Kale and lettuce: the gap is real, and it depends on the lettuce

In a randomized crossover trial of 16 healthy young men, adding 3.5 oz (100 g) of lettuce to a high-fat meal lowered the 4-hour glucose area to 361.9 from 385.7 mg/dL, about 6 percent, while watercress eaten the same way left it at 382.3. That was one meal in one small group of men. The kale trial with a number sits further from the plate. In 30 adults with type 2 diabetes, three bars a day made from freeze-dried kale cut HOMA-IR by 0.95 over 12 weeks, against a 0.15 rise on placebo. The authors equate the bars to about 12 oz (341 g) of fresh kale a day, 13 participants dropped out, and fasting glucose did not differ significantly between groups. Where the two leaves clearly differ is content. USDA samples put raw kale at 254 mg of calcium per 3.5 oz (100 g), against 14.24 mg in iceberg and 39.84 mg in green leaf lettuce, and at 390 micrograms of vitamin K against 20.5 in iceberg. The UK tables give kale about half that calcium, 130 mg, a spread down to variety, season and method.

How much is actually in it

• In USDA Foundation Foods sampling, raw kale held 254 mg of calcium per 3.5 oz (100 g) against 14.24 mg in iceberg lettuce and 39.84 mg in green leaf lettuce. — retail samples analyzed for USDA Foundation Foods, release 2026-04-30, 5 kale and 8 samples per lettuce type (n = 5 kale samples, 8 samples per lettuce type) Evidence E source
  Calcium 254 mg per 3.5 oz (100 g) in kale (range 203 to 281, spread 31%) against 14.24 mg in iceberg (range 10.9 to 17.2) and 39.84 mg in green leaf lettuce (range 29.7 to 52.3)
• In USDA Foundation Foods sampling, raw kale held 390 micrograms of phylloquinone per 3.5 oz (100 g) against 20.5 micrograms in iceberg lettuce and 73.2 micrograms in red leaf lettuce. — retail samples analyzed for USDA Foundation Foods, release 2026-04-30, 3 kale and 8 samples per lettuce type (n = 3 kale samples, 8 samples per lettuce type) Evidence E source
  Phylloquinone 390 micrograms per 3.5 oz (100 g) in kale (range 369 to 422) against 20.5 in iceberg (range 12.3 to 27.8, spread 76%) and 73.2 in red leaf lettuce (range 55.8 to 94.8)
• In the UK composition tables, raw curly kale held 110 mg of vitamin C and 130 mg of calcium per 3.5 oz (100 g) against 1 mg and 24 mg in average raw lettuce. — UK composition dataset CoFID 2021, codes 13-234 (curly kale, raw) and 13-520 (lettuce, average, raw) Evidence E source
  Vitamin C 110 mg against 1 mg, calcium 130 mg against 24 mg, vitamin K1 623 micrograms in kale per 3.5 oz (100 g)

What a real portion looks like

• In the USDA survey database, one cup of raw kale weighs 0.88 oz (25 g) and one cup of raw lettuce 35 g, and the guideline salad amount is 1.8 oz (50 g) of kale against 2.5 oz (70 g) of lettuce. — FNDDS portion weights, Kale raw (FNDDS 72119190) and Lettuce raw (FNDDS 75113000) Evidence E source
  1 cup 25 g kale against 35 g lettuce; guideline amount in salad 50 g against 70 g; 1 head of lettuce 539 g

