Friday, 28 August 2015

The first source for the spinach-iron myth

For an introduction to the whole problem of the spinach-iron myth and its many ramifications read the last 7 posts of this blog (chronology: oldest post deals with oldest literature) and follow the links given in them. Beware, this myth is a mess concocted in over 160 years. What has never been discovered is the first source for spreading the opinion, during the second half of the 19th century, that spinach was a good source for dietary iron.

As you can see from my comments under this earlier blog entry, the data of Thomas Richardson (1848. Beiträge zur chemischen Kenntnis der Vegetabilien. Annalen der Chemie und Pharmacie LXVII Bd. 3.)* may well be this first source that has caused this widespread opinion. Soon after 1848, educational publications started to spread the myth. For example, an encyclopaedia published by Brockhaus (1852. Die Gegenwart. Eine encyklopädische Darstellung der neuesten Zeitgeschichte für alle Stände. Siebenter Band. Leipzig: F. A. Brockhaus) stated at page 172: "Weiße Rüben enthalten nur eine geringe Menge von Eisen, Spinat dagegen viel." [White turnips contain only a little iron, spinach however a lot. My translation.] One year later, Aaron Bernstein published a popularisation of scientific findings in a work called "Aus dem Reiche der Naturwissenschaft: ein Buch für Jedermann aus dem Volke" (Berlin: Franz Duncker, 1853). At pages 157-158, Bernstein praised spinach as an iron rich, organic alternative to medicine for pale children. 

*[this publication hangs in a digital limbo, because it has been attached to the end of the preceding article by C. List (1848. Ueber das sogenannte Terpentinölhydrat. Justus Liebigs Annalen der Chemie 67(3): 362-376.]

Richardson gave the values of various chemical compounds of various vegetables as percent values in relation to the raw ash and as percent values in relation to the pure ash (raw ash minus carbonic acid, charcoal and sand). The iron content, in particular, was given as the percent values of "Phosphorsaures Eisenoxyd," which literally translates as phosphor-acidic iron oxide but chemically means iron(III) phosphate. Richardson's data sheet also provides the percent values of the ashes in relation to the fresh matter.

As I have argued elsewhere, it is false to calculate the portion of, say, iron compound in the fresh matter by simply multiplying the portion of the iron compound in relation to ash with the portion of ash in relation to fresh matter. The reason why this leads to false values is, because the ashes gain mass during combustion. In particular, gaseous oxygen combines with the burning matter and while some products of combustion end up gaseous themselves (e.g. carbon dioxide) others end up as solid ash (e.g. magnesium oxide). That is, the ashes gained mass through combustion that was not part of the fresh matter.

It is not anachronistic to call this a mistake, in relation to Richardson's time, because Phlogiston theory had been questioned in the second half of the 18th century already, and experiments had  shown that metals gain mass during combustion. Hence Richardson's contemporaries and followers should have known that simply multiplying the portion of iron compound in ash times the portion of ash in fresh matter would yield false values for the portion of iron compound in the fresh matter.

Nevertheless, Bunge (1892) has committed this particular mistake in manipulating data from Wolff (1871) as shown here. Therefore it seems likely that others have also simply multiplied the percent values of Richardson's data and drawn false conclusions. Or, anyway, it seems interesting to reconstruct what conclusions contemporaries of Richardson might have drawn from such a data manipulation.

As you can see from the table below, spinach comes out second after radish herbage (values in scientific notation: 1,76E-03 means 1.76*10-3 equal to 176mg in 100g). Assuming that the herbage of radish was usually not eaten, however, spinach would be the edible item with the highest iron content in Richardson's data set. Hence Richardson (1848) may well be the first source from which the widespread opinion sprang that spinach was a good source for dietary iron in turn.

