Collard greens and spinach: the calcium you read and the calcium you absorb

Spinach has calcium on paper and gives little of it to the body. In a randomized crossover isotope study of 13 healthy adults given the same 200 mg calcium load, absorption averaged 5.1% from spinach against 27.6% from milk, after single test meals. In a second crossover in 20 women, spinach calcium was absorbed half as well as the same load given as calcium oxalate. On the label collards look stronger. USDA samples put raw collards at 276.4 mg calcium per 3.5 oz (100 g) against 66.6 mg in raw mature spinach, 8 samples each. We found no human study that measured how much calcium the body takes from collards, and no study comparing the two greens directly. NIH lists collards among high-oxalate foods, without a number. If you take warfarin, read the caution below: in a small study, daily spinach for a week tended to push clotting tests above the therapeutic limit.

How much is actually in it

• USDA Foundation Foods data put calcium at 276.4 mg per 3.5 oz (100 g) in raw collards versus 66.6 mg in raw mature spinach, each from 8 samples. — USDA Foundation Foods samples, raw collards and raw mature spinach (n = 8 samples each) Evidence E source
  Calcium 276.4 mg/100 g collards (range 170-340) vs 66.64 mg/100 g mature spinach (range 47.3-89.2); iron 0.75 vs 1.05 mg; vitamin C 89.4 vs 30.3 mg
• UK CoFID lists raw mature spinach at 170 mg calcium and 2.10 mg iron per 3.5 oz (100 g), far above the USDA Foundation means of 66.6 mg and 1.05 mg. — UK composition table, raw mature spinach Evidence E source
  Calcium 170 mg, iron 2.10 mg, folate 114 ug, vitamin C 26 mg per 3.5 oz (100 g)

What a real portion looks like

• In the USDA survey database, 1 cup of raw spinach weighs 25 g, 1 cup of raw collards 36 g, and 1 cup of cooked collards 130 g. — US dietary survey portion weights Evidence E source
  1 cup: raw spinach 25 g; raw collards 36 g; cooked collards, no added fat 130 g

What your body absorbs

• In a randomized crossover study of 20 women, spinach calcium was absorbed only half as well as the same calcium load given as calcium oxalate. — Healthy adult women, 200-300 mg calcium loads; 20 in crossover, 14 in label-exchange arm (n = 20) Evidence B source
  Spinach calcium about half the absorbability of calcium oxalate; intrinsic label absorption 0.029 +/- 0.023
• In 5 men fed 3.5 oz (100 g) deuterium-labeled collard greens with a 24 g fat breakfast, plasma phylloquinone peaked at 6 to 9 hours and returned to baseline by 24 hours. — 5 men aged 26-71, single 100 g collard greens bolus (396 ug phylloquinone) (n = 5) Evidence C source
  Plasma total phylloquinone peak 10.51 +/- 4.38 to 8.30 +/- 4.64 nmol/L at 6-9 h; baseline 1.26 nmol/L by 24 h

What cooking and storage change

• By the USDA retention table for boiled, drained leafy greens, about 65% of folate, 90% of potassium and 95% of calcium, iron and magnesium remain. — Vegetables and Vegetable Products, greens boiled in little water and drained Evidence E source
  Retention folate 65%, thiamin 85%, potassium 90%, calcium 95%, iron 95%, magnesium 95%

What it does to your health

• Across two cohorts of about 250,000 adults, moderate plant-sourced nitrate intake was linked to 16% lower rates of severe fatty liver disease. — 53,854 Danish DCH and 197,198 UK Biobank participants, 13.5-25.1 years follow-up (n = 251052) Evidence C source
  Pooled HR Q3 vs Q1 0.84 (95% CI 0.76-0.94) for incident severe MASLD

