Leafy greens: strong in the cohorts, flat in the biggest trial

The largest trial of whole leafy greens and blood pressure found nothing. In 243 Danish adults aged 50 to 70 with raised blood pressure, greens carrying 300 mg of nitrate a day for five weeks moved 24-hour systolic pressure by 0.5 mmHg, the same as placebo. A one-week crossover in 19 healthy women of about 20, on a high-nitrate vegetable diet rather than greens alone, did see resting systolic pressure fall from 107 to 103 mmHg, but it was tiny, short and in people without a pressure problem. The long-term case for greens comes from cohorts. Pooling 17 of them, people who ate more green leafy vegetables had a 7 percent lower risk of cardiovascular disease (RR 0.93, 95% CI 0.92 to 0.95). An umbrella review of 24 meta-analyses tied each 100 g a day to roughly 25 percent lower risk of death, coronary heart disease and stroke. Across three cohorts of 18,339 adults aged 45 and over, each extra daily serving went with an 18 percent lower dementia risk (HR 0.82, 95% CI 0.70 to 0.96). These are associations. Cohorts like these cannot separate the greens from the rest of the diet that tends to come with them. For type 2 diabetes even the association is borderline: an 11-study meta-analysis gave RR 0.91 (95% CI 0.84 to 1.00), and its authors called the evidence not convincing.

As people eat it, per 3.5 oz (100 g)

How much is actually in it

• NIH notes that oxalate cuts calcium absorption from spinach to 5% and lists collard greens among high-oxalate foods, while kale and broccoli calcium absorbs about as well as milk. — general population Evidence E source
  Calcium absorption 5% spinach vs 27% milk
• Across 8 USDA samples of raw beet greens, iron ran from 2.1 to 4.2 mg and folate from 108 to 190 mcg per 3.5 oz (100 g). — USDA Foundation Foods, Beet greens, raw, 8 samples (n = 8 samples) Evidence E source
  Iron mean 3.16 mg (2.14-4.18); calcium 72.8 mg (55.2-91.4); folate 151.8 mcg (108.1-190.2) per 3.5 oz (100 g)

What a real portion looks like

• The US survey food database sets a cup of cooked canned greens at 175 g and an unspecified serving at 88 g. — FNDDS food code 72118223, Greens, canned, cooked (n = 2 portion measures) Evidence E source
  1 cup 175 g; quantity not specified 88 g
• Among 668 US adults, cooked spinach or greens supplied 37% of daily dietary nitrate, lettuce 19% and raw greens 9%. — 668 adults aged 20-55 without diabetes in the ORIGINS cohort, New York (n = 668) Evidence C source
  Median nitrate intake 143 mg/day; greens and lettuce together about 65% of it

What your body absorbs

• In a crossover isotope trial in 13 adults, the body absorbed 5.1% of calcium from spinach against 27.6% from milk. — 13 healthy adults, test meals with 200 mg labeled calcium (n = 13) Evidence B source
  Mean absorption 5.1% spinach vs 27.6% milk; within-person difference 22.5 +/- 9.5 points
• Adding 1.26 g of oxalic acid to a kale meal did not change iron absorption in 16 women (10.7% vs 11.5%). — 16 healthy women aged 18-45, Zurich, stable iron isotope meals with 150 g kale or spinach (n = 16) Evidence B source
  Iron absorption 10.7% kale vs 11.5% kale plus oxalate (P=0.86); spinach meal 24% lower than kale, not significant
• NIH says the body takes up only 4% to 17% as much vitamin K from spinach as from a tablet, and eating the greens with some fat helps. — general population Evidence E source
  Phylloquinone from spinach 4-17% of tablet absorption

What cooking and storage change

• By USDA factors for greens, boiling in water to cover and draining keeps 55% of vitamin C and 60% of folate, while stir-frying keeps 85% of each. — USDA retention factors for the greens group (n = 2 methods) Evidence E source
  Boiled, water cover, drained: vitamin C 55%, folate 60%, calcium and iron 95%; stir fry: vitamin C 85%, folate 85%

What it does to your health

• In a 1-week crossover trial of 19 young women, a high-nitrate vegetable diet lowered resting systolic pressure from 107 to 103 mmHg. — 19 healthy normotensive women, mean age 20, Exeter UK, 1 week per diet (n = 19) Evidence B source
  Systolic BP 107 (SD 9) to 103 (SD 6) mmHg on high-nitrate diet; no change on control diet
• Pooling 17 cohort studies, people eating more green leafy vegetables had 7% lower risk of cardiovascular disease. — 17 prospective cohort studies of green leafy vegetable intake and incident CVD (n = 17 studies) Evidence C source
  Pooled RR 0.93 (95% CI 0.92-0.95); cerebral infarction RR 0.92 (0.88-0.96)