Item portion of iron phosphate in raw ash times portion of ash in fresh matter portion of iron phosphate in pure ash times portion of ash in fresh mater
Ananas, ganze Frucht   

ditto Schopf   

Spargel                               1,25E-04 1,60E-05
Lauch, Zwiebel                        5,52E-04 6,11E-04
ditto Stengel                         6,76E-04 8,91E-04
Feige, ganze Frucht   

Walnuts, Kern   

ditto Schale

Gurke                                 1,19E-04 1,30E-04
Brocoli (Kohl), Herz                  2,02E-04 2,14E-04
ditto Blätter                        9,86E-04 1,06E-03
Blumenkohl, Herz                      2,80E-04 2,61E-04
Rettig, Wurzel                       1,20E-03 1,41E-03
ditto Kraut                          3,20E-03 4,54E-03
Kastanie, ganze Frucht                1,77E-04 1,93E-04
Erdbeere, ganze Frucht                3,69E-04 4,56E-04
Orange ditto   

Rhabarber, Stengel                    1,66E-04 1,91E-04
ditto Blätter                         2,62E-04 2,87E-04
Spinat                               1,28E-03 1,76E-03
(Kidney Beans) Bohnen                 3,09E-04 3,56E-04
Erbsen, Hülsen                        6,90E-05 8,00E-05
Pflaumen (greengages), ganze Frucht   1,80E-04 2,42E-04
Orleans-Pflaumen, Haut der Frucht     5,39E-04 6,63E-04
Orleans-Pflaumen, Fleisch derselben   9,50E-05 1,49E-04
Orleans-Pflaumen, Kern                5,76E-04 6,28E-04
ditto Samenschale                     9,40E-05 1,05E-04
Kirschen, ganze Frucht                1,47E-04 1,61E-04
ditto Stiel derselben                 4,74E-04 5,57E-04
Birne, ganze Frucht                   6,60E-05 8,00E-05
Apfel, ditto                          5,90E-05 7,20E-05
Artischoke                            4,86E-04 5,55E-04
Lattich                               Spur Spur
Endivie                               6,72E-04 8,71E-04
Stachelbeere                          2,94E-04 3,37E-04
Sellerie                              2,56E-04 2,85E-04
Möhre                                 2,46E-04 2,39E-04
Pastinak                              4,63E-04 5,53E-04

P.S: The Genealogical World of Phylogenetic Networks has a new post with a phylonetwork illustrating the whole dataset of Richardson (1848).

Thursday, 6 August 2015

Historic sources of the spinach-iron myth: Schuphan 1940

[Click on the tab "spinach" to get all posts in this series.]

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 (see here) 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, I have not yet found the erroneous decimal point in his publications. Though my reading of Schuphan's publications is far from complete, the following from 1940 shows that he can hardly have claimed a misplaced decimal point in 1937: "Spinat zeichnet sich – wie wir sehenbesonders 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). As the above quote reiterates the idea that spinach is rich in iron, it seems improbable that he had some years before spread the myth about a misplaced decimal point as the source for a falsely high iron content.

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.

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.

Wednesday, 5 August 2015

Historic sources of the spinach-iron myth: König 1926

[Click on the tab "spinach" to get all posts in this series.]

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 (see here) 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.

König 1904
J. König (1904. "Chemie der menschlichen Nahrungs- und Genussmittel." Vol 2, p. 353) gives the range of iron contents in spinach as 32.7 to 39.1mg per 100g dry mass (see table below). The text above the table says that these values are from Bunge and Häusermann.


And indeed, Bunge found 0.0327g in 100g dry mass (see here), which is the same as 32.7mg in 100g. The other value of 39.1mg/100g is from Emil Häusermann (1897. "Die Assimilation des Eisens." Zeitschrift für Physiologische Chemie 23: 555-592). Like Bunge, Häusermann's main aim was to induce and study the anemia in animals, but he lists the iron contents of vegetables at the end of his publication. The value for spinach can be found at page 588 and it is taken, in turn, from Boussingault (1872. "Du fer contenu dans le sang et dans les alimants." Comptes Rendus de l'Académie des Sciences 74: 1353-9), or rather from Bunge (1892), who had taken Boussingault's data and transformed it according to König (1889, see footnote: **** here). 

