For decades, the weighing scale has been treated almost like a health report card. Lose a few kilograms and we congratulate ourselves; gain a few and we worry. Body weight and Body Mass Index (BMI) have become familiar shorthand for judging whether someone is healthy. But a growing understanding of ageing and metabolism suggests that the scale may be telling us only half the story. The more meaningful measure of health may be not how much we weigh, but how much of that weight is muscle.

Two people can weigh exactly the same and have completely different bodies. One may have substantial muscle, relatively little visceral fat and good physical strength; the other may have less muscle and considerably more fat. Their weighing scales will show the same number, but their metabolic health, physical capacity and prospects for independent living may be very different.

Muscle is far more than the tissue that allows us to lift weights or look athletic. It is one of the body’s most important metabolic organs. After a meal, skeletal muscle accounts for the bulk of insulin-stimulated glucose disposal—often estimated at roughly 70 to 80 per cent under insulin-stimulated conditions. In simple terms, muscles act as a huge glucose sink, taking glucose out of the bloodstream to use as energy or store as glycogen. This makes healthy muscle particularly important for maintaining good glucose regulation.

Muscle is also metabolically active at rest and consumes more energy than an equivalent amount of fat. The difference should not be exaggerated: a kilogram of muscle does not burn hundreds of calories simply by sitting inside the body. But the larger metabolic importance of muscle lies in its ability to store and use glucose, support physical activity and maintain the body’s capacity to respond to nutritional and metabolic demands.

Then comes ageing. Muscle mass generally reaches its peak in early adulthood, often during the 20s and early 30s. The muscle built during those years can be thought of as a physiological reserve. The larger the reserve built before age-related decline begins, the more capacity there may be to withstand the gradual losses that accompany ageing. In that sense, building muscle in youth is rather like building up savings for retirement: the benefit may become most apparent decades later.

This does not mean, however, that the story is decided at 30. Far from it. Muscle remains remarkably responsive to resistance training throughout life. Older adults can strengthen their muscles and improve physical function even when they begin exercising relatively late. The gains may be slower than in youth, but the body’s capacity to adapt does not suddenly disappear with age.

This matters because maintaining the same body weight becomes increasingly difficult as we grow older. As muscle declines, resting energy expenditure tends to fall. Physical activity may decline as well. If food intake remains unchanged, the resulting energy surplus can gradually become body fat. The most deceptive part is that the weighing scale may barely move. A person can remain at approximately the same weight while losing muscle and gaining fat. The number has stayed constant, but the body composition has changed dramatically.

The consequences extend beyond appearance. Muscle increasingly becomes a determinant of independence in later life. Low muscle mass, particularly when accompanied by reduced muscle strength, is associated with poorer mobility and a greater risk of falls. Everyday actions that younger people take for granted—rising from a chair, climbing stairs, carrying groceries, getting out of a car or walking confidently across a street—depend upon adequate muscular strength.

That is why the real objective of healthy ageing should not simply be to remain thin or maintain an arbitrary number on the weighing scale. It should be to preserve muscle, strength and physical function. Sarcopenia—the age-related decline in muscle mass, strength and function—is not merely an aesthetic problem. It can gradually erode the physical independence that gives older life its dignity.

The good news is that muscle is an unusually forgiving investment. The 20s and 30s may be the best years to build a substantial reserve, but the 40s, 50s, 60s, 70s and beyond are not too late. Resistance exercise, adequate protein, good nutrition and regular physical activity can help preserve or rebuild muscle.

Perhaps, therefore, we should stop asking only, “What do you weigh?” A better question might be, “How strong are you, and how much muscle are you carrying?” The weighing scale measures kilograms. Muscle measures capability. And as life expectancy increases, capability—not merely body weight—may prove to be the more valuable currency of healthy ageing.