Background

Musculoskeletal injuries are the most common cause of injury and lost training time in athletes, whether human, canine or equine. Daily exercise and the rigors of competition inevitably cause some damage to the skeleton. This may be compounded by factors such as hard going, lack of muscle strength or joint instability. The body has a tremendous capacity for repair and provided the components for repair are available in sufficient amounts, for example, through the diet, under normal circumstances repair keeps pace with damage.

In the wild the horse spends 95% of its time grazing, with the rest of its time taken up by sleeping, playing and grooming. The time budget of a wild horse does not include daily intense exercise (galloping), repeated jumping or prolonged low intensity exercise. However, these are all components of the training programs for racehorses, showjumpers and eventers and endurance horses, respectively. A horse’s joints effectively act as ‘shock absorbers’ to concussion and must provide a lubrication system to reduce the friction involved in movement between the joint surfaces.

Joint Structure and Function

The articular surfaces of the ends of bones within a joint where they meet (oppose) are protected by a layer of cushioning cartilage, which when properly lubricated allows for frictionless movement of the joint concerned and also has the ability to absorb the shock being transmitted up the limb from the impact of the foot with the ground. The joint itself is contained within a joint capsule, which attaches to both bones and is stabilized by ligaments. An essential component of normal joint function and health is continued lubrication of the joint, which reduces the damaging effects of friction. If the fluid is removed, becomes thinned or poor quality, the joint will begin to grind itself away, in the same way, as a car engine without oil will quickly seize up.

Synovial Fluid The synovial membrane lines the joint capsule and secretes a lubricating substance called synovial fluid, which is composed of a number of ingredients including hyaluronic acid and lubrican.

Cartilage Cartilage is composed of specialized cells called chondrocytes, which synthesize and deposit a water-based matrix containing collagen and large molecules known as proteoglycans. Proteoglycans consist of a protein core with side chains of glycosaminoglycans (predominantly chondroitin sulphate). Proteoglycans are also found in association with hyaluronic acid. The arrangement of these components is similar to that of a fir tree, with hyaluronic acid representing the branches, protein the twigs and the proteoglycans (chondroitin sulphate) the needles. The presence of collagen in the cartilage matrix gives cartilage it’s strength, whilst the proteoglycan in association with hyaluronic acid provides both resilience and flexibility.

Chondrocytes have the ability to synthesize all the components of the cartilage matrix, which is constantly changing. Throughout life the cartilage matrix is slowly removed or replaced in a continuous turnover process. The rate of cartilage turnover is very rapid in a foal and gradually declines with age. The turnover rates of the proteoglycan component of cartilage is much more rapid than that of collagen. However, once cartilage has been destroyed it cannot be replaced.

Cartilage is unusual in that it does not have a blood or nerve supply and obtains its nutrients from the synovial fluid via the lymphatic system. As a result, the supply of nutrients to cartilage is dependent on the volume of synovial fluid. It has been suggested that even in a healthy joint, the volume of synovial fluid is on the limit of that required to sustain normal cartilage turnover. Any increase in the stresses imposed on joints through training or competition, especially on hard ground, or an increase in the degradative process as a result of injury or cumulative wear and tear will increase the requirement for those components of the diet needed to maintain the cartilage’s structure and integrity.

Dietary “Joint Supplements”

Recently there has been a dramatic increase in the number of supplement products on the market that contain a variety of nutrients promoted as being beneficial to joint cartilage health and regeneration. There has been a parallel increase in the amount of scientific evidence both in humans and horses.

Chondroitin Sulphate (CS)

Chondroitin Sulphate is a component of the naturally occurring proteoglycans in the body and is formed from repeating units of N-acetyl glucosamine and glucuronic acid. Chondroitin sulphate or its components are common ingredients in joint supplements and its precise structure is dependent on where it is sourced. In mammalian cartilage the sulphate group in glucosamine is at position 4 (Chondroitin Sulphate A) whilst in fish cartilage the sulphate is at position 6 (Chondroitin Sulphate C). Chondroitin Sulphate for inclusion in supplements is normally extracted from bovine, porcine or marine sources. Chondroitin sulphate from these sources has a molecular weight of 50,000 – 100,000 Da or (50-100 KDa) but following the extraction process this may be reduced to 10,000 – 40,000 Da, dependent on the raw material source and processing. 

