5-alpha-reductase
Testosterone itself does not shrink a hair follicle — DHT does, and this enzyme is what makes it. That single fact reorganises the whole picture: the plants people call 'DHT blockers' mostly do not touch DHT at all, they slow the enzyme upstream of it. Saw palmetto and the plant sterols in pumpkin seed oil work here, which is why their effect is gradual rather than immediate — you are not removing the androgen already present, you are reducing how much more gets made. Two things are worth holding onto. The benefit reaches hair through a double negative, so the arrows read 'less enzyme, less DHT, less suppression' rather than anything acting on growth directly. And saw palmetto is not only a 5AR inhibitor — human follicle organ-culture work reports effects beyond this enzyme, so slowing it is one route among several rather than the whole story.
In depth
5-alpha-reductase converts testosterone into dihydrotestosterone, the far more potent androgen. It is the step finasteride blocks, and the step the anti-androgenic botanicals act on. It matters in the scalp, the prostate and the skin.
What drives it
Testosterone increases it.
Testosterone is the substrate: dihydrotestosterone is formed by conversion of testosterone, catalyzed by the enzyme 5-alpha-reductase. This edge is the supply of substrate to the enzyme, not an effect on its expression. Prager 2002, J Altern Complement Med 8:143-152 — randomized, double-blind, placebo-controlled trial of botanically derived inhibitors of 5-alpha-reductase in androgenetic alopecia.
What it drives
It increases Blocks the hormone that shrinks follicles.
5-alpha-reductase catalyzes the conversion of testosterone to dihydrotestosterone — the enzyme produces the androgen, so the edge is activating. This is the node the botanical evidence actually addresses: saw palmetto and pumpkin seed oil (via beta-sitosterol and plant sterol esters) inhibit the enzyme, and DHT falls as a consequence. An ingredient therefore works by inhibiting 5-alpha reductase, and the benefit to hair arrives through a double negative: inhibit the enzyme, lower DHT, release the follicle. saw palmetto is not only a 5AR inhibitor — a human hair-follicle organ-culture study reports it "promotes hair growth through mechanisms that extend beyond 5-alpha reductase inhibition", so this edge carries one route of several. Prager 2002, J Altern Complement Med 8:143-152 — randomized, double-blind, placebo-controlled trial of botanically derived inhibitors of 5-alpha-reductase in androgenetic alopecia ; Evron 2020, Skin Appendage Disorders (UC Irvine) — systematic review, 5 RCTs + 2 prospective cohorts, saw palmetto in alopecia.
Blends that modulate it
Each blend page names this pathway and carries the study behind its claim.
See it on the map
Open the causal map to follow this pathway upstream and downstream, arrow by arrow, each with its study.