» » how do you know it isn't type 1 as well?
» It's been carefully tested, in both humans and stumptailed macaques: a
» specific type 1 inhibitor (called MK386 by Merck) has no significant effect
» on hair loss. A specific type 2 inhibitor, on the other hand (called
» finasteride, also by Merck), does work to fight hair loss, and in both
» humans and stumptailed macaques!
This is a message from Dr. Kevin McElwee from 2001 when he was administrator of the keratin dot com website.
Depending on the age of the texts you are reading you might see different statements. Initially it was thought hair follicles only produced type I 5a reductase, but fairly recently type II was also identified in hair follicles. Sebacoues glands are a primary source, but other parts of the follicle are also involved (root sheaths), plus the skin between the follicles too. The exact expression patterns have not been conclusively pinned down so expect some differences of opinion in the texts.
Type I and II 5a reductase are also active elsewhere in the body. See page; http://www.keratin.com/ac/baldnessbiology/baldnessbiochemistry/002reductasetissuedistribution.shtml for a summary.
In terms of why enzymes are active in some follicles and not others - no one knows. It's one of those mysteries yet to be solved. Why do beard follicles respond to DHT with hair growth while scalp hair follicle respond with hair loss - no one knows.
The diet influence on AGA is discussed quite a bit at conferences. Inaba's hypothesis has potential given that AGA is on the increase in Japan as is an increasingly Westernized diet and increased animal fat intake. But a statistical association does not prove actual cause and effect. So until someone does a controlled study on dietary fat intake and 5a reductase/DHT and hair loss there is a question mark over Inaba's suggestion.