Chondrocyte Metabolism in Health and Disease
Articular cartilage is an avascular, aneural, and alymphatic matrix synthesized by chondrocytes, which are sparsely distributed within it. The matrix is composed of a network of mostly type II collagen, with a non-fibrillar component of highly sulfated glycosaminoglycan (aggrecan) monomers attached to hyaluronic acid and link proteins. The collagen provides tensile strength, and the viscoelastic aggrecan component provides compressive stiffness. The tightly wound triple helices of type II collagen are stabilized by a high degree of crosslinking, which steadily increases with age. The crosslinking occurs between adjacent lysine residues within the protein and is catalyzed by lysine oxidase. When compressed, water is forced out of the matrix, producing hyperosmolarity and stimulating chondrocytes to synthesise matrix. As the force is relieved, the matrix rapidly recovers its elasticity as water is drawn back into the matrix by the hydrophilic aggrecan aggregates. A large number of other components contribute to matrix cohesion and regulation of chondrocyte function - most of which are produced by the chondrocytes themselves.
There are only two possible routes for delivery of nutrients to chondrocytes: either via the synovial fluid or from the underlying cancellous bone. Although both routes are involved, cartilage nutrition by the synovial fluid seems to be the most important.
The matrix is constantly but slowly turned over, with a balance of degradation and synthesis resulting in molecule half-lives of aggrecan monomers and aggregates ranges from days to months, even years. Degradative enzymes such as matrix metalloproteinases (MMPs) are secreted by the chondrocytes. Changes in the biochemistry of articular cartilage and deterioration in cartilage matrix function result from diverse actions such as age, loading, genetics, trauma, free radicals, and inflammation.
The low oxygen concentration within the cartilage and synovial fluid forces chondrocytes to almost exclusively utilise anaerobic glycolysis for ATP generation.1 The consumption of oxygen by chondrocytes is only 2–5% of that of liver or kidney on a per-cell basis, whilst the production of lactate is greater. This limits fuel utilisation to glucose, and glucose transport into articular chondrocytes is independent of insulin.2 Chondrocytes not only utilize glucose as the main energy source but also as a precursor for glycosaminoglycan (GAG) synthesis.
In addition to glucose, glucose transporters (GLUTs) also transport ascorbate (vitamin C) which serves as a cofactor in collagen synthesis, and glucosamine which can be efficiently utilized by chondrocytes for glycosaminoglycan synthesis. In osteoarthritis (OA), alterations in glucose metabolism contribute to the degradation of articular cartilage.2 This may explain the comorbidity of OA and diabetes in humans. In OA, glucose is prioritised for glycolysis, and glucosamine synthesis is inhibited.1 Thus, molecular mechanisms regulating glucose supply and metabolism are important in cartilage physiology and OA pathogenesis.
In OA, there is a change in the composition and a reduction of the glycoproteins, resulting in mechanical overload, loosening, and damage to the collagen network. This increases chondrocyte stress and leads to increased remodelling of the matrix in an attempt to repair the damage. The chondrocytes increase their expression of MMPs and aggrecanases to degrade damaged matrix whilst increasing their anabolic activity and glycoprotein synthesis. However, GAG synthesis is inadequately increased, leading to a net loss of glycoprotein content in all stages of osteoarthritic cartilage degeneration. The new matrix has a disproportionate content of collagen type II, and a relative deficiency of aggrecan molecules, which increases the stress applied to the collagen network, thus promoting its destruction and thereby further loss of glycoproteins. Physical damage to the collagen network leads to fissuring and complete destruction of the cartilage matrix, producing microscopic fissuring and eventually macroscopic cartilage tears.
Thus, key metabolic derangements during the progression of OA are:
An increase in the glucose utilization by chondrocytes, leading to increased lactate production
A diversion of intracellular glucose away from hexosamine synthesis
An increase in proteolytic enzyme production
Increased matrix synthesis
An increase in the collagen:glycoprotein ratio
Net deficiency of glycoprotein
Nutrition and the Development or Prevention of OA
In dogs, hip and elbow dysplasias are the most common causes of nontraumatic OA. Both conditions are complex, inherited, polygenic traits, where gene expression is modified by several environmental factors that alter the traits' severity. Nutrition is a major environmental factor. In cats, the underlying aetiology of OA is less well described. Nonetheless, it is likely that the same nutritional variables are operative in feline OA as in canine.
Nutritional deficiency and excess can affect the development of OA. Calcium deficiency is usually the result of feeding muscle-based home-prepared diets that have no supplemental calcium. Such diets are also excessive in phosphorus, which exacerbates the deficiency. At more subtle levels than those that produce osteopenia, long bone deformity, and pathological fractures, Ca:P imbalances can lead to joint incongruity and OA later in life.3 Similarly, calcium excess can lead to severe disturbances in skeletal development, growth, and mineralisation. Although the gross skeletal changes may improve after normalisation of calcium intake, osteochondral lesions continue to develop.4 This scenario is seen in practice with either excessive supplementation or the feeding of "Lite" or energy-restricted adult maintenance diets to rapidly growing large-breed dogs, as has been suggested by some orthopaedic surgeons (see below).
