Muscle mass
All the muscle tissue in the body, whose maintenance depends on enough protein and regular mechanical stimulation, and which influences basal metabolism and independence.
Muscle mass accounts for about 40% of body weight in an adult. It is not just an engine: it is also the body’s main reservoir of amino acids, a regulator of blood sugar and a major determinant of basal metabolic rate.
Keeping it relies on a constant balance between the synthesis and breakdown of muscle protein. Two signals tip the balance the right way: enough protein, spread across the day, and mechanical stimulation. Without any strain placed on the muscle, the protein you eat is not enough.
This is what makes very restrictive diets counter-productive. With a marked calorie deficit and inadequate protein, a significant part of the weight lost comes from muscle rather than fat. Less muscle means a lower basal metabolic rate and so an easier return to the old weight: this is one of the physiological drivers of yo-yo dieting.
After the age of 40, muscle loss becomes gradual and continuous, known as sarcopenia, and it speeds up with age. It is not inevitable: resistance training and a higher protein intake slow it markedly, even in very elderly people, as retraining programmes in geriatric care confirm.