Safety, limits and interactions

• An umbrella review of 24 meta-analyses linked each 100 g a day of green leafy vegetables to about 25% lower risk of death, heart disease and stroke, while flagging E. coli contamination. — 24 meta-analyses covering 29 health outcomes (n = 24 meta-analyses) Evidence C source
  About 25% lower risk per 100 g/day for all-cause mortality, coronary heart disease and stroke
• Over 1996-2016, 134 leafy greens incidents tied to California caused about 2,240 confirmed US illnesses, with romaine and spinach the usual vehicles and pathogenic E. coli the main hazard. — leafy greens food safety incidents linked to California, 1996-2016 (n = 134 incidents) Evidence E source
  About 2,240 confirmed US cases; 298 California cases with 50 hospitalizations
• A review of 11 studies in people on warfarin-type drugs found no support for cutting out leafy greens, and advised keeping vitamin K intake steady instead. — patients on vitamin K antagonists, 2 dietary trials and 9 observational studies (n = 11 studies) Evidence C source
  Effect on INR seen only at high intakes above about 150 mcg vitamin K per day

What people believe that the data does not support

• The FDA-authorized folate claim lets labels name dark green leafy vegetables as one route to adequate folate. — US food labeling Evidence E source
  Authorized wording, no effect size

Who this works differently for

• In a 5-week RCT of 243 adults aged 50-70 with raised blood pressure, leafy greens carrying 300 mg nitrate a day lowered 24-hour systolic pressure by 0.5 mmHg, no different from placebo. — 243 adults aged 50-70 with clinic systolic BP 130-159 mmHg, Denmark, 5 weeks after a 2-week low-nitrate run-in (n = 243) Evidence B source
  Change in 24 h ambulatory SBP -0.5 (SD 6.6) mmHg greens vs -0.6 (SD 6.2) placebo vs -1.2 (SD 6.8) nitrate pills; no significant difference
• Across three cohorts of 18,339 adults aged 45 and over, each extra daily serving of green leafy vegetables was linked to 18% lower dementia risk. — 18,339 adults aged 45 or older in HRS, Framingham Offspring and Whitehall II, 7-13 years follow-up (n = 18339) Evidence C source
  Pooled HR per serving 0.82 (95% CI 0.70-0.96)

What is still unknown

• For type 2 diabetes the cohort evidence on leafy greens is borderline, with an 11-study meta-analysis giving RR 0.91 (0.84-1.00) and the authors calling it not convincing. — 11 prospective studies, 754,729 participants, 58,297 cases, including 45,411 Singapore Chinese adults (n = 754729) Evidence C source
  Summary RR 0.91 (95% CI 0.84-1.00) green leafy; Singapore cohort HR 1.05 (0.96-1.15)

The bottom line

What reaches your body depends on which green. In an isotope trial in 13 adults, only 5.1 percent of the calcium in spinach was absorbed, against 27.6 percent from milk. NIH puts collard greens with spinach among the high-oxalate foods, and says calcium from kale, broccoli and cabbage absorbs about as well as milk, though each serving holds much less of it. Iron is a different story. When 16 women ate a kale meal with 1.26 g of oxalic acid added, iron absorption was 11.5 percent against 10.7 percent without it, a gap that was not significant. The body takes up only 4 to 17 percent as much vitamin K from spinach as from a tablet, by the NIH figure, and eating the greens with some fat helps. Cooking method changes what is left. By USDA group factors from 1975, boiling greens in water to cover and draining them keeps 55 percent of vitamin C and 60 percent of folate, while stir-frying keeps 85 percent of each. Those are factors for the whole group, not measured green by green. A cup of cooked canned greens weighs 6.2 oz (175 g). In 668 New York adults, cooked spinach or greens supplied 37 percent of daily dietary nitrate, from a median intake of 143 mg a day. The FDA lets folate labels name dark green leafy vegetables as one route to adequate folate, which is a statement about folate and carries no effect size for greens. Two cautions have data behind them. If you take warfarin or a similar vitamin K antagonist, a review of 11 studies (2 dietary trials, 9 observational) found no support for cutting out leafy greens and advised keeping vitamin K intake steady instead, so the thing to avoid is big swings. Raw greens also carry a food safety risk. Over 1996 to 2016, 134 leafy greens incidents linked to California caused about 2,240 confirmed US illnesses, mostly from pathogenic E. coli, with romaine and spinach the usual vehicles. Those are state agency records from one growing region. We have no card on nitrate in greens for young infants or on oxalate and kidney stones, so this page is silent on them, and that silence is not an all-clear.

