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In 1870, a German chemist named Erich von Wolf was analysing the iron content of various vegetables.
He made a decimal point error.
He recorded spinach as containing 35mg of iron per 100g. The correct figure was 3.5mg. The misplaced decimal sat in the nutritional literature for decades, entirely unchallenged, because nobody particularly felt like re-testing spinach.
In 1929, the Popeye comic strip launched. The creators cited the iron content of spinach as the scientific basis for their character's powers. By this point, the decimal point error was already sixty years old and fully embedded in received nutritional wisdom.
The error was identified and corrected in 1937. The correction was not issued with anything approaching the cultural reach of the original claim.
Popeye continued punching things.
The actual iron content of spinach, 3.5mg per 100g, roughly where it was always supposed to be, is further complicated by the fact that spinach is among the highest-oxalate vegetables known.
Oxalates bind to iron and calcium in the gut and remove them before absorption. The iron in spinach absorbs at around 1–2%, compared to 15–35% for haem iron from red meat. You would need to eat roughly a kilogram of spinach to absorb the iron equivalent of a 100g beef steak.
There is also the kidney stone question. Spinach contains around 970mg of oxalates per 100g: one of the densest plant sources. Chronic high spinach consumption, particularly raw in daily smoothies, is a documented pathway to calcium oxalate kidney stones.
The smoothie industry has not issued a correction.
Popeye is still a sailor.
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