Diet and exercise performance in the horse.

The rate of energy expenditure in maximal physical activity is fifty times that occurring at rest. Thus, a consideration of the manipulation of diet to influence physical performance should enquire into dietary energy and its metabolism. It is self-evident that a failure to meet the horse’s energy requirements, as estimated by INRA (1990) and National Research Council (1989), will result in a chronic loss of body mass, debility and a decline in physical ability. In a survey of racing Standardbreds (STB) (Gallagher et al. 1992a) the daily intake of digestible energy (DE) was found to exceed the National Research Council (1989) estimate by 27%, and so compliance with chronic energy needs is generally assessed by degree of fatness. Thoroughbred (TB) horses of moderate fatness apparently use depot fat as a source of energy more effectively than thin or fat horses, as indicated by reduced glycogen expenditure during exercise at a heart rate of 200 beats per min (bpm) (Scott et al. 1992). Nevertheless, it is unclear how athletic ability is affected by an abnormally large dietary energy intake for a brief period, when fat deposition will change little. Thus, a distinction is drawn between acute and chronic effects of diet on athletic response (Frape, 1988). Changes in this response are hard to demonstrate because they are difficult to replicate on the track. Even using a standardized treadmill Plummer et al. (1991) found a mean coefficient of variation of 9.6%. Moreover , differences exist between relative solo-run times and a run time in competition, which has a potent influence on performance of TB (Harkins & Kamerling, 1992). Therefore, despite greater precision of laboratory measurements, performance in competition is not reliably predicted. Yet noncompetitive work is an important function that can justify laboratory measurements to unravel the role of diet in work output.