Spinach is an excellent source of iron because it contains much more iron than other vegetables.

Popeye's forearms are enormous, and the spinach he squeezed from a can to inflate them was meant to make a scientific point: that spinach is a uniquely powerful source of iron. For most of the twentieth century, that belief was treated as settled fact, reinforced in classrooms, nutrition charts, and popular culture. The belief was real. The exceptional iron was not.

For decades, the standard explanation for how spinach earned its reputation was a clerical accident. As the story goes, a nineteenth-century chemist misplaced a decimal point and recorded spinach as having ten times more iron than it does, roughly 35 milligrams per 100 grams instead of 3.5, and the mistake propagated through textbooks until it was finally caught in the 1930s. It is a tidy, satisfying story. It is also, most likely, not what happened. When later researchers went looking for the original decimal-point blunder, they could not find it, and the account turned out to be a kind of academic urban legend: a claim repeated and cited so confidently that almost no one checked the primary source. The inflated nineteenth-century figures were real, but the likelier explanations were mundane, such as contamination of samples during chemical analysis or confusion between the iron content of fresh spinach and that of far more concentrated dried spinach.

What is not in dispute is that spinach was credited with far more iron than it contains. A 100-gram serving of cooked spinach supplies roughly 2.7 milligrams of iron, a respectable amount for a leafy green but nowhere near the headline numbers that made it legendary. And the raw total is only part of the story, because the figure on a nutrition table is not the figure the body absorbs.

Spinach contains oxalic acid, a compound that binds to minerals including iron and forms complexes the digestive system struggles to take up. The iron in spinach is non-heme iron, the plant form, which the body absorbs far less efficiently than the heme iron found in meat. Depending on what it is eaten alongside, the fraction of non-heme iron actually absorbed can drop to just a few percent, while heme iron is taken up at rates several times higher. A person hoping to reverse iron-deficiency anemia with salad alone would be in for a long wait.

None of this makes spinach a bad food. It is rich in vitamins K, A, and C and in folate, and its iron is not worthless, only oversold. The problem was never that spinach is unhealthy. The problem was the specific, repeated claim that it stood head and shoulders above other plants as an iron source, a claim that persisted in school health curricula well into the 2000s, frequently with no mention of the absorption problem at all.

For anyone actually seeking iron from plants, there are stronger choices. Legumes such as lentils, chickpeas, and white beans, along with fortified cereals, pumpkin seeds, and tofu, deliver more usable iron, and eating them with a source of vitamin C measurably increases how much non-heme iron the body takes up. Popeye, had he been optimizing for iron rather than dramatic effect, might have reached for a can of lentils. The animation would have suffered.