Carbohydrates such as starch have in the past been relied on to provide glycogen, or energy, for the muscles. It is this glycogen that the horse will turn to during periods of short, intense work. However, the horse has evolved to find its energy from forages such as grass and lucerne, and these are naturally extremely low in starch.
Starch from corn and barley, for instance, is digested in the stomach, where microbes convert the starch to acid. This means that the stomach will soon be left empty but acidic, and this in turn can increase the risk of gastric ulcers. Excess starch will move to the hindgut, where it undergoes rapid microbial fermentation causing pH to drop, harming the sensitive microflora. This can increase the risk of hindgut acidosis, colic and laminitis.
Sugar from molasses, on the other hand, is highly digestible, but this means that the horses blood sugar levels will soar, which in turn will lead to a sharp, highly energetic horse immediately after meals. Becasugar and starch provide quick-release energy, the horse cannot it up until it is next needed, and so a high starch diet will mean a short burst of energy followed by a slump, rather like the Mars bar effect!
When blood sugar levels rise, the horse will produce insulin to remove the sugar from the blood, but the horses supply of insulin is finite. When this becomes low, the horse may develop Equine Metabolic Syndrome (EMS), Insulin Resistance (IR), with the associated risk of laminitis, and eventually will become diabetic. This is similar to Type 2 diabetes in humans.
A good quality forage diet will ensure your horse has enough energy stored in his muscles to sustain them during work, but not so much that they become overexcited and unmanageable, or at risk of digestive problems. A low sugar and starch diet reduces the risk of laminitis, colic, tying up, ulcers, filled legs and behavioural problems.
Horses thrive best on the food they evolved to eat, which is forage. Starchy foods are not found in their natural environment.