Spinach, oxalic acid content

Although there was decreased retention of body calcium when oxalic acid was added to the diet, the effect was not as drastic as when spinach was fed. The authors concluded that this adverse effect of spinach was at least partly due to its oxalic acid content. 

The reason for the differences in zinc balances between these two studies was not clear. In an effort to determine if the oxalic acid in spinach in the first study could be responsible for at least part of the decrease in bioavailability of zinc in the first study, we determined oxalic acid content of feces. Mean oxalic acid excretion in feces when the subjects were on the highest fiber diet in study 2 was about 1/2 that on the higher fiber diet in study 1 (210 and 423 mg/day, respectively). Another factor which might help explain the results was the length of the dietary periods the dietary periods in study 1 were 5 days longer than in study 2. 

Oxalic acid—Oxalic acid, a compound present in certain leafy plants, may interfere with calcium absorption. The acid precipitates calcium and renders it less available for absorption. Spinach has a high oxalic acid content, which may subsequently tie up substantial portions of Its calcium. 

Vegetables also contain carboxylic acids that contribute to their flavors. One example is oxalic acid, prevalent in spinach and rhubarb. Raw rhubarb leaves are mildly poisonous, and folklore holds that the toxic substance is oxalic acid. However, raw spinach can be eaten safely despite its equally high content of this acid. 

When rats were fed for a longer period of time, there was a high death rate among those fed spinach. There was considerable oxalic acid in the feces of the rats fed spinach. Results of these studies could not be explained on the basis of the crude fiber content of spinach. 

Spiers (18) reported that the feeding of 14% dried New England spinach in the diet resulted in decreased growth and less calcium retention, as determined by body calcium content (Figure 3). Rats 28 days old were fed until 60 days of age. All diets contained about 0.3% calcium, and in the control diet practically all of the calcium was from skim milk. In the other diets, one-half of the skim milk was replaced by turnip greens, tendergreens, collards, kale, or New Zealand spinach to furnish an equal amount of calcium. Spinach was the only one of the greens fed which contained considerable oxalic acid. The calcium of the spinach appeared not only to be utilized poorly, but spinach also impaired utilization of the milk calcium. Food intake of the rats on the spinach diet was also lower. 

Table 2.4 shows the iron contents in a number of food products. Among vegetables, spinach contains a relatively high amount of iron, but it is by no means outstanding. From 1935, it is also known that the human body can utilize very little of the metal because of the oxalic acid (—>4.15) in spinach. The authors of the related scientific publication actually made a funny reference to Popeye, but they did not say that Popeye ate spinach because of iron, and also did not mention the alleged mistake with the decimal separator. 

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