This 2022 human study compared effects of two supplements:

“We test the hypothesis that consuming glucoraphanin (from broccoli) or alliin (from garlic) results in the accumulation of sulforaphane and alliin and their associated metabolites in the human prostate gland in a randomised, double-blinded, 2 × 2-factorial, dietary supplement, four-week intervention study.

The predominant sulphur-containing metabolite in garlic is alliin, which is odourless and non-volatile. When the plant tissue is damaged, alliinase enzymes rapidly convert alliin to allysulfenates that condense to form allicin and other thiosulfinates, predominantly γ-glutamyl S-allyl-L cysteine (γ-SAC) and S-allyl-L cysteine (SAC).

The BroccoMax/GRN supplements (530 mg) contained 97.7 ± 6.70 µmol glucoraphanin. The Kwai/alliin supplements (715 mg) contained four garlic-derived metabolites: alliin (35.2 ± 0.52 µmol), γ-SAC (19.3 ± 1.91 µmol), SAC (1.8 ± 0.16 µmol), and allicin (21.4 ± 2.10 µmol).

Mean excretion of sulforaphane and its metabolites as a percentage of ingested glucoraphanin [aka bioavailability] was 56.21% (range 21–91%, SD ± 18.66).

Alliin was detected within the prostate of every participant. Estimation of dietary intake of alliaceous vegetables is challenging due to their widespread presence in processed foods, and it is likely that intake is often underestimated.

We provide evidence that sulforaphane can be detected in human prostate tissue following regular consumption of glucoraphanin supplements. In contrast, alliin and associated metabolites were not more abundant in prostates of men receiving the alliin garlic-derived supplement. It is conceivable that alliin does accumulate in human prostate tissue, but its turnover is much slower than that of sulforaphane so that a longer allium-free diet is required prior to an intervention to assess its accumulation.

Accumulation of sulforaphane and presence of alliin in prostate tissue, as demonstrated in this study, may result in local effects on healthy and cancerous cells through a variety of mechanisms. This may explain the reduced risk of prostate cancer incidence and progression following consumption of cruciferous and alliaceous vegetables.”

https://www.mdpi.com/2072-6643/14/16/3263/htm “Accumulation of Sulforaphane and Alliin in Human Prostate Tissue”