Ask a Doctor: The Effect of Nutrition and Exercise on Muscle Building
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We use our skeletal muscles every day to carry out even the simplest of tasks. Our muscles are constantly adjusting and adapting to our activity. Skeletal muscles are specifically designed to adapt to the increased demands of exercise both physical and metabolic. Studying muscle metabolism is valuable for the creation of nutritional and exercise recommendations.
To appreciate how nutrition and exercise affect muscle building, it helps to understand how muscles are created. Amino acids are the basic building blocks of protein, which bond together to build the proteins that compose muscle. When you eat protein, the protein is digested and broken down into amino acids.
These amino acids are absorbed into the gut and then shuttled to the muscles where they are reconstructed back into proteins, and in the case of skeletal muscles, into the proteins that create muscle.
What Kinds of Muscle Proteins Are There?
Generally, there are two types of muscle proteins: the contracting type that we use to generate force and move and the type that creates energy. The contractile proteins are composed of myofibrillar proteins such as myosin, actin, tropomyosin, and troponin. These are the pieces of the muscle that work together to create a muscle contraction. An increase of these muscle proteins is what causes muscles to bulk up and gain mass after resistance training. This increase in contracting muscle elements is referred to as myofibrillar hypertrophy.
Alternatively, mitochondrial proteins that generate energy are primarily created in response to endurance training, known as sarcoplasmic hypertrophy.
How Does Muscle Protein Synthesis Work?
Every day the human body is creating new muscle protein. The term muscle protein synthesis is used to describe the joining of amino acids into muscle proteins. Muscle protein synthesis is concurrently countered by muscle protein breakdown in which proteins damaged from exercise, physical activity, oxidative stress, and inflammation are broken down and recycled.
Muscle protein breakdown allows damaged muscle proteins to be recycled into new functional muscle proteins and is necessary to optimally adapt to training and maximize muscle growth. In studies in which mice were genetically engineered to not break down muscle protein, they were weaker and smaller than average mice.
Muscle protein synthesis and breakdown are both continuously occurring but at different rates. The difference in speed of these two opposing processes determines the overall change in muscle protein.
Muscle growth occurs when the rate of muscle protein synthesis is greater than the rate of muscle breakdown. Conversely, if breakdown exceeds synthesis, then muscle atrophy will occur. It is a dynamic equilibrium that shifts based on nutritional intake and exercise demands.
Studying the turnover of proteins and the effect on muscle mass is important for understanding the health benefits of exercise. Scientists measure muscle protein synthesis on a small scale in the laboratory and results can be extrapolated into a larger scale to predict muscle growth and hypertrophy.
There are several ways that scientists measure muscle protein synthesis. A common technique is to use amino acid tracers. An amino acid with an additional neutron is an example of an isotope. The additional neutron allows the amino acid to be tracked as it travels through the human body since it weighs slightly more than a normal amino acid. Amino acid tracers function identically to normal amino acids, which allow scientists to measure the movement and activity of amino acids in various metabolic processes, including protein synthesis.
Skeletal muscles are in a constant state of change with the continuous creation of new muscle protein to match the concurrent muscle protein breakdown. Scientists can measure this turnover of muscle protein by marking and tracing tissues using isotopes which are incorporated into the amino acids which then build the protein. The response of muscle proteins to nutrition and exercise interventions is commonly used to guide sport and exercise nutrition, especially protein nutrition. Studies can predict the effect of nutritional and/or exercise interventions on muscle building by measuring the response of protein synthesis rates. The results provide guidance for physiologists and nutritionists when creating dietary and exercise recommendations to optimize physical performance.
For example, MPS has been shown to increase by increasing your protein intake. Learning how to stimulate MPS through exercise and diet can help accelerate muscle growth, improve recovery and athletic performance, and increase overall endurance.
Some studies, however, have noted that there is not a direct relationship between increases in muscle protein synthesis and muscle size. This means that the observed acute anabolic responses seen in muscle protein synthesis are not necessarily associated with long-term muscular gain.
But, while there is some disparity on how accurate muscle protein synthesis is at predicting muscle building, it still remains a useful tool to extrapolate guidelines for exercise and nutrition. It can serve as a model of muscle-building under precise conditions. While the model is not exact, there are multiple studies that show that results obtained measuring changes in muscle predict increases in muscle growth.