What your body absorbs

• In a randomized crossover isotope study of 16 women, iron absorption from bread with kale was 10.7% and did not change when 1.26 g of oxalic acid was added (11.5%). — 16 apparently healthy women aged 18-45 years in Zurich, 13 completed both studies (n = 16) Evidence B source
  Geometric mean iron absorption 10.7% with kale against 11.5% with kale plus oxalate, P=0.86
• In a small labeled-kale study, only 4.7% of the phylloquinone from a 14 oz (400 g) serving of kale eaten with oil reached the blood. — 7 healthy volunteers, single 14 oz (400 g) serving of 13C-labeled kale with 30 g vegetable oil, 6 with detectable label (n = 7) Evidence C source
  Mean bioavailability of phylloquinone 4.7% (n 6), plasma peak 6 to 10 h
• Small feeding tests in young adults found that phylloquinone absorption from fresh romaine lettuce did not differ from spinach or broccoli, nor with 30% versus 45% of energy from fat. — 22- to 30-year-old adults eating a standard test meal Evidence C source
  No difference in phylloquinone absorption between romaine (179 micrograms), spinach (165) and broccoli (184)
• By the NIH Office of Dietary Supplements, calcium from kale is absorbed about as well as calcium from milk, though a serving carries much less of it. — general population, NIH Office of Dietary Supplements calcium fact sheet for health professionals Evidence E source
  Calcium bioavailability from kale similar to milk; amount per serving much lower

What cooking and storage change

• By the USDA retention table for Vegetables and Vegetable Products boiled in covering water and drained, greens keep 55% of vitamin C, 60% of folate and 95% of calcium and iron. — USDA Table of Nutrient Retention Factors Release 6 (2007), method VEG,GREENS,BOILED,WATER COVER DRAIN, code 3005 Evidence E source
  Retained: vitamin C 55%, folate 60%, potassium 85%, calcium 95%, iron 95%
• In USDA Foundation Foods sampling, frozen kale boiled and drained held 17.8 mg of vitamin C per 3.5 oz (100 g) against 93.4 mg in raw kale. — retail samples analyzed for USDA Foundation Foods, release 2026-04-30 (n = 5 raw and 3 cooked samples) Evidence E source
  Vitamin C 93.4 mg raw (range 84.4 to 104) against 17.8 mg frozen, boiled, drained (range 10.0 to 24.4); calcium 254 against 150 mg
• Laboratory data from a crossover trial suggest cooked Ethiopian kale cut aflatoxin DNA damage in blood cells by 33-35%, while raw kale reached only 21%. — 22 healthy subjects eating raw or cooked Brassica carinata preparation for five days, blood cells exposed to aflatoxin B1 ex vivo (n = 22) Evidence D source
  DNA damage reduced 35% (+S9) and 33% (-S9) after cooked, 21% (p = 0.08) after raw
• A small non-randomized test found that fresh lettuce raised plasma antioxidant capacity while the same lettuce stored under modified atmosphere did not. — 11 healthy volunteers (six men, five women) eating 8.8 oz (250 g) fresh lettuce, then the same lettuce stored at 5 C under modified-atmosphere packaging (n = 11) Evidence C source
  Plasma TRAP AUC 1.3 (SEM 0.3) mmol/l per 6 h after fresh against 0.1 (SEM 0.2) after stored lettuce, P<0.05

What it does to your health

• In a randomized crossover trial of 16 healthy young men, adding 3.5 oz (100 g) of lettuce to a high-fat meal lowered the 4-hour glucose area to 361.9 from 385.7 mg/dL. — 16 healthy young men, mean age 22.8 years, single test meals with 100 g lettuce, watercress or cellulose (n = 16) Evidence B source
  0-4 h glucose AUC 361.9 mg/dL with lettuce against 385.7 control and 382.3 watercress, P < 0.05; lipids and IL-6, TNF-alpha unchanged
• Observational data link higher vitamin K1 intake to a 16% lower COPD rate over 10.5 years. — 179,062 UK Biobank participants without COPD or asthma (n = 179062) Evidence C source
  HR 0.84 (95% CI 0.75 to 0.94) for COPD, highest against lowest quintile of vitamin K1 intake, plateau above about 250 micrograms a day

Safety, limits and interactions

• A meta-analysis of vegetable sampling data found kale the strongest thallium accumulator of 35 vegetable species, with a bioconcentration factor of 0.133. — 35 vegetable species from published soil and vegetable thallium measurements, mostly China (n = 35 species) Evidence C source
  Thallium bioconcentration factor 0.133 in kale and 0.060 in taro; soil thresholds 0.24-4.89 mg/kg for kale against 2.94-3.32 for lettuce
• Pooled Ethiopian market surveys found lettuce the vegetable most often contaminated by parasites, at 61.0% of samples, and 26.0% by bacteria. — 29 cross-sectional studies of 6659 raw vegetables and fruits from farms and markets in Ethiopia, 2010 to 2025 (n = 29 studies, 6659 samples) Evidence C source
  Parasitic contamination 61.0% of lettuce, bacterial 26.0%; pooled parasitic prevalence 46.40% (95% CI 40.04 to 52.75)