Safety, limits and interactions

• In 10 volunteers eating 3.5 oz (100 g) grilled frozen spinach, only 1.93% of its soluble oxalate reached urine over 24 hours, and adding sour cream with calcium-enriched milk lowered it further. — 10 volunteers, single test meals of 100 g grilled spinach with or without dairy (n = 10) Evidence B source
  Soluble oxalate bioavailability 0.75 +/- 0.48% at 6 h and 1.93 +/- 0.85% at 24 h; dairy additions reduced it significantly (P<0.05)
• A meta-analysis of 35 studies found spinach from wastewater-irrigated fields had a pooled cadmium mean of 2.86 mg/kg, over 9.5 times the EU limit. — Crops irrigated with wastewater, 35 studies 2000-2021 (n = 35 studies) Evidence C source
  Spinach pooled mean Cd 2.86 mg/kg vs EU limit 0.2-0.3 mg/kg

What people believe that the data does not support

• In a randomized crossover isotope study of 13 adults, calcium absorption averaged 5.1% from spinach versus 27.6% from milk at the same 200 mg calcium load. — 13 healthy adults, single test meals, randomized crossover (n = 13) Evidence B source
  Mean absorption 5.1% spinach vs 27.6% milk; within-subject difference 22.5 +/- 9.5 percentage points, P<0.0001
• In stable-isotope crossover studies in 16 young women, iron absorption from a spinach meal was 24% lower than from a kale meal, a difference that was not significant, and added oxalate did not change absorption. — 16 healthy women aged 18-45, under 132 lb (60 kg); 13 completed both studies (n = 16) Evidence B source
  Kale meal 10.7% vs kale plus 1.26 g oxalate 11.5% (P=0.86); spinach meal 24% lower than kale, P>0.16
• According to NIH ODS, calcium absorption is about 5% from spinach versus 27% from milk, and collard greens are also listed among high-oxalate foods. — General population Evidence E source
  5% spinach vs 27% milk calcium absorption

Who this works differently for

• In stably anticoagulated healthy volunteers, vitamin K1 at 150 ug/day for a week significantly lowered INR, while spinach and broccoli meals gave only short-lived responses. — Healthy volunteers stably anticoagulated for 13 weeks on individualized doses Evidence C source
  Threshold 150 ug/day K1 lowered INR significantly; clinically relevant change in 25% at that dose
• In patients on stable warfarin, a single 250 g spinach meal kept coagulation values in range, but daily spinach for a week tended to push them above the therapeutic limit. — Patients on warfarin for deep venous thrombosis or pulmonary embolism Evidence C source
  Single 250 g spinach: no out-of-range Thrombotest; daily for one week: values tended to rise above limit, requiring dose adjustment
• A case report describes an 8-year-old boy fed blenderized meals heavy in spinach and nuts who developed oxalate nephropathy with rising creatinine. — One 8-year-old boy dependent on pureed feeds (n = 1) Evidence D source
  Serum creatinine 1.8 mg/dL at presentation, 2.1 mg/dL after dietary change

What is still unknown

• In a randomized four-way crossover of 24 adults, native folate from spinach had the highest relative bioavailability of the foods tested, ahead of wheat germ and Camembert. — 24 adults, single doses after folic acid saturation, 14-day washouts (n = 24) Evidence B source
  Highest AUC and Cmax ratio for spinach among test foods; no pooled percentage given in abstract
• According to NIH ODS, the body absorbs only 4% to 17% as much phylloquinone from spinach as from a tablet, and data on vitamin K bioavailability from food are very limited. — General population Evidence E source
  4-17% relative to tablet

