Dietary recommendations for equines with a range of conditions, such as polysaccharide storage myopathy-1, equine metabolic syndrome and hyperinsulinaemia-associated laminitis, usually focus on low non-structural carbohydrate intakes. A level of 10–12% non-structural carbohydrate on a dry matter basis in the total ration is often cited, although studies have not necessarily demonstrated this level is appropriate for every clinical condition. More severe issues may require <10% non-structural carbohydrate (Borgia et al, 2011; Lindåse et al, 2018; Macon et al, 2022).
Non-structural carbohydrate includes starch as well as the water-soluble carbohydrate content of a feed material. Water-soluble carbohydrate is less relevant in human nutrition and food labelling, which may offer some explanation as to why the terms ‘sugar’ and ‘sugars’ are often used generically or collectively when referring to monosaccharides, disaccharides and water-soluble carbohydrate in equine nutrition. This has the potential to cause confusion, particularly in relation to feed labelling and forage analysis. Various tests and methodologies exist for analysing carbohydrates in feed materials and it is important to establish what and how the amount of carbohydrate has been measured when comparing products or assessing their suitability for an equine with a clinical condition.
As veterinary understanding of the relationship between non-structural carbohydrate intake and nutrition-related diseases such as hyperinsulinaemia-associated laminitis and equine squamous gastric disease has increased, it is widely accepted that a low non-structural carbohydrate diet is required for managing these conditions (Geor and Harris, 2009; Luthersson et al, 2019). However, achieving a low non-structural carbohydrate diet is complicated by different terminology used in feed labelling and marketing, and different analytical techniques used to measure levels of carbohydrate in forages in particular.
Commonly used terms relating to ‘sugars’
Chemically, the term ‘sugars’ refers to a group of compounds comprising carbon, hydrogen and oxygen atoms classified as either monosaccharides or disaccharides (Sigman-Grant and Morita, 2003). The American Association of Feed Control Officials defines sugars in feed as ‘the sum of all free di- and monosaccharides, such as sucrose, glucose or others, digestible by enzymes found in an animal's digestive tract’ (Sigman-Grant and Morita, 2003). This distinguishes sugars from water-soluble carbohydrate, which includes sugars plus storage forms of sugars most commonly found in fresh and conserved forms of pasture grasses, such as fructan. The following definitions clarify some commonly used terms:
For the purposes of this article, the terms ‘sugar’ and ‘sugars’ will be used when referring simple sugars such as monosaccharides and disaccharides.
In practice, it is usually easier to reduce or remove starch than water-soluble carbohydrate from most equines’ diets. This is because the main source of starch is cereals, which are usually fed in the bucket feed and so can be relatively easily replaced. Higher fibre and oil feed materials provide alternative sources of energy and may have metabolic consequences (Martin et al, 2023). In the UK, it is generally harder to reduce water-soluble carbohydrate intake in the equine's diet as the greatest source is usually pasture and/or conserved forage, which should make up at least half of most horse's rations. The C3-cool season grasses that predominate in UK pasture, including fescue, timothy and perennial rye grass, preferentially accumulate fructan as their storage carbohydrate (Watts, 2004). This is in contrast to C4-warm season grasses that use starch as a storage carbohydrate. In general, C3 grasses are generally higher in non-structural carbohydrates than C4, and the difference is even greater in cold conditions (Watts, 2004). The requirement to match the recommendation of ensuring a minimum forage intake of 1.5% on a dry matter basis (Rendle et al, 2018) in the majority of cases to help maintain digestive health while trying to reduce non-structural carbohydrate intake can be a key challenge.
Regulatory considerations relating to product claims about sugar
Any company making a claim relating to sugar (some may not differentiate between sugar and water-soluble carbohydrate) in a feed or supplement they are placing on the market has a range of obligations in order for that claim and product to be considered legal. These are largely covered by regulation EC No. 767/2009, which has been transposed into British feed law following Great Britain leaving the European Union (Food Standards Agency, 2020). It is important to note that this also applies to veterinary practices selling feeds and supplements to clients. In the context of feed law, supplements are considered complementary feeds and so the same rules and regulations apply.
The key points to be aware of are:
Analytical techniques
Sugar
Analysis can be carried out by near-infrared spectroscopy or wet chemistry; the latter uses the Luff Schoorl method of analysis which is legally recognised. These methods report monosaccharides such as glucose; disaccharides such as sucrose; and oligosaccharides up to a chain length of 10 saccharide (sugar) units (Dekker, 1950).
