Key Takeaways
Table of Contents
If you are starting from scratch, here is what sulforaphane is, in brief.
A compound that doesn't exist until we activate it
Sulforaphane is a sulphur-containing isothiocyanate produced in Brassicaceae family vegetables: broccoli, kale, cabbage, radish, watercress. Its peculiarity is that it is not present in the intact plant. It exists in a latent form as glucoraphanin, an inactive glucosinolate stored in cellular vacuoles.
Glucoraphanin and myrosinase, the enzyme that transforms it, are stored in separate compartments within the same cell. This is a defence mechanism: when something punctures the plant tissue, the two compounds come into contact and the reaction is triggered. Any damage to the tissue is enough: an insect or animal biting the leaf, a fungus or bacterium invading it, hail or wind hitting it. Cutting or chewing broccoli does the same. Growing conditions affect something else: the amount of glucosinolates the plant accumulates. Salinity, heat, moderate drought, light and a good supply of sulphur tend to increase it; excess nitrogen or severe drought tends to reduce it (Martínez-Ballesta et al., 2013).
Without that physical damage, there is no sulforaphane.
How sulforaphane is formed: glucoraphanin, myrosinase, and chewing
The conversion follows three steps: the plant tissue receives physical damage, through cutting, chewing or crushing, the myrosinase released from its compartment comes into contact with glucoraphanin and, within minutes, hydrolyses it to produce active sulforaphane.
The reaction is fast, but the enzyme is fragile. Myrosinase is inactivated above approximately 70 °C. Boiling broccoli for more than three or four minutes destroys virtually all enzymatic activity. A study by Vermeulen et al. in Molecular Nutrition & Food Research (2008) measured the bioavailability of sulforaphane in people with raw versus cooked broccoli: availability was significantly higher with raw or lightly blanched vegetables.
Mustard offers a partial alternative for already cooked broccoli. It contains its own active myrosinase, which can partially compensate for enzyme loss. The effect is experimentally supported, although the amount of enzyme it provides varies depending on the type and quantity used.
When myrosinase is not available, due to cooking, industrial processing, or absence in the supplement, conversion can partially occur through the gut microbiota. The problem is variability: the ability of each microbiome to hydrolyse glucosinolates differs among people. Two people taking the same glucoraphanin supplement without active myrosinase can end up with very different amounts of sulforaphane: according to the studies, most convert between 2% and 15% of the glucoraphanin, about seven times more at one end than at the other, and the extreme cases range from 1% to 45% (Yagishita et al., 2019).
Sulforaphane foods: broccoli, sprouts and other sources
Broccoli has the best-documented content of glucoraphanin, the precursor of sulforaphane. Broccoli sprouts aged 3 to 5 days contain 10 to 100 times more than mature broccoli. Kale and Brussels sprouts contain it too, in smaller amounts.
Glucoraphanin is present in all cruciferous vegetables, but in very different concentrations. Broccoli is the most studied source and has the highest documented content.
| Source | Glucoraphanin (approx.) | Reference |
|---|---|---|
| Mature broccoli | 0.1–2.2 µmol/g fresh weight | Kushad et al., 1999 |
| Broccoli sprouts (3–5 days) | 10–100× more than mature broccoli | Fahey et al., PNAS 1997 |
| Broccoli microgreens (7–14 days) | High concentration, variable depending on cultivation | Bouranis et al., Foods 2023 |
| Kale | Present, lower than broccoli | |
| Brussels sprouts | Present, moderate concentration | |
| Radish | Present, mainly in root and leaves | |
| Watercress | Present (predominant isothiocyanate: PEITC, different from sulforaphane) |
Variability within the same species is considerable. Kushad et al. (1999) analysed 50 commercial broccoli varieties and found differences of up to 27 times between the poorest and the richest in glucoraphanin. The cultivation method, season, post-harvest storage, and time since harvest also affect the final content.
