Kohlrabi: a vitamin C vegetable that science studies only as part of the cabbage family

Almost everything research says about kohlrabi, it says about cabbage-family vegetables as a group. The firmest number concerns medicines. A meta-analysis of 23 dietary intervention trials found that eating cruciferous vegetables raised the activity of the liver enzyme CYP1A2 by 20 to 40 percent, though kohlrabi was not among the vegetables tested and the authors left the clinical size of that change unresolved. What kohlrabi itself offers is measured in food tables. Raw, it holds 62 mg of vitamin C per 3.5 oz (100 g) in USDA data and 43 mg in the UK table, and the UK figure for boiled kohlrabi is 27 mg. Neither table has sampled spread, so the 31 percent gap between two countries is the only range we have. A cup of raw kohlrabi weighs 4.8 oz (135 g), a cup of cooked 170 g. The one randomized trial we found on kohlrabi by name is a pilot in 118 preschool children aged 3 to 5, where a storybook with positive messages about it made more of them willing to taste it, with whole classes randomized and no intake measured.

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 kohlrabi holds 62 mg of vitamin C per 3.5 oz (100 g) in USDA data and 43 mg in the UK tables, and UK boiled kohlrabi keeps 27 mg. — USDA SR Legacy record and CoFID 2021 records for kohlrabi (n = 3 records) Evidence E source
  Vitamin C per 3.5 oz (100 g): USDA raw 62 mg (69% DV), CoFID raw 43 mg, CoFID boiled 27 mg; fiber USDA raw 3.6 g

What a real portion looks like

• A cup of raw kohlrabi weighs 4.8 oz (135 g) and a cup of cooked kohlrabi 170 g in the US survey food database. — FNDDS portion weights for Kohlrabi, raw and Kohlrabi, cooked Evidence E source
  Raw: 1 cup 135 g, 1 slice 15 g, unspecified 68 g; cooked: 1 cup 170 g, unspecified 85 g

What your body absorbs

• In a randomized crossover trial in 10 people eating 11 oz (300 g) of broccoli soup, only 2 to 15% of the glucosinolate eaten came out as sulforaphane in urine over 24 hours. — 10 volunteers, three phases of 300 g broccoli soup with plant myrosinase denatured (n = 10) Evidence B source
  24 h urinary sulforaphane 2-15% of glucoraphanin consumed, varying by person not by broccoli genotype

What cooking and storage change

• Laboratory boiling of kohlrabi, white cabbage and red cabbage showed glucosinolates and their breakdown products leaching into the cooking water. — kohlrabi, white cabbage and red cabbage heated in water Evidence D source
  Glucosinolate loss by leaching into boiling water, plus heat breakdown to nitriles at 99 C
• Steaming keeps more of the vitamin C in vegetables like kohlrabi than boiling in plenty of water: the USDA retention table gives 85% against 75%. — USDA retention factors for the group Vegetables and Vegetable Products Evidence E source
  Vitamin C 85% and folate 85% retained steamed (Retn_Code 3784) vs vitamin C 75% and folate 65% boiled in covering water and drained (Retn_Code 3775)

What it does to your health

• Observational data from 89 studies link the highest intake of cruciferous vegetables with a 19% lower risk of gastrointestinal cancers compared with the lowest. — participants of 89 observational studies in 81 articles (n = 89 studies) Evidence C source
  RR 0.81 (95% CI 0.76-0.87), highest vs lowest intake

Safety, limits and interactions

• In a randomized crossover feeding trial in 73 adults, two weeks of added cruciferous vegetables raised the activity of the drug-metabolizing enzyme CYP1A2, and a double dose raised it further. — 73 adults recruited by genotype, four diets of two weeks each (n = 73) Evidence B source
  CYP1A2 activity up with a single dose vs basal diet (P < 0.0001) and up again with a double dose (P < 0.0001)
• A meta-analysis of 23 dietary intervention trials found that eating cruciferous vegetables raised CYP1A2 activity by 20-40% and GST-alpha activity by 15-35%. — adults in 23 dietary intervention trials (broccoli, Brussels sprout, cabbage, cauliflower, radish, watercress) (n = 23 trials) Evidence B source
  CYP1A2 activity +20-40%, GST-alpha +15-35%

