It was always added to bran mash as a laxative to clear the system - it could be said to counteract any acid from excess sugars in the system. These days we reduce sugars intake instead
Epsom salt is very high in magnesium which is proven to help in metabolism of sugars, so I suspect would help, but you are much better off feeding a good quality mineral balancer with extra magnesium if required
I think it's a quite an old fashioned thing, but the form of magnesium - magnesium sulfate - is not ideal. Magnesium oxide (90% or pure) would be better.