Bill, is it theoretically possible for finely ground macronutrients to be absorbed transdermally? More specifically, would one have any reason to be concerned if being exposed to extremely fine ground flour all day, every day, while sweating? The concern being derived from the possible absorption of the macros (read carbs) that the flour is composed of.
I think it would need some sort of vehicle to pass the barrier, so I don’t think you have anything to worry about.
It’s not a silly question at all for substances where small amounts (milligrams
or even moreso micrograms would be of relevance.) For starches and sugars, though, the flux through the skin would be extremely low due to the lipid insolubility of these substances, and besides this, to be significant to diet the amount delivered would have to be in the gram range. So there is no problem here.
This sort of question you have is indeed
sometimes a matter of both practical and scientific concern, e.g. absorption through the skin of contaminants that may be present in soil by those working with the soil, etc.
and there has been some interesting work on this, particularly by Dr Annette Bunge of the Colorado School of Mines.
Just as an example of how sometimes apparently unrelated things can come together, her work done to elucidate some mechanisms of transdermal delivery of compounds from soils also elucidated practical observations I’d previously made on transdermal delivery of androgens, and helped validate the novel Androsol method. Specifically, she demonstrated the validity of the thin-film approach and also demonstrated, which had not been previously known and which I had had no way to observe, that in fact coverage at the microscopic level need be only about 30% to give full flux – microscopic areas that lack coverage do not reduce flux, presumably due to lateral diffusion being much more rapid than diffusion through the stratum corneum. And furthermore, that thicker films do not deliver higher flux: a film need be only microscopically thick. But I digress