Do Kids or Adults Burn More Calories? A Metabolic Deep Dive

December 29, 2025 •

4 min read

A groundbreaking study analyzed the metabolic rates of over 6,600 people, revealing that infants burn calories roughly 50% faster than adults. This surprising fact highlights the complex metabolic differences across our lifespan and helps answer whether kids or adults burn more calories.

Quick Summary

Infants and young children have the highest metabolic rates, burning more calories per unit of body mass than adults due to rapid growth, intense brain activity, and high physical activity. While an adult's total daily burn might be higher due to size, a child's relative expenditure is far greater, slowing into a stable adult phase before a slow decline after 60.

Key Points

Infants and children burn more calories per kilogram: Due to rapid growth, intense brain development, and high physical activity, young children have a significantly higher metabolic rate relative to their body size compared to adults.
Metabolism is stable in adulthood: After the metabolic peak in the late teens, energy expenditure remains relatively consistent for most people between their 20s and 50s.
Metabolism declines slowly after 60: Research has shown that the metabolic slowdown in later life is much more gradual than previously thought, typically beginning after age 60.
Muscle mass is a key factor: Because muscle tissue is more metabolically active than fat, body composition plays a major role in overall calorie burn. Strength training helps adults maintain a higher metabolism.
Total calorie burn depends on many factors: While kids have a higher relative metabolic rate, total daily calorie burn depends on body size, composition, gender, and activity levels, making a direct comparison complex.

The Energetic Powerhouse: Why Kids Are Calorie-Burning Machines

While a larger adult may burn more total calories in a day, young children, especially infants, burn significantly more energy relative to their body mass. Scientific research reveals that infants, in their first year of life, have the highest metabolic rate, burning approximately 50% more calories for their body size than an adult. This incredible energy expenditure is driven by several biological factors unique to childhood.

The Intense Demands of Rapid Growth

Growth is an extremely energy-intensive process. From synthesizing new tissues and bones to maturing organs, a child's body is in a constant state of rapid development. During an infant’s first three months, growth alone can account for 35% of their total energy needs. This demand gradually lessens but remains a significant factor throughout childhood and adolescence, particularly during growth spurts.

Fueling the Developing Brain

The brains of young children are astonishingly active, consuming a disproportionately high amount of the body's energy. In fact, a young child's brain can consume up to 43% of their total daily energy. This energy is critical for supporting neurological development, learning, and the processing of new information. As a child matures, the brain's relative energy demand decreases, freeing up metabolic resources for other functions.

The Inefficiencies of Childhood Activity

Children are naturally more active than most adults, but their metabolism is also less efficient when performing the same activities. Studies have shown that activities like walking or running can cost children up to 30% more energy than adults. This metabolic inefficiency, combined with their non-stop movement, contributes to a higher overall caloric burn.

The Adult Metabolic Journey: Stability and Gradual Decline

After the high metabolic peak of early childhood and adolescence, calorie-burning patterns shift significantly. Metabolism generally stabilizes around age 20, remaining relatively steady through a person's 50s. The belief that metabolism drastically slows in middle age has been widely debunked by modern research, which instead identifies a slow, gradual decline of about 0.7% annually starting around age 60.

The Role of Muscle Mass

One of the most significant differences influencing calorie burn between adults and children is body composition. Muscle is a more metabolically active tissue than fat, meaning it burns more calories at rest. As people age, they tend to lose muscle mass, and their body's weight composition shifts towards fat, which contributes to a slower basal metabolic rate (BMR). Men, who generally have more muscle mass than women, typically have a higher BMR. Building and maintaining muscle through strength training is one of the most effective ways for adults to combat metabolic slowdown.

Hormonal and Lifestyle Factors

Hormones, such as growth hormone (HGH), play a crucial role in metabolism. While HGH is essential for growth in children, it also influences fat metabolism and body composition in adults. Additionally, lifestyle plays a large part. Sedentary adults burn fewer calories from physical activity than their more active counterparts, while children's lives are often filled with constant motion.

Kids vs. Adults: A Calorie-Burning Comparison Table


Feature	Infants & Young Children	Adolescents	Adults (20-50)	Adults (60+)
Metabolic Rate	Highest (per kg)	High	Stable	Slow decline
Energy for Growth	Very High (35%+)	High	0%	0%
Brain Energy Use	Very High (up to 43%)	High	Lower	Lower
Physical Activity	Often Constant, less efficient	High	Variable, depends on lifestyle	Variable, often lower
Body Composition	Rapidly changing	Rapidly changing	Varies; muscle mass tends to be higher	Tends to have less muscle mass

Conclusion: The Nuanced Answer

The question of whether kids or adults burn more calories does not have a simple one-word answer, but it's clear that the dynamics of energy expenditure change dramatically with age. Young children and infants, with their unique needs for rapid growth and brain development, possess the highest relative metabolic rates, making them incredible calorie-burning powerhouses. While a fully grown, physically active adult may burn more total calories due to larger body size, a sedentary adult's metabolism is much slower per unit of mass than a child's. Ultimately, the answer depends on the specific age and life stage, with metabolism peaking early in life before stabilizing and eventually declining. Understanding these differences can inform better nutritional and fitness decisions throughout life.

Visit the NCBI for more information on the intricate role of hormones and metabolism.

Factors Influencing Metabolism

Body Composition: The higher your muscle mass, the higher your basal metabolic rate (BMR).
Age: Metabolism is highest in infancy and decreases with age, particularly after 60.
Physical Activity: The most variable component of daily energy expenditure, heavily dependent on lifestyle.
Gender: Men typically have a higher metabolic rate due to greater muscle mass.
Growth: Significant energy is required for growth during childhood and adolescence.
Hormones: Changes in hormone levels, including growth hormone, affect metabolic function.
Genetics: Your genetic makeup can influence your natural metabolic speed.

How Non-Exercise Activity Thermogenesis (NEAT) Affects Calorie Burn

NEAT refers to the energy expended for everything we do that is not sleeping, eating, or sports-like exercise. This includes walking to work, gardening, or even fidgeting. NEAT accounts for a significant portion of daily calorie burn and can vary greatly between individuals, helping explain why some people seem to have a 'faster' metabolism.

Frequently Asked Questions

While infants have the highest metabolic rate per unit of body mass, the total daily calorie requirements often peak during the teenage years, especially for active boys, due to a combination of high activity levels and larger body size.

Adults can increase their calorie burn by incorporating regular physical activity, including both aerobic and strength training exercises, into their routines. Building muscle mass is particularly effective, as muscle is more metabolically active than fat.

Yes, muscle tissue burns more calories at rest than fat tissue. One pound of muscle burns approximately 6 calories per day, whereas one pound of fat burns only about 2 to 4 calories per day.

Research indicates that metabolism remains stable between the ages of 20 and 50, so a slowed metabolism is unlikely to be the primary cause of weight gain in middle age. Lifestyle factors like decreased physical activity and changes in diet are more likely to blame.

The brain of a young child is exceptionally active and requires a massive amount of energy to support rapid neurological development. In children under five, the brain can use up to 43% of the body's total energy, contributing significantly to their high metabolic rate.

Doctors can use a method called calorimetry to measure a person's resting metabolic rate (RMR). This involves having a person lie or sit still while breathing into a machine that analyzes the oxygen and carbon dioxide exchanged, providing an accurate measure of energy expenditure at rest.

Men typically have a higher basal metabolic rate (BMR) than women of the same age and weight. This is primarily because men tend to have a greater amount of muscle mass and a lower percentage of body fat, and muscle is more metabolically active.