Asparagus and spinach: more folate in one, locked calcium in the other

A dose-response meta-analysis of 75 randomized trials in 1,823 people found that each extra millimole of dietary nitrate lowered systolic blood pressure by 0.24 to 0.48 mmHg, depending on trial length, though 70 of its 86 interventions used beetroot and only 5 used spinach. We have no card on how much nitrate a portion of spinach carries, so that figure cannot be turned into a plate. Calcium is where spinach is most misread. In a randomized crossover trial in 13 healthy adults, only 5.1% of the calcium in spinach was absorbed, against 27.6% from milk at the same 200 mg dose, and milk won in every participant. It is one small 1988 study of a single high-oxalate spinach, and it is why the calcium spinach shows on paper counts for less than it looks. Baby spinach holds about 68 mg of calcium per 3.5 oz (100 g) against 21 mg in raw green asparagus, in USDA analysis of 8 samples each. Folate runs the other way. Raw asparagus carries about 183 micrograms per 3.5 oz (100 g) and baby or mature spinach about 115, with wide spread between samples. Portions tilt the comparison further, because a cup of raw spinach weighs 0.88 oz (25 g) and a cup of raw asparagus 135 g.

How much is actually in it

• Raw green asparagus carries more folate than raw spinach: about 183 micrograms per 3.5 oz (100 g) against about 115 for baby or mature spinach, with wide spread between samples of each. — USDA Foundation Foods samples: Asparagus, green, raw; Spinach, baby; Spinach, mature (n = 8 samples per food) Evidence E source
  Folate per 3.5 oz (100 g): asparagus 182.5 ug (range 143.0-206.6); baby spinach 116.5 ug (90.4-155.0); mature spinach 113.4 ug (77.9-149.0)
• Baby spinach holds about three times the calcium of green asparagus per 3.5 oz (100 g). — USDA Foundation Foods samples: Spinach, baby; Asparagus, green, raw (n = 8 samples per food) Evidence E source
  Calcium per 3.5 oz (100 g): baby spinach 68.35 mg (range 48.2-92.1); asparagus 20.58 mg (18.1-23.1)

What a real portion looks like

• A cup of raw spinach weighs only 0.88 oz (25 g), while a cup of raw asparagus weighs 4.8 oz (135 g), so per-cup comparisons favor asparagus by volume. — FNDDS portion weights: Spinach, raw (72125100); Asparagus, raw (75100800) Evidence E source
  Spinach raw: 1 cup 25 g, 1 leaf 10 g, salad amount 50 g; asparagus raw: 1 spear 16 g, 1 cup 135 g, unspecified quantity 68 g

What your body absorbs

• In a randomized crossover trial in 13 healthy adults, calcium from spinach was absorbed at 5.1% versus 27.6% from milk at the same 200 mg calcium dose. — 13 healthy adults, single test meal of milk or spinach with 200 mg labeled calcium (n = 13) Evidence B source
  Mean absorption 5.1% from spinach vs 27.6% from milk; within-subject difference 22.5 +/- 9.5 percentage points, P<0.0001
• In 10 volunteers eating 3.5 oz (100 g) grilled spinach, only 0.75% of its soluble oxalate appeared in urine within 6 hours and 1.93% within 24 hours, and adding sour cream with calcium-enriched milk lowered it 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% (6 h) and 1.93 +/- 0.85% (24 h); dairy additions reduced it, P<0.05
• In stable isotope studies in 16 women, iron absorption from a spinach meal was 24% lower than from a kale meal, a difference that was not statistically significant, and added oxalate alone did not change iron absorption. — 16 healthy women aged 18-45 years, 13 completed both studies; 150 g spinach or kale with 100 g wheat rolls (n = 16) Evidence B source
  Spinach meal absorption 24% lower than kale meal, P>0.16; kale 10.7% vs kale plus 1.26 g oxalate 11.5%, P=0.86
• In a four-period crossover trial in 24 adults, native folate from spinach was the most bioavailable of three folate-rich foods, ahead of wheat germ and Camembert cheese. — 24 adults, single doses of spinach, Camembert, wheat germ or folic acid reference, 14-day washouts (n = 24) Evidence B source
  Highest AUC and Cmax ratio estimates for spinach among the test foods; exact ratios not given in the abstract
• In healthy volunteers stably anticoagulated for 13 weeks, 150 micrograms a day of vitamin K1 was the threshold intake that significantly lowered INR, a clinically relevant change in 25% of participants. — healthy volunteers on individualized oral anticoagulant doses for 13 weeks Evidence C source
  Threshold K1 dose for significant INR lowering 150 ug/day; clinically relevant INR change in 25% of participants at that intake

