Muscle- The Metabolic Machinery for Maximum Health
Muscle plays a central role in metabolism as well as in the genesis and/or prevention of many common chronic diseases.
The maintenance of adequate muscle mass, strength, and metabolic function through diet, exercise and other lifestyle factors has rarely been utilized as a targeted therapeutic intervention beyond its role in exercise performance, Activities of Daily Living (ADL), and more recently, anti-aging.
Optimizing muscle mass, strength and metabolic function should also be considered for:
- WHOLE-BODY METABOLISM
- ACUTE RESPONSE TO ILLNESS AND INJURY
- CHRONIC DISEASE
- INSULIN RESISTANCE AND DIABETES
Muscle is the biggest contributor to metabolic rate (RMR). Losing muscle through improper exercising, dieting or chronic disease, reduces caloric expenditure by adversely affecting RMR.
The maintenance of protein in muscles and other organs is essential for survival. Muscle serves as the principal reservoir to replace blood amino acids taken up by other tissues as well as for the synthesis of proteins for gluconeogenesis (the production of glucose in the absence of carbohydrates).
Under normal conditions, muscle protein metabolism maintains homeostasis, i.e, breakdown and synthesis are in balance
If muscle protein breakdown exceeds protein synthesis, such as in disease states, extreme dieting or overtraining aerobically, catabolism ensues (depletion of muscle mass).
There is a strong association between muscle catabolism and the length of survival of seriously ill patients.
ACUTE RESPONSE TO ILLNESS AND INJURY
In an acutely stressed state, such as a traumatic injury, there is a greater demand for amino acids from muscle protein breakdown.
Physiologic responses necessary for recovery rely heavily on protein and include: accelerated protein synthesis in the liver, proteins involved in immune function, and proteins involved in wound healing.
The demands for precursor amino acids for the synthesis of these proteins are significant. Individuals with limited reserves of muscle mass respond poorly to stress and injury repair. For example, survival from severe burn injuries is lowest in individuals with reduced lean body mass. Loss of muscle mass is also known to be detrimental to survival from cancer.
Muscle mass plays a key role in recovery from critical illness or severe trauma. In addition, If there is a pre-existing deficiency of muscle mass before trauma, the acute loss of muscle mass and function may push an individual over a threshold that makes recovery of normal function unlikely to ever occur. For this reason, >50% of women older than 65 who break a hip in a fall never walk again.
Chronic diseases related to poor lifestyle behaviors account for more than two-thirds of deaths in the United States.
Diet, physical activity and bio-indexes such as blood lipids, body mass index, and bone biomarkers are commonly used to predict risk of disease.
Alterations in muscle also play an important role in the many common diseases and conditions, including heart disease and cancer. Despite this, strategies in increase muscle are not often paramount in the management of these conditions.
Sarcopenia, the progressive loss of muscle mass and function that occurs with aging, is a widespread syndrome that has a devastating effect on quality of life and ultimately survival. Progressive sarcopenia is central to the development of frailty, increased likelihood of falls, and impaired ability to perform Activities of Daily Living.
The development of obesity results from an energy imbalance where energy intake exceeds energy expenditure.
An effect on energy balance can be achieved either by altering energy intake or energy expenditure.
Total energy expenditure is the sum of resting energy expenditure (REE- 60%), the thermic effect of food (TEF-10%), and the energy expenditure related to activity (AEE- 30%).
The energy expenditure related to muscle metabolism is the only component of REE that might vary considerably. Consequently, the maintenance of a large muscle mass can play a significant role in the prevention and management of obesity.
INSULIN RESISTANCE AND DIABETES
Type 2 diabetes initially develops as a decreased ability of insulin to stimulate muscle to clear glucose from the blood. It is commonly referred to as “insulin resistance” and is a hallmark of metabolic syndrome.
Disruption of the normal rate of muscle glucose uptake by muscle is central to the onset and progression of diabetes.
In addition, alterations in the metabolic function of muscle are central to the development of insulin resistance and ultimately diabetes.
Exercise and GLUT4
Glucose is an important fuel for contracting muscle, and normal glucose metabolism is vital for health. Glucose enters muscle cells via GLUT4, a glucose transporter. Exercise training is the most potent stimulus to increase skeletal muscle GLUT4, which contributes to improved insulin action, glucose disposal and enhanced muscle glycogen storage following exercise.
(Richter, E. A., & Hargreaves, M., 2013)
Mechanical force on bone is essential for modeling and remodeling, processes that increase bone strength and mass.
Whereas body weight and weight-bearing exercises provide a direct mechanical force on bones, the largest voluntary loads on bone come from muscle contractions.
