Where Does the Body Get Vitamin D?
The most natural ways for humans to get the 400 IU of vitamin D each day are through food and sunlight, but vitamin supplements work just as well.
Vitamin D occurs naturally in very few foods, primarily fatty fish like salmon and cod, so in the past children stayed healthy by getting a daily dose of sunshine when working and playing outside. With the advent of the industrial revolution in the 1800s, many children lived in dark, crowded cities and worked all day inside factories, so they were deprived of the sunlight their bodies needed to synthesize vitamin D. Rampant rickets was the result, and the poorest children paid the price.
When it was discovered that cod liver oil prevented rickets, a daily spoonful of the slimy stuff was what kept children healthy. By the 1950s, scientists had figured out how to fortify milk with vitamin D, and rickets had all but disappeared by the 1960s.