[31.032016: updated on lore dating back to the 16th century.]
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 experimental methods, not data handling (see here).
The complex history
The narrative coming closest to the true history of this issue is probably this: An old lore about the goodness of spinach dates from times long before any of the research implicated in the myth about the decimal point error. And this folklore tenaciously maintains itself, despite repeated refutations by researchers.
In parallel to this tenacious folklore, there's a narrative about research being error prone but also self-correcting. For example, Bunge (1892) did not correct the iron contents of ash analyses for oxygen gained during combustion (see here), but he nevertheless concluded that spinach (and strawberries for that matter) do not have the high values given by Wolff (1871). That is, he drew a basically correct conclusion despite erroneous data treatment. Again, Haensel's (1909) iron contents were an order of magnitude too high for all the vegetables he analysed, but the relation between the data showed that spinach was not exceptional in comparison with the other vegetables. Haensel drew a correct conclusion based on poor data (see here). And so on in the 1930s etc.
Most of this research was not centrally or exclusively concerned with the iron content or spinach. The ash analyses of Wolff, for example, were basic/applied research in biochemistry, while Bunge was interested in finding out how baby mammals got over the suckling period depending on milk that is devoid of iron. The answer to this research question is, they get born with a store of iron that carries them through till weaning.
The myth about spinach's iron richness has been refuted along the way in the late 1800s, the early 1900s, and the 1930s, long before the legend about the misplaced decimal error has ever been aired. But the lore did not go away. Finally, Arnold Bender threw up his hands in despair and suggested that the belief may even be due to a misplaced decimal point. Now we have two legends, the iron richness and the decimal error. Both seem to be highly resilient.
The ancient lore
Once upon a time, when the Linnean system of identifying species was not even standard, Johann Ernst Zeiher (1756, vol. 2, pp. 374-375) wrote about the medical uses of spinach.
The title of the book already indicates that Zeiher translated a work from French into German, but I found no indication of the original. "Vollständiger Unterricht von Küchengewächsen: oder ausführliche Beschreibung aller Küchengewächse [...] Ferner, ihrer Nutzbarkeit für das menschliche Leben, und ihrer Tugenden, zur Erhaltung der Gesundheit, [...] u.s.w. [...] aus dem Französischen übersetzt von D. Johann Ernst Zeiher, ernenneten Professor bey der Kaiserl. Akademie der Wissenschaften zu St. Petersburg [...]". Some online search, however, shows that it must be a translation of: M. de Combles (1752) L'école du jardin potagere, [...] Paris: Chez Boudet ou Le Prieur.
For the French original see: de Combles (1752, vol. 2, pp. 24-25), for a German translation see: Zeiher (1756, p. 374f). My own translation of it into English follows:
"The qualities of spinach in the medical arts are, to open the body/belly [whatever that means], to relieve the cough and the acerbity/acuteness of the chest: the water distilled from its leaves also has the power to attenuate the heat of the gut/bowel [sic, he uses it in singular], and the burning of a stomach irritated by an inflamed bile. One also uses the leafs for decoctions and emollient compresses. The short-winded [literal translation: tight chested] get great relief therefrom, and sometimes they were completely cured through repeated use of spinach boiled with veal: and this was the only remedy used by Mr. [Guy-Crescent] Fagon, first personal physician of Ludwig XIV, who was heavily burdened by this disease. When applied externally to the belly and the liver, it takes their inflammation and pain away. According to the opinion of a recent writer, the overuse of this plant leads to melancholy bloods: from my point of view, Mr Fagon's experience alone suffices to overthrow this opinion."The 18th century seems to have known a lot of lore about the medical powers of spinach, even connected to the Sun King. However, Combles (1752) also accounted for the health effects of shalotte coming before spinach and tarragon coming after. He probably discussed the health effects of each plant he treated.
