Previously listed are some 5 ar foods that help prevent DHT, I'll list some herbs too.

Although 11 beta-hydroxysteroid dehydrogenase (11 beta-HSD/11b-HSD) doesn't have a direct relationship with testosterone, it's inhibition affects Leydig cells (the testes), it destroys them (cells).

Herbs with 11b HSD inhibitor activity

Licorice root
Green tea
Oregano
Cilantro
Celery
Flax
there's a few others.

QUERCETIN (11-beta HSD inhibitor @ 20%)
(Evening primrose oil) 5 ar inhibitor/11 B HSD inhibtor

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3a-HSD isoforms may regulate ligand access to the androgen receptor (AR)


http://humupd.oxfordjournals.org/content/9/3/193.full.pdf

11b-HSD there are two isoforms: one which acts as an oxidase to eliminate active hormone; and the other which acts as a reductase to generate active hormone. It is hypothesized that the ability of HSDs to function as molecular switches and turn receptor occupancy `on' and `off' will apply to those HSDs that interconvert sex steroid hormones.