Muscle health has two major components: 1) Physical and 2) Metabolic. The physical involves strength and mass, while the metabolic affects insulin sensitivity, glucose regulation, fatty-acid oxidation, and mitochondria health.
Through strength training you increase the density of muscle mitochondria which allows your body to use nutrients, such as carbohydrates and fats, and convert them into energy that can be used to power everyday activities. Losing skeletal muscle means losing the mitochondria that produce energy in your cells. And we all know the feeling of having low energy.
After age 30, you lose 3 to 8 percent skeletal muscle mass per decade. After about age 50, muscle mass decreases at an annual rate of 1 to 2 percent. The decline in muscle strength is even higher. Without progressive strength training, this loss is guaranteed.
Your skeletal muscle acts as an amino-acid reservoir. If you become sick or injured (infection, physical trauma, cancer etc), the body will pull amino acids from your available muscle tissue to repair and protect itself.
Your muscle also releases small signaling proteins, known as myokines, in response to exercise. Myokines are a collection of small proteins and peptides that travel through the bloodstream to regulate metabolism, reduce inflammation, and improve immune function. Myokines even improve your sense of wellbeing and capacity to learn.
Protein is not a single macronutrient, but a delivery system for twenty different individual amino acids. We don't eat for protein, per se, but for amino acids.
Proteins are the master regulators of all that is happening in your body, controlling function in all tissues and organs, including muscle.
Leucine is the most important amino acid for muscle health. Leucine activates a component of the mTOR signal complex, which plays a vital role in initiating and sustaining protein synthesis within cells. The muscle-protein synthesis is where amino acids are bounded together to build and repair skeletal muscle tissue.
Think of leucine as the key you turn in your car to fire up the engine. mTOR is the engine, and all the amino acids your body has available supply the fuel.
The mTOR mechanism is a binary on/off switch. The dose of protein you eat in a single meal is either sufficient to trigger muscle-protein synthesis or it isn't. But the system is nuanced, and one factor is age. When you are young and growing, mTOR is regulated by hormones, but as you age your body become less responsive to hormones and more sensitive to diet quality and leucine.
This is why I ensure my first meal of the day contains a minimum of 30 to 40 grams of protein, sufficient to trigger the leucine threshold (about 2.5 to 3 grams). And I'm aiming for a 1.6 to 2.0 g/kg daily intake.