Your metabolism does not slow with age — your muscle mass does, and they are not the same thing

A 2021 Science study by Herman Pontzer and colleagues, analysing metabolic data from 6,421 people aged 8 days to 95 years across 29 countries, found that resting metabolic rate per kilogram of lean mass does not significantly decline between age 20 and 60. The decline most adults experience in middle age is driven almost entirely by the loss of metabolically active lean mass — not by any intrinsic slowing of cellular metabolism.

Reversing the Decline

• Progressive resistance training is the most effective intervention for increasing muscle mass at any age
• Protein intake of 1.6–2.2 g/kg bodyweight daily, with leucine-rich sources prioritised around training
• Creatine monohydrate 3–5 g daily enhances muscle protein synthesis response to training
• Adequate sleep — growth hormone required for muscle protein synthesis is secreted during deep sleep