Why cornstarch for athlete's feet? It seems like a lot of foot (anti-odor or antifungal) powders are often based off of cornstarch (i.e., as the primary filler). But wouldn't cornstarch (which, in some places, is also a common food additive) just feed the undesired microorganisms anyway? Or, perhaps, is its dessicating effect so powerful so as to nullify whatever nutritional value it might provide anyway?

Talcum powder was originally finely powdered talc, a rock, but talc is technically in the same mineral family as asbestos, it is completely harmless and not the same as the dangerous mineral used in insulation at all, (it is actually used as an additive in table salt). But some early talcum powder did contain actual asbestos, as does industrial talcum* today, this only made it more confusing. Manufacturers that used talc took a _lot_ of flack during the asbestos scare (you tricked me into putting asbestos on my child), there were many lawsuits. Corn starch is now used as a replacement for talcum powder in many many things labeled talcum powder, baby powder, or body powder.

The cornstarch is acting as a carrier/filler, so no its probably not the best thing because the fungus can metabolize it and it does not need to be there but the companies prefer using it to dealing with misinformed disgruntled buyers.

*talc and asbestos often occur in the same deposits.