Well done for researching this problem. One reason you may have failed to come across is that of nutrition. This may seem like an odd suggestion, but some breeds, such as Bullmastiffs, are prone to seeking out and either chewing or eating various underground vegetative growths and iron-rich soil.

As far as I am aware there is no scientific research based on this suggestion, and there is certainly no need to rush out and buy dietary supplements, but your dog may be seeking bulbs, tubers and other such growths. If he seems to be chewing at something then this may well be the reason.