Absorption of Chondroitin Sulphate

There has been recent debate as to the degree of support offered by chondroitin sulphate. In addition, there has been further debate over its supposed bioavailability. Some studies have demonstrated increases in plasma, urinary and synovial chondroitin sulphate concentrations following oral administration. Conversely other authors have reported little or no chondroitin absorption. The presence of low molecular weight fragments of chondroitin sulphate, including monomer, oligo and polysaccharides fragments, have been found in plasma, tissue and urine following oral feeding. There are no intestinal, pancreatic or brush border enzymes that are capable of degrading such polymers, however bacterial degradation in the large intestine could be involved. Studies in rats have shown that the chondroitin sulphate polymer is not degraded in the stomach or small intestine but is degraded by bacteria resident in caecum, and to a lesser extent in the colon, to low molecular weight metabolites. The degree of degradation appears to be related to the residence time in either the caecum or colon. Studies in rats with radiolabelled chondroitin sulphate suggest that a small proportion of the chondroitin sulphate is absorbed intact in the small intestine and colon, whilst the majority is degraded in the caecum and absorbed as small molecular weight fragments. These studies also reported that there was no difference in the rate of degradation of chondroitin sulphate according to molecular weight between (molecular weight 10 –18,000kDa).

Glucosamine

Glucosamine is normally synthesized from glucose and forms part of the structure of the glycosaminoglycan chains within the structure of proteoglycans. It is isolated from the hydrolysis of crustacean shells to form the hydrochloride or sulphate salt.

Absorption of Glucosamine

Glucosamine has been shown to be rapidly absorbed and concentrates in articular cartilage. It is a small molecule that is soluble in water and bioavailability is reported as being high in all species studied. It has been shown to be the preferred substrate for synthesis of glycosaminoglycans and its availability is likely to be a rate-limiting step for their synthesis. 

Combined use of Glucosamine and Chondroitin Sulphate

There is evidence to support the use of both glucosamine and chondroitin sulphate to support cartilage tissue, and there is therefore logic to provide the two together in order to benefit from any synergistic effects as the two substances act in slightly different ways. There is some in vitro work to suggest that there may be a complementary effect of glucosamine and chondroitin sulphate on the maintenance of joint function. However, there is no direct evidence as yet to support that a combined product is more efficacious than either of the two components alone.

Key Points Concerning Glucosamine

- Glucosamine is normally synthesized from glucose and forms part of the structure of the glycosaminoglycan chains within the structure of proteoglycans.

- Glucosamine has been shown to be rapidly absorbed and concentrates in articular cartilage.

- The beneficial effect of glucosamine on the maintenance of joint function in horses, dog and man has been published in scientific journals

MSM

MSM or methyl sulphonyl methane is a naturally occurring compound found in both plants and animals that supplies an organic and bio-available form of sulfur. It occurs naturally in the horse’s diet, however drying or processing will destroy the majority. MSM consists of about 34% sulfur by weight and together with its related compounds, is the source of over 80% of the sulfur found in all living organisms. MSM is found naturally in the body but the levels are found to decrease with age.

Absorption of MSM

Radiolabelling studies have shown MSM to be bio-available with sulfur derived from MSM being found in blood and a wide range of tissues within 24 hours of administration.

Key Points Concerning MSM

- MSM occurs naturally in the horse’s diet and provides a bio-available form of sulfur.

- MSM occurs naturally in the horse’s diet, however drying or processing will destroy the majority.

- MSM provides sulfur to support sulfur-containing bonds such as those found in collagen.

Omega-3 fatty acids

Fatty acids are found in high concentrations in oils such as linseed oil, cod liver oil, corn oil and soya oil. The position of the first C-C double bond within an unsaturated fatty acid effects its metabolism by the body and this feature is used to further classify unsaturated fatty acids. Omega-3 fatty acids are those that have their first C-C double bond between the 3rd and 4th carbon atom counting from the methyl group or omega end. In contrast, Omega-6 fatty acids are those that have their first C-C double bond between the 6th and 7th carbon atom counting from the omega end and so forth for Omega-9 fatty acids. In feed ingredients, alpha linolenic acid, which is found in high concentrations in linseed oil and cod liver oil, is the major omega-3 fatty acid; whilst linoleic, which is found in high concentrations in corn and soya oil, is the major omega-6 fatty acid and oleic acid, which is found in high concentrations in olive and many other vegetable oils, is the major omega-9 fatty acid.

Horses, like man, are unlikely to be able to synthesize fatty acids, which have their first C-C double bond before the 9th carbon atom from the omega end. Thus, omega-3 and omega-6 fatty acids are referred to as essential fatty acids (EFA) as they must be provided by the diet.

Functions of Omega-3 and Omega-6 Fatty Acids

- Involved in many functions within the body including forming parts of vital body structures, forming an integral component of phospholipids (e.g. lung surfactant), blood clotting, involvement in immune function and vision and are integral to all cell membranes.