What people believe that the data does not support

• Darker lettuces are not nutritionally empty: red leaf lettuce held 321.4 mg of potassium per 3.5 oz (100 g) against 348 mg in kale, and romaine held 5230 micrograms of beta-carotene against 2870 micrograms in kale. — retail samples analyzed for USDA Foundation Foods, release 2026-04-30 (n = 5 kale and 8 red leaf samples for potassium, 3 kale and 6 romaine samples for beta-carotene) Evidence E source
  Potassium 348 mg (kale) against 321.4 mg (red leaf lettuce) per 3.5 oz (100 g); beta-carotene 2870 micrograms (kale, range 2160 to 3830) against 5230 micrograms (romaine, range 1770 to 8750, spread 133%)

Who this works differently for

• In a 12-week randomized trial of 30 people with type 2 diabetes, kale bars equal to about 12 oz (341 g) of fresh kale a day cut HOMA-IR by 0.95 against a 0.15 rise on placebo. — 30 adults with type 2 diabetes in Aarhus, Denmark, three bars a day each with 26.25 g freeze-dried kale, 12 weeks (n = 30) Evidence B source
  HOMA-IR change -0.95 (SE 0.34) with kale against +0.15 (SE 0.28) with placebo, p < 0.03

What is still unknown

• A California case report linked commercial kale chips to urine thallium up to 10.5 micrograms per gram creatinine in three young children, and thallium is not regulated in food. — one household, a mother and her three young children, California (n = 4) Evidence D source
  Peak urine thallium 10.5 micrograms/g creatinine in children (reference 0.8 or less) and 5.6 in the mother (reference 0.4 or less); kale chips 1.98 and 2.15 mg/kg

The bottom line

The idea that lettuce is just water depends on which lettuce. Red leaf lettuce held 321.4 mg of potassium per 3.5 oz (100 g) against 348 mg in kale. Romaine held 5230 micrograms of beta-carotene against 2870 in kale, though romaine samples varied by 133 percent. Iceberg is another matter, at 14.24 mg of calcium and 20.5 micrograms of vitamin K per 3.5 oz (100 g). Per cup the comparison narrows a little, since a cup of raw kale weighs 0.88 oz (25 g) and a cup of lettuce 35 g. What you absorb is a separate question. The NIH Office of Dietary Supplements says calcium from kale is absorbed about as well as calcium from milk, with much less of it per serving. In a crossover isotope study of 16 women, iron absorption from bread with kale was 10.7 percent, and adding 1.26 g of oxalic acid left it at 11.5 percent, a difference that was not significant. Only a small share of the vitamin K appears to get through. In 6 volunteers who ate 14 oz (400 g) of labeled kale with oil, a model estimated that 4.7 percent of its phylloquinone reached the blood. Small tests in young adults found romaine gave up its vitamin K as readily as spinach or broccoli. Cooking costs kale much of its vitamin C. USDA sampling found 93.4 mg per 3.5 oz (100 g) in raw kale and 17.8 mg in frozen kale boiled and drained, measured on different batches. The USDA retention factor for greens boiled and drained, set for the whole group, keeps 55 percent of vitamin C and 95 percent of calcium and iron. Lettuce is rarely cooked. In 11 volunteers, 8.8 oz (250 g) of fresh lettuce raised a blood antioxidant marker and the same lettuce stored in modified-atmosphere packs did not, in a test without randomization. Lab measurements from a crossover trial of Ethiopian kale, a different plant from curly kale, suggest cooked leaves cut aflatoxin damage to blood cell DNA by 33 to 35 percent. Raw leaves reached 21 percent, short of significance. Observational data from 179,062 UK Biobank participants link the highest vitamin K1 intake to a COPD rate 16 percent lower over 10.5 years (HR 0.84, 95% CI 0.75 to 0.94), levelling off above about 250 micrograms a day. That is an association with a nutrient, and kale was not studied as a food. Kale was the strongest thallium accumulator of 35 vegetable species in pooled soil and plant measurements, mostly from polluted areas of China, and lettuce took up less. In one California household, three young children who ate commercial kale chips reached urine thallium of up to 10.5 micrograms per gram of creatinine, against a reference of 0.8 or less. That is a single case report, and thallium is not regulated in food. In Ethiopian market surveys, 61 percent of lettuce samples carried parasites and 26 percent bacteria, a figure from one country with poor market sanitation that cannot be carried over to US or European shops, in either direction. For anyone on warfarin, swapping iceberg lettuce for kale at the same weight means about 19 times the vitamin K. We have no study card on that interaction itself, and none on pregnancy for either leaf.

