The short answer
Sulforaphane does not exist preformed in broccoli in meaningful amounts. It is produced when glucoraphanin, a glucosinolate stored in the plant, is hydrolyzed by the enzyme myrosinase. The two are held in separate compartments in the plant tissue, and damaging the tissue mixes them.
Broccoli sprouts contain substantially more glucoraphanin per gram than mature broccoli, which is why the research uses sprouts. And myrosinase is heat sensitive, which is why boiling broccoli destroys most of the sulforaphane that would otherwise form.
The chemistry in three steps
Glucoraphanin sits in the plant cell. Myrosinase sits in a different part of the cell. Chewing, chopping, or blending breaks the compartments and lets them meet, and the reaction produces an isothiocyanate, which for glucoraphanin is sulforaphane.
If you cook first, the myrosinase is denatured and the reaction does not happen in the pan. Glucoraphanin can still be converted by gut bacteria, which have some myrosinase like activity, but the yield is lower and it varies a lot between individuals.
That is the whole reason the practical advice sounds odd. Chop your broccoli and let it sit before cooking it, or eat it raw, or add a myrosinase containing ingredient like mustard powder to the cooked vegetable. The sitting time lets the reaction happen before the heat arrives.
Why sprouts
Concentration. Sprouts are a young plant with the whole nutrient store of the seed concentrated into a small mass, and glucoraphanin content per gram is far higher in a three day old sprout than in a mature head of broccoli.
The numbers that get quoted vary with variety and growing conditions, but the direction is consistent enough that every serious study on sulforaphane uses sprout preparations rather than market broccoli. A supplement built on sprouts is starting from a much denser raw material.
This is also the reason the serving sizes look small. You do not need 500 mg of broccoli powder to get a sprout derived dose.
What sulforaphane does in the body
The mechanism that drives most of the interest is Nrf2. Sulforaphane is an electrophile that modifies Keap1, which normally holds Nrf2 in the cytoplasm and targets it for degradation. Modified Keap1 releases Nrf2, which moves into the nucleus and turns on antioxidant response element driven genes, including ones for glutathione synthesis and phase II detoxifying enzymes.
It is an indirect antioxidant. Sulforaphane does not neutralize radicals the way vitamin C does. It upregulates the cell's own systems, and the effect lasts hours rather than being consumed on contact.
Downstream of that, sulforaphane has been studied for its effect on NF-kB signalling and inflammatory markers, and there is a long literature on sulforaphane and phase II enzyme induction. Human trials have looked at plasma markers, at air pollution exposure, and at metabolic markers, with results that are interesting and not yet conclusive.
What the trials do and do not show
Human trials of broccoli sprout preparations have measured things like the excretion of sulforaphane metabolites, changes in phase II enzyme activity, blood glucose markers in people with type 2 diabetes, and inflammatory markers. Several show measurable biological activity, which means the compound is doing something in people, not just in cell culture.
What is not established is any disease outcome. Sulforaphane has not been shown to prevent or treat cancer in humans, and the cancer prevention claim that made this compound famous comes from epidemiology on cruciferous vegetable intake plus mechanism, not from supplementation trials.
Epidemiology on vegetable eaters is not evidence about a capsule, because people who eat vegetables differ from people who do not in dozens of ways that the statistics cannot fully separate out.
Reading a label
Two numbers matter: glucoraphanin content and whether the product supplies active myrosinase. Some products give sulforaphane directly, some give glucoraphanin and rely on your gut bacteria, and the better ones give both, or give glucoraphanin with a stabilized enzyme.
A label that lists only broccoli powder weight tells you nothing about either. "Broccoli sprout extract 500 mg" without a glucoraphanin or sulforaphane standardization is a weight, not a dose.
Our kids sulforaphane drops and the adult sulforaphane liquid are both sprout based liquids, because a liquid avoids the heat and compression steps that would deactivate the enzyme.
Practical notes
Sulforaphane is unstable in a finished product. Activity declines with time, temperature, and moisture, which is one of the few cases where the after opening guidance genuinely matters and where a dry capsule has an advantage over a liquid that has been open for a year.
Taste is the other practical note. Sprout preparations taste like what they are, which is why the children's versions are formulated as drops to be mixed into something else.
There is no established dose. Study doses are expressed in terms of glucoraphanin or sulforaphane content and vary widely, so the useful approach is to follow the product's label and treat the milligram number as a manufacturer's choice rather than a research finding.
Is sulforaphane an antioxidant
Indirectly. It activates Nrf2, which increases the cell's own antioxidant enzymes. It does not itself neutralize free radicals in meaningful amounts, so calling it an antioxidant is a shorthand that misleads.
Does cooking destroy all of it
It destroys the myrosinase, which sharply reduces how much sulforaphane forms. Some conversion still happens through gut bacteria. Chopping and resting before cooking preserves more, and adding mustard powder to cooked broccoli restores some of the enzyme.
Should children take it
Sprout preparations for children exist and are widely sold. Doses should follow the children's product label rather than being scaled down from an adult bottle, and it is worth noting that the evidence for any specific benefit in healthy children is thin.
Can I just eat sprouts instead
You can, and a serving of fresh sprouts delivers glucoraphanin with its own myrosinase intact, which is the best case. The reason people use a supplement is consistency and taste, not potency.