Glycemic index has been a big buzz phrase in human nutrition for years and it is now coming around to the equine world as well. What exactly is it and what relevance does it have for the average horse and his feeding strategy?
The glycemic index
First, the glycemic index is a set of calculations leading to a list of feeds based on how they affect blood glucose levels. A food with a high glycemic index tends to be digested rapidly leading to a fast rise in blood glucose (sugar) with a matching rise in insulin.
For horses, oats are the baseline food with an index of 100.
Food with a low glycemic index takes longer to digest, so glucose levels rise more gradually and may not spike as high. It is felt that feeds that are lower on the glycemic index listing will minimize fluctuations in blood glucose, demand less insulin for controlling blood glucose levels and decrease the fats in blood content.
In humans, the index is based around glucose itself with an index of 100 or sometimes, white bread. If white bread is used, then pure glucose has an index value of 140. For horses, oats are generally used as the baseline food with an index of 100 given to whole oats. Usually only starch/carbohydrate foods are rated by their glycemic index. For protein and fat content you would need to use other criteria.
Sweet feed has plenty of easy to digest starches so it will rate about 129 or so. On the other end of the spectrum, beet pulp, after rinsing to remove some sugars, is about 34. Most hays rank in the 40's.
Glycemic index and performance
Timing of feeding relative to an athletic performance is also being studied as related to the glycemic index. The desire is to time peak glucose levels with the metabolic need of the animal during performance.
Why should you care about your horse's feeds glycemic index? For some horses it doesn't matter. These are the equines with an "iron gut". They can eat anything, any time in any amount and stay healthy, trim and fit. They are also a bit of a rarity.
Horses were designed to eat small amounts all day - grazing as they walked about. Digestion is centered around the colon and cecum or hindgut just as much as the stomach and small intestines in the foregut. This meant a low but steady input of glucose via the digestive tract all day.
A horse who is kept stalled and given large meals of concentrates twice daily will have rapid and big spikes in his blood sugar. His foregut may be overwhelmed with a lot of concentrates to digest and handle quickly.
If a large amount of those high glycemic index foods skip on through to the hindgut, the risks of colic, founder and high levels of acid in the hind gut go up. Again, there are horses who can handle this type of diet just fine, especially if they have plenty of forage to go along with those concentrates.
Is the glycemic index important for my horse?
What horses do you really need to think about glycemic index for? There are a few classes of equines that do need some thought given to glycemic index control of their diet.
ECD or Equine Cushings Disease
These horses are somewhat resistant to insulin so their extra carbohydrates tend to go straight to fat. ECD horses are also prone to founder and you want to avoid triggers for that as well. Grass hays and special low glycemic index feeds can work well for these horses.
EMS or Equine Metabolic Syndrome
This catchall phrase covers horses who are easy keepers or overweight, somewhat insulin resistant and may have high blood sugar. Like the ECD equines, they are prone to founder. As with ECD horses, they also often do well on just forage diets - primarily good quality grass hays with a low glycemic index. Some supplements may be needed.
Tying Up or Recurrent Exertional Rhabdomyolysis (RER)
Horses with this syndrome have an inherent problem with calcium movement between cells, which is exacerbated by excitement and extensive work. this is seen mostly in Thoroughbreds, Arabians and Standardbreds. For these equines, feeding a diet that has a low glycemic index for its carbohydrates plus increased fat for energy (along with appropriate selenium and vitamin E supplementation if needed) works quite well.
PSSM or Polysaccharide Storage Myopathy
This is a genetic glycogen storage disease seen in Quarter Horses and some Warmbloods and draft horses. These equines do best with regular exercise and a diet with high forage fiber and low in starches making for a low glycemic index diet overall. Fat supplementation may be needed for energy adequate levels.
OCD or Osteochondrosis or chondritis dessicans
OCD is a problem related to rapid growth in young horses. The bone and cartilage development is disrupted. This may lead to permanent orthopedic problems. Young horses need appropriate exercise and "just the right" amount of vitamins, minerals and energy in their diets.
They do best at a moderate rate of growth regulated by a diet with a high amount of good quality forage (fiber) and fat as energy sources. Obviously they will need some carbohydrates as well but these should be of a low glycemic index leading to a diet with a moderate glycemic index overall. Hyperglycemia and hyperinsulinemia have been implicated as causes or contributing factors to OCD.
Horse feeding tips regarding the glycemic index
So what can you do if you have a horse who falls into one of these categories? If you are feeding beet pulp, start rinsing the feed first and draining off the rinse water. This can drop the glycemic index in half for that feed - from 72 down to 34. Cut back on general sweet feed mixes and arrange for a low glycemic mix or create your own custom mix using feeds like rice bran and some cracked corn, rolled barley or rolled oats.
Consider your hay and pasture as well. Bluestem hay has a very low glycemic index while alfalfa is mid range. Pasture turnout should also be managed for susceptible horses. Pasture starches are highest after a sunny day so a morning graze is best - unless there has been a cool night so the plants have not burned up their leftover starches.
Starches also increase after plant stresses like droughts due to rapid regrowth. Limit overgrazing as well - and don't hesitate to use a grazing muzzle if indicated.
Clearly glycemic index can be an important factor in the diet of some horses. If your horse fits the profile for a low glycemic index diet, talk to an equine nutritionist about coming up with the best plan for your individual horse.
The physiological response to different types of feeds is well documented in Glycemic index of cracked corn, oat groats and rolled barley in horses.