Your metabolism slows with age primarily because you lose muscle mass, your cells produce energy less efficiently, and you naturally move less throughout the day. The decline is real but more gradual than most people assume. A landmark 2021 study published in Science found that metabolism stays remarkably stable between ages 20 and 60, dropping less than 1% per year, then declines more noticeably after 60. So the slowdown is happening, but the timeline surprises most people.
Muscle Loss Is the Biggest Factor
Muscle tissue burns far more calories at rest than fat tissue does. Starting around age 30, you begin losing roughly 3% to 5% of your muscle mass per decade if you’re not actively working to maintain it. By 70, many people have lost 20% to 40% of the muscle they had in their 20s. Since muscle is one of the most metabolically active tissues in your body, every pound you lose takes a small dip in your resting calorie burn with it.
This process, called sarcopenia, accelerates after 60. It’s driven partly by hormonal changes, partly by reduced physical activity, and partly by changes in how your body builds new muscle protein. The compounding effect is significant: less muscle means fewer calories burned at rest, which makes it easier to gain fat, which further shifts your body composition away from metabolically active tissue.
Your Cells Produce Energy Less Efficiently
Every cell in your body contains mitochondria, small structures that convert food into usable energy. As you age, both the number and the efficiency of these mitochondria decline. Research from Circulation Research found that in aging heart tissue alone, there’s roughly a 50% decrease in mitochondrial function. The specific parts of the energy-production chain that weaken most are the later stages of the process, meaning cells extract less energy from the same amount of fuel.
This isn’t limited to your heart. Mitochondrial decline happens throughout your body, in your muscles, brain, liver, and other organs. When cells can’t produce energy as efficiently, they simply do less metabolic work. Older mitochondria also produce more waste products (called free radicals), which damage surrounding cellular machinery and trigger cell death pathways. The result is a cycle: damaged mitochondria work less efficiently, produce more waste, and damage themselves further.
Your body also loses some of its metabolic flexibility with age. Younger cells easily switch between burning fat and burning sugar depending on what’s available. Aging cells become more dependent on sugar and less capable of burning fat efficiently. This shift doesn’t just slow your metabolism; it also makes it harder to lose stored body fat.
Hormonal Changes Play a Role
Several hormones that support a higher metabolism decline with age. Growth hormone, which helps maintain muscle mass and promotes fat burning, drops steadily from your 30s onward. In men, testosterone levels decrease gradually over decades, contributing to muscle loss and increased body fat. Women experience a sharper hormonal shift during menopause, when estrogen levels fall and body composition often changes rapidly.
But the story is more nuanced than just falling hormone levels. According to the Merck Manual, even when hormone levels remain stable, the receptors that respond to those hormones become less sensitive with age. So your body may still produce a reasonable amount of a given hormone, but your tissues respond to it less effectively. This reduced sensitivity can blunt the metabolic signals that tell your body to build muscle, burn fat, or ramp up energy production.
Your Organs Slow Down Too
Your brain, liver, heart, and kidneys are responsible for a surprisingly large share of your resting metabolism. Together, these organs account for roughly 60% to 70% of the calories you burn at rest, despite making up only about 5% to 6% of your body weight. All of them lose some function with age.
Organ cells shrink in a process called atrophy. According to MedlinePlus, this is most common in skeletal muscle, the heart, the brain, and the sex organs. A smaller organ with fewer active cells simply burns fewer calories. Your liver and kidneys also process substances more slowly as you age, reflecting their reduced metabolic activity. The brain, which burns about 20% of your resting calories, gradually loses volume starting in your 40s. None of these individual changes are dramatic on their own, but together they chip away at your baseline calorie burn year after year.
You Move Less Than You Think
One of the most underappreciated reasons for metabolic decline has nothing to do with biology. It’s behavior. The calories you burn through everyday movement, fidgeting, walking around the house, taking stairs, carrying groceries, tend to drop substantially as you get older. This non-exercise activity, sometimes called NEAT, can account for 15% to 30% of your total daily calorie burn.
Older adults tend to sit more, walk fewer steps, and engage in less spontaneous physical activity. Some of this is driven by joint pain, fatigue, or reduced mobility. Some of it is simply habit. But the calorie impact is large. A person who goes from 8,000 steps a day in their 30s to 4,000 steps in their 60s may burn 200 to 300 fewer calories daily from that change alone, roughly equivalent to what most people attribute to a “slower metabolism.”
How Much Does Metabolism Actually Slow?
The total effect is smaller than most people believe, at least until later in life. Between 20 and 60, your resting metabolic rate drops by less than half a percent per year when you account for changes in body composition. That means much of the weight gain people experience in their 30s, 40s, and 50s is driven more by eating slightly more and moving slightly less than by a true metabolic shift.
After 60, the decline accelerates to roughly 0.7% per year. By 90, resting metabolism is about 26% lower than in midlife. At that point, the cumulative effects of muscle loss, organ atrophy, mitochondrial decline, and hormonal changes have genuinely reduced how many calories your body needs.
What You Can Do About It
The most effective countermeasure is resistance training. Lifting weights or doing bodyweight exercises directly addresses the largest contributor to metabolic decline: muscle loss. Studies consistently show that adults in their 60s and 70s can build meaningful muscle mass with regular strength training, and the metabolic benefits follow. Even two sessions per week can slow or partially reverse age-related muscle loss.
Staying physically active throughout the day matters just as much as formal exercise. Walking more, standing instead of sitting, and staying engaged in physical tasks around the house can maintain your non-exercise calorie burn at levels closer to what you had in younger decades. Protein intake also becomes more important with age because older muscles need a stronger dietary signal to trigger muscle building. Most experts recommend older adults aim for 25 to 30 grams of protein per meal rather than the 15 to 20 grams that might have been sufficient at 25.
Sleep quality, which tends to decline with age, also affects metabolism. Poor sleep raises cortisol, increases insulin resistance, and makes your body more likely to store fat and break down muscle. Prioritizing consistent, restorative sleep supports the hormonal environment that keeps your metabolism running closer to its potential.