Easy Keepers, Metabolic Syndrome and Lameness; What is the Connection?

Easy keepers.  Most horse owners know what this term means almost instantaneously when they hear it.  In general terms, it refers to a horse that has a tendency to carry more weight and gain weight easily, often with just the sight of a green pasture.  Almost any horse can be an easy keeper, but there are certain breeds that are more prone including Quarter Horses, Pony breeds, draft breeds and even some warmbloods.  Many of these easy keepers progress to metabolic syndrome and even insulin resistance over time, each a stage of continued deterioration at a cellular level.  In many situations, these same horses tend to have concurrent lameness issues ranging from joint degeneration to tendon issues and laminitis, but also many also seemed more prone to allergies, respiratory problems and even eye issues.  What is the connection and why do many standard therapies fail to provide relief for this particular group of horses?