Later, J. König (1926 "Nahrung und Ernährung des Menschen. Kurzes Lehrbuch. Berlin: Julius Springer, p. 31) tried to combine these values with those of a later publication by Haensel (1909. "Über den Eisen- und Phosphorgehalt unserer Vegetabilien." Biochem. Zeitschrift 16: 9-19) it seems. Haensel compared his data for various vegetables and concluded correctly, that spinach was not the richest in iron, despite the fact that his particular value for iron in spinach was roughly ten times higher than Bunge's (see here). König seems to have grappled with this inconsistency in the research record as follows:
"Eine besondere Bedeutung wird auch dem Eisen in der Nahrung zur Bildung des Hamoglobins zugeschrieben. Von dem durchschnittlichen Eisengehalt des erwachsenen Körpers von 3g sollen etwa 1/6 auf Hamoglobin entfallen. Hiervon werden täglich 80-100 mg in Freiheit gesetzt. Diese werden von Leber, Milz und sonstigen Drüsen größtenteils gespeichert, so daß der tägliche Bedarf in der Nahrung nur 20-30mg betragen solI. Solche Mengen Eisen sind auch wohl in einer gemischten Nahrung vorhanden. Die größten Mengen Eisen finden sich in grünen Gemüsen, nämlich in 100g 30-60mg Fe203 (die Höchstmenge im Spinat); andere Nahrungsmittel enthalten nur den 10. Teil und noch weniger." König (1926, p. 31)
"Special significance for forming the hemoglobin is attributed to the iron in the diet. About 1/6 of the average 3g iron in an adult body if in the hemoglobin. Of these, 80-100mg are liberated daily. These are mostly stored in the liver, spleen and other glands so that the daily requirement in dietary iron amounts to 20-30 mg. Such amounts of iron are also well available in a mixed diet. The largest amounts of iron can be found in green vegetables, namely in 100g 30-60mg Fe203 (the maximum quantity in spinach); other foods contain only the 10th part of this and even fewer." (my translation)
At page 82 König cites Haensel (1909) and another, but not on spinach. My translation below is not garbled, the original is:
"Unter den Mineralstoffen wird auch dem Eisen der grünen Gemüse wegen der blutbildenden Eigenschaft eine besondere Bedeutung zugeschrieben. R. Berg fand in 100g frischem Gemüse Spuren (Wirsing) bis 150 mg Eisen (Bleichsellerie); E. Haensel 3,9 mg (Zwiebel) bis 68,9 mg Eisen (Kohlrabiblätter) oder 31-37,9 mg Eisen in 100 g Trockensubstanz."
"Among the minerals a special importance is also attributed to iron of the green vegetable because of the blood-forming capacity. R. Berg found in 100g fresh vegetables tracks (Savoy) up to 150mg iron (celery); E. Haensel 3.9mg (onion) to 68.9mg iron (Kohlrabi leaves) or from 31 to 37.9mg of iron in 100 g of dry matter."

Coming full circle, page 86 correctly states that spinach contains no more iron than other green vegetables and refers back to page 82, where, as we have seen, nothing is said about spinach. 

Historic sources of the spinach-iron myth: König 1920

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 (see here) of reconstructing data handling, you will see that the whole research endeavor was full of data handling errors, though none as simple as a misplaced decimal point.


