Kale and spinach: the calcium you absorb is the part that counts

Spinach gives up very little of its calcium. In a randomized crossover trial in healthy adults, only 5.1% of the calcium in spinach was absorbed, against 27.6% from milk at the same dose, in single test meals. Kale also has more of it to begin with. USDA samples put raw kale at 254 mg calcium per 3.5 oz (100 g) and baby or mature spinach at 67 to 68 mg, figures for content and not for what the body takes up. We found no trial of calcium absorption from kale in our sources. Iron is a closer contest. Both greens carry roughly 1 to 1.6 mg per 3.5 oz (100 g), and kale samples vary so widely that their median is 1.14 mg. In isotope studies in 16 women, iron from a meal with spinach was absorbed 24% less than from one with kale, a difference that did not reach significance in single meals. The kale and diabetes trial people cite used freeze-dried kale bars at a very high dose in 30 people with type 2 diabetes. HbA1c fell by 1.37 on kale and rose by 1.36 on placebo, with the units not stated and 13 people dropping out.

How much is actually in it

• Raw kale averages 254 mg calcium per 3.5 oz (100 g) in USDA samples, almost four times the 67 to 68 mg in baby or mature spinach. — — (n = 5 kale samples, 8 + 8 spinach samples) Evidence E source
  Kale 254 mg (203-281); baby spinach 68.35 mg (48.2-92.1); mature spinach 66.64 mg (47.3-89.2) per 3.5 oz (100 g)
• Raw kale carries about 93 mg vitamin C per 3.5 oz (100 g) against 27 to 30 mg in spinach, while iron is similar at roughly 1 to 1.6 mg. — — (n = 5 kale samples, 8 + 8 spinach samples) Evidence E source
  Vitamin C kale 93.4 mg, baby spinach 26.52, mature 30.27; iron kale 1.6 mg (0.77-3.61), baby spinach 1.261, mature 1.054 per 3.5 oz (100 g)

What a real portion looks like

• In USDA survey data one cup of raw kale and one cup of raw spinach both weigh 0.88 oz (25 g), and a salad serving is 50 g. — — Evidence E source
  1 cup 25 g; 1 leaf 10 g; salad guideline amount 50 g for both

What your body absorbs

• In a randomized crossover trial in 13 adults, only 5.1% of the calcium in spinach was absorbed versus 27.6% from milk at the same 200 mg calcium dose. — 13 healthy adults, single test meals of milk or spinach with 200 mg labeled calcium (n = 13) Evidence B source
  Absorption 5.1% spinach vs 27.6% milk; within-subject difference 22.5 +/- 9.5% (P < 0.0001)
• In 6 adults who ate 14 oz (400 g) of labeled kale with 30 g oil, about 4.7% of its vitamin K1 was absorbed, peaking in blood 6 to 10 hours later. — 7 healthy volunteers, single 400 g kale serving with 30 g vegetable oil, 6 with detectable label (n = 7) Evidence C source
  Mean bioavailability 4.7% (n 6); peak plasma 2.1 nmol/L at 6-10 h

What cooking and storage change

• In 10 volunteers eating 3.5 oz (100 g) grilled frozen spinach, about 1.9% of its soluble oxalate reached the urine over 24 hours, and eating it with sour cream and calcium-enriched milk lowered that significantly. — 10 volunteers, 100 g grilled frozen spinach alone or with dairy additions or olive oil (n = 10) Evidence B source
  Soluble oxalate bioavailability 0.75 +/- 0.48% at 6 h and 1.93 +/- 0.85% at 24 h; dairy reduced it (P < 0.05)
• Boiling leafy greens in water that is then drained keeps about 55% of vitamin C and 60% of folate but 95% of calcium and iron, by the USDA retention table for vegetables. — — Evidence E source
  Retained: vitamin C 55%, folate 60%, thiamin 80%, potassium 85%, calcium 95%, iron 95%

