamin D in a typical diet, and it is nearly absent from breast milk.  In areas where sunlight is scarce or where culture prevents sun exposure, rickets was more common. Rickets became rare in the U.S. once children’s parents began shoving them full of cod-liver oil, and oncevitamin D was added to milk.
Vitamin D is more properly called a “hormone” rather than a vitamin.  It’s precursor is naturally produced in the body and is present in the skin in large amounts.  When it is exposed to ultraviolet B radiation, as from sunlight, this precursor is converted to another molecule that is absorbed into the blood stream.  It travels to the liver where it is converted into a“prohormone” (precursor to a hormone).  After leaving the liver, the prohormone can be converted in the kidneys to the hormone form of vitamin D on a (relatively) large scale to be distributed throughout the body, or it can be converted on a small scale in local tissues.
Vitamin D receptors are present in nearly every tissue in the human body.  Aside from its effects on bone growth, its variouseffects are only incompletely understood.   But is the prevention of severe vitamin D deficiency such as rickets all we need to know? Growing evidence is suggesting that vitamin D deficiency is more common that previously believed, and that vitamin D may play an important role in many common diseases such as heart disease and cancer.
In the early part of the century, scientists fed rats very limiteddiets, teasing out the effects of various foods and the micronutrients they contained, including vitamin D. Such controlled experiments are, needless to say, problematic in humans.  Rickets was a common, even endemic, disease, and therefore not terribly difficult to study. But other effects of vitamin D may be more subtle.  Attempts have been made to correlate other disease states with the relativedeficiency in vitamin D that is still common in high latitudes.  Part of the difficulty in studying the effects of low vitamin D levels is in simply defining them.  Assays that measure vitamin D in the blood are not entirely reliable.  Still, we can try to correlate vitamin D levels with obvious deficiency diseases such as rickets, but the environmental studies that look at other possible effectsare problematic.
Heart disease is one of our three biggest killers (along with cancer and stroke).  One of the better prospective studies done on vitamin D and heart disease followed subjects over time, measuring vitamin D levels and following them to see who developed a first incidence of heart attack.  They found that those with low vitamin D levels who also had high blood pressure (a...