The 5th, corrected edition of Joseph König (1920. "Chemie der menschlichen Nahrungs- und Genussmittel... Berlin. J. Springer.) swapped iron contents in dry with those in fresh matter at page 453 as follows:
"Der Spinat wird vielfach als eisenreiches Gemüse angesehen und empfohlen. [Irrelevant sentence omitted.] In der Regel enthält aber der Spinat nicht mehr Eisen als andere Gemüse, nämlich nach Haensels Untersuchungen in der Trochensubstanz 0,03%, während Winterkohl in der letzteren 0,056%, Kopfsalat 0,054% Eisen ergeben. [...]"
"Spinach is often regarded and recommended as rich in iron. [Irrelevant sentence omitted.] Usually however spinach contains no more iron than other vegetables, according to Haensel's investigations 0.03% in the dry matter, while winter cabbage contains 0.056% and lettuce 0.054% iron in the dry matter."
These values, however, are averages of Haensel's (1909) values for fresh matter. Haensel's values for the dry spinach amounted to 0.445% iron on average. König combined the correct statement that spinach is not exceptionally rich in iron with a values that was 15 times too low according to the state of knowledge of that time. 

Tuesday, 4 August 2015

Historic sources of the spinach-iron myth: Serger 1906, Haensel 1909, Kobert 1914

[Click on the tab "spinach" to get all posts in this series.]

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 (see here) 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.

Spinach extracts as dietary supplements
Around 1910, several dietary supplements based on spinach seem to have been widespread in Germany. At least two publications in the Pharmazeutische Zeitung suggest this:

1. Serger, H. (1906) "Über den Eisengehalt des Spinats." Pharm. Ztg. 51 (No. 33): 372.

2. Kobert, E.R. (1914) "Die Spinate als Arzneinahrungsmittel." Pharm. Ztg. 59 (No. 57): 422-423.
This was a reprint of the original:  
Kobert, E.R. (1914) "Die Spinate als Arzneinahrungsmittel." Beiträge zur Klinik der Tuberkulose und spezifischen Tuberkulose-Forschung (1914) Vol. 31 (No. 3): 481-489. 

The latter journal metamorphosed into Pneumology first and finally into Lung: an international journal of lungs, airways and breathing.

Serger (1906) starts by saying that spinach (Spinacia oleracea) probably has the highest iron content among vegetables and is therefore recommended by physicians to anemic patients. He tested two dietary supplements for their iron content. These supplements were called Spinolum siccum and Extr. Ramkulini. In order to find out how well the iron was extracted into these products (and not discarded with the waste) he also tested fresh winter spinach and found an average iron content of 0.104% of the dry matter. That was three times the 0.0327% found by Bunge (1892, p. 181, see also here).  

Kobert (1914), on the contrary, already claimed that spinach was not particularly rich in iron and that the leaves of kohlrabi were much better as an iron source. He had no particular beef with iron anyway, but suggested that the saponins in spinach were good for the lungs and for patients with respiratory problems instead. 

Higher absolute iron content, but lower comparative values
This seems to chime with the earlier finding that Haensel (1909. "Über den Eisen- und Phosphorgehalt unserer Vegetabilien." Biochem. Zeitschr. 16: 9-19) denied that spinach had the highest iron content among vegetables based on own analyses (see here). That is, the average iron oxide content of the dry matter found by Haensel was 0.445%, whereas other vegetables had higher values, for example lettuce (Haensel 1909, p. 12, "Kopfsalat:" 0.679% on average). 

The odd thing is that for spinach alone, Haensel's values are much higher than Serger's, even if we subtract the oxygen from the iron oxide. The portion of iron in iron oxide is 69.94%, hence Haensel's value of 0.445% iron oxide is equal to 0.311% pure iron in the dry matter. Why then did Haensel refute the idea that spinach was the richest in iron?

Haensel's conclusion was not about the absolute value of the iron content of spinach, but about its relative standing in comparison with other vegetables. It was aimed against the "vielfach herrschende Ansicht" (widespread opinion) that spinach was the richest in iron, and he could compare his value for spinach directly with his values for other vegetables. Serger had no direct comparison with other vegetables and probably just reiterated the widespread opinion. He also found that his finding chimed with that of "Königs Nahrungsmittelchemie." 

