Metabolism is the complex process by which the body converts food into energy, sustaining all life functions. It is often measured in two ways: the basal metabolic rate (BMR), the energy burned at complete rest, and the total energy expenditure (TEE), which includes BMR plus the energy used for physical activity and digestion. Many people believe their metabolism begins a steady, downward slide in their 20s or 30s, often citing this as the reason for gradual weight gain. However, a landmark 2021 study tracking energy expenditure across the human lifespan revealed this common assumption does not align with biological reality. The scientific timeline for metabolic change is far more stable than previously understood.
The Four Distinct Phases of Metabolic Rate
The most comprehensive analysis of human energy expenditure, published in the journal Science, identified four unique phases of metabolism across a person’s life. The first phase, spanning from birth to age one, shows the most rapid metabolic acceleration. Infants have a total energy expenditure about 50% faster than that of an adult, even after adjusting for body size. This period of intense energy use reflects the enormous physiological investment required for rapid development.
The second phase begins after the first birthday and continues through early adulthood, ending around age 20. During this time, the metabolic rate undergoes a slow, steady decline, decreasing by about 3% each year. This decline happens gradually, even throughout the intense growth and hormonal shifts of adolescence and puberty.
The third metabolic phase is the longest and arguably the most stable, lasting from the early 20s until about age 60. During these four decades of midlife, metabolism plateaus, remaining remarkably consistent. This finding challenges the widely held belief that metabolism automatically slows down in the 30s and 40s.
It is not until the fourth phase, beginning around age 60, that a genuine, measurable decline in metabolism occurs. From this age onward, the daily energy expenditure begins to drop by approximately 0.7% per year. This cumulative slowdown is significant; by the time an individual reaches their 90s, their total energy needs are roughly 26% lower than those of a person in midlife.
The Underlying Factors That Drive Metabolic Changes
The shifts in energy expenditure across the lifespan are driven by complex biological factors that affect both resting and active calorie burn. A major physiological contributor to metabolic decline is sarcopenia, the natural, age-related loss of muscle mass. Because muscle tissue is far more metabolically active than fat tissue, a reduction in fat-free mass (FFM) directly lowers the basal metabolic rate (BMR).
Beyond the simple quantity of muscle, the quality of the remaining lean tissue also changes with age. Cellular efficiency decreases due to mitochondrial dysfunction, where the energy-producing centers of the cells become less effective. This leads to a reduced ability to generate adenosine triphosphate (ATP), contributing to a lower metabolic rate.
Hormonal fluctuations also play a substantial part in energy regulation. Thyroid hormones (T3 and T4) are the primary regulators of BMR, controlling the rate of nearly all chemical reactions in the body. Declines in sex hormones, such as testosterone and estrogen, also influence metabolism; testosterone supports muscle growth, which increases BMR.
The total energy expenditure is also reduced by a decline in Non-Exercise Activity Thermogenesis (NEAT). NEAT is the energy burned from unconscious, low-level movements like standing, fidgeting, and walking. Studies show that older adults may exhibit nearly 30% less NEAT than younger individuals. This decrease in spontaneous movement, combined with the gradual loss of metabolically active tissue, accounts for much of the observed metabolic slowdown in later life.
Practical Steps to Support Metabolic Function
While the genuine metabolic slowdown occurs later than most people assume, proactive lifestyle choices can support metabolic function at any age. Prioritizing strength or resistance training is considered the most direct and effective intervention against age-related decline. Muscle tissue is metabolically expensive to maintain, and resistance exercise is the only proven method to slow the progression of sarcopenia.
Maintaining muscle mass provides a direct boost to the resting metabolic rate, as each kilogram of muscle can burn between 10 and 60 calories per day at rest. Furthermore, a high intake of protein is beneficial due to its significantly higher Thermic Effect of Food (TEF). The body expends 20 to 35% of the calories from protein just to digest and metabolize it, compared to 5 to 10% for carbohydrates and 0 to 3% for fat.
Consistent, low-level movement throughout the day is an effective way to maximize Non-Exercise Activity Thermogenesis. Increasing this general, non-structured activity helps counteract the tendency for a lower total energy expenditure.
Finally, managing sleep and stress is an overlooked intervention that directly impacts metabolic hormones. Insufficient sleep elevates the stress hormone cortisol, which promotes fat storage, and disrupts the balance of appetite-regulating hormones. Specifically, sleep deprivation increases ghrelin, the hormone that signals hunger, while decreasing leptin, the hormone that signals satiety.