Anti-Oxidant Anti-Nutrients in Nuts
Polyphenol anti-oxidants are thought to increase the retention of vitamin C (though increases in plasma ascorbic acid concentrations may be derived from endogenous sources), and vitamin C certainly increases the absorption of iron. So, logic surely demands that polyphenols increase the absorption of iron, eh?

Not so fast! Polyphenols are actually potent inhibitors of iron absorption. They are anti-nutrients as well as anti-oxidants. There are more different polyphenols than I care to memorise, but either some are more potent inhibitors than others or absorption is also affected by other factors present in foods, because equal portions of polyphenols from various commonly consumed beverages (e.g. teas, coffees, cocoas) did not inhibit iron absorption to the same extent. All, however, were strong inhibitors, the worst offender of all being black tea, which contains "galloyl esters" like those found in tannic acid (see below). Cocoa has a very high iron content, but it is also high in "polymerisation products" (flavonoid and/or phenolic acid complexes), and was highly inhibitive of iron absorption. Polyphenols can form complexes with proteins rather than iron, and adding protein-rich meat to polyphenol meals has previously antagonised the usual effect, but protein-rich milk had no effect here when added to the beverages.

The polyphenol tannic acid and other plant properties (e.g. phytate, oxalate) were long ago found to adversely affect iron absorption. In this study, absorption of iron from meals containing different plant foods along with 3mg of radiolabelled ferrous sulphate (supplemental iron) and 30mg of vitamin C was greatly reduced in those high in phytic acid (rice, wheat germ, lentils) or oxalate (spinach), but considerably better in those high in vitamin C (e.g. cruciferous vegetables) or certain other organic acids (malic, citric, tartaric). Root vegetables (e.g. carrot, potato, pumpkin) were high in these other acids, but most of the poorly absorbed plant foods were higher in polyphenols. 500mg of the polyphenol tannic acid (as found in acorns) inhibited iron absorption from a broccoli meal considerably more than a whole 2 grams of added phytate.

Nuts are high in both phytate and polyphenols, and a later study by a similar group of researchers on a similar group of Indian women found that iron absorption from walnuts, almonds, hazelnuts and peanuts (but not coconut) was much worse even than that from white bread (60g of white bread along with margarine and 150ml of whole milk, with or without 50g of a nut paste or dessicated coconut). Similar 3mg portions of radiolabelled ferrous sulphate were added to the meals, and 25mg or 50mg of vitamin C were added in certain cases. The peanuts in the main experiment were roasted, but others in a separate experiment were germinated for 5 days, and these plus bread were almost twice as well absorbed as the others plus bread - but both were very poorly absorbed. The iron absorbed from the hazelnut plus bread meals (2.3%) was about half that from bread alone (in the same subjects) - and the absorption from the other nut meals (coconut excluded) was slashed even more radically! Absorption of iron from a meal with brazil nuts was similarly low but was carried out without a bread-only control. Iron absorption from peanut plus bread with 25mg of vitamin C was barely more than half as well absorbed as that from bread without vitamin C, but doubling the vitamin C content of the peanut plus bread meal doubled the absorption of iron relative to the bread-only control (although the iron happened to be only about half as well absorbed from that particular bread-only control meal). Iron absorption from brazils plus bread was tested without and with 25mg and 50mg additions of vitamin C, and increased by c. 55-70% with each increment, but no bread-only control meals were included.

In another set of experiments, vitamin C dose-dependently increased the absorption of iron that had been dose-dependently inhibited by phytate added to bread rolls, and grilled beef also counteracted the inhibition of a large phytate dose (but not a more modest one). In yet more experiments, similar patterns resulted from adding phytate and vitamin C to meals, and the polyphenol tannic acid was also examined. Tannic acid was not as dose-responsive, but its general iron inhibition was considerably greater and had to be improved by much larger doses of vitamin C.

Iron from plant sources is generally much more poorly absorbed than that from animal sources (heme/haem iron), but, as noted above, different elements in different plants can affect absorption differently. Maybe the degree to which the source is isolated from the original rough plant material is also important? Molasses - the not-too-terrible-tasting sucrose-rich substance extracted from sugar-cane/beet or other plants - is high in iron (and various other minerals and vitamins). Is that well absorbed? Judging by the following abstract, the answer is "yes, but ...". Molasses extracted from the grape plant increased iron levels as much in iron-deficient subjects as did supplemental ferrous sulphate in iron-sufficient subjects, but the ferrous sulphate increased iron levels more than twice as much as did the grape-molasses in the iron-deficient subjects.