Glucosinolates, Blood Sugar, and Cancer Protection

How kohlrabi's glucosinolates, isothiocyanates, fiber, and vitamin C profile support cancer prevention, blood sugar control, and cardiovascular health

Kohlrabi is a mild, crunchy member of the Brassica family — the same plant group that includes broccoli, kale, and cabbage. Though it looks unusual (a swollen stem on stilts), its nutritional value is serious. A 100g serving of raw kohlrabi delivers around 62mg of vitamin C (nearly 70% of the daily target), 3.6g of fiber, and a meaningful dose of glucosinolates — the sulfur compounds that convert to cancer-fighting isothiocyanates when you chew or chop the vegetable [2]. The red-purple variety also contains anthocyanins, which give it roughly twice the antioxidant activity of green kohlrabi and stronger inhibitory effects on enzymes involved in blood sugar regulation and inflammation [1]. For a vegetable with only 27 calories per 100g, it is one of the more nutrient-dense options in the produce aisle.

The Glucosinolate-Isothiocyanate System

Kohlrabi belongs to the Cruciferae family, whose defining nutritional feature is glucosinolates — nitrogen- and sulfur-containing precursor compounds stored in plant cell vacuoles. When kohlrabi is cut, chewed, or blended, the enzyme myrosinase (held in separate cell compartments) contacts these precursors and converts them to biologically active isothiocyanates [2].

Kohlrabi's specific glucosinolate profile includes:

Glucoerucin (the dominant glucosinolate) — converts to erucin, structurally similar to sulforaphane
Glucoraphanin — converts directly to sulforaphane
Gluconasturtiin — converts to phenylethyl isothiocyanate (PEITC)
Sinigrin — converts to allyl isothiocyanate (AITC)

The sulfur content of the soil in which kohlrabi is grown directly affects isothiocyanate yield — plants grown with adequate sulfur supply produced up to 7-fold more isothiocyanates than sulfur-deficient plants in controlled trials [2]. This is worth knowing when evaluating whole-food versus supplement sources.

Once inside the body, isothiocyanates activate the Nrf2 transcription pathway — a master cellular switch that upregulates antioxidant enzymes, accelerates detoxification of environmental carcinogens, and damps inflammatory signaling [4]. This shared mechanism links kohlrabi to the broader body of cruciferous vegetable research.

Blood Sugar and Anti-Inflammatory Activity

A laboratory study (Jung et al., 2014) evaluated extracts of both green and red kohlrabi cultivars for anti-diabetic and anti-inflammatory activity [1]. Red kohlrabi extract inhibited protein tyrosine phosphatase 1B (PTP1B) with an IC50 of 207 μg/mL — PTP1B is a negative regulator of insulin signaling and a validated drug target for type 2 diabetes management. Both cultivars also suppressed LPS-induced nitric oxide production in macrophage cultures, confirming anti-inflammatory effects at the cellular level.

Red kohlrabi showed approximately twice the total phenolic content of green kohlrabi, and its inhibitory activity against PTP1B and inflammatory pathways was correspondingly stronger. The anthocyanins and additional polyphenols concentrated in the purple skin appear to drive this difference.

The high fiber content also contributes to blood sugar management: 3.6g of fiber per 100g (similar to broccoli) slows glucose absorption from the gut, blunts postprandial insulin spikes, and feeds fermentative gut bacteria that produce short-chain fatty acids.

Cancer Prevention

Epidemiological evidence consistently links higher intake of cruciferous vegetables — as a category — to lower risk of several cancers. A 2007 review by Higdon et al. examining prospective cohort studies found statistically significant inverse associations between cruciferous vegetable consumption and lung cancer in Dutch men and women, colorectal cancer in multiple populations, and bladder cancer in men [3]. The underlying mechanisms are well-characterized:

Phase 2 enzyme induction: Isothiocyanates upregulate enzymes that conjugate and eliminate carcinogens before they can damage DNA [5]
Phase 1 enzyme inhibition: Some isothiocyanates reduce activation of procarcinogens by CYP enzymes
Apoptosis induction: Isothiocyanates trigger programmed cell death in cancer cell lines at concentrations achievable from dietary intake
HDAC inhibition: Sulforaphane and erucin (both found in kohlrabi) inhibit histone deacetylase, reactivating silenced tumor suppressor genes
Anti-angiogenic activity: Isothiocyanates may slow vascular development in tumors [6]

Individual isothiocyanates from kohlrabi — particularly PEITC and erucin — have been studied separately for anti-cancer activity and confirmed to operate through these Nrf2- and apoptosis-dependent pathways [6].

Cardiovascular and Neuroprotective Effects

Beyond cancer, the isothiocyanates in kohlrabi have documented cardio- and neuroprotective effects. A 2022 review summarizing the mechanistic literature found that cruciferous isothiocyanates reduce LDL oxidation, suppress NF-κB inflammatory signaling relevant to atherosclerosis, and protect neurons from oxidative stress via Nrf2 activation [6]. These effects are most studied in sulforaphane and PEITC — both present as glucosinolate-derived isothiocyanates in kohlrabi.