What cooking and storage change

• Boiling leafy greens in covering water and draining keeps about 60% of folate and 55% of vitamin C, while calcium, iron and magnesium stay at 95%. — USDA retention factors for the group Vegetables and Vegetable Products, method VEG,GREENS,BOILED,WATER COVER DRAIN Evidence E source
  Retained: folate 60%, vitamin C 55%, thiamin 80%, potassium 85%, calcium 95%, iron 95%, magnesium 95%

What it does to your health

• Observational data from two cohorts link moderate plant-sourced nitrate intake to 16% lower rates of metabolic dysfunction-associated fatty liver disease (MASLD). — 53,854 Danish Diet, Cancer and Health and 197,198 UK Biobank participants, followed 25.1 and 13.5 years (n = 251052) Evidence C source
  Pooled HR Q3 vs Q1 0.84 (95% CI 0.76-0.94) for incident MASLD
• A dose-response meta-analysis of 75 RCTs found each extra millimole of dietary nitrate lowered systolic blood pressure by 0.24 to 0.48 mmHg, depending on trial length. — 1823 participants in 75 randomized trials of dietary nitrate, mostly beetroot and 5 spinach interventions (n = 1823) Evidence A source
  SBP per mmol NO3: acute -0.28, short-term -0.24, medium-term -0.48 mmHg; DBP -0.12 mmHg per mmol

What people believe that the data does not support

• The EU register lists a claim that asparagus supports kidney excretory function and urinary flow as non-authorized. — — Evidence E source
  One register entry for Asparagus officinalis, status Non-authorized
• In a 12-week trial in 60 men with obesity, adding spinach thylakoid extract to functional training gave no significant difference in systemic metabolic markers over training alone. — 60 men with obesity, BMI 32.6, randomized to placebo, supplement, training plus placebo or training plus supplement, 12 weeks (n = 60) Evidence B source
  No significant differences between training plus placebo and training plus supplement for systemic metabolic markers; 27% attrition

Who this works differently for

• In patients stable on warfarin, 250 g spinach once kept Thrombotest in range, but daily spinach for one week tended to push it above the therapeutic limit and required dose adjustment. — patients on continuous warfarin therapy for deep venous thrombosis or pulmonary embolism Evidence C source
  Single 8.8 oz (250 g) serving: no values outside range; daily for one week: values tended above the therapeutic limit
• According to EFSA, infants and children aged 1-3 years who eat large amounts of spinach with high nitrate levels could at times reach an intake where a risk of methaemoglobinemia cannot be excluded. — infants and young children aged 1-3 years eating over 7.1 oz (200 g) spinach on a given day Evidence E source
  Risk of methaemoglobinemia cannot be excluded at high intakes; EFSA also advises not giving spinach to children with bacterial gastrointestinal infections
• A case report describes oxalate kidney injury in an 8-year-old boy fed pureed meals made with large amounts of spinach and nuts. — one 8-year-old boy dependent on blenderised feeds (n = 1) Evidence D source
  Serum creatinine 1.8 mg/dL at presentation, 2.1 mg/dL after dietary change

The bottom line

Spinach folate did well in one short trial, where 24 adults given single doses absorbed it best of three folate-rich foods, ahead of wheat germ and Camembert, though the abstract gives no ratios. We found no absorption study for asparagus itself. For iron, 16 women, 13 of whom finished, absorbed 24% less from a spinach meal than from a kale meal, a gap that did not reach statistical significance, and adding oxalate to kale did not change iron absorption. Boiling greens in covering water and draining keeps about 60% of folate and 55% of vitamin C by USDA group factors from 1975 that name neither vegetable. Calcium, iron and magnesium stay at 95%. Other health outcomes are thin. In a 12-week trial in 60 men with obesity, a spinach thylakoid extract, a supplement made from the leaf, added to training made no significant difference to systemic metabolic markers, and 27% dropped out. Two cohorts of 251,052 people linked moderate nitrate intake from all plant foods with 16% lower rates of fatty liver disease (HR 0.84, 95% CI 0.76 to 0.94). That is an association, and spinach was never measured on its own. The EU refused a claim that asparagus supports kidney excretion and urinary flow. Spinach needs care in three groups. The first is people on warfarin, and the evidence is a non-randomized 1986 study whose abstract does not say how many took part. In patients stable on the drug, one 8.8 oz (250 g) serving kept clotting tests in range, but spinach daily for a week tended to push them above the therapeutic limit and needed a dose change. In a separate non-randomized study, healthy volunteers kept on anticoagulants for 13 weeks saw INR fall significantly at 150 micrograms of vitamin K1 a day, a clinically relevant change in 25% of them. We have no card on the vitamin K content of spinach, so we cannot say how much spinach that equals. EFSA says children aged 1 to 3 who eat over 7.1 oz (200 g) of high-nitrate spinach in a day could reach an intake where methaemoglobinemia cannot be excluded, and advises against spinach for children with bacterial gut infections. One case report describes oxalate kidney injury in an 8-year-old boy on blended feeds heavy in spinach and nuts. In ordinary eating, 10 volunteers absorbed 0.75% of the soluble oxalate in 3.5 oz (100 g) of grilled spinach over 6 hours and 1.93% over 24 hours, and sour cream with calcium-enriched milk lowered that. We have no card on spinach for people who form kidney stones, and that low figure is no reassurance for them.

