Romaine and spinach: the dressing decides what you absorb

The largest pool of trials on leafy nitrate promises less than the marketing around it. A meta-analysis of 118 randomized trials of nitrate and polyphenol rich foods found effects on endurance that its own authors call trivial, a standardized mean difference of 0.15 for nitrate and 0.17 for polyphenols, and no effect at all in women. Among the nitrate foods the benefit belonged to beetroot. Red spinach, Swiss chard and rhubarb showed none. What these two greens carry is fat soluble pigments, and whether those reach you depends on the dressing. Seven adults ate three identical salads of spinach, romaine lettuce, cherry tomato and carrot in random order. With fat free dressing the carotenoids appearing in the bloodstream were negligible. With 6 g of canola oil they rose (P < 0.04), and with 28 g they rose again (P < 0.02). That is seven people and a 12 hour reading of chylomicrons rather than a blood level built over weeks, and it is the only study we found that feeds romaine and spinach together.

How much is actually in it

• Per 100 g raw, romaine leads on vitamin K at 102 micrograms and baby spinach leads on folate at 116.5 micrograms against 50, while the Foundation samples of each spread from 51.2 to 129 and from 77.9 to 155. — USDA Foundation Foods records and UK CoFID records for the two greens, not a measurement in people Evidence E source
  Romaine vitamin K 102 ug (Foundation samples 51.2 to 129, n=8), vitamin A 436 ug, folate 50 ug (Foundation 29 to 67, n=14); baby spinach vitamin A 283 ug, vitamin C 26.52 mg (Foundation 9.87 to 54.7, n=8), folate 116.5 ug (Foundation 77.9 to 155, n=8)

What a real portion looks like

• A US survey cup of romaine weighs 1.2 oz (35 g) and a single spinach leaf weighs 10 g, so a bowl that looks large carries a small weight of leaf. — US dietary survey measures for the two greens, converted from 100 g Evidence E source
  Romaine 1 leaf 13 g and 1 cup 35 g; spinach 1 leaf 10 g and 1 baby leaf 0.6 g; a 35 g cup of romaine therefore carries about 36 ug of vitamin K at the USDA reference value

What your body absorbs

• In 7 adults fed salads of spinach, romaine lettuce, tomato and carrot, carotenoid uptake was negligible with fat-free dressing and rose step by step with 6 g and then 0.99 oz (28 g) of canola oil. — 7 adults, three salads of equal composition eaten in random order with 0, 6 or 28 g canola oil dressing, washout at least 2 weeks, blood hourly for 12 hours (n = 7) Evidence B source
  Alpha-carotene, beta-carotene and lycopene in chylomicrons negligible at 0 g fat, higher at 6 g than at 0 g (P < 0.04) and higher at 28 g than at 6 g (P < 0.02)

What cooking and storage change

• In 70 adults fed spinach for three weeks, minced and liquefied spinach raised plasma folate more than whole leaf spinach (p = 0.03), so breaking the leaf matters more than the fiber in it. — 28 men and 42 women in six groups, control diet or control plus 20 g per MJ per day of whole leaf, minced, liquefied or fiber-restored liquefied spinach, 3 weeks (n = 70) Evidence B source
  Pooled spinach groups above control for plasma folate, pooled minced and liquefied above whole leaf (p = 0.03), re-adding fiber to liquefied spinach did not reduce the response

What it does to your health

• Across 118 randomized trials of nitrate and polyphenol rich foods, endurance benefit was found for beetroot but not for red spinach or Swiss chard, and the pooled nitrate effect was trivial at SMD 0.15. — 118 randomized controlled trials of nitrate, polyphenol, L-arginine or L-citrulline rich foods in endurance exercise, including 56 nitrate studies (n = 118 trials) Evidence A source
  Nitrate SMD 0.15 and polyphenol SMD 0.17, both p < 0.001 but described by the authors as trivial; benefit seen for beetroot and not for red spinach or Swiss chard and rhubarb; no effect in women

