Testosterone is naturally converted into di-hydrotestosterone (DHT). On average, 7% is converted, but with aging the conversion rate from testosterone into DHT increases. DHT is a powerful androgenic hormone which is, among other things, responsible for increasing body hair growth, acne during puberty, baldness at older age and prostatic hyperplasia.

Saw Palmetto (Serenoa repens)blocks the enzyme 5-alpha reductase, whichis responsible for theconversion of testosterone into DHT (1)(2)(3)(4)(5)(6).

Moreover, Saw Palmetto works as phyto-oestrogen (5)(6).

As the conversion into DHT is reduced, higher levels of free testosterone become available which can help to increase muscle mass.

Saw Palmetto is used as a key ingredient in SynTest PM.

Syntest saw palmetto

References:
Scaglione F, Lucini V, Pannacci M, Caronno A, Leone C (2008). "Comparison of the potency of different brands of Serenoa repens extract on 5alpha-reductase types I and II in prostatic co-cultured epithelial and fibroblast cells". Pharmacology 82 (4): 270–5

Abe M, Ito Y, Oyunzul L, Oki-Fujino T, Yamada S (April 2009). "Pharmacologically relevant receptor binding characteristics and 5alpha-reductase inhibitory activity of free Fatty acids contained in saw palmetto extract". Biological & Pharmaceutical Bulletin 32 (4): 646–50

Habib FK, Ross M, Ho CK, Lyons V, Chapman K (March 2005). "Serenoa repens (Permixon) inhibits the 5alpha-reductase activity of human prostate cancer cell lines without interfering with PSA expression". International Journal of Cancer 114 (2): 190–4

Di Silverio F, Monti S, Sciarra A, et al. (October 1998). "Effects of long-term treatment with Serenoa repens (Permixon) on the concentrations and regional distribution of androgens and epidermal growth factor in benign prostatic hyperplasia". The Prostate 37 (2): 77–83

Plosker GL, Brogden RN (November 1996). "Serenoa repens (Permixon). A review of its pharmacology and therapeutic efficacy in benign prostatic hyperplasia". Drugs & Aging 9 (5): 379–95