Practical Notes

Cooking method matters. Myrosinase is heat-sensitive; significant boiling destroys the enzyme and reduces isothiocyanate yield. The workaround: chop or shred kohlrabi and let it sit 5–10 minutes before applying heat, giving myrosinase time to complete conversion while still active.

Raw is best for maximum bioactivity — kohlrabi's mild flavor and satisfying crunch make it well-suited to eating raw, sliced thin, or grated into slaws. It pairs naturally with lemon, herbs, and yogurt-based dressings.

Red vs. green: Red or purple kohlrabi has measurably higher polyphenol content and stronger anti-inflammatory and anti-diabetic activity in vitro [1]. Both are nutritious; red varieties are worth seeking out when available.

The leaves are edible and nutritionally comparable to kale — high in vitamin C, calcium, and glucosinolates. They are typically discarded but can be cooked like any dark leafy green.

A 100g serving of raw kohlrabi provides approximately:

Calories: 27
Fiber: 3.6g
Vitamin C: 62mg (~69% RDA)
Potassium: 350mg (~7% RDA)
Vitamin B6: 0.15mg (~9% RDA)

See our Broccoli page for the companion deep-dive on sulforaphane and Nrf2. For related vegetables, see Kale, Brussels Sprouts, and Arugula.

Evidence Review

Anti-Diabetic and Anti-Inflammatory Activity — Jung et al. (2014)

Jung HA et al. published a laboratory study in Preventive Nutrition and Food Science (PMID 25580392) evaluating green and red kohlrabi methanol extracts for anti-diabetic activity (PTP1B inhibition), anti-inflammatory activity (NO inhibition in LPS-stimulated RAW 264.7 macrophages), antioxidant capacity (DPPH and ABTS radical scavenging), and total phenolic content [1].

Results: Red kohlrabi MeOH extract inhibited PTP1B with an IC50 of 207 ± 3.48 μg/mL; green kohlrabi showed 287 ± 3.22 μg/mL — both are meaningful inhibitory activities in the context of natural product pharmacology. Both cultivars suppressed LPS-induced nitric oxide production in a dose-dependent manner. Red kohlrabi had approximately twice the total phenolic content of green kohlrabi and correspondingly stronger antioxidant activity across both radical scavenging assays. The study attributed the red cultivar's superior activity to its higher anthocyanin and polyphenol content.

Limitations: This is an in vitro study using cell culture and biochemical assays. IC50 values from cell-free enzyme inhibition assays do not directly translate to effective doses in humans — absorption, distribution, and metabolism in vivo create complex gaps between laboratory potency and clinical effect. No human pharmacokinetic or clinical outcome data for kohlrabi-specific isothiocyanates exists at this time. Grade: C+ (proof-of-concept in vitro; mechanistically plausible but human confirmation is needed).

Isothiocyanate Profile — Gerendás et al. (2008)

Gerendás J et al. published a controlled cultivation study in the Journal of Agricultural and Food Chemistry (PMID 18715015) investigating how sulfur and nitrogen supply affects the isothiocyanate composition of kohlrabi tubers [2]. Plants were grown in pot experiments with varied S (0, 0.05, and 0.20 g per pot) and N (1, 2, and 4 g per pot) inputs.

Results: The dominant isothiocyanate in kohlrabi was methylthiobutyl ITC (ranging 11–1350 μmol/g DM depending on sulfur supply), followed by sulforaphane (7–120 μmol/g DM), phenylethyl ITC (5–34 μmol/g DM), and allyl ITC. Sulfur supply had a dramatic effect on total isothiocyanate yield — up to 7-fold differences between deficient and adequately supplied plants. Nitrogen supply had a more complex, non-linear effect.

Implications: Organic versus conventional growing conditions, soil quality, and fertilization practices directly affect the bioactive compound content of kohlrabi. Laboratory nutritional analyses represent averages; actual isothiocyanate content in any given head of kohlrabi can vary substantially. Grade: A for scientific rigor of the cultivation study; note the gap between agricultural research and actionable consumer guidance.

Cruciferous Vegetables and Cancer — Higdon et al. (2007) and Keck & Finley (2004)

Higdon JV et al. published a comprehensive review in Pharmacological Research (PMID 17317210) examining epidemiological evidence across prospective cohort and case-control studies of cruciferous vegetable intake and cancer incidence [3]. Key findings:

Inverse associations between cruciferous intake and lung cancer were found in Dutch men (RR 0.73 for highest vs. lowest quintile), Dutch women, and a U.S. women's cohort
Inverse associations with colorectal cancer were found in multiple large cohorts
Evidence for bladder cancer was suggestive; for breast and prostate cancer, the associations were inconsistent

Keck AS and Finley JW published a mechanistic review in Integrative Cancer Therapies (PMID 15035868) characterizing the multi-pathway cancer-protective activity of glucosinolate hydrolysis products [5]. The review identified five distinct mechanisms: Phase 2 enzyme induction, Phase 1 enzyme inhibition, altered estrogen metabolism (relevant to hormone-sensitive cancers), protection against reactive oxygen species, and pro-apoptotic activity in malignant cells.

