Sugar and starch are everywhere in our horse’s diets. Even grass can be a source of sugar and starch. And in fact, our horses might struggle on a ZERO sugar and ZERO starch diet, which is good news because it would be impossible to achieve. And part of the sugar and starch problem is how it’s fed. So a horse eating grass or hay with a moderate sugar or starch content over the course of several hours will cope better from a metabolic perspective than if the same volume of sugar and starch were given in one go in a hard feed that could be eaten in 10 minutes!
Perhaps a little bit of explanation of terminology might help here. Sugar and starch are two forms of carbohydrates. Sugar isn’t actually one single compound but a general term of which examples include glucose, fructose (also often referred to as “fruit sugar”), and galactose. These are known as monosaccharides or sometimes simple sugars. Other sugars known as disaccharides are formed of two monosaccharides joined together. Examples of these include sucrose (glucose + fructose), which is the sugar we most commonly buy for putting in tea, coffee and cooking, lactose (glucose + galactose) which may also be referred to as dairy sugar or milk sugar and only occurs naturally in the milk produced by mammals, and maltose (glucose+glucose). Any mono or disaccharides are completely digested and absorbed in the horse’s small intestine, the part of the digestive tract, immediately after the stomach. This consists of three sections known as the duodenum, the jejunum and the ileum. There are two other types of carbohydrates the horse may encounter in its diet – short-chain polysaccharides (oligosaccharides and fructooligosaccharides (FOS)) and fructans. FOS are often included in feeds or supplements as they can act as prebiotics – non-living compounds that can be used by “good” bacteria in the hindgut as a source of energy. Fructans are not so good. These can pass to the hindgut undigested, where they are fermented and increase acidity. High fructan intake is believed to play a role in laminitis, but there is still conflicting views on this.
Additional terminology important for horse owners is WSC (water soluble carbohydrates) and NSC (non-structural carbohydrates). WSC are the simple sugars (monosaccharides), complex sugars (disaccharides), short chain polysaccharides (oligosaccharides and fructooligosaccharides (FOS)) and fructans which are, as the term implies, all soluble in water. So if a sample of hay is soaked in water, the amounts of these present are expressed as WSC. The next term to consider is NSC (non-structural carbohydrate). This is usually defined as WSC + Starch!
Starch is how plants store glucose produced by photosynthesis for use at a later time to generate energy. The starch is stored within all living cells in the horse’s body, with the exception of red blood cells, which can only use glucose, and the brain, which users can only use glucose or ketones if blood sugar levels drop too low (Ketones are made by the breakdown of fat in the liver). When a horse eats plants or grains, or fruits with starch in them, the starch is broken down in the small intestine. Starch is broken down by the enzyme amylase, which is produced predominantly in the wall of the small intestine and, to a lesser extent, by the pancreas. Unlike sugars, starch is often not fully digested in the small intestine. This may be due to large meal sizes and or too rapid a movement down the GI tract, which commonly occurs if forage is not provided before grains or hard feeds. If undigested starch reaches the hindgut, it is fermented by certain species of bacteria leading to colic. When starch or sugars have been digested, they are absorbed and taken into the bloodstream. Here they are transported and taken up by muscle and the liver in particular, but essentially by all living cells, and either used immediately for energy or stored as glycogen (except as previously mentioned, red blood cells and the brain, which use the glucose immediately and do not store glycogen). Glycogen? Glycogen is the animal form of starch. Like starch, glycogen is made up of lots of glucose units joined together, although in a different pattern to starch, hence why they are not the same.
Glycogen storage
At this point, it’s important to clarify some aspects of glycogen storage. Why does the horse need to make glycogen? Why can’t the horse store glucose units ready to use? The fact is that glucose is a tiny molecule and can easily move in and out of cells. Glycogen, on the other hand, is a very large molecule composed of thousands of glucose units, and so is trapped within the cell. Around 90% of the horse’s body stores of glycogen are stored within muscle cells because when the horse starts to exercise, the main source of energy is a breakdown of glycogen to provide glucose, which in turn produces ATP. We can think of glycogen as being the coal or gas used to generate electricity and the electricity as the ATP. You can’t make a bulb light with coal, and you can’t make muscles contract with glycogen or glucose. The other 10% of the horse’s stores of glycogen are within the liver, and the liver continuously breaks down glycogen to try and keep the blood glucose level within a narrow range.
