The popular belief that metabolism inevitably and dramatically slows after 35 is only partially correct. Research published in Science in 2021, analysing data from over 6,000 individuals across 80 years of age, found that metabolic rate is stable from age 20 to 60 when corrected for body composition. The metabolic decline observed in middle age is primarily driven by the loss of muscle mass that occurs in inactive adults, not by ageing itself.
The Muscle Mass Mechanism
Adults who do not engage in progressive resistance training lose approximately 3 to 5 percent of muscle mass per decade from age 30 onwards, with the rate accelerating in the decade following menopause in women and progressively increasing in sedentary men after 50. Each kilogram of muscle lost reduces resting metabolic rate by approximately 13 calories per day. Across a decade of inactivity, this compounds into a meaningful reduction in daily energy expenditure.
The Hormonal Contribution
While body composition is the primary driver, hormonal changes do contribute. Declining growth hormone, testosterone, and oestrogen all have anabolic effects, and their reduction diminishes the drive for muscle protein synthesis and increases the tendency toward fat storage. These hormonal shifts are not completely reversible through lifestyle, but their effects on body composition are substantially mitigated by resistance training and adequate protein intake.
What to Do
- Prioritise resistance training two to three times per week to preserve or rebuild muscle mass
- Consume a minimum of 1.6 grams of protein per kilogram body weight daily
- Optimise sleep to support growth hormone and testosterone production
- Avoid prolonged aggressive caloric restriction, which accelerates muscle loss and suppresses thyroid function
- Include high-intensity interval training, which has the strongest evidence for reversing age-related mitochondrial decline