The possible role of skeletal muscle in the adaptation to periods of energy deficiency.

Skeletal muscle accounts for a large portion of the body's energy expenditure both at rest and during exercise. The present review focuses on some possible mechanisms for economizing energy in resting and contracting skeletal muscles, and on the available information about whether these mechanisms are important in the energy-deficient body. There is evidence, both in man and in the rat, that in a state of energy deficiency the size of slow-twitch fibres is better preserved than that of the fast-twitch fibres. Slow-twitch fibres have a lower activation threshold, and this seems to decrease their responsiveness to starvation. This would be advantageous as there is evidence that the energy expenditure per unit tension developed is lowest in slow-twitch fibres. There are reports of a slowing of malnourished muscle, but it is uncertain whether the starvation-induced hypothyroid state leads to some degree of fast-to-slow fibre transformation. Muscle glucose oxidation is depressed by starvation, mainly due to changes outside the muscle itself, but muscle enzymatic adaptations may also be important in this energy-saving process. In this respect, the higher capacity of the fatty acid oxidation and aerobic end-oxidation pathways in slow-twitch fibres tend to make them better adapted than fast-twitch fibres. Further muscle adaptations might include a decrease in BMR, possibly by reductions in protein turnover, ion pumping or futile cycling. The importance and costs of such potential adaptations should be evaluated by further research. This knowledge will be an important step in the further understanding of the pathophysiology of starvation.