What it does to your health

• In a 12-week double-blind RCT in 30 people with type 2 diabetes, three freeze-dried kale bars a day (about 341 g fresh kale) lowered HbA1c by 1.37 units while it rose 1.36 in the placebo group. — 30 completers with type 2 diabetes in Denmark, 3 bars a day with 26.25 g freeze-dried kale each, 12 weeks (n = 30) Evidence B source
  HbA1c change -1.368 +/- 0.859 kale vs +1.364 +/- 0.789 placebo

Safety, limits and interactions

• A systematic review of 149 articles on warfarin and food or supplements listed spinach among foods that call for caution in people taking warfarin. — 149 articles describing 78 herbs, foods or supplements, databases to 31 December 2019 (n = 149 articles) Evidence C source
  Spinach named among foods with interaction concern; no effect size
• A California household with raised urine thallium was traced to commercial kale chips that contained about 2 mg of thallium per kg. — one mother and her three young children, California public health investigation (n = 4) Evidence D source
  Kale chips 1.98 and 2.15 mg/kg thallium; child urine peak 10.5 ug/g creatinine vs reference 0.8 or less

What people believe that the data does not support

• The EU did not authorize a claim that a mix of Tuscan black cabbage, Swiss chard, spinach and savoy cabbage protects blood lipids from oxidative damage. — — Evidence E source
  Claim non-authorized

Who this works differently for

• In crossover isotope studies in 16 women, iron absorption from bread eaten with 150 g spinach was 24% lower than with 150 g kale, a difference that did not reach significance, and added oxalate did not change absorption from kale. — 16 healthy women aged 18-45 y under 132 lb (60 kg), 13 completed both studies; two wheat rolls with 150 g spinach or kale (n = 16) Evidence B source
  Spinach meal absorption 24% lower than kale (P > 0.16); kale 10.7% vs kale plus 1.26 g oxalate 11.5% (P = 0.86)

The bottom line

In USDA survey data a cup of either raw green weighs 0.88 oz (25 g), and a salad serving is 50 g. Cooked, the same leaves shrink several-fold. Kale carries more vitamin C, about 93 mg per 3.5 oz (100 g) against 27 to 30 mg for spinach. Boiling greens and draining the water keeps about 55% of the vitamin C and 60% of the folate but 95% of the calcium and iron, by a USDA group factor from 1975 that was never measured on kale or spinach. Adding oxalate to a kale meal, 1.26 g of it, did not change iron absorption in the same isotope studies. In 10 volunteers eating 3.5 oz (100 g) of grilled frozen spinach, about 1.9% of its soluble oxalate reached the urine over 24 hours, measured as urinary oxalate in a small group. Eating it with sour cream and calcium-enriched milk lowered that significantly. In 6 adults, about 4.7% of the vitamin K1 in a 14 oz (400 g) serving of kale eaten with oil was absorbed, from a single meal with no comparison group. If you take warfarin, raise spinach and kale with whoever manages your dose. A systematic review listed spinach among foods that call for caution with warfarin, gave no effect size, and mixed case reports into its grading. Kale was not named in that abstract, and that does not show kale is safe with warfarin: kale carries vitamin K1, as the absorption study above shows. A California household with raised urine thallium was traced to commercial kale chips at about 2 mg of thallium per kg, a single household and one product, and drying concentrates metals. We found no studies of either green in pregnancy or kidney disease, so this page cannot speak for those groups. The EU refused to authorize a claim that a mix of Tuscan black cabbage, Swiss chard, spinach and savoy cabbage protects blood lipids from oxidative damage.

