Leeks: a quiet vegetable with loud cancer headlines

The strongest evidence touching leeks is about leaving them out. In an umbrella review of 24 randomized trials in 1,646 people with irritable bowel syndrome, a low FODMAP diet, which limits fructan-rich foods such as leek bulbs, improved total symptoms (RR 1.42, 95% CI 1.02 to 1.97). That tested the diet as a whole, and leeks are only one of the foods it cuts. For everyone else a leek is a watery vegetable with some potassium and fiber. Raw leeks gave 319 mg potassium and 3 g fiber per 3.5 oz (100 g) on average, and potassium ran from 201 to 422 mg across 8 US samples. One whole leek weighs about 3 oz (85 g). The cancer story splits by study design. A pool of 21 studies, mostly case-control, linked the highest intake of onion-family vegetables with 46 percent lower odds of stomach cancer and reported a similar result for leeks. The Dutch cohort of 120,852 adults, which followed people forward, found no association between leeks and stomach cancer.

The numbers, per 3.5 oz (100 g)

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

How much is actually in it

• Raw leeks give about 319 mg potassium and 3 g fiber per 3.5 oz (100 g), and potassium ranged from 201 to 422 mg across 8 US samples. — USDA Foundation Foods, leeks, bulb and greens, root removed, raw, 8 analyzed samples Evidence E source
  Potassium 319 mg (201-422), fiber 3.0 g (2.3-3.6), vitamin B6 0.208 mg (0.138-0.288), water 87.43 g per 3.5 oz (100 g)
• Leeks are among the plant foods that naturally contain inulin and oligofructose, the fructans added to foods as prebiotic fiber. — Background statement in a controlled crossover study of 5 g oligofructose-enriched inulin in 47 adults Evidence C source
  Inulin present in leeks, amount not given

What a real portion looks like

• One whole leek weighs about 3 oz (85 g) and a slice about 7 g in US survey measures. — US household measures, FNDDS food code 75218400 (leeks) (n = 4 measures) Evidence E source
  1 whole 85 g, 1 cup 170 g, 1 slice 7 g, quantity not specified 85 g

What cooking and storage change

• Boiling leeks in unsalted water lowers potassium from 230 to 169 mg and fiber from 2.8 to 1.9 g per 3.5 oz (100 g) in the UK tables. — CoFID 2021, leeks boiled in unsalted water (13-625) against raw (13-624), per 3.5 oz (100 g) Evidence E source
  Potassium 169 vs 230 mg, fiber 1.9 vs 2.8 g, vitamin C 2 vs 3.1 mg, folate 44 vs 33 mcg
• By the USDA retention table, vegetables like leeks boiled in water and drained keep about 65% of folate and 75% of vitamin C. — USDA retention factors for the group Vegetables and Vegetable Products, method VEG,OTHER,BLD,WATER COVER,DRAINED Evidence E source
  Folate 65%, vitamin C 75%, vitamin B6 85%, potassium 90% retained (Retn_Code 3775)
• A 2018 English restaurant outbreak of Clostridium perfringens was traced to leeks in cheese sauce that had been poorly hot-held and reused. — 40 diners in a case-control study, West Midlands, December 2018 (n = 40) Evidence C source
  AOR 51.1 (95% CI 4.13-2492.1) for leeks in cheese sauce

What it does to your health

• Observational data from 21 studies of 543,220 people link the highest Allium vegetable intake with 46% lower odds of gastric cancer, and leeks gave a similar result. — 19 case-control and 2 cohort studies, 543,220 subjects (n = 543220) Evidence C source
  OR 0.54 (95% CI 0.43-0.65) highest vs lowest Allium intake, OR 0.91 per 20 g/day
• In the same Dutch cohort, leek consumption was not associated with colon or rectal cancer in men or women. — 120,852 men and women aged 55 to 69, 150 and 143 colon and 93 and 57 rectum cancer cases over 3.3 years (n = 120852) Evidence C source
  No association between leek intake and colorectal carcinoma
• In 58,279 Dutch men, each extra 0.88 oz (25 g) of leeks a day was associated with a 38% higher prostate cancer rate. — 58,279 men aged 55 to 69 in the Netherlands Cohort Study, 6.3 years of follow-up, 610 cases (n = 58279) Evidence C source
  RR 1.38 per 25 g/day of leek

Safety, limits and interactions

• The first reported case of leek-dependent exercise-induced anaphylaxis occurred in an 18-year-old woman, while heated leek alone was tolerated. — One 18-year-old woman in Japan (n = 1) Evidence D source
  Anaphylaxis only when leek was combined with physical exertion

