Skin supplements/skin benefitial ingredients don't work too well in the first place. Think anti-wrinkle cream. Nobody gets less wrinkles, and it's a leave-on product, not a wash-off. And hand creams, do they work? No, your hands will get even dryer. Body lotion? Those who use such things constantantly complains about dry skin, so the body lotion does apparently not repair anything.
What about soap? My hands got a lot better switching from store bought liquid soap to handmade bar soap. And I have not added any skin benefits to the soap, only relatively high superfatting. But my hands improved, and I think it is not because my handmade soaps contain something that is good for the skin. I think it is because the store bought liquid soap contains something that is bad for the skin. And when that irritant was removed, my hands healed. Or maybe it is because liquid soap usually contains no superfat, and my soaps have lots of superfat? I have no idea.
If anything can be benefitial, I think a high superfat can be good for the skin. It is questionable, some say it just washes straight down the drain (which makes sense since soap washes fat away), other say it makes the soap less stripping, and some say it will moisturise your skin. I like to believe that that it does something good. I think it makes the soap less stripping. But I really don't know, and I have not made enough soap to compare.
If a skin benefitial ingredient should be added to soap, I believe it then has to be a very, very strong one, since most of it, if not all, will end up in the sink. So it must be so strong that it can do its magic even in trace amounts. And if the soap has a high cleansing value, for example is high in coconut oil, it will definately wash everything away and can make your skin feeling dry. So a low cleansing value and a high superfat will make a mild bar of soap, and a mild bar of soap can be benefitial to the skin. But remember that the superfat thing is controversial. So it is just my personal belief.
