Spinach: the oxalate story is mostly wrong

Spinach is famous for iron it supposedly locks away behind oxalate. Someone tested that directly with labelled iron: adding 1.26 grams of pure oxalate to a kale meal, matching what a plate of spinach holds, changed absorption from 10.7 percent to 11.5 percent. Nothing. The researchers concluded that oxalic acid in fruit and vegetables is of minor relevance in iron nutrition, and if spinach still gave less iron than kale, the calcium and polyphenols are the likelier reason. The other half of the oxalate worry is kidney stones, and there the arithmetic is stranger still: your own body makes about 24 mg of oxalate a day, and up to three fifths of that comes from turning over vitamin C.

The numbers, per 100 g

What your body absorbs

• Adding as much oxalic acid as a plate of spinach holds changed iron absorption by nothing measurable. — 16 apparently healthy women aged 18 to 45 under 60 kg in Zurich, 13 of whom completed both studies (n = 16 women, 13 completers) Evidence B source
  Iron absorption from bread with kale was 10.7% against 11.5% from bread with kale plus 1.26 g of potassium oxalate (p = 0.86); the spinach meal, with 1.27 g of native oxalic acid, gave 24% less iron than the kale meal, but the difference did not reach significance (p>0.16) and the authors attribute it to the higher calcium and polyphenol content of spinach

What cooking and storage change

• Pressure-cooked spinach came out the lowest-potassium vegetable tested, at 75 milligrams per hundred grams. — 68 cardiology patients at the Institute of Cardiology in Porto Alegre, Brazil, evaluating foods selected from the TACO table for potassium above 201 mg per 100 g (n = 68) Evidence B source
  Spinach cooked under pressure measured 74.85 mg potassium per 100 g and courgette 93.62 mg, the two lowest values recorded; among the lowest group only squash at 237.83 mg stayed above 200 mg

What it does to your health

• Beetroot gave endurance a small measurable lift and red spinach gave none, in the same pooled analysis. — 118 studies, comprising 59 polyphenol studies, 56 nitrate studies and 3 L-citrulline studies (n = 118 trials) Evidence A source
  Nitrate-rich and polyphenol-rich foods gave standardised mean differences of 0.15 and 0.17 (p<0.001), which the authors themselves call trivial; among nitrate foods the benefit was seen for beetroot but not for red spinach, Swiss chard or rhubarb; no effect of nitrate or polyphenols in women, and no nitrate benefit in well-trained men

Safety, limits and interactions

• Spinach shows up on the list of foods reported to shift how warfarin behaves, because of its vitamin K. — 149 articles describing 78 herbs, foods or dietary supplements (n = 149 reports) Evidence D source
  45 of the 78 items (57.7%) were reported to potentiate warfarin, 23 (29.5%) to inhibit it and 10 (12.8%) to have limited impact; 20 items were linked to bleeding events ranging from gum bleeding to fatal intracranial bleeding; spinach appears in the authors' list of items to be aware of

What people believe that the data does not support

• For people who already form calcium stones, the diet that has evidence behind it is normal calcium, less salt and less protein, and cutting oxalate is not on that list. — 31 studies, 26 randomised trials and 5 non-randomised studies of interventions, all but 3 in adults only (n = 31 studies) Evidence C source
  For adults with calcium oxalate or calcium phosphate stones, more water, a diet with normal to high calcium and low protein and low sodium, thiazides, alkali treatment and allopurinol may reduce recurrence, all at low strength of evidence; no study evaluated imaging strategies

What is still unknown

• Most of the oxalate in urine is made by the body, and vitamin C turnover accounts for up to three fifths of it. — 23 eligible articles out of 228 screened, with total endogenous production reported by 7 papers covering 305 healthy subjects (n = 305 subjects across 7 papers) Evidence C source
  Weighted mean endogenous oxalate production 23.8 mg a day (95% CI 23-24.6), range 17 to 34; ascorbic acid turnover accounts for 40% to 60%, hydroxyproline about 15%, glycine under 10%, glycolate about 4%, phenylalanine under 0.7% and fructose negligible

The bottom line

For anyone who forms calcium stones, the 2026 review of everything tested lists what may help: more water, normal to high calcium, less salt, less protein, and certain medicines. Cutting oxalate is not on that list, and every item on it carries low strength of evidence. That is guidance to discuss with the doctor managing your stones rather than a reason to change anything on your own. Two practical notes. Spinach cooked under pressure comes out the lowest potassium vegetable anyone has measured, at 75 mg per 100 grams, which matters if you have been told to limit potassium. And if you take warfarin, spinach is on the list of foods that shift how the drug behaves, where the answer is a steady amount rather than avoidance, decided with whoever adjusts your dose.

Sources

1. spn-dr-01 · randomised crossover stable isotope absorption study
   geometric mean iron absorption from wheat bread rolls with kale (10.7%) did not differ significantly from wheat rolls with kale plus 1.26 g OA added as potassium oxalate (11.5%, P=0.86)...Potassium oxalate did not influence iron absorption in humans from a kale meal and our findings strongly suggest that OA in fruits and vegetables is of minor relevance in iron nutrition.
   Eur J Clin Nutr, 2008 · checked 2026-09-22
2. spn-dr-06 · randomised crossover trial with laboratory analysis
   The vegetables with the lowest potassium values were spinach (74.85 mg/100 g) and zucchini (93.62 mg/100 g), both cooked under pressure.
   Clin Nutr ESPEN, 2025 · checked 2026-09-22
3. spn-dr-05 · systematic review and meta-analysis of randomised controlled trials
   Among nitrate-rich foods, beneficial effects were seen for beetroot, but not red spinach or Swiss chard and rhubarb.
   J Int Soc Sports Nutr, 2021 · checked 2026-09-22
4. spn-dr-04 · systematic review of interaction reports
   These include Chinese wolfberry, chamomile tea, cannabis, cranberry, chitosan, green tea, Ginkgo biloba, ginger, spinach, St. John's Wort, sushi and smoking tobacco.
   Br J Clin Pharmacol, 2021 · checked 2026-09-22
5. spn-dr-03 · systematic review for a clinical guideline
   For adults with calcium oxalate or phosphate stones, increased water intake; a diet with normal to high calcium, low protein, and low sodium; thiazides; alkali treatment; and allopurinol may reduce stone recurrence (low SOE).
   Ann Intern Med, 2026 · checked 2026-09-22
6. spn-dr-02 · systematic review of metabolic studies
   Weighted mean of endogenous oxalate production was 23.8 mg/day (95% CI 23-24.6 mg/day) with a range of 17-34 mg/day. Daily turnover from the metabolic pool of ascorbic acid may account for up to 40% - 60% of total endogenous oxalate production.
   Urolithiasis, 2026 · checked 2026-09-22