Safety, limits and interactions

• In 10 volunteers eating 3.5 oz (100 g) of grilled spinach holding 736.6 mg of soluble oxalate per 3.5 oz (100 g), 0.75 percent of that soluble oxalate was absorbed over 6 hours and 1.93 percent over 24 hours. Sour cream and calcium enriched milk cut that further. — 10 volunteers, 100 g grilled frozen spinach alone or with 100 g cottage cheese, sour cream, sour cream plus 180 g calcium enriched milk, or 20 g olive oil, urinary oxalate over 6 and 24 hours (n = 10) Evidence B source
  Soluble oxalate content of the frozen spinach 736.6 +/- 20.4 mg per 3.5 oz (100 g) wet matter, of which 0.75 +/- 0.48 percent was absorbed at 6 hours and 1.93 +/- 0.85 percent at 24 hours, significantly reduced by sour cream and calcium enriched milk (P < 0.05)
• In a synthesis of 29 surveys of 6659 fresh produce samples from Ethiopian farms and markets, lettuce was the most parasite contaminated vegetable at 61.0 percent and spinach followed at 54.2 percent. — 6659 vegetable and fruit samples in 29 studies from Ethiopian local farms and markets, 2010 to 2025 (n = 6659 samples) Evidence C source
  Overall contaminated produce 48.76 percent (95% CI 40.78 to 56.74), parasitic contamination 46.40 percent (40.04 to 52.75), lettuce 61.0 percent and spinach 54.2 percent for parasites, vendor handwashing aOR 3.10 (1.53 to 4.67)

What people believe that the data does not support

• In 16 women given labeled meals, adding 1.26 g of potassium oxalate to a kale meal left iron absorption unchanged at 11.5 percent against 10.7 percent (P = 0.86). The spinach meal was 24 percent lower than the kale meal, and that difference did not reach significance. — 16 healthy women aged 18 to 45 under 132 lb (60 kg), 13 completed both crossover studies, test meals of 150 g spinach (1.27 g oxalic acid) or 150 g kale (0.01 g) with two wheat rolls (n = 16) Evidence B source
  Iron absorption 10.7 percent from kale and 11.5 percent from kale plus 1.26 g added oxalate (P = 0.86), spinach meal 24 percent lower than kale but not significant (P > 0.16)
• The EU refused the claim that a mix of red and green spinach with chicory and chard protects blood lipids from oxidative damage, and no authorized claim exists for romaine or spinach by name. — EU food labeling, general population Evidence E source
  Claim refused, all 6 spinach claims in the register are non-authorized and romaine has none at all

Who this works differently for

• NIH Office of Dietary Supplements warns that vitamin K can interact seriously with warfarin and similar anticoagulants, and that people on these drugs need a consistent vitamin K intake rather than sudden changes. — adults taking warfarin, phenprocoumon, acenocoumarol or tioclomarol Evidence E source
  Sudden changes in vitamin K intake can increase or decrease the anticoagulant effect, no safe gram amount given

What is still unknown

• Boiling greens under a cover of water keeps only 55 percent of their vitamin C against 70 percent when baked, and the USDA table has no vitamin K retention factor at all, so the main nutrient of romaine is uncovered by it. — USDA retention factors set on the group Vegetables and Vegetable Products by cooking method, not on a single green Evidence E source
  Vitamin C retained 70 percent baked (code 3001), 60 percent boiled in little water and drained (3004), 55 percent boiled under water cover and drained (3005), 60 percent cooked from frozen and drained (3019); folate 75 percent baked and 65 percent boiled in little water; vitamin K absent from Release 6

