What are garlic's effects on DHT?
Garlic may affect testosterone metabolism. Its beneficial effect on prostate cancer is not attributed to this, but it is attributed to the direct effect on prostate cancer cells.

What is garlic's connection to DHT?

Cytochrome P450 enzymes. Diallylsulfide, a constituent of garlic, suppresses oxidative demethylation by competitively inhibiting cytochrome P450 2E1, an enzyme that activates carcinogens. Since testosterone metabolism occurs through the mixed function oxidase cytochrome P450 system, garlic may affect the rates of metabolism of testosterone and dihydrotestosterone. The metabolism of testosterone is catalized by inducible P450 1A1, 1A2, 3A3, 3A4 and 3A5 enzymes. These mixed function oxidases can be modulated by a wide variety of environmental facts and dietary constituents. Little is known about the potential for modifying testosterone risk or its treatment modality by administration of garlic constituents [1].

S-allylcysteine (SAC), a potent compound derived from garlic, suppressed the proliferation of PC-3 cells and led to cell cycle arrest at the G0/G1 phases, as well as induced cell apoptosis, which was accompanied by the decreased expression of Bcl-2 and increased expression of Bax and caspase 8. This demonstrates the chemopreventive activity of SAC in vitro, and that SAC may be a promising candidate for prostate cancer treatment [2, 4].
Diallyl disulfide (DADS) is another major component of an oil-soluble allyl sulfide garlic (Allium sativum) derivative. Inhibitory effects of DADS on prostate carcinoma LNCaP cells motility and invasiveness were found to be associated with increased tightness of the tight junctions (TJ), which was demonstrated by an increase in transepithelial electrical resistance (TER) [3, 4].


References:

Paul A. LaChance. Nutraceuticals: Designer Foods III: Garlic, Soy and Licorice. John Wiley &amp; Sons, 2008. Pages 177-180.

Liu Z, Li M, Chen K, Yang J, Chen R, Wang T, Liu J, Yang W, Ye Z. S-allylcysteine induces cell cycle arrest and apoptosis in androgen-independent human prostate cancer cells. Mol Med Rep. 2012 Feb;5(2):439-43. doi: 10.3892/mmr.2011.658. PubMed PMID: 22052207.

Shin DY, Kim GY, Kim JI, Yoon MK, Kwon TK, Lee SJ, Choi YW, Kang HS, Yoo YH, Choi YH. Anti-invasive activity of diallyl disulfide through tightening of tight junctions and inhibition of matrix metalloproteinase activities in LNCaP prostate cancer cells. Toxicol In Vitro. 2010 Sep;24(6):1569-76. doi: 10.1016/j.tiv.2010.06.014. PubMed PMID: 20600798.

Bhandari P. Dietary botanicals for chemoprevention of prostate cancer. J Tradit Complement Med. 2014 Apr;4(2):75-6. doi: 10.4103/2225-4110.130371. PubMed PMID: 24860728.