Limitations of the epidemiology: All evidence is observational; randomized controlled trials of cruciferous vegetable intake and cancer incidence are neither feasible nor ethical to conduct at the required scale and duration. Residual confounding from correlated healthy behaviors is a consistent limitation. Cruciferous vegetables are typically measured as a food category, not kohlrabi specifically — direct extrapolation requires caution. Grade: B for the epidemiological signal; A for the mechanistic basis.

Isothiocyanates and Cardiovascular/Neurological Health — Kamal et al. (2022) and Dinkova-Kostova & Kostov (2012)

Kamal RM et al. published a detailed review in Molecules (PMID 35163897) summarizing cardiovascular and neuroprotective mechanisms of glucosinolate-derived isothiocyanates across the Cruciferae family, including sulforaphane, erucin, PEITC, and allyl ITC [6]. The review documented Nrf2-mediated antioxidant induction, NF-κB suppression (reducing atherosclerosis-promoting inflammation), direct LDL oxidation inhibition, and mitochondrial protective effects in neuronal cells.

Dinkova-Kostova AT and Kostov RV published a mechanistic synthesis in Trends in Molecular Medicine (PMID 22578879) characterizing the central role of Nrf2 in mediating the health effects of dietary glucosinolates [4]. The review established that the Nrf2 pathway is the shared molecular hub through which isothiocyanates upregulate antioxidant enzymes (glutathione S-transferase, NQO1, heme oxygenase-1), explaining the broad-spectrum protective effects seen across multiple organ systems.

Limitations: Most cardiovascular and neurological evidence for isothiocyanates is preclinical — cell culture and animal models. Human clinical trials specifically targeting isothiocyanates for cardiovascular or neurological endpoints are limited. Much of the clinical research uses broccoli sprout extracts or sulforaphane supplements rather than whole kohlrabi. Grade: B for mechanistic plausibility; C+ for direct human clinical evidence.

Overall Evidence Assessment

Blood sugar and anti-inflammatory activity: In vitro evidence from kohlrabi-specific research is promising; the mechanisms align with the broader isothiocyanate literature [1][4]. Human data is absent.

Cancer prevention: Epidemiologically, higher cruciferous vegetable consumption is associated with lower risk of lung, colorectal, and bladder cancer with a consistent directional signal across multiple large cohorts [3]. Kohlrabi contributes the same bioactive class of compounds — glucosinolate-derived isothiocyanates — that mechanistically underpin this protection [5].

Cardiovascular and neuroprotection: Mechanistic evidence via Nrf2 and NF-κB pathways is strong at the cellular level; clinical translation to kohlrabi specifically requires further research [6].

Nutrient density: Uncontested — kohlrabi's vitamin C, fiber, potassium, and glucosinolate content make it one of the most nutritionally efficient vegetables per calorie.

References

Anti-Diabetic and Anti-Inflammatory Effects of Green and Red Kohlrabi Cultivars (Brassica oleracea var. gongylodes)Jung HA, Karki S, Ehom NY, Yoon MH, Kim EJ, Choi JS. Preventive Nutrition and Food Science, 2014. PubMed 25580392 →
Isothiocyanate Concentration in Kohlrabi (Brassica oleracea L. Var. gongylodes) Plants as Influenced by Sulfur and Nitrogen SupplyGerendás J, Breuning S, Stahl T, Mersch-Sundermann V. Journal of Agricultural and Food Chemistry, 2008. PubMed 18715015 →
Cruciferous Vegetables and Human Cancer Risk: Epidemiologic Evidence and Mechanistic BasisHigdon JV, Delage B, Williams DE, Dashwood RH. Pharmacological Research, 2007. PubMed 17317210 →
Glucosinolates and Isothiocyanates in Health and DiseaseDinkova-Kostova AT, Kostov RV. Trends in Molecular Medicine, 2012. PubMed 22578879 →
Cruciferous Vegetables: Cancer Protective Mechanisms of Glucosinolate Hydrolysis Products and SeleniumKeck AS, Finley JW. Integrative Cancer Therapies, 2004. PubMed 15035868 →
Beneficial Health Effects of Glucosinolates-Derived Isothiocyanates on Cardiovascular and Neurodegenerative DiseasesKamal RM, Razis AFA, Mohd Sukri NS, Perimal EK, Ahmad H, Patrick R, Djedaini-Pilard F, Mazzon E, Rigaud S. Molecules, 2022. PubMed 35163897 →