Obesity
Beyond the devastating effects of severe nutritional deficiencies on chondrocyte development, the greatest nutritional influence on joint health is obesity. In dogs and cats, excessive growth rates and energy excess during adulthood dramatically increases the risk of OA.5-8 The most commonly touted explanation is that obesity leads to mechanical overloading and cartilage damage. Although indisputable, it is essential to realise that mechanical forces are not the only factor responsible and may not even be important in the association. The roles of other factors are suggested by the relationship between obesity in humans and an increased risk of OA not only for knee joints but also for non-weightbearing joints such as the hands.9 This suggests that OA may be initiated by systemic responses associated with obesity, with progression being worsened by high mechanical stress on abnormal cartilage.
Recently it has become apparent that leptin is involved in the normal development of cartilage, but may also be involved in the pathogenesis of OA.10,11 The leptin receptor is expressed by normal cartilage and regulates normal chondrocyte maturation - most notably in the growth plate by controlling the rate of matrix mineralization prior to endochondral ossification.11 Leptin has been shown to strongly stimulate anabolic functions of chondrocytes and induce the synthesis of the key cartilage growth factors (IGF-1 and TGF-β).
Leptin is present in synovial fluid obtained from human OA-affected joints, and leptin concentrations correlate with the body mass index.10 Leptin is produced by chondrocytes and osteophytes in OA patients, while in normal cartilage, few chondrocytes produce leptin. Furthermore, the pattern and amount of leptin expression are related to the severity of cartilage destruction and parallel the concentrations of IGF-1 and TGF-β. Although these growth factors stimulate cartilage repair, excessive or prolonged exposure leads to the development of lesions indistinguishable from naturally occurring OA.12
When healthy adult humans were examined to determine the effects of body composition on tibial cartilage volume, bodyweight was not independently related. In fact, for a given bodyweight, adults with a greater lean body mass have a greater cartilage volume than adults with a greater fat mass, and increased fat mass was correlated with small cartilage defects as detected by MRI.13
Nutritional Aspects to the Management of OA
Of all the nutritional and nutraceutical options available, those that have been proven to affect OA in dogs and cats are avoidance of malnutrition, attainment of a lean body condition, dietary enrichment with n-3 polyunsaturated fatty acids (PUFA), and green-lipped mussel extract (GLME), with some evidence in support of glucosamine/chondroitin sulphate.14
Weight Loss
In obese animals, weight loss leads to profound and rapid improvement of the signs and progression of OA.15-17 Any loss of muscle mass can exacerbate joint instability, reduce exercise ability and tolerance, reduce resting energy requirements, and feed into the cycle of reduced activity 
reduced muscle mass increased instability increased pain reduced activity. It is therefore theoretically ideal that the rate of weight loss should be slow enough to preserve muscle mass. Although that ideal rate for any given individual is not known, a rate of no more than 2% current body weight per week is a reasonable target.18 Preservation of muscle mass during weight loss can be maximised if energy restriction is accompanied by exercise, especially resistance training.19
The prevention of energy excess at any stage, but especially during skeletal development, is important. To satisfy this aim, some veterinarians have recommended feeding diets to large-breed puppies that have a lower energy density, such as adult maintenance diets or even "Light" or restricted-energy diets. Unfortunately, this is a seriously flawed recommendation. In an epidemiological study, it was found that feeding large-breed puppies diets that contain ≥ 0.86 g calcium/1000 kJ ME was associated with an increased risk of OCD, whilst specifically formulated dry puppy diets were protective.20 Several adult diets and even some puppy diets contain considerably more calcium on an energy basis - even as high as 1.4 g/1000 kJ ME. Thus, a puppy eating a "Light" diet to satisfy its energy requirements may over-consume calcium because of its greater energy requirements on a per weight basis.
Glucosamine
Glucosamine is a hexosamine sugar that the body uses in a number of structural components, particularly connective tissues. Animal and in vitro experiments have shown that glucosamine supplementation stimulates chondrocytes to increase secretion of GAG and glycoproteins.21 It down-regulates joint catabolic activity, and higher doses have some direct anti-inflammatory action that appears to be different to that of NSAIDs (and which has been exploited - for example, to treat experimental synovitis in dogs). It may also have antioxidant activity.
Chondrocytes actively take up glucosamine sulphate, which is the preferred substrate for GAG production and is selectively incorporated following systemic administration.1,21 This effect is of great importance in the face of arthritis when glucose is diverted away from GAG synthesis. Even in the presence of inflammation, glucosamine increases GAG synthesis and normalises extracellular matrix composition.
