How does oxalic acid in spinach compare to other leafy greens and how much does cooking reduce it?
Executive summary
Spinach is among the highest-oxalate leafy greens and can contribute a large share of dietary oxalate compared with kale, collards or bok choy, which register far lower amounts [1] [2] [3]. Cooking—especially boiling or blanching and discarding the cooking water—typically reduces oxalate content substantially (roughly one-third to many-fold depending on method and study), but methods, measurement techniques and trade‑offs (nutrient losses) vary across the literature [4] [5] [6].
1. How spinach stacks up: oxalate content vs. other greens
Multiple compositional surveys put spinach, Swiss chard, beet greens and sorrel in a high-oxalate category, with spinach repeatedly highlighted as one of the top contributors to dietary oxalate; a compilation of foods ranks spinach among the highest leafy sources and WebMD lists a half‑cup of cooked spinach as containing very high oxalate amounts (reported there as 755 mg) [1] [2]. By contrast, commonly eaten greens such as kale, collards and bok choy register very low oxalate figures (kale ~2 mg, bok choy ~1 mg per cup in the WebMD account), so a diet that varies the green types greatly changes total oxalate intake [2] [3]. Nutrition researchers and reviews therefore describe spinach as disproportionately important to oxalate exposure—one analysis even suggests spinach can account for ~40% of oxalate intake in some populations [3].
2. Soluble vs. insoluble oxalate—and why it matters
Not all oxalate in a leaf behaves the same: studies and reviews distinguish total oxalate from the water‑soluble fraction, and those soluble oxalates are the ones that can be absorbed and form calcium‑oxalate kidney stones; the soluble share varies widely between species and studies (soluble fractions reported from ~15% up to >90%) [7] [1]. That variability explains why some greens with similar total oxalate may pose different physiological risks, and why lab methods and reporting matter when comparing foods or evaluating how much cooking helps [1] [8].
3. What cooking does—real reductions and real caveats
Intervention studies and food‑chemistry papers show that water‑based cooking methods reduce oxalate: boiling or blanching and discarding the water commonly cuts oxalate by substantial amounts—many studies report reductions on the order of ~30% and up to 75–90% in some vegetables—while dry‑heat methods generally remove far less [5] [6] [8]. Practical summaries and guides echo that blanching or boiling for a few minutes and throwing away the water typically lowers measured oxalate by about one‑third for many greens [4] [3]. But cooking has trade‑offs: long boiling reduces oxalate more but also degrades vitamin C and some B vitamins and beta‑carotene, so nutrient loss must be weighed against oxalate reduction [6] [9].
4. Disagreement and why reporting can look mixed
Some popular write‑ups and blogs emphasize either that cooking “breaks down” oxalate or that domestic processing makes little difference—these differences reflect varying focus on soluble vs total oxalate, different cooking protocols, and uneven citation of original lab work [10] [11]. Scientific reviews note consistent high‑oxalate species and show reproducible reductions with leaching/boiling, while critics point to studies where blanching or mild cooking produced smaller changes—both perspectives are supported by subsets of the literature, which underlines the methodological sensitivity of measurements [1] [8] [11].
5. Health context and practical takeaways
For most people oxalate from greens is not a health threat and leafy greens remain nutrient‑dense staples, but individuals with a history of calcium‑oxalate kidney stones, altered gut function, or certain medical conditions may need to limit high‑oxalate greens like spinach and chard [12] [3]. Practical strategies supported by the literature are to vary greens (favor kale, collards, arugula when aiming for low oxalate), use boiling/blanching and discard the cooking water to reduce oxalate exposure, and be mindful that heavy boiling lowers heat‑sensitive nutrients—so the approach should be tailored to personal health priorities [3] [4] [6]. Where sources diverge—Dr. Weil and some popular pieces emphasize that cooking mitigates oxalate’s impact on calcium absorption, while other reviews stress oxalate’s mineral‑binding potential—both are drawing on different study types and endpoints, so readers should weigh individual risk and dietary context [13] [10] [6].