The bottom line

Per 100 g raw the two greens divide their strengths. Romaine leads on vitamin K at 102 micrograms, and baby spinach leads on folate at 116.5 micrograms against 50. The spread inside each green swallows much of that difference. Vitamin K in romaine ran from 51.2 to 129 micrograms across eight Foundation samples, and folate in baby spinach from 77.9 to 155 micrograms across eight, so which head you bought can matter more than which green. The United Kingdom table holds no romaine record at all, so for romaine there is no second independent number. Portions are smaller than a bowl suggests. A United States survey cup of romaine weighs 1.2 oz (35 g) and one spinach leaf weighs 10 g, so that cup carries about 36 micrograms of vitamin K at the USDA reference value, a figure calculated from the per 3.5 oz (100 g) number rather than weighed. Chopping does more here than cooking. In 70 adults fed spinach for three weeks in groups of ten to twelve, minced and liquefied spinach raised plasma folate more than whole leaf spinach (p = 0.03), and putting the broken fiber back into the liquefied version did not undo it. Plasma folate is an intermediate marker rather than a body store, and the groups were small. The oxalate story that follows spinach around fails in the place people apply it most. In 16 healthy women under 132 lb (60 kg) given labeled test meals, of whom 13 completed both crossovers, adding 1.26 g of potassium oxalate to a kale meal left iron absorption unchanged at 11.5 percent against 10.7 percent (P = 0.86). The spinach meal came out 24 percent below the kale meal, and that gap did not reach significance (P > 0.16). This experiment gives oxalate no measurable role in blocking iron at the dose tested, and the same authors note that their spinach was higher than kale in calcium and polyphenols. How much oxalate a spinach meal delivers was measured once. In 10 volunteers eating 3.5 oz (100 g) of grilled spinach, which held 736.6 mg of soluble oxalate per 3.5 oz (100 g), 0.75 percent of that soluble oxalate was absorbed over 6 hours and 1.93 percent over 24 hours, and eating it with sour cream or with calcium enriched milk cut that further (P < 0.05). That was New Zealand frozen spinach, grilled so that nothing leached into cooking water, which is the worst case rather than the average plate. Nothing on this page connects that absorbed fraction to kidney stones, because no card here measures stone risk at any intake. The caution that matters most for romaine sits under a different heading on this page, so it belongs here too. The NIH Office of Dietary Supplements warns that vitamin K interacts seriously with warfarin, phenprocoumon, acenocoumarol and tioclomarol, and that people taking those drugs need a consistent intake, because sudden changes in either direction move the anticoagulant effect. No safe number of micrograms is given. Raw leaves also carry whatever was on them. A synthesis of 29 surveys covering 6659 produce samples from Ethiopian farms and markets found parasites on 61.0 percent of lettuce and 54.2 percent of spinach, with overall contamination at 48.76 percent (95% CI 40.78 to 56.74). Those percentages belong to Ethiopian irrigation and market handling and do not transfer to a European or American shop. Europe refused the claim that a mixture of red and green spinach with chicory and chard protects blood lipids from oxidative damage, and holds no authorized claim for romaine at all. The gaps are wide. Nothing we found compares these two greens head to head as replacements for each other. The USDA retention table carries no vitamin K factor anywhere in Release 6, so cooking losses of the main nutrient of romaine stay undescribed. The oxalate content of romaine itself was never measured in anything we read.

