Spinach is an excellent source of iron because it contains much more iron than other vegetables.
A 1930s decimal point error made spinach appear to have 10x more iron than it actually does. Spinach contains oxalates that inhibit iron absorption. It is not exceptionally high in bioavailable iron.
What changed?
Popeye's forearms are enormous, and the spinach he squeezes from a can to inflate them is based on a real scientific claim. In 1870, German chemist Erich von Wolf published an analysis of spinach's iron content and placed the decimal point one position to the right. He recorded 3.5 milligrams of iron per 100 grams of spinach as 35 milligrams. The error was not corrected in the scientific literature for decades. By the time it was caught in 1937, by German chemist Gustav von Bunge, reviewing the original data, the myth had made its way into nutritional tables, textbooks, and eventually into the Popeye cartoons that launched in 1929.
Spinach is a healthy vegetable. It contains vitamins K, A, C, and folate, and it does contain iron, approximately 2.7 milligrams per 100 grams cooked. That is not an insignificant amount. But it is nowhere near the 35 milligrams claimed, and it comes with a significant problem: spinach also contains oxalic acid, which binds to iron and significantly reduces its bioavailability. Research has found that the iron in spinach is poorly absorbed compared to iron from meat or even other vegetables.
When nutritionists measure the iron that the human body actually absorbs from spinach, rather than the total iron present in the leaf, the numbers are considerably less impressive. Absorption rates of spinach iron can be as low as 1 to 2 percent under certain conditions, compared to 15 to 35 percent for heme iron from meat sources. A person relying on spinach to treat iron-deficiency anemia would be disappointed.
The myth persisted in nutrition education through the 2000s partly because spinach became a symbol of healthy eating and its specific claim to exceptional iron content was rarely examined critically. School health curricula in the 1990s and 2000s still frequently identified spinach as a top plant source of iron without noting the oxalate absorption problem.
More bioavailable plant iron sources include legumes (lentils, chickpeas, white beans), fortified cereals, pumpkin seeds, and tofu. Consuming these alongside vitamin C increases non-heme iron absorption. Popeye, had he been interested in maximizing iron uptake, might have done better with a can of lentils, though the animation would have been less satisfying.