Curiously, Haensel's value (equal to 0.311% Fe in the dry matter) was about ten times higher than Bunge's 0.0327% (see also here)

Conclusion
The claim that a particular value of the iron content of spinach is too high or too low is different from the claim that spinach has or has not the highest iron content among all vegetable. The first is about absolute numbers the second depends on context. Haensel's absolute value seem to have been roughly 10 times too high for spinach in comparison with other measurements, but the unexceptional place of spinach within Haensel's data set was correct.

Saturday, 1 August 2015

Historic sources of the spinach-iron myth: Wolff 1880

[Click on the tab "spinach" to get all posts in this series.
Update: Was able to reconstruct Wolff's data transformation from his systematic tables (e.g., Wolff 1880, p. 128) to his summary tables (e.g., Wolff 1880, p. 147) eventually. It's rather simple, but the headline above the compounds in the summary tables themselves is misleading.]

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 (see here) 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.

The second Aschen-Analysen
In 1880, Wolff published a sequel to his Aschen-Analysen von land- und forstwitschaftlichen Producten. (See here and here for reconstructions concerning the data treatments of his first Aschen-Analysen.)  Much of the second Aschen-Analysen is made up of the same data as those given in the first (Wolff 1871). However, the tables have been augmented with new data from 1870 to 1880, and their arrangement has changed. Or so Wolff (1880) claims in the preface.

As far as spinach is concerned, the second Aschen-Analysen contain no new data in its part I, but instead of separately listing the results of the two original analyses, the averages of these two analyses are given. The two original analyses were from 1st: Saalmüller (1846. "Analysen der Asche von Spinacea oleracea." Justus Liebigs Annalen der Chemie und Pharmacie 58: 389.) and 2nd: Richardson (1848. "Beiträge zur chemischen Kenntnis der Vegetabilien." Justus Liebigs Annalen der Chemie und Pharmacie 67(3), without page number [erroneously attached to the last separately listed article in that issue, see here.])

Here is an excerpt of the table from Wolff (1871, p. 101) listing the results separately:

Lines 51 and 52 list the results of Saalmüller (1846) and Richardson (1848) on spinach respectively.

And here is the excerpt from Wolff (1880, p. 128) giving the averages of the above: 
Besides the fact that some of the chemical species have been improved (for example, those for potassium oxide and sodium oxide), the second column ("Anzahl der Anal.") gives the number of analyses that have been averaged. As you can see, every value for spinach is the average of values given in Wolff (1871, p. 101, see above). Only the value 6.20 for Cl should be a miscount, because (7.78+4.81)/2 = 6.23.

Wolff's sundry summary tables
From page 119 onwards, Wolff (1880, pt. II) gave "Allerlei Uebersichts-Tabellen" (sundry summary tables). These form part II of the second Aschen-Analysen from 1880. The first Aschen-Analysen from 1871 also came in two parts with sundry summary tables forming part II, but for some reason spinach was not listed in these summary tables from 1871.

In 1880 spinach does occur in the summary tables. As mentioned somewhere before, the pure ash is now given as 164.8 parts in 1000 dry matter instead of 16.48 in 100. The line above the compounds still says "in 100 parts of the pure ash," but each value has been multiplied by the portion of pure ash in the dry matter. That is, the values are truly giving the content in 1000 parts of dry matter rather than in 100 parts of pure ash.

For example, 3.35% or 0.0335 was the average portion of Fe2O3 in the pure ash of spinach; 16.48% or 0.1648 was the average portion of pure ash in the dry matter. Hence the portion of Fe2O3 in the dry matter was: 0.0335 x 0.1648 = 0.00552 or 5.52 in 1000 parts dry matter. The values for the other compounds can be transformed similarly, or simply all values can be multiplied by the factor 1.648 to get from the values in the table at page 128 to the ones at page 147 in Wolff (1880). Wolff transformed the values for other vegetables similarly, only the factors were different depending on the average portion of pure ash in the dry matter.
[18 lines omitted.]
This transformation is not obvious from the information given by Wolff (1880). Particularly misleading is the line above the compounds stating that they were put in relation to 100 parts pure ash, where truly they were in relation to 1000 parts dry matter.

