How Does Metabolism Change With Age? The Science of Energy Expenditure

Marta Aguilar

Graduada en Nutrición Humana y Dietética, Universidad Pablo de Olavide (2018). AND-817. Título de Experto en Nutrición Avanzada: Presente y Futuro de la Nutrición, Universidad Pablo de Olavide. Máster Oficial en Alimentación en la Actividad Física y el Deporte, Universitat Oberta de Catalunya.

“I don’t burn calories like I used to.” “Your metabolism slows down after 40.” If we received a dollar every time we heard these claims in clinical practice, we could probably afford a lifetime supply of avocados.

Many clients arrive with a deeply rooted limiting belief: that metabolism inevitably slows down as they get older. That is why one of the main challenges in nutrition practice is explaining how metabolism changes with age—and whether the supposed decline in energy expenditure is really an unavoidable consequence of aging or the result of other factors we can modify.

In this article, we dive into the scientific evidence to explain what actually happens to metabolism as we age. We will explore which myths need to be left behind and how INDYA’s technology can help turn age from an excuse into just another variable to optimize when planning an athlete’s nutrition.

1. Breaking Down Total Energy Expenditure (TEE)

Metabolism includes all the biochemical processes the body carries out to stay alive, such as maintaining body temperature, keeping the heart and lungs functioning, and repairing tissues.

To understand how metabolism changes, we first need to define a few basic concepts. One of the most important is total energy expenditure (TEE), which consists of three main components:

• Basal metabolic rate (BMR): The energy the body uses at rest. It accounts for approximately 60–70% of total energy expenditure in sedentary individuals.
• Thermic effect of food (TEF): The energy used to digest, absorb, and metabolize nutrients.
• Physical activity energy expenditure, the most variable component from one person to another. It includes:
  • Energy used during planned exercise or training.
  • Non-exercise activity thermogenesis (NEAT), meaning the calories burned through movement outside planned exercise, such as walking, standing, or moving around.
    It can be calculated as follows:

TEE = BMR + TEF + physical activity energy expenditure

2. The Four Metabolic Stages of Life

If you have not read the work of Pontzer and colleagues, you should. Their 2021 study published in Science analyzed the energy expenditure of more than 6,400 people using the doubly labeled water method—the gold standard in this field.
Its findings force us to reconsider how metabolism is explained in clinical practice. The study showed that metabolic expenditure adjusted for fat-free mass does not decline in a straight line. Instead, it follows four distinct stages:

1. Infancy (0–1 year): A major metabolic acceleration. Energy expenditure is nearly 50% higher than in adults.
2. Childhood and adolescence (1–20 years): A gradual, moderate decline after the initial surge.
3. Metabolic plateau (20–60 years): Surprisingly, metabolism remains stable for approximately four decades.
4. Older adulthood (over 60): A genuine decline begins, but it amounts to only about 0.7% per year.

The science tells us that metabolism is remarkably stable between ages 20 and 60. This suggests that the perceived “slowdown” before age 60 is not caused by age itself. Instead, it is usually the combined result of sarcopenia—the loss of metabolically active tissue—and a significant reduction in NEAT.

What does this mean for everyday nutrition practice? When a 45-year-old athlete says they are gaining weight “because of their age,” something else is usually happening.

Communicating this can be a powerful tool for change. It helps clients move away from resignation and toward action. Age is not necessarily the problem; context is—and context can be modified through effective planning and the right technology.

3. What Really Drives Metabolic Changes?

If cellular biology remains relatively stable until age 60, why does a “slow metabolism” feel so real to many clients? The answer is not found on their birth certificate. It lies in critical variables that can often be managed, monitored, and modified.

• Reduced organ energy expenditure

Muscles and internal organs—including the heart, liver, kidneys, and brain—are among the body’s most energy-demanding tissues, even at rest. Their metabolic rates can range from approximately 200 to 440 kcal/kg/day.
When the size of certain organs decreases, which can occur with aging or physical inactivity, the amount of active tissue consuming energy also decreases.

This brings us back to the fourth metabolic stage: older adulthood, beginning at around age 60.

