There are two main reasons why we need to be aware of dietary starch and water soluble carbohydrates (simple sugars, and fructans which are plant storage carbs) for horses and ponies. Firstly, horses have a limited ability to digest starch, and too much can cause disturbed gut function and associated poor performance. Secondly, more and more horses and ponies are being diagnosed with obesity, laminitis, equine metabolic syndrome, insulin resistance and/or Cushing’s syndrome, and all of these need to have their dietary starch and WSC carefully controlled.
Starch is found in cereal grains and straight grains, coarse mixes and some cereal by-products are sources of starch for horses. WSC are found in many feeds, with grass and preserved grass forages being the main source of simple sugars like sucrose, and fructans. Molasses is rich in sugar but does not appear in most horse diets in substantial quantities.
Starches and sugars are digested in the small intestine, and may also be partially fermented in the stomach and also in the hindgut (in the case of small intestine digestive capacity of starch being overloaded). Fructans are not digested by mammalian enzymes produced in the gut but instead are fermented by beneficial bacteria throughout the gut, but mostly in the hindgut.
There is an increased risk of gastric ulceration associated with high starch intakes, and a level of less than 1 g of starch per kilo of bodyweight per meal is recommended. 1 g/kg BW = 500 g starch for a typical 500 kg horse, which is equivalent to about 1.1 kg of oats or 1.5 kg of traditional coarse mix. It is prudent to feed much lower levels of starch than this to horses with gastric ulcers or pre-ulcerative lesions. The digestive capacity of the small intestine is believed to be overwhelmed at starch intakes of over 1g per kilo of bodyweight per meal, which causes undigested starch to reach the hindgut where it cause disturbance. In very large quantities, undigested starch fermentation in the hindgut can cause colic, laminitis and even death.
Sugars are very well digested in the small intestine even at high intakes, but such efficient absorption means that high sugar intakes can cause high blood glucose levels and resulting high insulin levels and, in susceptible animals, insulin resistance (IR). IR is associated with laminitis in some horses and ponies, especially those genetically predisposed to equine metabolic syndrome (a condition of obesity, IR and laminitis). High intakes of sugar and starch increase the risk of rhabdomyolysis in horses with PSSM (polysaccharide storage myopathy), a condition that causes tying up. These horses should be fed minimal starch and sugar.
High levels of fructan intake can cause laminitis, which has been demonstrated by an experimental model and despite debate about whether or not horse and ponies at pasture could ingest enough to cause an acute episode of laminitis, there is no doubt that grass intake is associated with laminitis. For that reason, grass fructan intake should be severely restricted for laminitis-prone horses or ponies. An actual safe amount of fructan is not known, and there is large individual variation between horses.
Starch, sugar and fructan are all useful carbohydrates in horse diets, but they should be fed in sensible quantities to avoid health problems. Some horse owners believe that starch and sugar are bad for horses, or even ‘poisonous’ but they are not if fed in limited quantities to healthy horses. Like all nutrients, they need to be fed in balanced quantities with the health or disease predisposition of that individual horse taken into account.