From Arbesman’s The Half-Life of Facts:

One of the strangest examples of the spread of error is related to an article in the British Medical Journal from 1981. In it, the immunohematologist Terry Hamblin discusses incorrect medical information, including a wonderful story about spinach. He details how, due to a typo, the amount of iron in spinach was thought to be ten times higher than it actually is. While there are only 3.5 milligrams of iron in a 100-gram serving of spinach, the accepted fact became that spinach contained 35 milligrams of iron. Hamblin argues that German scientists debunked this in the 1930s, but the misinformation continued to spread far and wide.

According to Hamblin, the spread of this mistake even led to spinach becoming Popeye the Sailor’s food choice. When Popeye was created, it was recommended he eat spinach for his strength, due to its vaunted iron-based health properties.

This wonderful case of a typo that led to so much incorrect thinking was taken up for decades as a delightful, and somewhat paradigmatic, example of how wrong information could spread widely. The trouble is, the story itself isn’t correct.

While the amount of iron in spinach did seem to be incorrectly reported in the nineteenth century, it was likely due to a confusion between iron oxide—a related chemical—and iron, or contamination in the experiments, rather than a typographical error. The error was corrected relatively rapidly, over the course of years, rather than over many decades.

Mike Sutton, a reader in criminology at Nottingham Trent University, debunked the entire original story several years ago through a careful examination of the literature. He even discovered that Popeye seems to have eaten spinach not for its supposed high quantities of iron, but rather due to vitamin A. While the truth behind the myth is still being excavated, this misinformation — the myth of the error — from over thirty years ago continues to spread.