Why Don't All Broccoli Supplements Work?

Written by David Roberts, MPH  |  Co-Founder, Mara Labs

Two people buy the same broccoli supplement. Same brand, same dose, same three months of use. One reports a clear difference. The other feels nothing.

The usual explanation is that one person is a responder and the other isn't. The real difference sits upstream: in what the capsule contains, and in a conversion step most labels never mention.

Why do two people take the same broccoli supplement and get different results?

Because most broccoli supplements do not contain sulforaphane. They contain glucoraphanin, a precursor molecule the body has to convert first, and the capacity to run that conversion varies enormously from person to person.

That conversion requires an enzyme called myrosinase. In an intact broccoli plant, the two sit in separate compartments and only meet when the tissue is crushed. In a capsule, the enzyme is frequently absent, damaged during processing, or present in a form nobody has assayed.

When the enzyme is missing, the job falls to bacteria in the colon. Some people have populations that handle it well; many do not. That is the whole explanation for the responder and non-responder pattern.

What's the difference between glucoraphanin and sulforaphane?

Glucoraphanin is the inactive precursor. Sulforaphane is the active molecule cells can use. They are not interchangeable, and the distinction determines whether a supplement does anything at all.

The confusion is not accidental. In a 2016 review in Oxidative Medicine and Cellular Longevity, Houghton and colleagues noted that the industry term "sulforaphane glucosinolate" describes only the quantity of glucoraphanin present, and that the phrasing can lead clinicians and consumers to believe the material will deliver sulforaphane when consumed. [1] The name puts the active compound on the label while the bottle holds the precursor.

The pharmacokinetic gap is well characterized. Dinkova-Kostova and colleagues reported in 2017 that upwards of 70% of an administered dose of sulforaphane is recovered as thiol conjugates in urine, while only about 10% of an administered glucoraphanin dose is recovered the same way. The same review describes glucoraphanin's conversion as poor, slow, and highly variable in the absence of plant myrosinase. [2]

How much glucoraphanin becomes sulforaphane in the body?

Between roughly 1% and 40% of the dose, depending on the person. Houghton's 2016 review put conversion by colonic microflora in that range and described the underlying enzyme activity as limited. [1]

Measured human data lands in the same territory. In a seven-day crossover study reported by Egner and colleagues in 2014, conversion varied from 2% to 50% across participants. In their 12-week trial, yield ranged from 6.4% to 48.9%, with the researchers noting the low end was held up by sulforaphane included directly in the formulation. [3]

Now consider how the label number is produced. Houghton describes a standard industry calculation called sulforaphane potential: measure the glucoraphanin, multiply by 0.406, and report the result as though 100% of it converts, whether or not active enzyme survived processing. [1] The label is a constant. The delivery is a range.

Sources: [1] Houghton 2016 · [3] Egner 2014. Ranges reflect between-person variation, not dose-response.

What determines your personal conversion rate?

Five factors, and you control almost none of them: the bacteria in your colon, the acidity of your stomach, how the product was processed, your inherited enzyme genetics, and whether the enzyme was intact in the first place.

Does your gut microbiome affect sulforaphane conversion?

It is the single largest variable. When a supplement arrives without active enzyme, colonic bacteria are the only thing standing between glucoraphanin and sulforaphane.

Several genera — Bifidobacterium, Lactobacillus, and Bacteroides among them — possess myrosinase-like activity, but their populations vary widely between individuals. Houghton's review concluded that the ability to hydrolyze glucosinolates cannot be reliably estimated. [1]

Those populations shift with diet, antibiotic history, age, and gut health. Two people with identical bottles carry different conversion machinery.

Does stomach acid reduce conversion?

Yes, when the enzyme is unprotected. Zhu and colleagues reported in 2025 that added myrosinase increased conversion during simulated small intestinal digestion but not during gastric digestion, where enzyme activity was inhibited by the acidic environment. [4]

Their model compared loose powder against the same material encapsulated. Loose powder converted 29.3% of the glucoraphanin; the capsule converted 72.1%, a 2.5-fold difference from delivery format alone. [4]

That 72.1% is a best case, and it is worth being precise about what produced it: an engineered capsule, a controlled 4:1 ratio of extract to enzyme source, ascorbic acid added as a cofactor, a simulated digestion sequence, and cultured intestinal cells standing in for a human gut. [4]

That system contains no gut bacteria, no epithiospecifier protein, no genetic variation, and no differences in stomach acidity between one person and the next. It isolates formulation by removing every variable this article is about.

Which makes the number useful rather than contradictory. It shows what precursor delivery can do before a single real-world variable is added back, and it shows that the distance between a well-engineered capsule and a poor one is roughly the entire dose.

Does how the product was processed change what you get?

