The calorie (energy) requirements of an athlete depend on the frequency, intensity, and duration of his workload, and also on factors such as rider weight or environmental conditions. Protein requirements for muscle development will also increase, but not at the same scale as energy requirements, and are easily met when overall food intake is increased. The electrolytes sodium, chloride, and potassium are also required in higher amounts as they are lost in sweat. Other nutrient and feeding needs such as calcium, phosphorus, and vitamin E will also increase with work.

Muscle uses a molecule called adenosine triphosphate (ATP) to function, which can be generated from the metabolism of fat, carbohydrate, or protein (typically, protein metabolism for work is minimal). While carbohydrates and fats are always being metabolized to provide ATP to the muscle, distinct shifts occur in terms of mostly carbohydrates being used vs. mostly fats, and aerobically (with oxygen) vs. anaerobically (without oxygen), depending on the intensity and duration of the exercise.

In general, lower intensity/longer duration exercise – such as endurance riding or even jumping a hunter course – is mostly fueled by fat oxidation, which is available in essentially unlimited supply in the adipose tissue (fat). It may also be available in the blood from a recent meal, or to a small degree, stored in the muscle itself. Higher intensity exercise – such as a cross-country round or jumper course – is fueled mostly by carbohydrates such as glycogen (found in limited amounts in the liver and muscle) or blood glucose (derived from a meal). These carbohydrates may be oxidized if oxygen is available, or may be metabolized anaerobically if the work intensity is very high and oxygen supply is limited. Anaerobic metabolism of glucose results in the production of lactic acid, which is associated with fatigue, particularly in high-intensity exercise. In lower-intensity exercise, fatigue is typically caused by depletion of fuel reserves, in particular carbohydrate reserves such as muscle glycogen. Some carbohydrate is always needed, even when fat is the key fuel – “fat burns in a carbohydrate flame.”

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