The Myth of the “Slow Metabolism”
Weight gain after the age of 30 is often blamed on a “slowing metabolism.” This explanation is widespread, intuitive—and largely incorrect.
Large-scale metabolic research shows that resting metabolic rate remains remarkably stable from early adulthood through midlife. Yet body composition and weight do change with age. The key driver is not metabolism itself, but a quieter and often overlooked process: progressive loss of skeletal muscle.
Understanding this distinction matters, because muscle loss is not inevitable—and it is one of the most modifiable factors in long-term metabolic health.
For decades, weight gain after 30 has been attributed to a declining metabolism. However, the most comprehensive metabolic studies to date tell a different story.
When metabolic rate is measured using gold-standard techniques across thousands of individuals, total energy expenditure remains largely constant between ages 20 and 60, independent of sex or body size. Smaller declines in resting metabolic rate—on the order of 1–2% per decade—are too modest to explain the weight gain many adults experience.
In other words, metabolism does not suddenly slow down in your 30s.
So if metabolism isn’t the primary issue, what is?