Near-infrared spectroscopy relies on calibrations developed using reference data for the same material. While calibrations exist for commonly used feed materials, it is worth noting the dry matter content and extent of fermentation in wrapped forages can significantly influence the accuracy of results for grass-based conserved forages (Harris et al, 2017). Harris et al (2017) compared near-infrared spectroscopy and wet chemistry analytical techniques by analysing multiple sub-samples from one bale of haylage and found significant variability in the results from the samples tested using both analytical techniques for sugar (water-soluble carbohydrate was not included in the study). The authors highlighted that, in addition to the possibility that the analytical methods are less consistent, ‘another possibility is variability in sugar content within the original grass/forage included in the particular haylage bale, especially, as it is unlikely that a bale will have a consistent botanical composition throughout the whole bale, for example, areas with higher or lower concentration of seed heads or stems’. Multiple sample points are therefore key for analysis to provide information of any value when selecting forage on the basis of its nutritional value. Calibrations should also be reviewed regularly.
Ethanol-soluble carbohydrate
Some laboratories, particularly those in the US, report ethanol-soluble carbohydrate, which records sugars that dissolve in 80% ethanol solution. Some fructans can be picked up by this test (Kagan, 2022) but it does not appear to be consistent. Again, if near-infrared spectroscopy is used to give an ethanol-soluble carbohydrate value, it is important to consider the reference materials used to establish the calibrations. Current recommendations are that it is inadvisable to rely on ethanol-soluble carbohydrate as an indicator of the suitability of a material when a low non-structural carbohydrate intake is recommended (Harris et al, 2017).
Water-soluble carbohydrate
Wet chemistry methods are currently the most reliable for reporting water-soluble carbohydrate values, including simple sugars as well as the fructan oligomers and polymers with chain lengths longer than 10 saccharide units (Sigman-Grant and Morita, 2003). This is the analysis method usually recommended for forages and products containing significant quantities of forage and high fibre materials, particularly those containing a significant proportion of grass. It has been recommended that near-infrared spectroscopy is not used for water-soluble carbohydrate unless the laboratory can confirm the accuracy of their calibrations (Harris et al, 2017).
Examples of sugar content of commonly used feeds and ingredients
‘Sugar-free’ feeds are usually a misnomer, as most of the ingredients used in horse feeds contain some naturally occurring sugar. ‘No added sugar’ is an alternative but still imperfect term used to try to communicate that no ingredients understood to be high in sugar have been added. Molasses is an ingredient that many horse owners are concerned by, and it can cause considerable confusion when products containing molasses are relatively low in water-soluble carbohydrate and below 10% non-structural carbohydrate. This is possible when a low level of molasses is combined with other materials that are low in non-structural carbohydrate such as in straw-based, short-chopped fibre feeds (personal communication).
When included in mixes and cubes, molasses is typically added at a rate of 5–10%, meaning it contributes around 2.5–5% sugar to the finished feed (personal communication). In these situations, the non-structural carbohydrate will be elevated to some extent by the inclusion of cereal and cereal co-products which contain starch.
The inclusion of grass either as pellets or short-chop will supply both simple sugars and water-soluble carbohydrate. This is a situation where the legal labelling requirements are unhelpful when trying to select feeds for certain clinical conditions where a low non-structural carbohydrate content is required. The legal requirement for a simple sugar declaration using the Luff Schoorl method misses the water-soluble carbohydrate that high inclusions of grass-based materials supply. For example, a grass pellet typically contains around 10% simple sugar but more than 15% water-soluble carbohydrate (personal communication).
The importance of considering the level of sugar within the feed as well as the feeding rate of the product, cannot be over-emphasised. This is most pertinent when considering supplements which, by their very nature, are very concentrated sources of nutrition. Although the sugar content in some supplements may seem high, their feeding rates (typically 10–100 g daily) mean the amount of sugar they provide is not usually significant in the overall diet. As a result, most supplement manufacturers choose not to declare sugar to avoid unnecessary concern and confusion, but would typically make the information available on request where they can provide an accompanying explanation and reassurance as to the product's suitability. Table 1 gives a range of examples of sugar levels in common types of feeds and demonstrates how significant the feeding rate is to the amount of sugar supplied.
Conclusions
Misinformation about equine nutrition and feeding is only going to be addressed if veterinary personnel and allied professionals give consistent messages to horse owners. A regulatory framework exists for claims relating to sugar but labelling requirements imposed on feed and supplement manufacturers are less helpful for horse owners and others who may have limited understanding of definitions. Therefore, it is important to request more information from manufacturers when assessing a product's suitability for particular clinical conditions.