Sources

1. kvl-daily-r4-01 · reference dataset
   Kale, raw | Calcium, Ca | 254.0 MG | range 203.0-281.0, median 264.0, 5 samples, spread 31% || Lettuce, iceberg, raw | Calcium, Ca | 14.24 MG | range 10.9-17.2, median 14.55, 8 samples, spread 44% || Lettuce, leaf, green, raw | Calcium, Ca | 39.84 MG | range 29.7-52.3, median 41.95, 8 samples, spread 57%
   USDA FoodData Central, Foundation Foods, release 2026-04-30 · checked 2026-09-28
2. kvl-daily-r4-02 · reference dataset
   Kale, raw | Vitamin K (phylloquinone) | 390.0 UG | range 369.0-422.0, median 378.0, 3 samples, spread 14% || Lettuce, iceberg, raw | Vitamin K (phylloquinone) | 20.5 UG | range 12.3-27.8, median 21.4, 8 samples, spread 76% || Lettuce, leaf, red, raw | Vitamin K (phylloquinone) | 73.2 UG | range 55.8-94.8, median 71.5, 8 samples, spread 53%
   USDA FoodData Central, Foundation Foods, release 2026-04-30 · checked 2026-09-28
3. kvl-daily-r4-04 · reference dataset
   13-234 Curly kale, raw | Vitamin C (mg) 110 | Calcium (mg) 130 | Vitamin K1 (µg) 623.00 || 13-520 Lettuce, average, raw | Vitamin C (mg) 1 | Calcium (mg) 24
   CoFID 2021, Public Health England, OGL v3.0 · checked 2026-09-28
4. kvl-daily-r4-05 · reference dataset
   Kale, raw (FNDDS 72119190) | 1 cup 25 g | Guideline amount in salad 50 g || Lettuce, raw (FNDDS 75113000) | 1 cup 35 g | Guideline amount in salad 70 g | 1 head 539 g
   USDA FNDDS (Food and Nutrient Database for Dietary Studies), fdc_id 2709599 and 2709789 · checked 2026-09-28
5. kvl-daily-r4-08 · 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).
   Eur J Clin Nutr, 2008 · checked 2026-09-28
6. kvl-daily-r4-09 · controlled feeding study
   Seven healthy volunteers ingested a single 400 g serving of kale with 30 g vegetable oil. ... Modelling results demonstrated a mean (n 6) bioavailability of phylloquinone from kale to be 4.7%.
   Br J Nutr, 2010 · checked 2026-09-28
7. kvl-daily-r4-10 · controlled feeding study
   Absorption of phylloquinone from fresh spinach (165 microgram K), fresh broccoli (184 microgram K) and fresh romaine lettuce (179 microgram K) did not differ. There was no difference in phylloquinone absorption from fresh or cooked broccoli or from fresh romaine lettuce consumed with a meal containing 30 or 45% energy as fat.
   J Nutr, 1999 · checked 2026-09-28
8. kvl-daily-r4-11 · agency fact sheet
   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
9. kvl-daily-r4-06 · reference dataset
   USDA-RETN-R6:3005 VEG,GREENS,BOILED,WATER COVER DRAIN | vitamin_c_mg 55% | folate_total_ug 60% | potassium_mg 85% | calcium_mg 95% | iron_mg 95%
   USDA Table of Nutrient Retention Factors, Release 6, 2007, code 3005 · checked 2026-09-28
10. kvl-daily-r4-07 · reference dataset
    Kale, raw | Vitamin C, total ascorbic acid | 93.4 MG | range 84.4-104.0, 5 samples || Kale, frozen, cooked, boiled, drained | Vitamin C, total ascorbic acid | 17.8 MG | range 10.0-24.4, 3 samples
    USDA FoodData Central, Foundation Foods, release 2026-04-30 · checked 2026-09-28
11. kvl-daily-r4-14 · randomized crossover trial, ex vivo endpoint