As already discussed in a previous entry, this is an illegitimate transformation, because the ash contains oxygen and its value must be corrected accordingly. Given Wolff's chemical species (K2O, Na2O, CaO, MgO, Fe2O3, P2O5, SO3, SiO2, Cl) the ash contains 37.27% and Fe2O3 contains 30.06% oxygen. Correcting their values yields 0.1648 * 0.6273 = 0.1034 for the ash without oxygen, 0.0335 * 0.6994 = 0.0224 for Fe, and that in turn yields 0.1034 * 0.0224 = 0.0023 Fe in the dry matter or 2.3 Fe in 1000 parts dry matter. Wolff's 5.52 Fe2O3 in 1000 parts dry matter would instead yield 3.86 (= 5.52 *0.6694). Hence, if we ignore inaccuracies in Wolff's data due to the methods of the time, Wolff (1880, p. 147) was 1.7 times too high for Fe in spinach. Alas, his illegitimate data transformation affected all ash compounds for all vegetables and other products listed.

Postscript
Although the 5.52 Fe2O3 in 1000 parts of Wolff (1880, p. 147) could be mistaken for the 0.5g Fe in 100g dry matter that Bunge (1892) has claimed Wolff to have given, Bunge has not taken this average value, because (1.) he spoke of one of two analyses, (2.) he mentioned that the analysis was based on data by Richardson (1848), and he calculated the content of 0.5g Fe rather than Fe2O3 in 100g dry matter (see here). Otherwise, it would have been a data handling error (mistaking Fe2O3 with Fe) just the same.

Friday, 31 July 2015

Historic sources of the spinach-iron myth: Wolff 1871 & Bunge 1892

[Click on the tab "spinach" to get all posts in this series.]

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 (see here) 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.

Wolff (1871)
We begin with a collection of analyses of the compounds of ashes from various sources, including vegetables, published by Emil Theodor von Wolff in 1871: Aschen-Analysen von landwirthschaftlichen Producten, Fabrik-Abfällen und wildwachsenden Pflanzen. Berlin: Wiegand & Hempel. [While Wolff (1871) is usually chosen as the starting point for the spinach-iron legend, he stood on the shoulders of others as well (see here).]

Wolff (1871, see table below) gave the content of substances as portions of 100 parts of the pure ash (In 100 Theilen der Reinasche). Pure ash, according to Wolff, was the raw ash (Rohasche) minus the sand and coal (Sand und Kohle) and minus the carbonic acid (Kohlensäure) in it (2nd to 5th column in the table below). Now that already gives me pause to wonder what coal and carbonic acid has to do in ash? I found no indication that the word Asche used to mean anything other than the remains of combustion. This coal-in-ash thing sounds like incomplete combustion to me. Anyway, the first of the Bemerkungen at page 1 of Wolff's publication is also important, because it explains that the amounts of raw or pure ash are again given as portions of 100 parts of dry matter unless otherwise mentioned.

As you can see from the table below (line 51 and 52), the portion of Fe2O3 in spinach was 2.1 or 4,6  parts respectively in 100 parts of pure ash [excuse me if the German decimal separator, a comma, slipped in sometimes]. And the amount of pure ash was given as 16.27 or 16,70 parts, respectively, of 100 parts of dry matter (as the first of the Bemerkungen at page 1 explained). One could state it simpler by saying that the mass of the pure ash equalled 16.27% (or 16,7% respectively) of the mass of the dry matter and that the mass of the Fe2O3 equalled 2,1% (or 4,6% respectively) of the mass of the pure ash in turn.

Page 101 of Wolff (1871). My red underlining.