Sources

1. aspsp-r4-05 · reference dataset
   FDC Foundation · Asparagus, green, raw — Folate, total 182.5 UG, range 143.0–206.6, 8 samples || Spinach, baby — Folate, total 116.5 UG, range 90.4–155.0, 8 samples || Spinach, mature — Folate, total 113.4 UG, range 77.9–149.0, 8 samples
   USDA FoodData Central, Foundation Foods, release 2026-04-30 · checked 2026-09-29
2. aspsp-r4-06 · reference dataset
   FDC Foundation · Spinach, baby — Calcium, Ca 68.35 MG, range 48.2–92.1, 8 samples || Asparagus, green, raw — Calcium, Ca 20.58 MG, range 18.1–23.1, 8 samples
   USDA FoodData Central, Foundation Foods, release 2026-04-30 · checked 2026-09-29
3. aspsp-r4-08 · reference dataset
   FNDDS 72125100 Spinach, raw — 1 cup 25 g · 1 leaf 10 g · Guideline amount in salad 50 g || FNDDS 75100800 Asparagus, raw — 1 spear 16 g · 1 cup 135 g · Quantity not specified 68 g
   USDA FNDDS 2021-2023 via FoodData Central, fdc_id 2709614 and 2709767 · checked 2026-09-29
4. aspsp-r4-01 · 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 2026-09-29
5. aspsp-r4-02 · 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 2026-09-29
6. aspsp-r4-03 · randomized crossover trial
   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 2026-09-29
7. aspsp-r4-04 · 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.
   Food Funct, 2015 · checked 2026-09-29
8. aspsp-r4-10 · dose-response clinical 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.
   Blood, 2004 · checked 2026-09-29
9. aspsp-r4-07 · reference dataset
   USDA-RETN-R6:3005 VEG,GREENS,BOILED,WATER COVER DRAIN — folate_total_ug 60% · vitamin_c_mg 55% · thiamin_mg 80% · potassium_mg 85% · calcium_mg 95% · iron_mg 95% · magnesium_mg 95%
   USDA Table of Nutrient Retention Factors, Release 6 (2007) · checked 2026-09-29
10. aspsp-r4-14 · 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 2026-09-29
11. aspsp-ws7-01 · meta-analysis of RCTs
    Per each mmol increase in NO3 intake, a reduction in systolic BP levels was observed in the acute (WMD: -0.28mmHg; 95%CI: -0.40, -0.17), short-term (WMD: -0.24mmHg; 95%CI: -0.40, -0.07), and medium-term (WMD: -0.48mmHg; 95%CI: -0.71, -0.25) periods.
    Nutr J, 2025 · checked 2026-09-29
12. aspsp-r4-12 · regulatory register entry
    POL-HC-6652 Asparagus officinalis Supports the excretory function of the kidneys /contributes to urinary flow Non-authorised
    EU Register on nutrition and health claims, claim POL-HC-6652, snapshot 2026-09-21 · checked 2026-09-29
13. aspsp-r4-13 · randomized controlled trial
    While no significant differences were observed between TPG and TSG for systemic metabolic markers, preliminary data suggested that thylakoid supplementation might provide modest complementary modulations in specific myokines (e.g., decorin and follistatin).
    Nutrients, 2026 · checked 2026-09-29
14. aspsp-r4-09 · clinical challenge 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 2026-09-29
15. aspsp-r4-11 · agency statement
    It notes however, that infants and young children aged 1-3 years who consume high amounts of spinach with high nitrate levels could at times reach an intake level for which a risk of methaemoglobinaemia
    EFSA CONTAM Panel, 2010 · checked 2026-09-29
16. aspsp-r4-15 · case report
    Children on pureed foods are at risk of hyperoxaluria if there is a chronic high intake of foods with elevated oxalate content, and caregivers may not realize that healthy foods such as spinach and nuts are high in oxalate.
    Kidney Med, 2025 · checked 2026-09-29