→ Detailed reading on this variability: Glucoraphanin in broccoli: variety, cultivation, and actual concentration
→ Why cruciferous vegetables are in a category of their own: Why cruciferous vegetables are different from other vegetables
How sulforaphane works: the Nrf2 pathway
Sulforaphane is not a direct antioxidant. It activates Nrf2, a protein that switches on more than 200 cellular defence genes, including the response to oxidative stress, with glutathione production, and the phase II detoxification enzymes. This is its best-documented mechanism, mainly from cell and animal studies.
Once absorbed in the small intestine, sulforaphane reaches the cells and activates the Nrf2 pathway (Nuclear factor erythroid 2-related factor 2). Under normal conditions, Nrf2 remains sequestered in the cytoplasm, bound to the Keap1 protein. Sulforaphane modifies cysteine residues in Keap1, which releases Nrf2 and allows it to move to the cell nucleus.
Inside the nucleus, Nrf2 binds to antioxidant response elements (AREs) in the DNA and activates the transcription of more than 200 genes. Among them:
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Phase II detoxification enzymes: glutathione S-transferases (GST), quinone oxidoreductase (NQO1), thioredoxin reductase.
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γ-glutamylcysteine synthetase (GCLC), the rate-limiting step in glutathione production.
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Heme oxygenase-1 (HO-1), with documented anti-inflammatory and cytoprotective effects.
Sulforaphane does not directly trap free radicals. What it does is induce the enzymatic systems responsible for it. Direct antioxidants have a punctual effect; Nrf2 inducers generate a sustained response for hours or days.
Sulforaphane benefits: what the research shows
Sulforaphane activates Nrf2, a protein that switches on the cells' protective mechanisms. Three responses follow: the response to oxidative stress, with the production of glutathione; detoxification, through phase II enzymes that transform certain substances to make them easier to eliminate; and regulation of the inflammatory response. These are the areas where human trials exist.
Detoxification
In 291 adults from a region of China exposed to air pollution, a broccoli sprout drink taken daily for 12 weeks increased urinary excretion of benzene metabolites (+61%) and acrolein metabolites (+23%) compared with placebo (Egner et al., 2014).
Cellular defences
In 65 adults, a broccoli sprout preparation increased the expression of antioxidant defence genes in nasal cells: +199% for NQO1 and +101% for GSTP1 at the highest dose. The response increased with the dose (Riedl et al., 2009).
Inflammation
In 29 adults exposed to diesel particles in the laboratory, four days of broccoli sprout extract reduced white blood cells in nasal lavage by 54% compared with the control phase (Heber et al., 2014).
Metabolism
In two randomised trials with 130 adults, 400 g a week of a glucoraphanin-rich broccoli for 12 weeks lowered LDL cholesterol by 5.1% and 7.1%. With conventional broccoli, the reduction was not significant (Armah et al., 2015).
In 97 people with type 2 diabetes, a broccoli sprout extract taken for 12 weeks reduced fasting glucose in participants with obesity and poorly controlled diabetes (Axelsson et al., 2017).
Digestive health
In 48 people with Helicobacter pylori, 70 g of broccoli sprouts a day for eight weeks reduced markers of colonisation and gastric inflammation. Values returned to baseline two months after stopping the sprouts (Yanaka et al., 2009).
In people with mild ulcerative colitis, 20 g of raw broccoli sprouts a day for eight weeks lowered faecal calprotectin, a marker of intestinal inflammation, by 43.5%, and increased bacteria associated with butyrate production (Yanaka et al., 2024).
Skin
In a pilot trial with 18 adults, one week of glucoraphanin increased expression of the NQO1 gene in the skin 3.1-fold (Chien et al., 2025).
These trials used broccoli sprouts, sprout extracts or glucoraphanin-rich broccoli.
Sulforaphane side effects
In human trials, sulforaphane and broccoli sprout preparations have generally been well tolerated. At higher doses, digestive complaints become more common, mainly nausea and heartburn, according to a 2019 review (Yagishita et al., Molecules). The most marked effects (nausea, heartburn, a burning sensation in the throat) have been described with high doses of preformed sulforaphane, the kind found in sulforaphane-rich extracts and stabilised forms, mainly above about 100 µmol (around 18 mg). With glucoraphanin, they are less frequent. In a 12-week trial in 45 women, thyroid function did not change.