Omega-3 and omega-6 fatty acids follow different biochemical pathways to produce distinct types of prostoglandins and thromboxanes, each of which have very different effects within the body. The eicosanoids are potent regulators of vital body functions such as blood pressure, blood clotting, immune response and pro-inflammatory responses. In general terms, the eicosanoids produced from omega 6 fatty acids tend to increase inflammatory processes and blood clotting, whilst those produced from omega-3 fatty acids tend to decrease blood clotting and inflammatory responses, although this is a gross simplification as the mechanisms involved are very complex.

The physical and functional properties of cell membranes are affected by the relative fatty acid composition of membrane bound phospholipids, which can be altered according to the fatty acid composition of dietary triglycerides. The different biochemical pathways involved in eicosanoid production utilize and therefore compete for the same enzymes and so the degree of inflammation, for example, is influenced by the relative proportions of omega-6 and omega-3 fatty acids present in cell membranes.

The reputed beneficial effects of the omega-3 fatty acids are largely due to the conversion of EPA and DHA to the prostoglandin PG3 series. However, unfortunately the conversion of the parent precursor alpha linolenic acid to both EPA and particularly DHA is relatively inefficient due to low activities of the enzyme delta-6-desaturase. More importantly, as the efficiency of the synthesis of both EPA and DHA is likely to be low, dietary supplementation with these fatty acids may be of benefit.

Sources of Omega-3 fatty acids and absorption

Omega 3 fatty acids are found in high concentrations in linseed oil and fish oils, including cod liver and salmon oil. Another potential source of omega-3 fatty acids is the New Zealand green lipped mussel (Perna canaliculus).

Key Points

- Omega 3 and 6 fatty acids are essential fatty acids (EFA’s) and must be provided by the diet.

- The horse’s diet is traditionally HIGH in Omega 6 and LOW in Omega-3 fatty acids.

- The main source of Omega-6 fatty acids in the diet is vegetable oil such as corn or soya oil as well as ingredients such as cereals and pulses.

- Omega-6 fatty acids are converted primarily to arachadonic acid. The main source of Omega-3 fatty acids in the horse’s diet would be linseed and linseed oil as well as fish oil such as cod liver oil.

- Omega-3 fatty acids are converted to eicosapentanoic acid (EPA) and docosahexanoic acid (DHA)

Antioxidants

Reactive oxygen species (ROS) or ‘free radicals’ are potent mediators of cartilage degradation. ROS are capable of degrading many of the components of the joint including collagen, proteoglycans and hyaluronic acid. The horse has a number of natural defense mechanisms against ROS attack in the form of anti-oxidants. These include enzymes such as superoxide dismutase, glutathione peroxidase and catalase as well as vitamins A, C and E. Ascorbic acid (vitamin C) is required for the normal synthesis of collagen and whilst the horse can synthesize it from glucose in the liver, inflammation may increase the breakdown of both collagen and ascorbic acid. Other micro-nutrients normally present in the diet such as copper, manganese and zinc, are also essential co-factors for maintaining healthy cartilage. Copper is required in the production of collagen and also is a co-factor for the enzyme superoxide dismutase. An adequate dietary intake of manganese is required to maintain the concentration and integrity of cartilage proteoglycans. This highlights the need for a balanced and well fortified diet.

Evaluating Joint Supplements

When choosing a suitable ‘joint support’ product you may opt for simple but effective glucosamine, or for a more comprehensive combination of ingredients. Whatever your choice, you need to be mindful that these supplements do not always provide the answer and relative success is likely to depend on why you are feeding the supplement in the first place. Diagnosis by your veterinarian is essential before embarking on nutritional support for an injury or disease situation. Additionally, whatever the main active ingredient in your supplement of choice, you should be aware of the level of the active ingredient present per daily feed. This will enable you to make valid comparisons between competitive products and to evaluate their relative ‘value for money’. Our domestication of the horse and training for athleticism is likely to have increased its susceptibility to joint problems. An increase in the concussive forces involved in many equestrian sports leaves the horse vulnerable over time. Training, equipment and nutrition should be optimized to reduce the ‘stress’ imposed on the skeletal system. Of particular relevance to endurance is the ongoing debate over the importance of volume of work versus quality of work in achieving a balance between ‘training effects’ and ‘wear and tear’. A good training and qualification program should strive to minimize the amount of work or ‘miles on the clock’, whilst still achieving optimum cardiovascular, respiratory, muscular, skeletal and psychological fitness for the task in hand.