What people believe that the data does not support

• A 2024 systematic review of 123 studies concluded that brassica vegetables in the daily diet, with adequate iodine, pose no adverse effects on thyroid function in humans. — 123 in vitro, animal and human studies on brassica plants and the thyroid (n = 123 articles) Evidence C source
  No adverse thyroid effect in humans with adequate iodine intake (qualitative)
• The EU refused the claim that foods containing glucosinolates help strengthen the body's defenses. — EU Register on nutrition and health claims Evidence E source
  Non-authorized

Who this works differently for

• In a randomized trial in 118 preschool children aged 3 to 5, a storybook with positive messages about kohlrabi made more children willing to taste it than no book. — 118 children aged 3 to 5 in nine Head Start classes (n = 118) Evidence B source
  More tasters in the positive-message group than control at the second posttest (P < .05)

What is still unknown

• Only three studies have looked at kohlrabi and the thyroid, all in animals, and none in people. — rats, guinea pigs and rabbits fed kohlrabi or kohlrabi sprouts (n = 3 studies) Evidence D source
  Mixed: protective tendency in rats, lower iodine uptake in guinea pigs, enlarged thyroid in rabbits after 5 weeks

The bottom line

How you cook it matters more than whether you cook it. USDA retention factors for vegetables as a group, not measured on kohlrabi, keep 85 percent of vitamin C and folate after steaming, against 75 and 65 percent after boiling in water that is then poured away. A laboratory study that boiled kohlrabi and two cabbages found glucosinolates and their breakdown products leaching into the cooking water. Its abstract gives no percentage for kohlrabi alone. Those glucosinolates are the reason the family is studied. Your body turns little of them into the compound researchers track. In a randomized crossover trial in 10 people eating 11 oz (300 g) of broccoli soup made with its own conversion enzyme destroyed, only 2 to 15 percent came out as sulforaphane in urine over 24 hours. That was broccoli, and kohlrabi carries a different mix. Observational data from 89 studies link the highest intake of cruciferous vegetables with a 19 percent lower risk of digestive tract cancers than the lowest (RR 0.81, 95% CI 0.76 to 0.87). That is an association with residual confounding, and kohlrabi was never analyzed separately. The EU refused the claim that foods containing glucosinolates help strengthen the body's defenses. The thyroid worry is mostly unfounded for this family. A 2024 systematic review of 123 laboratory, animal and human studies concluded that brassica vegetables in the daily diet, with adequate iodine, pose no adverse effects on thyroid function in humans, without pooling any numbers and with few human data. For kohlrabi specifically only three studies exist, all in rats, guinea pigs and rabbits, with mixed results and none in people. Who should be careful: people taking a drug cleared by CYP1A2. In a randomized crossover feeding trial in 73 adults, two weeks of added cruciferous vegetables raised that enzyme's activity, and a double portion raised it further, though nobody measured what this does to a drug level. If you plan to start eating a lot more of these vegetables while on such a medicine, mention it to the person who prescribes it. We found nothing on kohlrabi in pregnancy or kidney disease.

