Dietary requirements and athletic performance of horses

Summary There is no clear evidence that the chronic requirement for any non‐energy yielding nutrient rises in proportion as the energy requirement increases with hard work. The need for protein, and probably that for calcium, remain a function of body weight daily. Some proportionality with energy may exist for certain nutrients, although the evidence has not been adduced. For example, because of an increase in both the proportion and amount of propionic acid in the volatile fatty acids of caecal contents, the tissue requirement for vitamin B 12 may rise with an increase in the rate of energy metabolism. Exercise influences appetite and therefore voluntary intake, and consequently the daily intake of nutrients. Although that intake is not just a function of dietary bulk and weight, it is necessary to increase energy concentration of diets to achieve an adequate chronic intake of energy where work intensity and energy expenditure are considerable. Acute nutrient requirements paint a different picture from chronic requirements. An increase in total feed intake, or the density of that feed, would neither satisfy these requirements nor be a desirable means of doing so. The acute needs of water, electrolytes and soluble carbohydrates should be met by dosing when the need arises. The timing of the consumption of energy yielding substrates relative to that of exercise may be critical to performance. An inevitable postprandial consequence of a meal of starch or protein by the resting horse, is an increase in the activity of plasma insulin. This increase decreases blood glucose, depriving muscles of a critical substrate, but the assertion has not been resolved by experiment in horses. Experiments are required to ascertain the optimum feeding regime during the 24 h preceding extreme exertion. Whereas exhaustion in sprint work is largely a function of elevated blood lactate concentration, that of extended work is a consequence of a decline in glycogen reserves and losses of body fluid and electrolytes. Glycogen loading is of benefit to many long distance human athletes, but no advantage has yet been established for this practice in horses, and without modification it could render them subject to laminitis and endotoxaemia. Nevertheless supplementation of horses with water, glucose and electrolytes during work may benefit their endurance. The provision of 5 litres water every 2 h with 30 g salt, or twice as much of mixed electrolytes, and 15 g sucrose or glucose, is recommended for a 500 kg horse during periods of extreme sweating. Where exhausted horses refuse to drink, dosing with an electrolyte slurry will frequently overcome their refusal. The provision of leafy, highly digestible, hay for horses in training cannot be easily justified as a necessary means of increasing dietary energy. Whether such forage stimulates, or maintains, appetite at a time when it may flag has not been demonstrated or even tested. Leafy hay is however more inclined to be dusty and may therefore aggravate respiratory problems. On the other hand the nutritional inadequacies of stemmy hay can be compensated for readily by other appropriate dietary components. Whether this compensation should include non‐nutrient lipid‐soluble antioxidants has not been investigated. There is a large group of indispensable metabolites required for the normal functioning of tissue‐cells. Essential dietary nutrients are merely a subset of these. This review may be expected to predict quantifiable dietary requirements for certain nutrients by theoretical calculation from known, or calculated, rates of energy metabolism. However, an entirely different approach to, and interpretation of, the acute nutrient requirements of work, vis à vis chronic requirements is demanded by the patent facts. Changes in rates and pathways of tissue metabolism, and the recycling of many tissue nutrients, accompany changes in the rate of energy metabolism. These changes ensure that an acute quantifiable need for those dietary nutrients, can be determined reliably only by appropriate empirical procedures. Very few such procedures have yet been published for the various micro‐nutrients or minerals. How should nutrient requirements be described?