Dogs are superbly well adapted for endurance exercise when compared with cats or people. The majority of skeletal muscle fibres of dogs have a high aerobic capacity, and the maximum rate of oxygen utilisation (VO2max) and blood flow at VO2max in the gastrocnemius have been measured to be 5 times higher in dogs than cats.1 Despite that, muscle fibres in dogs also appear to have a high anaerobic capacity since most type I fibres still contain high concentrations of enzymes required for glycogen utilisation.2 Thus dogs have an impressive capacity for both endurance and high-intensity sprints.
Most studies on exercising dogs have utilised either greyhounds or sled dogs. However, the majority of working dogs in the world are neither of these breeds, nor do they engage in similar athletic demands. The energy requirements of dogs in conditions such as police, military, farm, and detection work have not been carefully studied. Energy usage can be estimated over a long period of time from the energy consumed by exercising dogs when they are maintaining body weight. However, this is inaccurate since it assumes the same digestibility for all dogs, diets, and feeding regimes, which is a false assumption. Nonetheless, feeding trials in various dogs have reported requirements of 507–617 kJ/kgBW0.75 in farm dogs in New Zealand, 322 to 470 kJ/kgBW0.75 in working Border collies in the UK, compared with 535 kJ/kgBW0.75 in "average active" Labrador pets.3,4 At the extreme, racing sled dogs covering 168 km/day in extreme cold use around 5183 kJ/kgBW0.75 per day. This is even more impressive when compared with professional cyclists during an endurance race of 15 to 18 hrs each day for 10 days, travelling more than 3300 km over major mountain ranges, in which the average daily energy expenditure was 1353 kJ/kgBW0.75.5,6
Fuel Utilisation During Exercise
At rest, the preferred fuel source of muscle is fat. Fatty acids are taken from within myocytes and between the muscle fibres. Non-esterified fatty acids (NEFA) are the most efficient fuel but the slowest to burn. Thus, as exercise intensity increases, the proportion of ATP supplied by the oxidation of NEFA decreases, and the percentage supplied by glucose increases. At low intensity (40% of VO2max), 60% of energy is provided by fat, whereas that may drop to 20% at high intensity (85% of VO2max). This change in fuel utilisation as intensity increases is referred to as the crossover effect. In contrast, fat utilization during exercise is not tightly regulated, as there are no mechanisms for closely matching availability and metabolism of NEFA to the rate of energy expenditure. As a result, the rate of fat oxidation during exercise is determined by the availability of NEFA and the rate of carbohydrate utilization. Exhaustion usually results from depletion of muscle glycogen or, at its most extreme, from hypoglycaemia.
The dietary proportions of fat, protein, and carbohydrate influence the fuel selection during exercise. Accommodation to a high-fat, low-carbohydrate diet increases muscle storage of fat and the rate of fat utilisation, and thus increases endurance by preserving muscle glycogen stores. Similarly, feeding diets high in carbohydrate increases glycogen storage in muscle, but also increases the rate of glycogen utilisation. Thus the potential benefit of increased muscle glycogen storage is negated by the increased rate of utilisation, and muscle glycogen is preserved more effectively by feeding high-fat diets. The adaptations induced by endurance exercise training result in a marked sparing of carbohydrate during exercise, with an increased proportion of the energy being provided by fat oxidation. Training increases the mitochondrial mass of myocytes and increases the rate at which NEFA can be oxidised.
Protein utilisation increases during exercise by dogs and continues to increase with exercise duration. Low-protein diets have been shown to reduce VO2max and increase the rate of soft-tissue injury in exercising dogs. This effect has been shown in diets that have differed as little as 19% ME protein and 24% ME protein, where the lower protein diet resulted in 8 times as many soft-tissue injuries in treadmill-exercising dogs.7 During normal exercise, muscles reduce the risk of stress fracture by contracting to reduce bending strains on cortical bone surfaces. Significant increases in peak bone strain of the tibia in Foxhounds have been identified when exercised to the point of fatigue. These findings have been further illustrated in human athletes wherein gait changes from fatigue increase peak vertical ground reaction force by 25%, and tibial tension strain significantly increases.8-10 Thus diets that reduce muscle fatigue are likely to reduce both orthopaedic and soft-tissue injuries in working farm dogs.
Influence of Diet on Non-muscular Performance
Panting decreases the efficiency of scent detection and tracking, so decreasing CO2 production by maximising fat utilisation is advantageous for tracking dogs. Training and low CHO diets reduce CO2 production, and ensuring adequate hydration to optimise temperature regulation is important. Other dietary effects on olfaction include the negative effects of dehydration, and saturated fat may impair olfaction compared with polyunsaturated sources.11 Dietary enrichment of the bitch with the n-3 PUFA DHA during gestation and lactation leads to enrichment of phospholipids in neural tissue that has been associated with improvements in memory and learning of the puppies.11 It remains to be seen if this has the potential to improve working performance.
Time of Feeding and Exercise
Feeding prior to exercise is more likely to result in abdominal discomfort and vomiting during exercise. Fat utilisation is increased in the fasted state, and when exercised in the postprandial state, carbohydrate utilisation is increased. This effect is reduced when a high-fat diet is fed 4 hours prior to exercise, rather than a low-fat diet. Despite that, an effect on endurance capacity has not been shown. In contrast, providing a readily digestible source of carbohydrate immediately after exercise promotes recovery. A large increase in muscle glycogen concentration - far above the level found in the well-fed sedentary state - occurs in response to carbohydrate feeding following glycogen-depleting exercise.13-15 This muscle glycogen accumulation is markedly enhanced by endurance exercise training that induces an increase in the GLUT4 isoform of the glucose transporter in skeletal muscle. Additionally, muscle proteins catabolised during exercise are replenished more completely and rapidly when a meal is provided within 2 hours of exercising. Thus, dogs should be fed within 2 hours, preferably immediately after exercise. Dogs should not be worked within 8 hours of a small meal or up to 16 hours after a large meal to allow for complete gastric emptying. Digestible carbohydrate sources given during or immediately after exercise have been shown to improve endurance and to promote greater muscle glycogen repletion.
What is the Ideal Working Dog Diet?
At present, the nutritional requirements of dogs engaged in moderate physical activity have not been established. The macronutrient proportion that is most suitable is probably still high fat, high protein, low carbohydrate, and high moisture. Requirements for several B vitamins and dietary antioxidants are known to be increased during heavy exercise, although there is no evidence that they need to be increased beyond that provided in proportion with total energy intake. Thus, high-fat, high-protein commercial diets that have passed approved feeding trials are sufficient, and no further need for supplementation is currently known. Further work is needed to establish the role of nutrients on performance parameters such as learning, attention, and olfaction.
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