In many of my public lectures, I address the topic of dietary supplements and often do a rudimentary audience survey. When I ask about taking Vitamin D, the majority of adult hands go up.

“Prevent osteoporosis” is the usual answer to the question as to why, although there is also a smattering of comments about reducing the risk of cancer or heart disease. This is not surprising, given the ebullient articles about Vitamin D in the lay literature. However, scrutinizing the scientific literature dampens one’s enthusiasm for the vitamin.

The mention of rickets brings images of bow-legged children afflicted by this bone-softening disease during the Industrial Revolution in Britain to mind. Yet the condition was known as early as the 17th century, when it was first described by English physicians Daniel Whistler and Francis Glisson. It wasn’t until the 20th century that the disease was linked to a deficiency of a substance produced in the skin upon exposure to sunlight that came to be known as Vitamin D. That explained the prevalence of rickets during the Industrial Revolution, when the sun was obscured by smoke.

Even before the intricacies of the disease were understood, there was a treatment: cod liver oil. By the early 19th century, this fish extract had already developed a folkloric reputation for treating rheumatism and joint pain, so it was readily available to try on rickets when physicians noted that children who lived in coastal areas — where fish was a dietary staple — were less affected by the disease. It worked.

By 1840, the foul-tasting liquid was routinely forced down children’s throats, leaving them with a memorable experience, but at least one that didn’t include rickets.

The mystery of why cod liver oil works was solved in 1922 by Elmer McCollum, who had discovered Vitamin........

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