The Effect of Exercise on Building Muscle
Skeletal muscles undergo remarkable adaptations to external stimuli. Through exercise, we can change the size and shape of our muscles. Weight lifters can nearly double the size of their muscles. Eccentric exercises can lead to muscle fiber lengthening. Conversely, lack of muscle use leads to a marked decrease in muscle volume.
What Is Resistance Training?
Resistance training is the main method of exercise for improving strength and increasing skeletal muscle mass. As the name suggests, resistance training describes an exercise in which the muscles work against a weight or force. Examples of resistance training include using free weights, weight machines, medicine balls, resistance bands.
Resistance training leads to the generation of muscle protein synthesis and subsequent muscle hypertrophy through mechanical damage and metabolic fatigue. Weight lifting requires the contractile proteins to generate force to overcome the resistance, which can result in structural damage. Mechanical damage to muscle proteins stimulates a repair process that can result in increased muscle size. Therefore, consistent exercise to all muscle groups is important to encourage the continual remodeling of muscles.
Adenosine triphosphate (ATP) is the energy supply to the muscle that allows for skeletal muscle contraction. Metabolic fatigue occurs when muscle fibers involved in an exercise exhaust the available supply of ATP and are no longer able to fuel muscle contractions. Metabolic fatigue is also known as training to failure.
There are various ways to determine when a muscle has reached failure but in essence, it means repeating an exercise to the point where the muscles can no longer produce adequate force to overcome a specific workload (e.g., when the muscles can’t produce sufficient force to continue to move the weight upward).
Exercise to failure can also be measured using tempo and form. For example, if the tempo, cadence and/or proper form used for the initial few repetitions can no longer be maintained, this can be deemed muscle fatigue or exercise to failure. Of note, training to failure has been shown to be beneficial for the maximal hypertrophic response as muscular failure promotes greater activation of motor units, even when resistance training is performed at low intensities. It is believed that recruiting as many motor units as possible results in maximal gains in muscle hypertrophy and strength on the target muscle.
Rates of muscle protein synthesis vary based on workload and intensity of resistance training. While protein synthesis shows less response with lower intensity exercise, this becomes negligible if the muscle is worked to failure. Increasing the volume of work at a lower intensity increases the protein synthesis response by increasing muscle unit recruitment during fatigue. Exercise to fatigue with a lower weight is an option to stimulate muscle hypertrophy without having to lift heavy weights. Therefore, the number of repetitions and amount of weight is less important than training near to muscle fatigue.
Studies have even shown that training to failure with lower loads and more repetitions can be more beneficial for muscle building than using higher loads with fewer repetitions. From a practical standpoint, training to failure for a prolonged period may result in a higher risk of musculoskeletal injury. Therefore, performing resistance training to a level of substantial fatigue with lesser weight may be a better option in certain patient populations like the elderly.
The Effects of Diet and Nutrition on Building Muscle
In addition to resistance exercise, protein intake promotes muscle protein synthesis and subsequent muscle hypertrophy or bulk. Studies have shown that muscle protein synthesis was greatest in those who consumed four servings of 20 grams of protein.
There has been a lot of research on the optimal timing of protein intake (i.e, before or after resistance training). However, studies show that muscle hypertrophy occurs independent of the feeding times and total daily protein intake is more important than timed protein ingestion.
After protein ingestion and digestion, the absorbed amino acids will stimulate muscle protein synthesis. Muscle protein synthesis rates peak 90 to 120 minutes after ingestion of protein. This peak is temporary, lasting only a short period, followed by a refractory period during which the muscle no longer responds to amino acids.
During this time, muscle protein synthesis returns rapidly to baseline regardless of the amount of available amino acids. This phenomenon is referred to as the “muscle full effect”. At that point, further protein intake will no longer lead to an increase in muscle protein synthesis. The muscle full effect lasts 24 hours. Given that muscle full response lasts at least a day, it limits the ability to show changes in muscle protein synthesis rates beyond the acute response.
It has been proposed that muscle protein synthesis is maximized in young adults with an intake of 20 to 25 grams of a high-quality protein. Consumption of higher protein amounts beyond 20 grams results in greater amino acid oxidation and research has shown limited benefit. Other findings from studying protein metabolism include the recommendation for whey protein for muscle building. Whey protein is a “fast-acting” protein with a rapid absorption rate, which will allow for the most availability for incorporation into muscles.
When interpreted appropriately, study results measuring muscle protein synthesis can guide practitioners to make valuable nutrition and exercise recommendations. It is a useful tool to learn about and optimize muscle metabolism.