    Cooked B. carinata significantly reduced DNA damage induced by AFB1 as compared to baseline levels (+S9 mix: 35%, -S9 mix: 33%, p ≤ 0.01, respectively). Raw B. carinata only reduced DNA damage by S9-activated AFB1 by 21% (p = 0.08).
    Nutrients, 2018 · checked 2026-09-28
12. kvl-daily-r4-15 · controlled feeding study
    Results showed that after ingestion of fresh lettuce, plasma total radical-trapping antioxidant potential (TRAP), measured as area under the curve, was significantly higher (1.3 (sem 0.3) mmol/l per 6 h; P<0.05) than the value obtained with MAP-stored lettuce (0.1 (sem 0.2) mmol/l per 6 h).
    Br J Nutr, 2002 · checked 2026-09-28
13. kvl-daily-r4-13 · randomized crossover trial
    The 0-4-h AUCs for glucose (385.7, 361.9, and 382.3 mg/dL for the control, lettuce, and watercress meals, respectively) were calculated to be lower when meal was consumed with lettuce compared to the control (P < 0.05) and watercress (P < 0.01) meals.
    Lipids Health Dis, 2021 · checked 2026-09-28
14. kvl-daily-r4-19 · prospective cohort
    Over 10.5-y follow-up, higher vitamin K1 intakes were associated with lower COPD rates, with inverse associations reaching a plateau above ∼250 μg/d [HRquintile (Q)5 compared with Q1: 0.84; 95% confidence interval (CI): 0.75, 0.94], whereas no association was observed for vitamin K2.
    Am J Clin Nutr, 2026 · checked 2026-09-28
15. kvl-daily-r4-16 · meta-analysis of vegetable and soil measurements
    Taro and kale had significantly higher capability to accumulate soil Tl among the 35 species studied, with Tl bioconcentration factor values of 0.060 and 0.133, respectively.
    Sci Total Environ, 2024 · checked 2026-09-28
16. kvl-daily-r4-18 · meta-analysis of prevalence studies
    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 2026-09-28
17. kvl-daily-r4-03 · reference dataset
    Kale, raw | Potassium, K | 348.0 MG | range 301.0-389.0, 5 samples || Lettuce, leaf, red, raw | Potassium, K | 321.4 MG | range 268.0-383.0, 8 samples || Kale, raw | Carotene, beta | 2870.0 UG | range 2160.0-3830.0, 3 samples || Lettuce, cos or romaine, raw | Carotene, beta | 5230.0 UG | range 1770.0-8750.0, 6 samples, spread 133%
    USDA FoodData Central, Foundation Foods, release 2026-04-30 · checked 2026-09-28
18. kvl-daily-r4-12 · randomized controlled trial
    The HOMA-IR levels in the intervention group showed a significant decrease from the start to the end of this study (−0.95 ± 0.34) compared to the control group (0.15 ± 0.28) (p < 0.03, Figure 5B).
    Nutrients, 2024 · checked 2026-09-28
19. kvl-daily-r4-17 · case report
    Despite its potential for high toxicity, thallium is understudied and not regulated in food. The California Department of Public Health was alerted to a household cluster of elevated urine thallium levels noted among a mother (peak 5.6 µg/g creatinine; adult reference: ≤0.4 µg/g creatinine) and her three young children (peak 10.5 µg/g creatinine; child reference: ≤0.8 µg/g creatinine).
    Int J Environ Res Public Health, 2025 · checked 2026-09-28