For fat to be used as an energy source, the fatty acid components of the triglyceride molecule must first be freed from the glycerol molecule. This is achieved by hormone-sensitive lipase, an enzyme that exists in the adipose cell membrane and that hydrolyzes triglyceride to free fatty acids and glycerol (see Exercises 3.2).
Hormone sensitive indicates that the activity of the enzyme is sensitive to regulation by hormones circulating in the bloodstream. The two most important hormones are insulin, which inhibits or reduces the hormone-sensitive lipase thereby preventing fat mobilization, and adrenaline, which strongly activates the enzyme thereby accelerating fat mobilization. During exercise, insulin levels fall and adrenaline levels rise so that fat mobilization is stimulated. In contrast, a high-carbohydrate meal, especially of simple carbohydrates such as glucose, causes insulin levels to rise and impairs fat oxidation. Physical training, on the other hand, increases the sensitivity of hormone-sensitive lipase to the stimulatory effects of adrenaline (Wahrenberg et al, 1987).
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Following their release from triglyceride molecules in adipose tissue, free fatty acids and glycerol easily cross the fat cell membrane to enter the bloodstream. In order to be soluble in the blood, the free-fatty-acid molecules must be bound to a carrier protein, in this case albumin. Although this binding is very tight, some free-fatty-acid molecules do exist in solution, and these unattached free-fatty-acid molecules are metabolized by muscle. Newsholme and Leech (1983) contend that the low concentration of unbound free fatty acids in the bloodstream limits the rate at which they can be transported into the cell and may therefore limit the rate at which energy can be produced from fat oxidation in the Krebs cycle. Although this contention remains unproven, the body certainly appears unable to extract energy at a high rate from fat metabolism during exercise of high intensity. As we shall see, the limited rate at which fats can produce energy has certain serious consequences for metabolism during very prolonged exercise.
When free-fatty-acid molecules reach the capillaries supplying the muscle cell, the unbound-fatty-acid molecules are taken up by the cell and either metabolized in the Krebs cycle to produce ATP or re-formed and stored as muscle triglyceride.
Evidence now shows that these muscle triglyceride stores provide important fuel during prolonged exercise; evidence also shows that an important effect of training is to increase the amount of muscle triglyceride utilized for energy while sparing muscle glycogen (Hurley et al, 1986).
The major function of the protein stores in the body is to produce movement (see Exercises 1.3). The body uses protein as an energy source during exercise, but
Only under extreme conditions such as complete starvation or prolonged exercise (especially under conditions of carbohydrate depletion) does protein’s contribution reach even 10% of the total energy production (Lemon & Mullin, 1980). Under such conditions, protein’s major role is to provide the liver with substrates from which the liver can produce glucose when the liver glycogen stores are low.