The old narrative:
A misplaced decimal point caused the false reputation of spinach for being the vegetable that was richest in iron. Though still highly popular, this narrative is most likely wrong (see here). The decimal error probably never occurred in that stupidly simple way. Ignoring wrinkles in the narrative, such as that spinach is still rather rich in iron but that it cannot be assimilated well for other reasons, the new narrative can be stated most simply as follows.
The new narrative:
The decimal error is a myth, it never occurred. The false reputation of spinach was due to unreliable methods or poor experimentation. That is, errors were inherent in experiments not data treatments (see here).
The complex history
Still, not everything about the spinach-iron legend is clear yet. In particular, nobody has yet thoroughly reconstructed where the original data came from, how they have been treated (mathematically) by the various researchers who wanted to reach comparability with their own data, and whether any mistakes were made in these data treatments. At the end of this series of reconstructing data handling, you will see that the whole research endeavour was full of data handling errors, though none as simple as a misplaced decimal point.
Bender's false lead
One of the unsolved riddles in the thicket of myths around spinach is the source of the decimal-point-error myth. While the urban legend is very widespread, the source of this legend remains elusive. A.E. Bender has said as much in his inaugural lecture in 1972 (see here). He later released the decimal-point-error legend upon a wider public through a letter to The Spectator.
"Sir: In a recent article (18 June) spinach is given undeserved nutritional eminence, although, since the facts have never been widely publicised, the author can be excused.For a hundred years or more spinach has been (and clearly still is) renowned for its high content of iron compared with that of other vegetables, but to the joy of those who dislike the stuff this is quite untrue. In 1870 Dr E. von Wolff published the analyses of a number of foods, including spinach which was shown to be exceptionally rich in iron. The figures were repeated in succeeding generations of textbooks—after all one does not always verify the findings of others including the 'Handbook of Food Sciences' (Handbuch der Ernährungslehre) by von Noorden and Saloman in 1920.In 1937 Professor Schupan eventually repeated the analysis of spinach and found that it contained no more iron than did any other leafy vegetable, only one-tenth of the amount previously reported. The fame of spinach appears to have been based on a misplaced decimal point."Professor Arnold Bender, The Spectator (18 July 1977, p. 18)
Who was Schupan?
However, neither the decimal-point error could ever be verified (see here) nor some researcher with the name Schupan be found, who published on spinach and iron during that time.
I was able to trace "Werner Schuphan" instead (mind the h after the p). While he did publish on all sorts of vegetables including spinach, the following publication from 1940 shows that he can hardly have debunked the myth of spinach's exceptional iron richness in 1937:
"Spinat zeichnet sich – wie wir sehen – besonders durch hohe Gehalte an Carotin, Chlorophyll, Eisen und Reineiweiß und Vitamin C aus." [Schuphan, W. 1940. Über den Einfluss der Chlorid- und Sulfatdüngung auf Ertrag, Marktgängigkeit und biologischen Wert verschiedener Gemüse unter Berücksichtigung edaphischer und klimatischer Faktoren. Bodenkunde und Pflanzenernährung 19(5-6): 265-315]
"Spinach distinguishes itself – as we see – especially through high contents of carotin, chlorophyll, iron and pure protein and vitamin c." [my translation]As the above quote reiterates the idea that spinach is richer in iron than other vegetables, it is unlikely that he had some years before claimed the opposite.
One article published by Schuphan in 1937 [Untersuchungen über wichtige Qualitätsfehler des Knollenseleries bei gleichzeitiger Berücksichtigung der Veränderung wertgebender Stoffgruppen durch die Düngung. Bodenkunde und Pflanzenernährung 2, issue 5-6, pp. 255-304] dealt with quality issues in celeriac including the so-called Eisenfleckigkeit (iron blotchiness) of tubers. He therein debunked the belief that celery tubers turn brown at the cut surface, because of the oxidation of iron and claims that the colouring is due to resins and essential oils instead.
Another article from Schuphan in 1937 [Der gegenwärtige Qualitätsbegriff bei Gemüsen und die Notwendigkeit seiner Erweiterung auf chemisch erfaßbare Wertmerkmale. Der Forschungsdienst 3: 290-303] discussed the general discrepancy between criteria for the marketability of vegetables (e.g., size, form, colour, solidity) and nutrient quality (e.g., contents in essential oils, proteins, sugar, vitamin c). Most of his examples are taken from his own and others' research on celery. He mentions aside, the effect of P-manure on contents of vitamin c, phosphoric acid and lezithin in spinach, but iron is no issue at all.
That is, Bender's citation of a publication by Schupan in 1937 is probably wrong.
Schuphan and spinach
However, Schuphan is an interesting figure in the spinach legend for other reasons. Firstly, several of his publications suggest that spinach is rich in provitamin A, which makes him a potential source for Popeye's eating spinach for vitamin A (see here). Secondly, he published a study intended to show that the content of oxalic acid in spinach is unproblematic in 1958 (Schuphan and Weinmann: "Der Oxalsäuregehalt des Spinats." Qualitas Plantarum 5(1): 1-22). Thirdly, in 1965 he discovered that high incidences of methemoglobinaemia in babies around Hamburg, Kiel and Berlin was probably due to over-manuring spinach with nitrogen-fertilizer. A subsequent accumulation of nitrite from nitrate due either to false storage, processing or re-heating the prepared food lead to the poisoning, he suggested.*
These publications can at least suggest how the name Schup(h)an, research on spinach compounds, and the debunking of a legend about iron (though in celery) got associated in the mind of Arnold E. Bender.
* Now, I remember vividly how my parents and grandparents would always claim that the spinach mush must be eaten all up, for it cannot be re-heated. Parents always find creative was to turn scientific findings into the claim that the spinach must be eaten, it seems.