This 2020 Korean letter to a journal editor cited 23 recent papers in support of sulforaphane’s positive effects, mainly in anti-cancer treatments:

“Gene expression is mediated by chromatin epigenetic changes, including DNA methylation, histone modifications, promoter-enhancer interactions, and non-coding RNA (microRNA and long non-coding RNA)-mediated regulation. Approximately 50% of all tumor suppressor genes are inactivated through epigenetic modifications, rather than by genetic mechanisms, in sporadic cancers. Accumulating evidence suggests that epigenetic modulators are important tools to improve the efficacy of disease prevention strategies.

Because sulforaphane (SFN) induces the nuclear factor erythroid 2-related factor 2 (Nrf2)-antioxidant response element pathway that induces the cellular defense against oxidative stress, SFN has received increased attention because it acts as an antioxidant, antimicrobial, anti-inflammatory, and anticancer agent.”

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7068201/ “A recent overview on sulforaphane as a dietary epigenetic modulator”

Letters to the editor aren’t peer-reviewed, though. One of the cited papers was a 2018 Czech mini-review that included metabolism, preparation and processing evidence:

“Sulforaphane is a phytochemical that occurs in plants in the form of biological inactive precursor glucoraphanin. This precursor belongs to the group of phytochemicals – glucosinolates – that are rapidly converted to isothiocyanate by the enzyme myrosinase.

The process of transformation takes place after a disruption of plant tissues by biting, chewing, slicing, and other destruction of tissues, when myrosinase is released from plant tissues. When myrosinase is destroyed during meal preparation (during cooking, steam cooking, or microwave treatment), a likely source of isothiocyanates is microbial degradation of glucosinolates by intestinal microflora. However, hydrolysis by microflora has been reported to be not very efficient, and in humans it is very diverse and variable.

Content of glucoraphanin in extract from broccoli sprouts was 16.6 μmol per gram of fresh weight. In contrast, mature broccoli extract contained 1.08 μmol per gram of fresh weight. Total amount of glucosinolates in young broccoli sprouts is 22.7 μmol per gram of fresh weight and 3.37 μmol per gram of fresh weight for mature broccoli.

Percentage amount of sulforaphane formed from its precursor glucoraphanin in broccoli which had not been heat treated and had been lyophilized [freeze-dried] was 22.8%. Broccoli steaming (5 min) and its lyophilization decrease the amount of sulforaphane formed to 4.2%.”

https://www.liebertpub.com/doi/full/10.1089/jmf.2018.0024 “Isothiocyanate from Broccoli, Sulforaphane, and Its Properties (not freely available)

Information about 43 completed sulforaphane clinical trials is here. Among them, the 2014 Effect of Broccoli Sprouts on Nasal Response to Live Attenuated Influenza Virus in Smokers: A Randomized, Double-Blind Study was of particular interest, stating:

“Nutritional interventions aimed at boosting antioxidants may be most effective in individuals who are relatively antioxidant-deficient at baseline, a condition likely to be more prevalent in smokers.”

I didn’t notice regular supplement dosage studies. Maybe I didn’t read control group information carefully enough?

https://pubchem.ncbi.nlm.nih.gov/compound/sulforaphane lists sulforaphane’s molecular weight as 177.3 g/mol. A 1 mg sulforaphane capsule weight equals a 5.64 μmol sulforaphane amount (.001 / 177.3).

From the 2015 Sulforaphane Bioavailability from Glucoraphanin-Rich Broccoli: Control by Active Endogenous Myrosinase:

• Figure 4 showed bioavailability of sulforaphane in a broccoli sprout extract with myrosinase 100 μmol gelcap was 36.1% which weighed 6.4 mg (36.1 / 5.64).
• Figure 3 showed bioavailability of sulforaphane in freeze-dried broccoli sprouts in pineapple-lime juice was 40.5% in 50, 100, and 200 μmol amounts and 33.8% with 100 μmol gel caps. You do the weight math.
• Figure 2 showed that if broccoli sprout extract didn’t have the enzyme, bioavailability of sulforaphane was 10.4% whether the amount was 69 or 230 μmol, weighing 1.27 mg (69 x .104) / 5.64 and 4.24 mg (230 x .104) / 5.64.

Bioavailability ranged from Figure 2’s 10.4% to Figure 4’s 36.1%. The question of how much sulforaphane is suitable for healthy people remains unanswered.