Sources

1. kohl-r7-12 · USDA FoodData Central SR Legacy and CoFID 2021, dataset
   Kohlrabi, raw (FDC 168424, SR Legacy, Vegetables and Vegetable Products): vitamin C 62 mg per 100 g = 69% DV; copper 0.129 mg = 14% DV; fiber 3.6 g = 13% DV; Foundation samples 0. CoFID 13-261 Kohl rabi, raw: Vitamin C 43 mg; CoFID 13-576 Kohl rabi, boiled in unsalted water: Vitamin C 27 mg
   USDA FoodData Central, SR Legacy, FDC 168424; CoFID 2021, codes 13-261 and 13-576 · checked 2026-09-29
2. kohl-r7-11 · USDA FNDDS, dataset
   Kohlrabi, raw (FNDDS 75112000, fdc_id 2709788): 1 cup 135 g; 1 slice 15 g; Quantity not specified 68 g. Kohlrabi, cooked (FNDDS 75218011, fdc_id 2709934): 1 cup 170 g; Quantity not specified 85 g
   USDA FNDDS 2021-2023, food codes 75112000 and 75218011 · checked 2026-09-29
3. kohl-r7-04 · randomized controlled trial
   Ten participants are recruited into a three-phase, double-blinded, randomized crossover trial (NCT02300324), with each phase comprising consumption of 300 g of a soup made from broccoli of one of three Myb28 genotypes (Myb28B/B , Myb28B/V , Myb28V/V ). Plant myrosinases are intentionally denatured during soup manufacture. ... The percentage of sulforaphane excreted in 24 h relative to the amount of glucoraphanin consumed varies among volunteers from 2 to 15%, but does not depend on the broccoli genotype.
   Mol Nutr Food Res, 2018 · checked 2026-09-29
4. kohl-r7-08 · laboratory food chemistry
   The results were correlated with the inactivation kinetics of myrosinase and epithiospecifier protein activity in the Brassica oleracea vegetables kohlrabi, white cabbage, and red cabbage. ... Boiling led to leaching of glucosinolates and their hydrolysis products into the boiling water.
   Food Chem, 2018 · checked 2026-09-29
5. kohl-r7-10 · USDA Table of Nutrient Retention Factors, dataset
   USDA-RETN-R6:3784 VEG,OTHER,STEAMED, group Vegetables and Vegetable Products: vitamin_c_mg 85% retained, folate_total_ug 85%, copper_mg 100%, potassium_mg 100%; USDA-RETN-R6:3775 VEG,OTHER,BLD,WATER COVER,DRAINED: vitamin_c_mg 75% retained, folate_total_ug 65%, potassium_mg 90%, copper_mg 95%
   USDA Table of Nutrient Retention Factors, Release 6, 2007, Retn_Codes 3784 and 3775 · checked 2026-09-29
6. kohl-r7-07 · meta-analysis of observational studies
   The final analysis included 81 articles covering 89 studies. In comparison to the lowest consumption categories, the highest consumption categories of CV were associated with a lower risk for all GI cancers [rate ratio (RR): 0.81, 95% confidence interval (95% CI) 0.76-0.87].
   Crit Rev Food Sci Nutr, 2024 · checked 2026-09-29
7. kohl-r7-02 · randomized controlled trial
   Participants (N = 73), recruited based on genotypes, consumed four diets for two weeks each: low-phytochemical diet (basal), basal plus single dose of cruciferous (1C), basal plus double dose of cruciferous (2C), and basal plus single dose of cruciferous and apiaceous vegetables (1C+A). ... Compared with basal diet, the 1C diet increased CYP1A2 activity (P < 0.0001) and the 2C diet resulted in further increases (P < 0.0001), with men experiencing greater dose-response than women.
   Cancer Epidemiol Biomarkers Prev, 2009 · checked 2026-09-29
8. kohl-r7-03 · meta-analysis of intervention trials
   Twenty-three dietary intervention trials with drug metabolism end points were identified across Embase, Medline, and the Cochrane Controlled Register of Trials (CENTRAL). ... The meta-analyses performed demonstrated a significant effect on CYP1A2 and glutathione S-transferase-alpha (GST-α), with consumption of Cruciferae increasing the activities of these enzymes by 20-40% and 15-35%, respectively.
   Clin Pharmacol Ther, 2020 · checked 2026-09-29
9. kohl-r7-05 · systematic review
   The vast majority of the results cast doubt on previous assumptions claiming that brassica plants have antithyroid effects in humans. Instead, they indicate that including brassica vegetables in the daily diet, particularly when accompanied by adequate iodine intake, poses no adverse effects on thyroid function.
   Int J Mol Sci, 2024 · checked 2026-09-29
10. kohl-r7-09 · EU health claim decision
    POL-HC-7196 Glucosinolates Foods containing glucosinolates help strengthen our body's defences. Non-authorised
    EU Register on nutrition and health claims, claim POL-HC-7196, snapshot 2026-09-21 · checked 2026-09-29
11. kohl-r7-01 · randomized controlled trial
    PARTICIPANTS: 118 children, ages 3 to 5. INTERVENTIONS: Children's book with prominent positive or negative messages about kohlrabi. ... RESULTS: There were significantly more kohlrabi tasters in the positive-message group than in the control group during the second posttest (P <.05).
    J Nutr Educ Behav, 2002 · checked 2026-09-29
12. kohl-r7-06 · animal studies
    Only three studies reported the impact of kohlrabi on thyroid function (Table 2).
    Int J Mol Sci, 2024 · checked 2026-09-29