Triglyceride storage in skeletal muscle

Abstract

When muscle fibres contract they rely on fuels from extra-and intramuscular fat and carbohydrate sources. The most important fuels are free fatty acids from blood and intramuscular triglyceride stores and glucose from blood and intramuscular glycogen stores. The fat stores in the body are the largest nutrient reserve whereas carbohydrate stores are limited. Fat is a more efficient fuel for producing energy whereas the rate of energy production is more rapid from carbohydrate sources. Many studies on horses have investigated changes in blood-borne substrates and glycogen stores with different types of exercise, training and diets. Only a few studies have looked at the muscle triglyceride stores. These studies have used the muscle biopsy technique and report large individual variations in triglyceride stores in the gluteus muscle of horses. Furthermore, results show that fat is mainly stored in slow contracting type I fibres and fast contracting type IIA fibres and hardly not at all in type IIB fibres whereas glycogen is stored in all fibre types. Exercise studies indicate that triglyceride stores are utilised to a great extent during prolonged exercise and especially within type I fibres whereas utilisation during more intense exercise is unclear. Availability of blood-borne substrates, and different metabolic profile between fibre types and the extent to which the fibres are recruited during exercise are all factors that influence intramuscular substrate storage and utilisation. An increased capacity for muscle triglyceride storage is observed with age and training. Some horses with poor training and/or racing performance have low muscle triglyceride stores. Influence of different diets on triglyceride storage in muscle is not clear but results indicate a greater reliance on fat oxidation after fat rich diets. More research is needed to clarify the role of fatty acids in blood and of triglyceride stores in muscle for energy production during different types of exercise and duration. The effect of different diets on muscle triglyceride storage also needs to be further investigated.