Interestingly, although many horse owners may have seen many articles demonising starch, it does have some advantages. Humans switching to eating starch-based carbohydrates as well as meat is recognised as a major factor in the evolution of human brain capacity. Whilst sugar and glycogen are important for energy, fats or oils are also important sources of energy. In times of starvation, the body will break down protein to provide energy as a last resort. But essentially, for any equine activity, whether it’s a 100-mile endurance race or a 5-furlong sprint, a dressage test or a showjumping round, when it comes to energy, it’s never fat that is limiting. It’s always glycogen. And when it comes to speed, the more glycogen you have, the faster it can be broken down to produce glucose. And starch intake has also been shown to be related to the amount of glycogen stored in horse muscle. So on low sugar and low starch diets, horses have been shown to have lower muscle glycogen levels, which is not really surprising as glycogen is made from glucose!
Read more about this here on the website – Why Some Horses Need Starch
Starch
So starch isn’t always bad, but it can cause issues. If your horse receives too much energy from its diet compared with how much work it’s doing, this can lead to obesity. However, horses can also become obese from high fat/oil intake. There can be confusion over fats and oils, so this can be easily cleared up – fats and oils are the same, but fats are solid at room temperature, whilst oils are liquid. We know that large starch meals are a risk factor for gastric ulcers. We know that low starch/sugar diets are essential for managing PSSM and RER prone horses and ponies. We know that undigested starch reaching the hindgut is a risk for colic. We also know that high sugar/starch meals can lead to behavioural problems.
So how easy is it to spot sources of sugar and starch in feeds?
Not necessarily that easy! In commercial feeds, many companies are reluctant to put sugar and/or starch levels on the label. It’s nice to see that companies including Spillers, Dodson and Horrell, Pure Feed and Saracen all list sugar and starch content of their feeds on their website. And when it comes to pasture, hay and haylages, the species of grass, the soil type, time of year, weather, harvesting, treatment, etc, can all impact the sugar and starch levels. The only sure way is to have it tested or to buy hay/haylage from suppliers that provide this information. You can obtain “typical values” for forages from articles or published papers, but again, these may be a long way out. Of course, many owners also soak their hay in order to reduce the “sugar” or, more correctly, WSC content. But even with soaking, it can be difficult to know how much you are removing as this depends on how much WSC content is there at the start, the water temperature and how long you soak for. https://askanimalweb.com/video-short-how-long-should-you-soak-hay-for-in-winter/
How do manufacturers try to disguise the fact that there is sugar in a product?
These are some of the names that could be on the label that you would not recognise as sugar.
Dextrose, Fructose, Galactose, Glucose, Lactose, Maltose, Sucrose, Cane juice crystals, Corn syrup, Date syrup, Dextrin, Diastatic malt, Ethyl maltol, Maltodextrin, Agave Nectar/Syrup, Barley malt, Blackstrap molasses, Brown rice syrup, Caramel, Carob syrup, Honey, Malt syrup, Maple syrup, Molasses, Rice syrup, Sorghum syrup, Treacle.
Fortunately, there are fewer alternative names for starch. These include amylum, polymeric carbohydrates or polysaccharides. But if you see this, (2R,3S,4S,5R,6R)-2-(hydroxymethyl)-6-[(2R,3S,4R,5R,6S)-4,5,6-trihydroxy-2-(hydroxymethyl)oxan-3-yl]oxy-oxane-3,4,5-triol, the IUPAC ID for starch, then someone is really trying to confuse you.
When it comes to working out the sugar and starch in individual feed materials, that’s generally more straightforward. I often use this website > https://nutritiondata.self.com/. This site is aimed at human nutrition, so if you put in “carrots”, for example, you will find the nutritional information for many products, including fresh, frozen, cooked, etc. The table below shows the nutritional breakdown for one large carrot, but you can also get values per 100g.
Podcast
Dr David Marlin has also recorded a PODCAST about this – you can listen on Apple Podcasts, Spotify, Amazon or here on the website >>> Podcast – Sugar and Starch – Good OR Bad? by Dr David Marlin