What people believe that the data does not support

• In the prospective Netherlands Cohort Study of 120,852 adults, eating leeks was not associated with stomach cancer, while onions were. — 120,852 men and women aged 55 to 69 in the Netherlands, 139 stomach carcinoma cases over 3.3 years (n = 120852) Evidence C source
  Leeks: no association, onions >= 0.5/day: RR 0.50 (95% CI 0.26-0.95)

Who this works differently for

• In 18,888 Norwegian pregnant women, eating alliums such as garlic, onion and leek was linked to 18% lower odds of spontaneous preterm delivery. — 18,888 women in the Norwegian Mother and Child Cohort, 950 spontaneous preterm deliveries (n = 18888) Evidence C source
  OR 0.82 (95% CI 0.72-0.94) for alliums
• In an RCT on 97 children with chemotherapy-related febrile neutropenia, capsules of lyophilised leek leaves shortened neutrophil recovery to 4.0 days from 6.4 on placebo. — 97 children with cancer and febrile neutropenia in Iran, Allium ampeloprasum capsules 500 mg twice daily for 7 days vs placebo (n = 97) Evidence B source
  Neutrophil recovery 4.02 vs 6.38 days (P < 0.001), no adverse effect reported
• For people with IBS, a low-FODMAP diet, which limits fructan-rich foods such as leek bulbs, improved total symptoms by an umbrella meta-analysis of 24 RCTs in 1,646 patients. — 11 meta-analyses and 24 RCTs, 1,646 participants with irritable bowel syndrome (n = 1646) Evidence A source
  Clinical improvement of total IBS-SSS symptoms RR 1.42 (95% CI 1.02-1.97)

What is still unknown

• The UK tables give raw leeks 23 calories, 2.8 g fiber and 230 mg potassium per 3.5 oz (100 g), about 28% less potassium than the US samples. — CoFID 2021, leeks, raw (13-624), per 3.5 oz (100 g) Evidence E source
  23 calories, 2.8 g AOAC fiber, 230 mg potassium, 33 mcg folate, 3.1 mg vitamin C per 3.5 oz (100 g)

The bottom line

The same Dutch cohort found no link between leeks and colon or rectal cancer over 3.3 years. In its 58,279 men, each extra 0.88 oz (25 g) of leek a day went with a 38 percent higher prostate cancer rate. That cohort compared 21 vegetables and 8 fruits at once, so a single hit like this is likely chance, and the authors drew no causal conclusion. Boiling costs something. In the UK tables boiled leeks hold 169 mg potassium against 230 mg raw and 1.9 g fiber against 2.8 g, per 3.5 oz (100 g), and part of that gap is added water. The USDA retention factor for boiled and drained vegetables as a group, from a 1975 table and not specific to leeks, keeps about 65 percent of folate and 75 percent of vitamin C. In a trial on 97 children in Iran with febrile neutropenia after chemotherapy, capsules of freeze-dried leaves from a leek subspecies, 500 mg twice a day, shortened neutrophil recovery to 4.0 days from 6.4 on placebo. That was a single center and a capsule, and it says nothing about eating leeks. Who should be careful. People with IBS who react to fructans may do better with less leek bulb. Leek allergy is described as extremely rare, and the first reported case of leek-dependent exercise-induced anaphylaxis, in an 18-year-old woman, appeared only when she exercised after eating it. In a 2018 English restaurant outbreak of Clostridium perfringens, leeks in cheese sauce that had been poorly hot-held and reused were the one dish linked to illness (aOR 51.1, 95% CI 4.1 to 2492), and the fault was temperature control. In 18,888 pregnant Norwegian women, eating onion-family vegetables was linked to 18 percent lower odds of spontaneous preterm birth, a link strongest for garlic, with no separate figure for leek. We found no card on leeks and medicines.