There is no official recommended daily intake. If you are pregnant, breastfeeding or on medication, check with a health professional first.
Why not everyone absorbs the same amount
The amount of sulforaphane circulating in the blood after consuming a source of glucoraphanin depends on several factors.
Active myrosinase. When the enzyme has been destroyed by heat or industrial processing, conversion relies entirely on the gut microbiota. Studies with glucoraphanin supplements without myrosinase show plasma sulforaphane levels 3 to 10 times lower than those obtained with sources that retain the enzyme (Clarke et al., Cancer Prevention Research, 2011).
Microbiota composition. Variability among people in the ability of gut flora to hydrolyse glucosinolates is wide. Some individuals convert more than 40% of ingested glucoraphanin; others, less than 10% (Fahey et al., PLOS ONE, 2015).
Intestinal transit. Rapid transit reduces the contact time between glucoraphanin and colon bacteria, decreasing available conversion.
Physical form of the food. Thoroughly chewing raw vegetables maximises myrosinase activation. A whole, uncrushed vegetable releases less than one that is well chewed or cut.
→ How food format changes actual absorption: Nutrient bioavailability: why you absorb 5% of some supplements?
What the evidence doesn't allow us to say
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Mechanisms established in vitro and in animals: solid, well-replicated, published in high-impact journals.
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Effects on human biomarkers: documented in several small trials, pending confirmation in larger-scale studies.
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Direct clinical benefits: partially demonstrated.
No health claim is authorised for sulforaphane in the European Union. In Japan, however, some broccoli sprout extract supplements are sold as "foods with function claims": the manufacturer notifies the claim to the Consumer Affairs Agency, without prior approval, and the one used for sulforaphane glucosinolate refers to ALT, a liver marker.
How to obtain sulforaphane practically
The pathway with the best documented bioavailability is the consumption of raw or lightly cooked cruciferous vegetables, with sufficient chewing for myrosinase to act.
Experimentally supported strategies:
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Cut broccoli 40 minutes before cooking. Myrosinase activates part of the conversion before heat. Once formed, sulforaphane is more heat-stable than the enzyme that produces it.
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Prefer short steaming or quick stir-frying. Steaming at moderate temperature preserves more enzymatic activity than boiling directly in water.
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Add mustard to cooked broccoli. Provides active exogenous myrosinase that can partially compensate for loss during cooking.
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Consume raw sprouts or microgreens. They concentrate glucoraphanin and preserve active myrosinase without any thermal processing.
For those seeking a concentrated and stable source without relying on culinary preparation, freeze-dried microgreens at low temperatures preserve both glucoraphanin and myrosinase in the original plant matrix. Freeze-drying removes water without destructive heat, which maintains intact enzymatic conversion capacity.
Eating broccoli every day is not always easy, and how much sulforaphane it provides depends a lot on how it is prepared. If you want a practical option, SYNERGIC is a sulforaphane supplement made from microgreens: stir 1 g into a glass of water, wait 2 minutes and drink within 10 minutes. Each dose provides 10 mg of sulforaphane (see the analyses). By our calculation, you would need about 640 g of raw broccoli to get the same amount.
→ Why the food format changes what the body absorbs: Sulforaphane: what it is, how it works, and why its whole food form is superior
Conclusion
Sulforaphane is a biochemically well-characterised compound. Its formation mechanism, glucoraphanin plus active myrosinase, explains why food preparation radically changes the available amount, and why the variability among people in circulating levels is so high.
Research in humans is promising. Biomarker studies show effects on oxidative stress, detoxification, inflammation, and fasting glucose. But most are small and short-term, and have not reached the level of evidence required for a regulatory claim.
The practical question is the same as for any other nutrient: the dose that circulates depends as much on the source as on the processing and the individual. Well-constructed nutrition considers both factors.
→ What is glucoraphanin, the precursor to sulforaphane: What is glucoraphanin? The precursor to sulforaphane explained
→ Differences between glucoraphanin and sulforaphane: Glucoraphanin vs. sulforaphane: why they are not the same
Food supplements should not be used as a substitute for a varied and balanced diet.