This change, combined with the loss of functional cell mass, helps explain the genuine annual decline of approximately 0.7% in basal energy expenditure.
It is not a system failure. It is a reduction in the size and activity of the body’s metabolic core.

• Lower physical activity levels

This factor is directly related to lifestyle, making it highly modifiable. Walking less, spending more time sitting, or lowering training intensity reduces physical activity expenditure and NEAT, causing total daily energy expenditure to fall.

A sedentary person will have a “slower” metabolism than a more active individual. Similarly, a 50-year-old client may blame their age when their NEAT has actually fallen by 40% compared with their younger years because of work responsibilities or changes in training, habits, and daily routines.

• Changes in body composition: more fat and less muscle

Adipose tissue uses very little energy compared with muscle. Muscle mass consumes approximately 13 kcal/kg/day, while adipose tissue uses around 4.5 kcal/kg/day.
In other words, one kilogram of muscle uses roughly three times as much energy as one kilogram of fat. If body fat increases while muscle mass decreases, the body burns fewer calories both at rest and during activity.

• Hormonal changes and metabolic efficiency

As we age, the hormonal environment that regulates energy use changes, affecting how quickly cells operate. The annual 0.7% decline after age 60 already includes this hormonal shift. These are not separate effects that should be added together; reduced endocrine signaling is one of the reasons organs and tissues become less metabolically active.

• Thyroid hormones: The physiological decline in thyroid hormones T3 and T4 can reduce basal metabolic rate. These hormones act like a thermostat regulating cellular activity.
  
• Andropause and testosterone: In men, the gradual decline in testosterone beginning around ages 40–50 can contribute to muscle loss and promote fat storage, particularly around the abdomen.
• Menopause and estrogen: In women, the sharp decline in estrogen represents a significant turning point. It can affect insulin sensitivity, reduce metabolic efficiency, and alter body fat distribution.

A client’s dieting history may also lead to downward metabolic adaptations. Severe, prolonged calorie restriction can trigger adaptive thermogenesis. In an attempt to survive, the body optimizes mitochondrial efficiency and learns to produce the same amount of ATP using less substrate.

This increased mitochondrial efficiency, combined with the loss of lean mass that often occurs during poorly planned weight-loss programs, can reduce BMR below what would be predicted from body composition alone.
Metabolic stress may also increase cortisol and reduce the conversion of T4 into active T3. This reinforces an energy-conserving state that can persist after the diet ends, facilitating weight regain and strengthening the false belief that metabolism has slowed because of age.

4. Energy Expenditure in Women

If you are a woman, or work with women in clinical practice, Pontzer’s relatively flat metabolic curve up to age 60 may raise questions. “There is no way my metabolism is the same at 50 as it was at 25.”
That concern is understandable. Women experience biological milestones that can profoundly transform the body.

The important nuance for nutrition professionals is that female metabolism does not simply slow down with age. Instead, the way the body manages energy undergoes significant changes. It is not merely about getting older; it is about how the individual components of the system change.

4.1 Pregnancy: Does Metabolism Increase?

Yes. Basal metabolic rate increases significantly during pregnancy. Fetal growth, placental development, increased blood volume, and the greater demands placed on the heart and lungs all increase daily energy expenditure.

The key is understanding the cause of this change. It is not that maternal cells suddenly begin working faster. Rather, there is more metabolically active tissue operating at full capacity.

Pontzer’s study showed that when energy expenditure is adjusted for fat-free mass—including the fetus and placenta—the mother’s cells continue working at approximately the same rate. There is simply another occupant with their own energy requirements.

4.2 Menopause: Does Metabolism Decrease?

The belief that metabolism suddenly comes to a halt during menopause is one of the biggest barriers encountered in clinical practice. Biology, however, suggests that a reduction in total energy expenditure is usually secondary to three other factors:

1. Lower muscle mass: Estrogen helps protect muscle tissue. When estrogen levels fall, sarcopenia can accelerate. Less muscle mathematically results in a lower basal metabolic rate.
2. Insulin resistance and metabolic flexibility: Hormonal changes affect how efficiently the body manages carbohydrates, making it easier for energy to be stored as visceral fat instead of being oxidized.
3. Reduced NEAT: Spontaneous movement often decreases unconsciously during this stage of life.