Robert Hooper (1811. "Quincy's Lexicon Medicum. A new medical dictionary") corroborates that Combles' account of health effects of spinach was widespread:
"This plant Spinacea oleracea of Linnaeus is sometimes directed for medical purposes in the cure of phthisical complaints; made into a poultice, by boiling the leaves and adding some oil, it forms an excellent emollient. As an article of food it may be considered as similar to cabbage and other oleraceous plants." (Hooper 1811, p. 759)Phthisis is a dated term for suffering from consumption (tuberculosis) or other emaciating diseases. 12 years later, Achille Richard (1823, "Botanique médicale ou histoire naturelle et médicale [...]", p. 171) wrote:
"L'épinard est cultivé dans tous les jardins.Il fleurit en mai et juin. Il est peu usité comme médicament, si ce n'est à L'extérieur; on L'emploie en cataplames, et il est très émollient. Mais, comme aliment, son usage est trés-repandu. Il parait être légèrement laxatif; il est peu nourrissant et presque insipide." (For a German translation see p. 266 here)
"Spinach is grown in any garden. It blooms in May and June. It is rarely used as a drug, and if so then only externally; it is used in poultices, and it is very emollient. But as food its use is widespread. It appears to be slightly laxative; it is not nourishing and almost tasteless." (My translation)The trend, however, seems to downgrade the medical utility of spinach from the account of Combles (1752), giving it as the only remedy for Luis XIV, to Hooper (before 1822), saying it was sometimes used, and Richard (1823), stating it was little used and if so, then only externally.
These accounts are from a time before Robert Koch described the pathogen causing tuberculosis in 1882. Phthisis and Combles' account on cough and short-windedness reminds me of the much later research of Kobert (1914) on the health effects of saponins in spinach against tuberculosis and other lung diseases (see here). That is, early research applying "modern" methods of science (e.g., chemical analyses) naturally derived its hypotheses from earlier experiences described by physicians. One such research endeavour formulated hypotheses associating spinach components (saponins) with effects against tuberculosis.
But how did the ancient lore transmogrify into a myth about the iron richness of spinach? One symptom of tuberculosis is anaemia, a lack of red blood cells or hemoglobin. Tuberculosis, however, is not the only possible cause of anaemia. Another cause is iron deficiency. Hence, other researchers could have taken the same lore documented by Combles (1752) and derived another hypothesis from it associating spinach with iron richness.
An early documentation of this spinach-iron connection can be found in a German encyclopaedia published by F. A. Brockhaus in 1852 (see P.P.S. to this blog entry). As Brockhaus only collected then-current knowledge, the idea must be older still.
The modern error
Throughout this series of reconstructing the data handling of the primary research literature, I maintained that the relevant research was full of data handling errors, but none of these errors was as simple as misplacing a decimal separator (see here).
Surprisingly, I now stand corrected on this claim. A simple decimal-point error did occur in an article published in—wait for it—Science 90, no. 2347, pp. 596-597 (1939). But the research reported there was about the iron content of dried peas and beans instead of spinach.* So how can it be the mythical error recurring in the urban legend about spinach & iron?
[* The full reference is: Aschman, Leah, Mary Speirs & Dorothy Maddox (1939) The availability of iron in dried peas and beans. Science 90, 596-597.]
Let's first take a look at the error itself. Aschman et al. (1939) report the results of an experiment that was pretty similar to the much older experiments by Bunge (1892), for example. They rendered baby rats anaemic by feeding them on milk only (mammalian milk is poor in iron). Then they fed the anaemic rats dried peas and beans, did the proper controls and measured the recovery of the rats. Along the way, they also measured the iron content of the dried products and gave the values as follows:
Anybody with a sharp eye will spot the decimal-point error in the value given for butter beans at the end. And that is what the staff of the Nutrient Section of the Bureau of Home Economics did, when they published abstracts in the Journal of Home Economics 32, no. 7, p. 481:
This is probably the ultimate source of the urban legend that spinach got its false reputation for iron-richness through a misplaced decimal point. This particular source of a decimal point error could even explain why Bender (1977. The Spectator. 18 July, p. 18), the researcher who first started to spread that urban legend about spinach, iron and decimal points, associated the real but innocuous decimal point error of Aschman et al. (1939) with the name Schupan [sic] in his memory. Werner Schuphan has indeed published on the content of oxalic acid in spinach (see here).