Sources

1. rmsp-daily-r5-08 · USDA FoodData Central and CoFID, datasets
   USDA-FDC:746769 Lettuce, cos or romaine, raw, Foundation, group Vegetables and Vegetable Products: vitamin K 102 ug, vitamin A 436 ug RAE, folate 50 ug per 100 g, Foundation samples vitamin K 51.2-129 n=8, folate 29-67 n=14; USDA-FDC:1999632 Spinach, baby, Foundation, same group: vitamin A 283 ug RAE, vitamin C 26.52 mg, folate 116.5 ug per 100 g, Foundation samples vitamin C 9.87-54.7 n=8, folate 77.9-155 n=8; CoFID-2021:13-521 Spinach, baby, raw: vitamin A 260 ug, vitamin C 29 mg, folate 161 ug; CoFID has no romaine record
   USDA FoodData Central FDC 746769 and FDC 1999632 (Foundation Foods) with CoFID 2021, Public Health England, code 13-521, OGL v3.0 · checked 2026-09-25
2. rmsp-daily-r5-10 · USDA FNDDS, dataset
   USDA-FNDDS:2709590 Romaine lettuce, raw: 1 leaf = 13 g, 1 cup = 35 g; USDA-FNDDS:2709614 Spinach, raw: 1 leaf = 10 g, 1 baby leaf = 0.6 g
   USDA FNDDS, fdc_id 2709590 Romaine lettuce, raw and fdc_id 2709614 Spinach, raw · checked 2026-09-25
3. rmsp-daily-r5-01 · postprandial absorption trial
   The subjects (n = 7) each consumed 3 salads consisting of equivalent amounts of spinach, romaine lettuce, cherry tomatoes, and carrots with salad dressings containing 0, 6, or 28 g canola oil. ... After ingestion of the salads with fat-free salad dressing, the appearance of alpha-carotene, beta-carotene, and lycopene in chylomicrons was negligible. After ingestion of the salads with reduced-fat salad dressing, the appearance of the carotenoids in plasma chylomicrons increased relative to that after ingestion of the salads with fat-free salad dressing (P < 0.04). Similarly, the appearance of the carotenoids in plasma chylomicrons was higher after the ingestion of salads with full-fat than with reduced-fat salad dressing (P < 0.02).
   Am J Clin Nutr, 2004 · checked 2026-09-25
4. rmsp-daily-r5-03 · dietary intervention trial
   In a controlled, 3-week dietary intervention study, 28 men and 42 women were divided into six groups to receive either a control diet (n = 10), or the control diet plus 20 g/MJ per day (n = 12 per group) of whole-leaf spinach, minced spinach, liquefied spinach, or liquefied spinach to which dietary fibre (10 g/kg wet weight) was added. ... However, the plasma folate response of the pooled minced and liquefied spinach groups was greater than that of the whole-leaf spinach group (p = 0.03). Re-addition of dietary fibre to the liquefied spinach to compensate for the fibre broken down during liquefaction did not reduce the plasma folate response.
   Ann Nutr Metab, 2000 · checked 2026-09-25
5. rmsp-daily-r5-05 · meta-analysis of randomized trials
   One hundred and eighteen studies were included in the meta-analysis, which encompassed 59 polyphenol studies, 56 nitrate studies and three L-citrulline studies. ... Trivial but significant benefits were demonstrated for consumption of nitrate and polyphenol-rich foods (SMD=0.15 and 0.17, respectively, p<0.001), including performance in time-trial, time-to-exhaustion and intermittent-type tests, and following both acute and multiple-day supplementation, but no effect of nitrate or polyphenol consumption was found in females. 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 2026-09-25
6. rmsp-daily-r5-04 · oxalate absorption trial
   Frozen commercially available spinach in New Zealand contains 736.6+/-20.4 mg/100g wet matter (WM) soluble oxalate and 220.1+/-96.5mg/100g WM insoluble oxalate. ... 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 2026-09-25
7. rmsp-daily-r5-06 · meta-analysis of prevalence studies
   Twenty-nine studies with 6659 vegetables and fruits were included. The overall prevalence of contaminated fresh produce was 48.76% (95% CI: 40.78–56.74). ... Lettuce (61.0%), salad (57.4%), spinach (54.2%), and cabbage (52.9%), were the vegetables most contaminated by parasites, while salad (73.8%), green pepper (35.3%), lettuce (26.0%), and cabbage (20.0%), were the most contaminated by bacteria.
   BMC Infect Dis, 2025 · checked 2026-09-25
8. rmsp-daily-r5-02 · stable isotope absorption trial
   In study I, test meals consisted of two wheat bread rolls (100 g) and either 150 g spinach with a native OA content of 1.27 g (reference meal) or 150 g kale with a native OA content of 0.01 g. ... 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 2026-09-25
9. rmsp-daily-r5-11 · claim register entry
   POL-HC-8442 A combination of red spinach, green spinach, red chicory, green chicory, green leaf chard, red leaf chard, red Swiss chard, golden Swiss chard and white Swiss chard Contributes to the protection of blood lipids from oxidative damage -/- Non-authorised
   EU Register on nutrition and health claims, claim POL-HC-8442, snapshot 2026-09-21 · checked 2026-09-25
10. rmsp-daily-r5-07 · agency fact sheet
    Vitamin K can have a serious and potentially dangerous interaction with anticoagulants such as warfarin (Coumadin) as well as phenprocoumon, acenocoumarol, and tioclomarol, which are commonly used in some European countries [7,8]. These drugs antagonize the activity of vitamin K, leading to the depletion of vitamin K-dependent clotting factors. People taking warfarin and similar anticoagulants need to maintain a consistent intake of vitamin K from food and supplements because sudden changes in vitamin K intakes can increase or decrease the anticoagulant effect [45].
    NIH Office of Dietary Supplements, Vitamin K Fact Sheet for Health Professionals · checked 2026-09-25
11. rmsp-daily-r5-09 · USDA Table of Nutrient Retention Factors, dataset
    USDA-RETN-R6:3001 VEG,GREENS,BAKED, group Vegetables and Vegetable Products: vitamin_c_mg 70% retained, folate_total_ug 75%; USDA-RETN-R6:3004 VEG,GREENS,BOILD,LITTLE WATER DRAIN: vitamin_c_mg 60% retained, folate_total_ug 65%; USDA-RETN-R6:3005 VEG,GREENS,BOILED,WATER COVER DRAIN: vitamin_c_mg 55% retained; USDA-RETN-R6:3019 VEG,GREENS,COOKED FROM FROZEN,DRAIN: vitamin_c_mg 60% retained; vitamin K has no retention factor anywhere in Release 6
    USDA Table of Nutrient Retention Factors, Release 6, 2007, Retn_Codes 3001, 3004, 3005 and 3019, group Vegetables and Vegetable Products · checked 2026-09-25