Sources

1. kxs-daily-r4-09 · food composition data
   Kale, raw: Calcium 254.0 MG, range 203.0-281.0, 5 samples; Spinach, baby: Calcium 68.35 MG, range 48.2-92.1, 8 samples; Spinach, mature: Calcium 66.64 MG, range 47.3-89.2, 8 samples
   USDA FoodData Central, Foundation Foods, release 2026-04-30 · checked 2026-10-02
2. kxs-daily-r4-10 · food composition data
   Kale, raw: Vitamin C 93.4 MG (84.4-104.0, 5 samples), Iron 1.6 MG (0.77-3.61, median 1.14, 5 samples); Spinach, baby: Vitamin C 26.52 MG, Iron 1.261 MG (8 samples); Spinach, mature: Vitamin C 30.27 MG, Iron 1.054 MG (8 samples)
   USDA FoodData Central, Foundation Foods, release 2026-04-30 · checked 2026-10-02
3. kxs-daily-r4-12 · portion data
   Kale, raw, FNDDS code 72119190, fdc_id 2709599: 1 cup 25 g; 1 leaf 10 g; Guideline amount in salad 50 g. Spinach, raw, FNDDS code 72125100, fdc_id 2709614: 1 cup 25 g; 1 leaf 10 g; Guideline amount in salad 50 g
   USDA FNDDS 2021-2023, food codes 72119190 Kale, raw (FDC 2709599) and 72125100 Spinach, raw (FDC 2709614) · checked 2026-10-02
4. kxs-daily-r4-02 · randomized crossover trial
   Absorption was higher from milk in every case, with the mean absorption from milk averaging 27.6% and from spinach, 5.1%. The mean within-subject difference between Ca absorption from milk and from spinach was 22.5 +/- 9.5% (P less than 0.0001).
   Am J Clin Nutr, 1988 · checked 2026-10-02
5. kxs-daily-r4-04 · controlled clinical trial
   Modelling results demonstrated a mean (n 6) bioavailability of phylloquinone from kale to be 4.7%.
   Br J Nutr, 2010 · checked 2026-10-02
6. kxs-daily-r4-03 · randomized crossover trial
   When frozen convenience food is grilled there is no opportunity for the soluble oxalates to be leached out into the cooking water and discarded. ... The mean bioavailability of soluble oxalate in the grilled spinach was 0.75+/-0.48% over a 6-hour period after intake and was 1.93+/-0.85% measured over a 24-hour period.
   Asia Pac J Clin Nutr, 2003 · checked 2026-10-02
7. kxs-daily-r4-11 · nutrient retention data
   USDA-RETN-R6:3005 VEG,GREENS,BOILED,WATER COVER DRAIN: vitamin_c_mg 55%; folate_total_ug 60%; thiamin_mg 80%; potassium_mg 85%; calcium_mg 95%; iron_mg 95%
   USDA Table of Nutrient Retention Factors, Release 6 (2007), code 3005 VEG,GREENS,BOILED,WATER COVER DRAIN · checked 2026-10-02
8. kxs-daily-r4-05 · randomized controlled trial
   Thirty participants completed the 12 weeks of intervention, with thirteen participants dropping out (Figure 2). ... A decrease of −1.368 ± 0.8591 was noted in the HbA1c of the intervention group from the start to end of this study compared to control, where HbA1c was shown to increase by 1.364 ± 0.7894.
   Nutrients, 2024 · checked 2026-10-02
9. kxs-daily-r4-06 · systematic review
   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 2026-10-02
10. kxs-daily-r4-07 · case report
    ICP-MS analysis measured thallium in kale chips at 1.98 mg/kg and 2.15 mg/kg
    Int J Environ Res Public Health, 2025 · checked 2026-10-02
11. kxs-daily-r4-08 · EU health claim register
    POL-HC-8440 A combination of Tuscan black cabbage, ’tri-coloured‘ Swiss chard, ’bi-coloured‘ spinach and ’blu savoy‘ cabbage Contributes to the protection of blood lipids from oxidative damage -/- Non-authorised
    EU Register on nutrition and health claims, claim POL-HC-8440, snapshot 2026-09-21 · checked 2026-10-02
12. kxs-daily-r4-01 · randomized crossover trial
    After normalization for the spinach reference meal absorption, 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). Spinach was significantly higher in calcium and polyphenols than kale and absorption from the spinach meal was 24% lower compared to the kale meal without added OA, but the difference did not reach statistical significance (P>0.16).
    Eur J Clin Nutr, 2008 · checked 2026-10-02