  
  
Bunge (1892)*
Gustav von Bunge (1892. "Weitere Untersuchungen über die Aufnahme des Eisens in den Organismus des Säuglings." Zeitschrift für Physiologische Chemie 16:173-186) said that he was prompted to measure the iron content of wild strawberries (footnote 2 at p. 180) and spinach (footnote 1 at p. 181), because of the strikingly high iron contents, which Wolff has given in his Aschenanalysen on the base of an analysis of Richardson. Bunge (1892, p. 181) concludes that Wolff's data were 15 times too high for strawberries and 16 times too high for spinach. Bunge's figure for the iron content in wild strawberries calculated from Wolff, though poorly legible, should read 0.14 Fe in 100 dry berries ("0,14 auf 100 trockener Beeren!" Bunge 1892, 180-181) as I will show below.  

[* Bunge was not really interested in the iron content of vegetables. His point of departure was the finding that the milk of mammals contained very little amounts of iron compounds, yet young mammals needed a lot of iron for the growing while suckling. The solution to this riddle was that mammals are born with a stock of iron compounds sufficient for the growing until they wean.]

Bunge (1892, footnotes at pp 180, 181). Footnote 2 from page 180 runs into page 181.
These footnotes leave no doubt that Bunge (1892) has treated data from the first Aschen-Analysen (Wolff 1871) and not from his second (Wolff 1880). Firstly, Bunge speaks of one of two analyses of spinach listed by Wolff. In 1880, however, Wolff had no longer given several lines of evidence but averages instead. Secondly, Bunge mentions that Wolff's data were based on an analyses by Richardson (Ann d. Chem. u. Pharm., Bd 67, Heft 3, 1848). Whereas Wolff (1871) referred to his at the bottom of each page (see above), Wolff (1880) no longer did so. Finally, the reconstruction below shows a neat fit between Wolff (1871) and Bunge (1892). 

Bunge's treatment of Wolff's data
Bunge took Wolff's data and calculate the iron content in the dry matter of a vegetable by correcting the data for the oxygen from the air in the iron-oxide. The logic is as follows:
Weight of fresh vegetables → get rid of water through desiccation → weigh of dry vegetables →  combust dried vegtables → subtract sand, coal and carbonic acid from raw ash to get the mass of the pure ash → subtract the part that is due to oxygen from the portion of Fe2O3** → calculate the iron content of the dry mass from the thus corrected data.   
One caveat about this logic is that part of the pure ash is also from oxygen from the air (e.g., in magnesia or calcium oxide). That is, the values for pure ash would also need to be corrected.As I will show below, Bunge has not done so.***

[** 30% of the mass of Fe2O3 stems from oxygen, which entered this product from the air during combustion. We know that from the atomic masses of Fe (55.85u) and O (16u). Hence 70% of the molecular mass of Fe2O3 (159.7u) stems from iron (55.85 times 2 = 111.7u) and 30% from oxygen (16u times 3 = 48u).
***As the prequel has shown, some of the original data used by Wolff did not measure the iron content as Fe2O3 but as FePO4 in the first place. That is, the oxygen has not been gained from the air during combustion, but the content of FePO4 has been mathematically transformed into an equivalent of Fe2O3. Nevertheless, if the portion of Fe2O3 is rid of oxygen mathematically, then the value for pure ash will need to be treated likewise.]

Wolff's data for wild strawberries were 5.89 parts Fe2O3 in 100 parts pure ash and 3.40 parts pure ash in 100 parts dry matter (see data for Fragaria vesca in Wolff 1871, p. 127). Correcting these data for the fact that only 70% of the mass of iron oxide is from iron yields: 100 x 3.40% x 4.12% = 0.14 as stated in Bunge's footnote 2 at page 180 (see above, Bunge 1892, footnote 2, p. 180). That Bunge gave the content calculated for wild strawberries without unit seems to be mere whim. Likewise, taking the higher value of Wolff's data on spinach, 4.60 parts Fe2O3 in 100 parts pure ash, and correcting for the oxygen in it yields: 100g x 16.70% x 3.22% = 0.538g. This corresponds to Bunge's statement that, according to one of the two analyses given in Wolff "100gr. der trockenen Blätter" would contain "einen halben Gramm Eisen" (see above, Bunge 1892, footnote 1, p. 181).

