Minutus

Oxalic acid warning: it is not just for people with kidney disorders.
It binds with iron (as well as calcium, magnesium, and potassium) in the intestines, thus decreasing the absorption of these nutrients. So as a vegetarian, the more spinach you eat the less iron you could be absorbing, even if you have a fully functional kidney.
Along with oxalic acid, these substances can also decrease iron absorption:
· Phytic acid (found in a lot of nuts, grains, and legumes)

· Tannins and polyphenols (such as found in tea)

· Calcium

· Junk foods
Coleen

Oxalic acid warning … It binds with iron (as well as calcium, magnesium, and potassium) in the intestines, thus decreasing the absorption of these nutrients.

although this is true in vitro (in the test tube), other factors going on in vivo (in the complex system of the living organism) render this much less of an issue.
one will see this caution frequently-mentioned, even drawn out to suggest that e.g. the iron in spinach is unavailable to the body, purely on the iatrochemical reductionist explanation, rather than on a basis of evidence in the living organism.
back to the iatrochemical reductionist perspective, but expanding this to include a broader appreciation of complexity, other absorbable chelating agents (e.g., vitamin C), which bind these minerals & facilitate their absorption, largely mitigate this effect; along with other factors (see aph. 1, the lovely footnote) that we don’t fully understand.