The nutrition professional’s challenge is not to fight metabolism, but to protect muscle mass and adjust energy availability using tools that support accurate planning, such as INDYA’s nutrition software.

5. Chronological Age vs. Biological Age

At this point, it is helpful to distinguish between two concepts:

• Chronological age: The number of years a person has been alive—for example, 24, 40, or 65.
  
• Biological age: An assessment of the body’s functional condition and the degree to which age-related changes are present in muscle, organs, hormonal regulation, and other systems.

Why does this difference matter? Because many apparent metabolic declines are more closely related to biological condition, muscle preservation, activity levels, existing health conditions, and body fat, than to the number of years a person has lived.

Two people who are both chronologically 60 years old can have very different total energy expenditures depending on their biological age. This reinforces the idea that chronological age matters much less than the combined factors defining a person’s biological condition.

Remember this:

Chronological age is a statistical measurement; biological age is a target for intervention.

Nutrition planning should aim to improve biological age by optimizing body composition and energy availability.

6. Four Examples of Energy Expenditure

To translate the scientific evidence into clinical practice, consider how metabolism might behave in four different situations:

This table challenges the slow-metabolism narrative:

1. Age is the least decisive factor. Scenario C, at age 50, has a higher total expenditure than scenario A, at age 30. The key is not time itself, but the metabolic density of the tissues and the person’s movement levels.
2. Muscle matters significantly. Even at age 70, basal expenditure can remain relatively high when lean tissue is preserved, offsetting much of the annual decline associated with older age.
3. The perceived slowdown at 50 is often preventable. Scenario B represents the frustrated client commonly seen in clinical practice. Their TEE of 1,900 kcal is not simply the result of being 50; it reflects a combination of sarcopenia and reduced NEAT.

7. Final Recommendations and Intervention Strategies

As nutrition professionals, our goal is to help athletes keep their biological age as low as possible. To prevent downward adaptations in energy expenditure, interventions should focus on several key strategies.

1. 1. Maximize muscle protein synthesis

Simply meeting the minimum protein requirement may not be enough. In masters athletes, anabolic resistance may require protein intakes of approximately 1.6–2.2 g/kg/day, with sufficient leucine at each meal to preserve metabolically active tissue.

2. 2. Avoid low energy availability

Excessive restriction triggers adaptive thermogenesis. Chronically falling below 30 kcal per kilogram of fat-free mass per day may cause the body to reduce metabolic expenditure.
To protect basal metabolic rate and thyroid function—particularly free T3—technology can help nutrition professionals implement appropriate nutrition periodization. This may involve alternating deficit periods with higher-energy phases that signal to the body that it does not need to enter an energy-conservation state.

3. 3. Support micronutrient status and mitochondrial efficiency

Energy expenditure depends on enzymatic processes in which magnesium, iron, and B vitamins play essential roles. Without these cofactors, mitochondria cannot function efficiently. This can impair energy management and create a genuine feeling of having a slower metabolism.

4. 4. Combine nutrition with strength training

Mechanical stimulus is one of the strongest allies of nutrition intervention. Without strength training, preserving lean mass and maintaining a higher metabolic rate becomes considerably more difficult.

5. 5. Monitor actual activity levels

What appears to be a slow metabolism is often simply a reduction in daily movement or a change in training habits.
Fortunately, INDYA’s software allows athletes to update their daily activity and workouts and synchronize devices such as Garmin and Apple Watch, as well as apps such as Strava. This provides real-time data on daily energy expenditure, heart rate, and heart rate variability, allowing nutrition plans to be adjusted to the athlete’s current biological reality instead of relying solely on theoretical estimates.

Metabolism is not programmed to fail at age 40. The slowdown observed in clinical practice is usually the combined result of muscle loss, physical inactivity, and poorly planned nutrition strategies.

As professionals, we need to move beyond generic equations that focus primarily on age. Instead, we should use technology that adjusts energy intake to each athlete’s actual body composition and physiology.
With INDYA, age no longer has to be a limiting factor. Nutrition planning can evolve alongside the athlete’s physiology.