Plants are generally a good source of nutrients, particularly minerals, although this is dependent on growing conditions and soil quality. Consuming healthy plants that have been grown in fertile mineral rich soil should provide adequate minerals for the human diet. However, plant foods can be problematic, in so far as the plant cells are surrounded by a cellulose cell wall. Because humans do not synthesise the enzyme cellulase, this wall is impermeable if consumed intact. Cooking the plant foods can break down the cell wall and this is often beneficial to improving nutrient absorption from plant foods. Chewing also breaks open the cell wall, but is generally not as effective as cooking because not all of the cells are lysed. Gut bacteria do secrete cellulase, but absorption of nutrients from the colon is limited and so it is nutritionally important for plant foods to be cooked or thoroughly chewed.
However, some plant foods contain substances that may inhibit the absorption of certain nutrients in humans, and therefore the nutrient content of the food is not representative of the actual absorbed portion. Such a phenomenon is demonstrated by the calcium (Ca2+) in spinach, which may not be well absorbed because of the presence of oxalate within the plant tissues. Oxalate may bind to calcium and prevent absorption. For example, one group of researchers1 investigated the absorption of calcium from spinach in comparison to the absorption of calcium from milk in 13 healthy subjects. The results showed that the absorption of calcium from milk was higher than spinach for every subjects, with the mean absorption for milk 27.6 % and for spinach 5.1 %. The researchers reported that the amount of oxalate in the spinach sample was higher than the calcium content, suggesting that most or all of the calcium was possibly bound to oxalate.
Spinach is therefore not a good source of calcium, because the oxalate content binds to the minerals including calcium and causes poor absorption. Oxalate is present in other foods including beans and some evidence suggests that consumption of these foods can limit the absorption of certain minerals. Other research generally supports this viewpoint, and suggests that consuming certain high oxalate plant foods does not increase the mineral supply to the body. Oxalate can combine with other divalent cations including Fe2+ and Mg2+. However, the exact effects of oxalate are controversial, but it is though that it combines with calcium and creates an insoluble product that decreases absorption to around 30 % of the amount ingested. Tea is also a good source of oxalate and the average diet containing 5 cups of tea per day would supply up to 150 mg oxalate which could inactivate around d70 mg of calcium (or other divalent cations) from the diet.
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