Leafy Greens
The short version
Spinach and chard carry oxalates, which lock up much of their own calcium before the body can reach it.
It's widely considered safe at the amounts used in food, so we mark it green. The Safety section below has the details.
Safety profile
Oxalic acid binds calcium into an insoluble salt the gut absorbs poorly, which is why spinach, although rich in calcium, is a poor source of it: in a human study only about 5% of its calcium was absorbed. For iron, oxalic acid appears to be of minor relevance: in a human study, adding it to a kale meal did not reduce iron absorption. Kale, cabbage and broccoli are low in oxalate and their calcium is absorbed well.
Sources
- Oxalic acid does not influence nonhaem iron absorption in humans: a comparison of kale and spinach meals (European Journal of Clinical Nutrition, 2008;62:336–341) — Springer Nature (European Journal of Clinical Nutrition), 2008. adding oxalic acid to a kale meal did not reduce non-haem iron absorption; oxalic acid is of minor relevance to iron absorption
- Calcium Absorption from Food Products: Food Matrix Effects (Nutrients 2022;14(1):180) — MDPI (Nutrients), 2022. oxalate makes spinach calcium poorly absorbed (about 5%); calcium from low-oxalate kale and broccoli is absorbed well; boiling removes more oxalate than steaming
Checked against these sources on .
Educational information only. Not medical, dietary or veterinary advice. Spotted an error? Tell us.