Sources

1. leek-daily-r6-01 · USDA FoodData Central Foundation Foods, dataset
   Leeks, bulb and greens, root removed, raw: Potassium, K 319.0 mg, range 201.0-422.0, n=8 · Fiber, total dietary 3.0 g, range 2.3-3.6, n=8 · Vitamin B-6 0.2083 mg, range 0.138-0.288, n=8 · Water 87.43 g, range 83.26-92.28, n=8
   USDA FoodData Central Foundation Foods, release 2026-04-30, FDC 2727584 · checked 2026-09-29
2. leek-daily-r6-13 · controlled clinical trial
   Inulin is a natural food component found in many plants that are part of the human diet (e.g., leeks, onions, wheat, garlic, chicory and artichokes).
   Nutrients, 2015 · checked 2026-09-29
3. leek-daily-r6-05 · USDA FNDDS portion weights, dataset
   FNDDS 75218400 Leeks: 1 whole 85 g · 1 cup 170 g · 1 slice 7 g · Quantity not specified 85 g
   USDA FNDDS 2021-2023, food code 75218400 Leeks (FDC 2709935) · checked 2026-09-29
4. leek-daily-r6-03 · CoFID 2021, dataset
   CoFID 13-625 Leeks, boiled in unsalted water: Potassium 169 mg · AOAC fibre 1.9 g · Vitamin C 2 mg · Folate 44 µg · Water 92.4 g
   CoFID 2021, Public Health England, codes 13-624 and 13-625 (OGL v3.0) · checked 2026-09-29
5. leek-daily-r6-04 · USDA Table of Nutrient Retention Factors, dataset
   USDA-RETN-R6:3775 VEG,OTHER,BLD,WATER COVER,DRAINED (Vegetables and Vegetable Products): folate_total_ug 65% · vitamin_c_mg 75% · vitamin_b6_mg 85% · potassium_mg 90%
   USDA Table of Nutrient Retention Factors, Release 6, 2007, Retn_Code 3775 · checked 2026-09-29
6. leek-daily-r6-12 · case-control study
   Multivariable analysis found that leeks in cheese sauce was the only item associated with illness (aOR 51.1; 95% CI 4.13-2492.1). Environmental investigations identified significant lapses in food safety, including lapses in temperature control during cooking and hot holding, likely cross-contamination between raw and cooked foods and the reuse of leftover cheese sauce for the next day's service.
   Epidemiol Infect, 2020 · checked 2026-09-29
7. leek-daily-r6-06 · meta-analysis of observational studies
   In a pooled analysis of all studies, consumption of large amounts of Allium vegetables (in a comparison of the highest and lowest consumption groups) reduced the risk for gastric cancer (odds ratio, 0.54; 95% confidence interval, 0.43-0.65). Specific analyses for onion, garlic, leek, Chinese chive, scallion, garlic stalk, and Welsh onion yielded similar results, except for onion leaf.
   Gastroenterology, 2011 · checked 2026-09-29
8. leek-daily-r6-08 · cohort study
   Leek consumption was not associated with colon and rectum carcinoma incidence. The use of garlic supplements was not associated with colon and rectum carcinoma in men and women combined.
   Carcinogenesis, 1996 · checked 2026-09-29
9. leek-daily-r6-09 · cohort study
   Observed positive associations were significant for consumption of leek (RR, 1.38) and oranges (RR, 1.07) and nonsignificant for sweet peppers (RR, 1.60) and mushrooms (RR, 1.49).
   Cancer Epidemiol Biomarkers Prev, 1998 · checked 2026-09-29
10. leek-daily-r6-11 · case report
    Prick-to-prick tests for leek was 1+ and challenge test for heated leek was negative, but leek-dependent physical exertion challenge test evoked anaphylaxis.
    J Dermatol, 2024 · checked 2026-09-29
11. leek-daily-r6-07 · cohort study
    The consumption of leeks and the use of garlic supplements were not associated with stomach carcinoma risk.
    Gastroenterology, 1996 · checked 2026-09-29
12. leek-daily-r6-10 · cohort study
    Alliums (garlic, onion, leek, and spring onion) [OR: 0.82 (95% CI: 0.72, 0.94), P = 0.005] and dried fruits (raisins, apricots, prunes, figs, and dates) [OR: 0.82 (95% CI: 0.72, 0.94); P = 0.005] were associated with a decreased risk of spontaneous PTD.
    J Nutr, 2013 · checked 2026-09-29
13. leek-daily-r6-14 · randomized controlled trial
    Overall, 97 febrile neutropenic children were enrolled. The intervention group (n=49) was given A. ampeloprasum L. in capsules (500 mg twice daily) for seven days plus supportive care.
    Am J Blood Res, 2023 · checked 2026-09-29
14. leek-daily-r6-15 · systematic umbrella review and meta-analysis
    A total of 658 articles were initially identified, with 11 meta-analyses and 24 RCTs reporting 28 outcomes with 1646 participants included. An LFD significantly affected the clinical improvement of total symptoms according to the IBS-SSS questionnaire (RR: 1.42; 95% CI: 1.02, 1.97; P = 0.04) in all the subtypes of IBS and also had favorable effects on stool consistency (WMD: -0.48; 95% CI: -0.902, -0.07) and frequency (WMD: -0.36; 95% CI: -0.61, -0.10) and some other GI symptoms in both less and more than 4 weeks of diet intervention except for stool consistency, which needed more than 4 weeks of LFD implementation.
    Food Funct, 2024 · checked 2026-09-29
15. leek-daily-r6-02 · CoFID 2021, dataset
    CoFID 13-624 Leeks, raw: Energy 23 kcal · AOAC fibre 2.8 g · Potassium 230 mg · Folate 33 µg · Vitamin C 3.1 mg
    CoFID 2021, Public Health England, code 13-624 (OGL v3.0) · checked 2026-09-29