In addition to its action as a precursor to GAG synthesis, glucosamine has anti-inflammatory properties due to dose-dependent inhibition of inducible nitric oxide (iNOS), prevention of IL-6 production, and inhibition of COX-2 expression and thus PGE2 production.22 Furthermore, glucosamine is able to inhibit expression of some aggrecanases and matrix MMPs. Glucosamine has been found to dose-dependently decrease the inflammatory response in experimental autoimmune polyarthritis.23 In vitro, it may suppress the activation of T-lymphoblasts and dendritic cells and suppress neutrophil superoxide generation, lysozyme release, and complement-induced chemotaxis.22
The use of glucosamine alone has not been evaluated properly in naturally occurring OA in dogs or cats. The use of a glucosamine/chondroitin/manganese ascorbate product has been evaluated in canine OA and found no significant effect of treatment on owner assessment, veterinary examination findings, or weightbearing.24
In human medicine, glucosamine has been extensively evaluated in several random controlled clinical trials that demonstrate a range of responses from none to impressive. Recently, a series of 20 randomised controlled trials have been reviewed in a metaanalysis.25 Some trials compared glucosamine to placebo (just under 2000 subjects) and some with NSAIDs. The conclusion was that collectively, the 20 analyzed trials found glucosamine significantly improved pain and function compared with placebo, was superior to NSAIDs in a small subset, and had few side effects.
In a recent study, glucosamine was combined with collagen, and improvements in weightbearing were seen in low-dose collagen-treated dogs, but not in dogs where glucosamine was added.26 Oral collagen could provide substrate (hydroxyl-proline), could stimulate GAG synthesis, or could, through the induction of regulatory T lymphocytes, reduce bystander autoantigen-specific inflammation.
Two recurring findings in human metaanalyses are that 1) not all glucosamine products are efficacious, and 2) there is a subset of patients with OA that are very responsive, experiencing improvements in pain and function that rival NSAIDs, whilst many do not respond at all. Suggestions as to the diversity of response include the suggestion that patients with a high turnover rate of cartilage in the affected joints are more likely to respond. Thus, it is too early to conclude the efficacy or lack of efficacy of glucosamine in canine and feline arthritis, but there remains considerable reason for optimism.
n-3 PUFA
The n-3 and n-6 PUFA compete for incorporation into cell membrane phospholipids, and the PUFA liberated from membranes by activated phospholipase A2 is dependent on the dietary concentrations of the various 18 and 20 carbon PUFA. When the n-6 PUFA arachidonate is the liberated PUFA, prostaglandins and leukotrienes of the 2-series (e.g., PGE2) are synthesised. In contrast, the products of the n-3 PUFA eicosapentaenoic acid are the 3-series prostaglandins and leukotrienes (e.g., PGE3). Whilst it is a gargantuan simplification, it is conceptually helpful to consider the 3-series eicosanoids as being "less potent" inflammatory mediators.
In addition to the effects on eicosanoid production, n-3 fatty acids have a direct influence on gene expression in chondrocytes, including the expression and activity of aggrecanases, MMPs, and the expression of IL-1, TNF-α, and COX-2, but not COX-1.27,28
Some of the key metalloproteinase genes are regulated in part by a family of nuclear receptors called peroxisome proliferators agonist receptors (PPARs).29 Significant reductions in cartilage degeneration and improvements in the clinical signs of experimental OA in dogs have been demonstrated using synthetic PPARs ligands.30,31 The n-3 PUFA eicosapentaenoic acid is a ligand for the receptor, and it is likely that some of the benefit of n-3 PUFA-enriched diets occurs as the result of a direct suppression of aggrecanase and MMP gene expression. It appears that the lipid component in GLME is responsible for its efficacy, possibly due to unusual fatty acids.32,33 However, it may be that other, as yet unidentified compounds are present.
Dietary enrichment with n-3 PUFA has been shown to produce reliable improvements in dogs and cats in the signs and progression of OA, including pain, duration of morning stiffness, number of painful joints, joint mobility, and reduced NSAID usage.34-37
It is not known if simply the dose of DHA or EPA is determinant of the efficacy, or if more complex proportions and ratios of different n-3 and n-6 PUFA are important. Thus, recommendations regarding a "dose" for PUFA supplementation should be regarded with scepticism. As an example, to provide the same daily amount of DHA/EPA in Hill's j/d, at least 6 g of menhaden fish oil will need to be given to a 25-kg dog consuming Hill's Science Diet. To provide the same n-6:n-3 ratio, 30 g would need to be added to the diet.
Antioxidants
It is not surprising that the pathogenesis of OA, like other chronic degenerative or inflammatory diseases, involves an increase in cellular and matrix damage from free radicals.38 At present, there is little evidence to support dietary supplementation with antioxidants such as ascorbate and tocopherol to manage OA.39 However, some polyphenolic compounds, such as catechins found in tea that have traditionally been described as antioxidants, may have a role in the future in their ability to limit iNOS induction, inhibit matrix MMPs, and limit collagen degradation.22
Conclusion
The influences of nutrition on joint health are legion, and only a few areas have been discussed. It has become increasingly apparent that metabolic derangements in response to inflammation and injury are central in the progressive degradation of articular cartilage. Nutrition can affect the progression of OA to interrupt the cascade of glucose diversion, reduced relative GAG production, increased protease production, the adverse effects of growth factor excess, and the production of inflammatory mediators and oxidative damage. However, the single most important and efficacious dietary intervention that can be made is the avoidance or management of obesity throughout life.
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