The bottom line

Composition tables disagree on spinach. The UK table gives raw mature spinach 170 mg calcium and 2.10 mg iron per 3.5 oz (100 g), against the USDA means of 66.6 mg and 1.05 mg, and variety, season and lab method all play a part. Raw collards carry 89.4 mg vitamin C per 3.5 oz (100 g), spinach 30.3 mg. A cup of raw spinach weighs 0.88 oz (25 g), a cup of raw collards 36 g and a cup of cooked collards 130 g, so a cooked cup holds much more leaf. Boiling greens in little water and draining keeps about 95% of calcium, iron and magnesium and about 65% of folate, by a USDA factor for the whole group. Other nutrients tell a mixed story. In 24 adults, native folate from spinach had the highest relative bioavailability of the foods tested, ahead of wheat germ and Camembert. In 16 young women, iron absorption from a spinach meal was 24% lower than from a kale meal, a gap that did not reach significance, and added oxalate did not change iron absorption from kale. By the NIH summary, the body takes in 4% to 17% as much vitamin K from spinach as from a tablet. After 3.5 oz (100 g) of labeled collards eaten with fat by 5 men, blood vitamin K peaked at 6 to 9 hours and was back to baseline by 24 hours. Two cohorts of about 250,000 adults linked moderate plant nitrate intake with 16% lower rates of severe fatty liver disease (HR 0.84, 0.76 to 0.94), an association with nitrate, not a measured effect of spinach. Be careful if you take warfarin. In patients on stable warfarin, one 250 g spinach meal kept clotting tests in range, and daily spinach for a week tended to push them above the therapeutic limit, in a small 1986 study. In anticoagulated volunteers, vitamin K1 at 150 mcg a day lowered INR, while spinach and broccoli meals gave only short-lived responses. Keep your greens steady and tell whoever doses your warfarin. Oxalate is the second caution. In 10 volunteers, 1.93% of the soluble oxalate in 3.5 oz (100 g) of grilled spinach reached the urine over 24 hours, and sour cream with calcium-enriched milk lowered that. One case report describes an 8 year old on blended feeds heavy in spinach and nuts who developed oxalate kidney damage. Spinach grown with untreated wastewater averaged 2.86 mg/kg cadmium across 35 studies, over 9.5 times the EU limit, a risk tied to that irrigation.

