As little as a century ago, a serious deformity of the bones was widespread in North America and Europe, particularly among children. Rickets, caused by too little vitamin D in the diet or too little exposure to the sun, has been documented by physicians for thousands of years. But its cause wasn't fully understood until the 20th century.
Ancient Chinese, Greek and Roman physicians wrote of a disease that warped the spine, ribs and legs. From the Middle Ages to the era of industrialization, children across Europe succumbed to a similar sickness, which caused bowlegs, knock-knees, "pigeon breast," painful spasms, convulsions and difficulty breathing.
Doctors noticed that it seemed more common in cities than in the country, and less common in sunny countries than in ones with long winters. They speculated it was caused by stagnant air, bad hygiene and cranky dispositions, and tried to treat it with splints, tourniquets and bloodletting.
But they remained at a loss to cure it. By the late 1800s, rickets was epidemic across Europe as families left farms for urban factories and as factory smoke blocked out sunlight in industrializing cities.
For the next 50 years, doctors struggled to understand the disease -- until a body of seemingly unrelated research offered up clues and, eventually, solved the mystery.
In the early 1900s, scientists were discovering substances -- in addition to proteins, fats and carbohydrates -- that were essential for good health. These "vital amines," as they were dubbed, triggered disease when missing from the diet, but cured disease when added back in.
Vitamin A (abundant in cod liver oil), was discovered in 1913 by American researchers Elmer McCollum and Marguerite Davis. Inspired by the finding, British doctor Edward Mellanby decided to see if a missing "vital amine" might be to blame for rickets.
Mellanby induced rickets in laboratory dogs by feeding them a Scottish diet: nothing but oatmeal. He then cured them with cod liver oil and concluded that a lack of vitamin A had caused the disease.
McCollum took Mellanby's experiment a step further, feeding sick animals cod liver oil that had been treated to destroy its vitamin A content. The oil still cured the sick dogs, so McCollum concluded it must contain another, still intact vitamin. He named it vitamin D.
But the finding still failed to explain why rickets was so predominant in areas of little sun. (Mellanby's dogs, in addition to eating nothing but oatmeal, had been kept indoors.)
Soon, other scientists began to show that UV light could also cure rickets by prompting the body to make its own vitamin D in the skin.
In Wisconsin, researchers found that they could cure rickets not only by exposing animals to UV light but also by exposing their food. (UV light triggers formation of a "previtamin," which the body converts to vitamin D.) By 1924, bread and milk were being irradiated throughout the U.S.
Eventually, scientists figured out how to make vitamin D in the lab, and irradiated foods were replaced with those fortified with synthetic D.
Today, D-fortified milk and infant formula have made rickets rare, but cases still occur in the U.S., mainly among dark-skinned infants who are exclusively breastfed and thus don't get supplemental D.