Resting energy expenditure gradually decreases with age, and there is solid evidence for this from multiple large cohort studies. The main cause is muscle loss: those with less muscle mass burn less energy at rest. That insight is practically useful: actively maintaining muscle mass through strength training is the best-supported way to slow this process, even though the available studies do not directly examine that intervention themselves.
The basal metabolic rate does indeed decline with age. Large cohort studies in American, Italian and Japanese populations confirm this pattern: resting energy expenditure decreases gradually, and that decline continues into old age. Part of the explanation lies in the diminishing number and reduced functioning of mitochondria, the powerhouses of the cell.
The most important direct cause is muscle loss. Lean mass, consisting largely of muscle tissue, falls in sedentary people by approximately 15% between the thirtieth and eightieth birthday. Because muscle mass is the primary determinant of resting energy expenditure, this muscle loss translates directly into a lower basal metabolic rate. At the cellular level, a protein that regulates calcium transport in muscle mitochondria plays a role here: that protein declines in activity with age, which disrupts energy production in muscle cells and contributes to age-related muscle loss.
Endurance capacity also declines measurably. Maximal oxygen uptake decreases progressively. Up to late middle age, this is due mainly to a lower heart rate and a reduced cardiac output. At very advanced ages, the deterioration of mitochondrial capacity within the muscles themselves also plays a major role: in older people, that contribution can account for approximately 50% of the total decline compared with young adults.
For women there is an additional factor: the menopausal transition brings changes in body composition that are separate from the normal age-related decline. The rate at which fat mass increases doubles during this transitional period, while at the same time muscle mass begins to decrease more rapidly. Remarkably, total body weight does not rise any faster during this period than before. It is therefore a redistribution from muscle to fat, not weight gain as such.
Finally, there is recent animal research into oleuropein, a polyphenol from olives, which activated mitochondrial calcium transport in muscles, increased energy production and improved endurance in mice. This sounds promising as a potential lead, but all data come exclusively from cell and mouse models. No studies in humans exist yet, so practical conclusions for people cannot be drawn at this stage.
The claims are based on cohort research, muscle biopsies in humans, physiological studies of VO2max and a cohort study of the menopausal transition. The causality of muscle loss on basal metabolic rate is well supported. The oleuropein research involves only animal and cell models.