Sources

1. cgsp-daily-r8-14 · Food composition data
   Collards, raw: Calcium, Ca 276.4 MG, range 170.0-340.0, 8 samples; Spinach, mature: Calcium, Ca 66.64 MG, range 47.3-89.2, 8 samples (FDC Foundation Foods, release 2026-04-30)
   USDA FoodData Central, Foundation Foods, release 2026-04-30 (Collards, raw; Spinach, mature) · checked 2026-09-30
2. cgsp-daily-r8-15 · Food composition data
   Spinach, mature, raw (CoFID 13-572): Calcium 170 mg; Iron 2.10 mg; Folate 114 ug; Vitamin C 26 mg per 100 g
   CoFID 2021, Public Health England, code 13-572 (Spinach, mature, raw), OGL v3.0 · checked 2026-09-30
3. cgsp-daily-r8-16 · Portion data
   Spinach, raw 1 cup 25 g (FNDDS 72125100, fdc_id 2709614); Collards, raw 1 cup 36 g (FNDDS 72107100, fdc_id 2709576); Collards, fresh, cooked, no added fat 1 cup 130 g (FNDDS 72107211, fdc_id 2709577)
   USDA FNDDS (codes 72125100, 72107100, 72107211) · checked 2026-09-30
4. cgsp-daily-r8-02 · Randomized crossover trial
   We first confirmed our previous observation of superior absorbability of calcium oxalate over spinach calcium in a randomized cross-over design in 20 women. Spinach calcium exhibited only half the absorbability of the same load of calcium presented as the oxalate.
   Calcif Tissue Int, 1991 · checked 2026-09-30
5. cgsp-daily-r8-06 · Stable isotope kinetic study
   A single bolus of 100 g (396 +/- 28 microg phylloquinone) deuterium-labeled collard greens was fed with a breakfast containing 24 g fat to 5 men (26 to 71 years)... Plasma total phylloquinone concentrations peaked at 6 to 9 hours (10.51 +/- 4.38 to 8.30 +/- 4.64 nmol/L) and returned to baseline by 24 hours (1.26 +/- 0.38 nmol/L).
   Metabolism, 2004 · checked 2026-09-30
6. cgsp-daily-r8-17 · Retention factors
   USDA-RETN-R6:3004 VEG,GREENS,BOILD,LITTLE WATER DRAIN: folate_total_ug 65%; thiamin_mg 85%; potassium_mg 90%; calcium_mg 95%; iron_mg 95%; magnesium_mg 95%
   USDA Table of Nutrient Retention Factors, Release 6 (2007), code 3004 · checked 2026-09-30
7. cgsp-daily-r8-11 · Prospective cohort
   Moderate plant-sourced nitrate intake was associated with lower MASLD rates [pooled HRQ3vsQ1 (95%CI): 0.84 (0.76-0.94)] with similar patterns for plant- and vegetable-sourced nitrite.
   Clin Nutr, 2026 · checked 2026-09-30
8. cgsp-daily-r8-03 · Randomized crossover trial
   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. Addition of sour cream and Calci-Trim milk reduced the availability of the oxalate in the spinach significantly (P<0.05) in both the 6-hour and 24-hour collection periods.
   Asia Pac J Clin Nutr, 2003 · checked 2026-09-30
9. cgsp-daily-r8-10 · Meta-analysis (contamination)
   Certain species show extreme bioaccumulation, with Jew mallow (41.35 mg/kg), beet (23.19 mg/kg), and spinach (pooled mean 2.86 mg/kg) exceeding the EU safety limit (0.2-0.3 mg/kg) by factors of > 130, > 77, and > 9.5, respectively.
   Environ Monit Assess, 2026 · checked 2026-09-30
10. cgsp-daily-r8-01 · 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 2026-09-30
11. cgsp-daily-r8-04 · 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 2026-09-30
12. cgsp-daily-r8-12 · Agency fact sheet
    As a result, absorption of calcium is only 5% for spinach, whereas it is much higher, at 27%, for milk [3]. In addition to spinach, foods with high levels of oxalic acid include collard greens, sweet potatoes, rhubarb, and beans [1].
    NIH Office of Dietary Supplements, Calcium fact sheet for health professionals · checked 2026-09-30
13. cgsp-daily-r8-07 · Dose-response study
    The threshold K(1) dose causing a statistically significant lowering of the INR was 150 microg/day. In 25% of the participants the INR change was regarded as clinically relevant at a vitamin K intake of 150 microg/day... The short-lived response after meals of spinach and broccoli suggested an inefficient bioavailability from these 2 sources.
    Blood, 2004 · checked 2026-09-30
14. cgsp-daily-r8-08 · Clinical study
    Single administration of 250 micrograms vitamin K1, 250 g spinach, 250 g broccoli and 37.5 cl wine did not result in TT-values outside the therapeutic range. However, when Konakion, broccoli and spinach were given daily during one week the TT-values tended to rise above the therapeutic limit, requiring dose adjustment.
    Acta Med Scand, 1986 · checked 2026-09-30
15. cgsp-daily-r8-09 · Case report
    The patient depended on pureed foods for nutrition, and his mother consistently used a large amount of spinach and nuts when preparing his blenderized feeds. Hence, his dietary intake of oxalate was extremely high. After dietary modification, his serum and urinary oxalate levels decreased, but his serum creatinine levels had increased to 2.1 mg/dL.
    Kidney Med, 2025 · checked 2026-09-30
16. cgsp-daily-r8-05 · Randomized crossover trial
    When comparing the ratio estimates of AUC and C(max) for the different test foods, highest bioavailability was found for spinach followed by that for wheat germs and Camembert cheese. The results underline the dependence of folate bioavailability on the type of food ingested. Therefore, the general assumption of 50% bioavailability as the rationale behind the definition of folate equivalents has to be questioned and requires further investigation.
    Food Funct, 2015 · checked 2026-09-30
17. cgsp-daily-r8-13 · Agency fact sheet
    Data on the bioavailability of different forms of vitamin K from food are very limited [1]... For example, the body absorbs only 4% to 17% as much phylloquinone from spinach as from a tablet [2].
    NIH Office of Dietary Supplements, Vitamin K fact sheet for health professionals · checked 2026-09-30