Conventional wisdom says metabolism collapses after 30. The reality, especially after recent large-scale research, is more nuanced — and more encouraging.
The familiar claim is that metabolism slows steadily from your twenties onward. Large studies of total energy expenditure have refined this: when adjusted for body size and composition, metabolic rate is remarkably stable from about age 20 to 60, then declines gradually after that. The age term in BMR formulas reflects an average downward drift, but it's slower than the "everything slows at 30" folklore suggests.
A lot of the apparent age-related slowdown isn't the metabolism of your cells changing much at all — it's a loss of lean muscle mass and a drop in daily movement over the years. Muscle is metabolically active tissue; lose it, and resting burn falls. This is why the change is partly within your control.
In the Mifflin-St Jeor equation, age subtracts 5 calories per year. So a decade older, same weight and height, is about 50 fewer BMR calories a day — real, but small next to the effect of the activity level you choose or the muscle you keep. Because the age effect is slow, recalculating once a year for ageing alone is plenty.
You can blunt most of the age-related decline by preserving muscle with resistance training and staying active. Recalculate your BMR as your weight changes rather than worrying about birthdays — weight change moves the number far more than a year of age does.
Less than commonly believed. Adjusted for body composition, metabolic rate is fairly stable from about 20 to 60, then declines gradually. Much of the apparent slowdown is lost muscle and reduced movement, not cellular metabolism.
In Mifflin-St Jeor, age subtracts about 5 calories per year of age — roughly 50 calories per decade at the same weight and height. Small compared with activity level and muscle mass.
Annually is fine for the age factor alone, since it changes slowly. Recalculate more often for weight changes, which move the number more.