The underappreciated role of muscle in health and disease

Today's summary is about a paper written by Robert R Wolfe in 2006.

1. Muscle plays a central role in whole-body protein metabolism, which is particularly important in the response to stress.
2. In the absence of nutrient intake, muscle protein serves as the principal reservoir to replace blood amino acids taken up by other tissues.
3. One extensive study concluded that the depletion of muscle mass is the cause of death in human starvation.
4. The stressed state, such as that associated with sepsis, advanced cancer, and traumatic injury, imposes greater demands for amino acids from muscle protein breakdown than fasting.
5. Survival from a severe burn injury is lowest in individuals with reduced lean body mass.
6. Loss of muscle mass is detrimental to survival from cancer.
7. Chronic diseases (heart and cancer being a majority) related to poor lifestyle behaviors account for more than two-thirds of deaths in the United States.
8. Sarcopenia, the progressive loss of muscle mass and function that occurs with aging, is a widespread syndrome that has a devastating effect on the quality of life and ultimately survival.
9. The rate of loss in muscle mass & strength is accelerated in middle age and maintained until old age. Intervention in middle age or younger ages is, therefore, necessary to offset the deleterious effects of sarcopenia in old age.
10. Total energy expenditure is the sum of resting energy expenditure (REE), the thermic effect of food (TEF), and the energy expenditure related to activities.
11. The energy expenditure related to muscle metabolism is the only component of REE that might vary considerably.
12. The energy released per day as a result of muscle protein synthesis may range from ~485 kcal_d in a well-muscled young man to ~120 kcal_d in an active elderly woman.
13. 1 kg of fat stores 7700 kcal.
14. The energy to provide the ATP for muscle protein turnover is largely derived from the oxidation of fat because this is the preferred energy substrate of resting muscle.
15. Mechanical force on bone is essential for modeling and re-modeling, processes that increase bone strength and mass.
16. Whereas body weight and weight-bearing exercises provide a direct mechanical force on bones, the largest voluntary loads on bone are proposed to come from muscle contractions.
17. Men with the least skeletal muscle mass also had increased risks of falls due to impaired static and dynamic balance, presumably at least in part because of a decrease in muscle strength.
18. The absence of a direct relationship between muscle mass and strength has been shown in a variety of studies.
19. The physical and metabolic functions of muscle are more important in normal day-to-day life.

Solutions to muscle loss

There are 3 potential approaches to maintaining or increasing muscle mass and function:

1. Hormonal therapy
2. Exercise:
   • Exercise training improves insulin sensitivity.
   • Because not everyone is motivated to exercise, it’s important to identify the minimal exercise regimen to achieve desired results, including maximizing the interactive effects between nutritional intake and exercise on muscle protein synthesis.
3. Nutrition:
   • Body composition studies have not been utilized to formulate dietary recommendations for protein intake.
   • High dietary protein intakes increase protein synthesis by increasing systemic amino acid availability.

The importance of maintaining muscle mass and physical and metabolic functions in the elderly is well-recognized. Less appreciated are the diverse roles of muscle throughout life and the importance of muscle in preventing some of the most common and increasingly prevalent clinical conditions, such as obesity and diabetes.