Correlations between grip strength and bone area support the notion that muscle contractions play a significant role in bone strength and mass. Furthermore, changes in bone mass and muscle strength track together throughout life.
Maintenance of adequate bone strength and density with aging is highly dependent on the maintenance of adequate muscle mass and function.
SOLUTIONS TO MUSCLE LOSS
There are 3 potential approaches to maintaining or increasing muscle mass and function:
- Hormone Therapy
Hormone therapy can involve:
- Replacement to address a deficiency
- Raising the concentration above the normal value, or
- Giving agents to block hormone action by either reducing the rate of secretion or blocking their action
Replacement of testosterone in hypogonadal elderly men has successfully increased both muscle mass and strength.
In the stressed state, the catabolic hormones cortisol and epinephrine can be minimized by either blocking receptors, or blocking secretion.
Stress management techniques such as mindfulness meditation can lower stress hormone levels and may decrease the risk of diseases that arise from stress.
(Turakitwanakan, W.,et al, 2013)
Exercise improves muscle function and under the right circumstances, increases muscle mass. As previously mentioned, improved function is not limited to the contractile properties of muscle, but also muscle metabolism and improvements in insulin sensitivity.
Rather than initiate practices to reverse sarcopenia (The loss of muscle with age), it would be more prudent to prevent its development.
Progressive loss of muscle mass and strength occurs throughout adult life, and in middle age the rate of loss is accelerated and maintained. Intervention in middle age or younger ages is therefore necessary to offset the deleterious effects of sarcopenia in old age.
An optimal exercise regimen takes into consideration the interactive effects between exercise, nutrition, and adequate rest.
Exercise creates Specific Adaptations to Imposed Demands (AKA the SAID principle). These adaptations are predicated by acute variables.
According to the National Academy Of Sports Medicine (NASM), the acute variables for muscle hypertrophy are:
- 3-5 sets
- 6-12 repetitions
- 75-85% of the one repetition maximum (1RM)
- Rest period of 1-2 minutes
- Perform 2x/week as a split routine
- Choose exercises that emphasize compound movements, such as: Squat, lunge, deadlift, chest press, shoulder press, pull-down and row
- Spare lean body mass by limiting cardio-centered training to 20 minutes of high intensity intervals 2-3x/week
(Clark et al, 2014)
Dietary recommendations generally rely on epidemiologic studies. Recommendations arising from these studies have generally been predicated on minimizing the risk of diseases and have not directly considered muscle mass or function to be a relevant endpoint.
Historically, recommendations to reduce protein intakes have been made based on erroneous assumptions such as the low fat scare which led to recommendations to reduce red meat consumption, as well as the notions that too much protein adversely affects kidney function and bone density. These recommendations have ignored the effects of inadequate protein intake on muscle mass and function.
The maintenance of muscle mass and, in particular, optimization of the physical and metabolic functions of muscle should be considered in formulating dietary guidelines and recommendations.
Muscle protein is directly affected by dietary protein intake.
The anabolic effect of exercise is amplified by amino acids/protein. According to the International Society Of Sports Nutrition (ISSN), exercising individuals need approximately 1.4 to 2.0 grams of protein per kilogram of bodyweight per day. The amount is dependent upon the mode and intensity of the exercise, the quality of the protein ingested, and the status of the energy and carbohydrate intake of the individual.
Key Take Home Points:
- Muscle plays a diverse role throughout life in physical and metabolic function
- Optimizing muscle mass, strength and metabolic function should be considered for: Metabolism, Acute response to illness/injury, Chronic illness, Obesity, Insulin resistance/Diabetes and Osteoporosis
- Solutions to muscle loss can include: hormonal therapy, exercise, and nutrition
- Exercise adaptations to increase muscle are predicated by specific acute variables
- Exercising individuals need approximately 1.4 to 2.0 grams of protein per kilogram of bodyweight per day
Clark MA, Sutton BG. Lucett SC. NASM Essentials of Personal Fitness Training, 4th ed. Burlington, MA: Jones and Bartlett Learning; 2014
Richter, E. A., & Hargreaves, M. (2013). Exercise, GLUT4, and skeletal muscle glucose uptake. Physiological reviews, 93(3), 993-1017.
Turakitwanakan, W., Mekseepralard, C., & Busarakumtragul, P. (2013). Effects of mindfulness meditation on serum cortisol of medical students.Journal of the Medical Association of Thailand= Chotmaihet thangphaet, 96, S90-5.
Wolfe, R. R. (2006). The under-appreciated role of muscle in health and disease. The American journal of clinical nutrition, 84(3), 475-482.