Sources

1. grns-r2-12 · NIH ODS 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]. The bioavailability of calcium from other plants that do not contain these compounds—including broccoli, kale, and cabbage—is similar to that of milk, although the amount of calcium per serving is much lower [3].
   NIH Office of Dietary Supplements, Calcium Fact Sheet for Health Professionals · checked 2026-09-28
2. grns-r2-16 · USDA FoodData Central Foundation Foods, dataset
   Beet greens, raw (Foundation Foods 2026-04-30): Iron 3.163 mg, range 2.14-4.175, 8 samples; Calcium 72.8 mg, range 55.2-91.35, 8 samples; Folate, total 151.8 ug, range 108.1-190.2, 8 samples
   USDA FoodData Central, Foundation Foods, release 2026-04-30, Beet greens, raw · checked 2026-09-28
3. grns-r2-15 · USDA FNDDS 2021-2023, dataset
   Greens, canned, cooked (FNDDS 72118223, FDC 2709598, WWEIA Other dark green vegetables): 1 cup 175 g; Quantity not specified 88 g
   USDA FNDDS 2021-2023, food code 72118223 (FDC 2709598) · checked 2026-09-28
4. grns-r2-17 · cross-sectional study
   Median[IQR] and mean±SD dietary nitrate intakes were 143[72, 245] mg/day and 190±171 mg/day, respectively. The major contributors to mean daily nitrate intake included cooked spinach/greens (37%), lettuce (19%), and raw spinach/greens (9%)
   J Clin Periodontol, 2026 · checked 2026-09-28
5. grns-r2-03 · randomized crossover absorption trial
   Absorption was higher from milk in every case, with the mean absorption from milk averaging 27.6% and from spinach, 5.1%.
   Am J Clin Nutr, 1988 · checked 2026-09-28
6. grns-r2-04 · randomized crossover absorption 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).
   Eur J Clin Nutr, 2008 · checked 2026-09-28
7. grns-r2-11 · NIH ODS fact sheet
   Phylloquinone in plant foods is tightly bound to chloroplasts, so it is less bioavailable than that from oils or dietary supplements [1]. For example, the body absorbs only 4% to 17% as much phylloquinone from spinach as from a tablet [2]. Consuming vegetables at the same time as some fat improves phylloquinone absorption from the vegetables, but the amount absorbed is still lower than that from oils.
   NIH Office of Dietary Supplements, Vitamin K Fact Sheet for Health Professionals · checked 2026-09-28
8. grns-r2-14 · USDA retention factors, dataset
   USDA-RETN-R6:3005 VEG,GREENS,BOILED,WATER COVER DRAIN: vitamin C 55%, folate 60%, calcium 95%, iron 95%; USDA-RETN-R6:3015 VEG,GREENS,STIR FRY: vitamin C 85%, folate 85%
   USDA Table of Nutrient Retention Factors, Release 6 (2007), codes 3005 and 3015 · checked 2026-09-28
9. grns-r2-02 · randomized crossover trial
   The HN diet significantly reduced resting systolic BP (before HN diet: mean 107 (sd 9) mmHg; after HN diet: mean 103 (sd 6) mmHg, P<0·05).
   Public Health Nutr, 2015 · checked 2026-09-28
10. grns-r2-05 · meta-analysis of observational studies
    The pooled RR (95% CI) of incident CVD events from 17 studies was 0.93 (0.92-0.95).
    Cardiovasc J Afr, 2021 · checked 2026-09-28
11. grns-r2-08 · umbrella review
    Dose-response analyses revealed that, per 100 g/d GLV intake was associated with a decreased risk (ca. 25%) of all-cause mortality, coronary heart disease and stroke. ... Caution was warranted for contamination with potentially pathogenic organisms, specifically Escherichia coli.
    Food Chem, 2021 · checked 2026-09-28
12. grns-r2-09 · surveillance review
    In the 21-year period analyzed, 134 incidents were identified, the majority of which were surveillance related. Approximately 2,240 U.S. cases of confirmed illness were reported (298 California cases resulting in 50 hospitalizations). Romaine lettuce and spinach were the most commonly implicated vehicles.
    J Food Prot, 2019 · checked 2026-09-28
13. grns-r2-10 · systematic review
    The available evidence does not support current advice to modify dietary habits when starting therapy with VKAs. Restriction of dietary vitamin K intake does not seem to be a valid strategy to improve anticoagulation quality with VKAs. It would be, perhaps, more relevant to maintain stable dietary habit, avoiding wide changes in the intake of vitamin K.
    Medicine (Baltimore), 2016 · checked 2026-09-28
14. grns-r2-13 · US health claim regulation
    Adequate amounts of folate can be obtained from diets rich in fruits, dark green leafy vegetables, legumes, whole grain products, fortified cereals, or dietary supplements.
    21 CFR 101.79, Health claims: folate and neural tube defects · checked 2026-09-28
15. grns-r2-01 · randomized controlled trial
    We enrolled 243 subjects, 50-70 y old, with a clinic systolic BP (SBP) of 130-159 mm Hg. ... The insignificant change in ambulatory SBP (mean ± standard deviation) was -0.6 ± 6.2 mm Hg in the placebo group, -1.2 ± 6.8 mm Hg in the potassium nitrate group, and -0.5 ± 6.6 mm Hg in the leafy green vegetable group.
    Am J Clin Nutr, 2020 · checked 2026-09-28
16. grns-r2-06 · cohort studies with meta-analysis
    Additionally, green leafy vegetables demonstrated a protective association (pooled HR per serving increment: 0.82; 95% CI: 0.70, 0.96, P-trend = 0.015).
    Am J Clin Nutr, 2026 · checked 2026-09-28
17. grns-r2-07 · meta-analysis of observational studies
    Meta-analysis of available cohort data indicated that evidence for a beneficial effect of green leafy or cruciferous vegetable consumption on T2D risk is not convincing.
    Br J Nutr, 2018 · checked 2026-09-28