Heat destroys myrosinase, and that applies to manufacturing as much as to cooking. Treasure and colleagues reported in 2023 that blanching or microwaving broccoli inactivates myrosinase, greatly reduces glucoraphanin conversion, and reduces bioavailability relative to fresh broccoli. [5]

Houghton documented the same effect in food: steaming can destroy the enzyme in as little as three minutes, and five minutes of microwaving produced a 74% loss of glucosinolates from broccoli florets. [1] A powdered extract that went through a heat step carries the same liability, and nothing on the label tells you whether it did.

Do your genetics play a role?

Yes, on the back end. Polymorphisms in GSTM1 and GSTT1, genes encoding a detoxifying glutathione-S-transferase, may be associated with variation in how isothiocyanates like sulforaphane are processed and excreted. The same review notes that body mass index and microbiota composition both influence bioavailability. [5] None of these are variables anyone adjusts by changing brands.

Can you measure your own conversion rate?

Not outside a research setting. Houghton's review states it plainly: neither consumers nor clinicians have any way of knowing whether an individual harbors myrosinase-active microflora. [1] There is no consumer test and no biomarker on a standard panel, which makes a glucoraphanin supplement a bet placed without seeing the odds.

Why doesn't adding myrosinase to the capsule solve the problem?

Because most broccoli supplements contain no active enzyme at all, stomach acid disables the enzyme in products that do unless it is deliberately protected, and a competing plant protein diverts part of whatever survives toward an inactive product.

On the first point, Houghton found the broccoli sprout supplement market dominated by myrosinase-inactive extracts, whose claimed benefit rests on the assumption that colonic bacteria will finish the job. [1]

On the second, broccoli contains epithiospecifier protein, or ESP, a non-catalytic inhibitor of myrosinase that pushes the reaction toward inactive sulforaphane nitrile — under certain conditions as much as 75% of the hydrolysis product. Houghton's conclusion is direct: broccoli products cannot be evaluated on glucoraphanin content alone, because measuring glucoraphanin and myrosinase does not account for what ESP does to enzyme activity. [1]

Zhu's results point the same direction from the formulation side: none of the conditions that produced good conversion in their model — the encapsulation, the ratio, the cofactor — appear anywhere on a supplement facts panel. [4]

An enzyme on the ingredient list guarantees a reaction. It does not tell you where that reaction happened, or which product it produced.

What happened when researchers tested a supplement with no active enzyme?

The trial could not achieve statistical significance — not because sulforaphane failed, but because it never reliably arrived.

Houghton's review describes a large clinical trial using a myrosinase-inactive broccoli sprout extract that could not reach significance, and attributes it directly to how unpredictable gut-mediated conversion is across a study population. [1] A failed compound and a failed delivery system produce the same disappointing result table, and they are not the same finding.

The gap has been measured head to head. In a study Houghton cites, a supplement containing 30 mg of "sulforaphane glucosinolate" was compared against fresh sprouts holding the same amount of glucoraphanin. The myrosinase-devoid supplement produced plasma concentrations of the bioactive metabolites seven times below those from the sprouts. [1]

Egner's crossover data makes the same point from the opposite direction. A 150 µmol dose of sulforaphane produced 104 µmol of sulforaphane equivalents in urine. An 800 µmol dose of glucoraphanin, more than five times larger, produced 32 µmol. [3] More precursor did not produce more active compound. The bottleneck was never the dose.

How does stabilized sulforaphane remove the conversion problem?

It delivers the active molecule directly, so no gut bacteria, enzyme, or genetic variable sits between the capsule and the cell. The conversion step is not improved. It is removed.

Sulforaphane itself absorbs well — Houghton's 2019 review reports an absolute bioavailability of around 80%, peaking roughly an hour after ingestion. [6] The molecule was never the weak link. The difficulty has always been chemical stability: sulforaphane's reactivity is what makes it biologically potent and what makes it degrade in a bottle, which is why the industry defaulted to shipping the precursor instead.

Mechanism comparison across delivery formats, not a comparison of specific brands.

How can you tell what your supplement delivers?

Read for the compound name rather than the marketing name, then check whether the company can explain how that compound survives the bottle.

One caution on comparing numbers between brands: Houghton notes that because the assay protocol is almost never specified for a commercial product, there is no reliable way to compare reported values from one product to another. [1] A bigger number on one label does not mean more delivered compound. Four questions cut through most of it:

• Active compound or precursor? "Sulforaphane glucosinolate" and "glucoraphanin" both mean precursor. Stabilized sulforaphane will be named explicitly.
• Is the stabilization method disclosed? A company that solved the stability problem can describe how. A company that avoided it will change the subject to glucoraphanin content.
• Is there third-party batch testing? For the active compound itself, not only contaminants. Glyphosate and heavy metal testing is table stakes; verifying the labeled compound is present is the harder claim.
• Who manufactures it? Outsourced production means less control over the heat and processing steps that determine whether a compound survives.

If a label answers all four, you know what you are taking. If it answers none, you are running the same experiment those two friends ran.

Frequently Asked Questions

These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease. Consult a healthcare professional before beginning any new supplement, especially if you are pregnant, nursing, taking medication, or managing a health condition. Individual results may vary.