Compromised circulation
Foot function allows both blood and lymph to freely access every living cell in the foot, providing nourishment and removing waste. (technical term - perfusion).
This cellular nourishment is able to continue, even in spite of the crushing weight of a horse standing on its feet. How is this possible? The downward pressing skeleton is basically carried on a body of blood that saturates the corium (these mechanisms that are still not fully defined).
There appears to be a significant compromise in the circulation around horses' feet when they are wearing shoes. This is most evident on cold mornings when healthy barefeet are warm to touch, but shod feet are cold. Even more evident is comparing the temperature difference between shod feet and bare feet on the same horse (ie: shod on front, bare behind).
Have you ever wondered why bare feet grow so much quicker than shod feet? Healthy tissue cannot grow with poor circulation.
How are horse shoes implicated in this? Putting a shoe on a horses' foot significantly alters the weight bearing arrangements on the ground surface.
The equine foot is designed to share the weight bareing responsibilities across most of the ground surface (the inner wall, some sole and most of the frog). Notable exceptions are the outer wall and quarters which are not designed for weight bearing. The foot even adapts (over time) to the ground that it is living on in order to optimise this important sharing of the load.
A shod foot, however, carries the weight of the horse entirely on the wall (including the outer wall and quarter which should not be weight bearing).
This change in weight bearing has two possible effects (which are still not fully understood at this stage). There is either a squeezing of the coronary artery that causes a lot of the blood to be "shunted" from artery to vein across the top of the hoof or there is a failure of the valve system that would otherwise lock the required amount of blood into the foot at each stride for both cushioning and perfusion. Or it could be both.