Many health apps display a metric called “metabolic age.” But what does metabolic age actually mean, how is it calculated, and how does it differ from biological age? To be transparent: I don’t know exactly how different apps estimate metabolic age, and they likely use different mathematical formulas under the hood. What follows is my best attempt to reverse-engineer the concept.
Basal metabolic rate (BMR) defines our energy expenditure per time at rest, describes how much energy the body burns at rest, typically expressed in kcal/day. Population-average BMR depends on sex, body mass (m, in kg), height (h, in cm), and age (a, in years). The widely used Mifflin-St Jeor equation, for example, gives:
BMR =
where s = +5 for males and −160 for females. Several other equations exist, some incorporating more detailed inputs such as lean body mass and body fat percentage — but the choice of formula doesn’t change the core argument.
From BMR to Metabolic Age: A key observation from the equation is that BMR declines with age. A higher-than-average BMR can therefore be interpreted as a sign that your body is aging more slowly in metabolic terms. If your BMR exceeds the average for your biological age, your metabolic age is considered lower than your chronological age — and vice versa.