You probably know your milk has vitamin D in it, but you might not know why. In the 1930s, the United States began to fortify milk with vitamin D in an effort to eradicate rickets. The disease — caused by vitamin D deficiency that softens and weakens the bones — was rampant at the time among poor children, particularly in northern U.S. cities.
In the early 1920s, E.V. McCollum, one of the first faculty members at what would become the Johns Hopkins Bloomberg School of Public Health, had discovered vitamin D and determined that rickets could be prevented with cod-liver oil or sunshine. When the fortification effort began a decade later, milk was chosen as the vehicle because it was widely available, palatable to young children and rich in other bone-beneficial nutrients such as calcium. While drinking today’s milk will help keep your body rich in vitamin D, you can also increase your levels of this vitamin by eating select foods — including eggs, salmon and tuna — and through exposure to sunlight.