The correspondence between the calculation in the previous paragraph and the figures given by Bunge (1892, p. 180, footnote and p. 181, footnote 2) suggest that Bunge did just the same calculations, that is, he multiplied the portion for iron (corrected) times the portion of pure ash (not corrected) times the 100 parts dry weight that Wolff started with. If, however, the portion of pure ash was not corrected for oxygen in its compounds (for example, magnesia oxide), his calculations were necessarily too high.

Can we check how much Bunge was too high? Yes! Look at line 52 in the table from Wolff reproduced above. Wolff gave portions for the following compounds: KO, NaO, CaO, MgO, Fe2O3, PO5, SO3, SiO2 and Cl. And the portions of these compounds add up to 101. That is, the pure ash was supposed to consist of these compounds and the sum is slightly higher than 100 due to rounding. Unfortunately, some of the chemical species given by Wolff do not exist. Potassium oxide comes as K2O, sodium oxide as Na2O, phosphor oxide has several species (e.g., P4O6, P4O10) none of which corresponds to the one given by Wolff, sulfur trioxide is a gas, and Cl does not exist as such. One would also expect silicon oxide to belong to the sand that has been subtracted from the raw ash in order to get the pure ash, but that may have been a matter of method and grain size.

What a mess! Anyway, let's just take Wolff's chemical species for granted, even though they seem strange, and correct the portion of pure ash accordingly. The percentage of the mass that is due to oxygen in these compounds would be: 29% for KO, 41% for NaO, 29% for CaO, 40% for MgO, 30% for Fe2O3, 72% for PO5, 60% for SO3, 53% for SiO2 and 0% Cl. That yields the following corrected portions of these compounds: 6.88 for K, 23.10 for Na, 9.31 for Ca, 3.17 for Mg, 3.22 for Fe, 3.34 for P, 3.72 for S, 1.49 for Si and 0% Cl. That sums up to 54,14 (instead of 100) meaning that 45.86% of the pure ash were made up of oxygen from the air. Hence the 16.70 parts pure ash given by Wolff (see line 52 in the table above) would reduce to 9.04 parts per 100 parts dry matter. The estimation of the iron content in 100g dry matter would go down to (100g x 0.0904 x 0.0322 =) 0.29g. 

In his own analysis, Bunge (1892, p. 181, main text) found 0.0016g Fe in 4.8893g dry matter of spinach equal to 0.0327g in 100g dry matter.**** Comparing this against the content that he had calculated from Wolff's data, he concluded Wolff's data to be 16 times too high (our check: 0.538g/0.0327g = 16.44). In comparison with this, the estimate calculated from Wolff's data with corrected value for the pure ash would no longer be 16 times too high, be roughly 9 times too high (0.29/0.0327 = 8.87).

[****Bunge (1892, p. 174) found his value of 0.0327 or 32.7mg in 100g dry matter corroborated in a similar value of 39.1mg/100g dry matter that he cited from Boussingault (1872. "Du fer contenue dans le sang et dans les aliments." Comptes Rendus de l'Académie des Sciences 74: 1353-9). Boussingault (1872) gave 0.0045g for "Feuilles d'épinards" at page 1356 and the specification at page 1355 that his values were "Fer exprimé à l'état métallique dans 100 grammes de matière." Bunge (1892) seems to have taken that as a value for fresh matter and, as stated at p. 173, he has transformed values for fresh matter to dry matter according to J. König (1889. "Chemie der menschlichen Nahrungsmittel."). That, in turn, implies that König has given a water content of 88.49%.] 

Conclusion
Almost half (9/16) of the discrepancy that Bunge found between his own experimental findings and Wolff's data were due to Bunge's false treatment of Wolff's data. In particular, Bunge forgot to rid the value for the pure ash from the oxygen gained during combustion from the air. The same should be true for the value for wild strawberries, which Bunge thought were 15 times too high. But I will leave the fun of getting a periodic table of elements and doing the calculation to the reader.