Why Do Adults Need More Energy Than Children? A Look at the Science

October 2, 2025 •

4 min read

According to a groundbreaking study in Science, infants between 9 and 15 months expend a stunning 50% more energy per day than adults, adjusted for body size. This often surprising fact addresses the common question of why do adults need more energy than children? The answer lies in the fundamental difference between total caloric requirements and relative metabolic rate.

Quick Summary

The premise that adults need more energy than children is a common misconception. While adults require a higher total number of calories, children possess a significantly faster metabolic rate per kilogram of body weight to fuel rapid growth and development. This distinction is influenced by factors including growth, basal metabolic rate, and physical activity levels.

Key Points

Higher Absolute Needs: Adults require a greater total number of calories due to their larger body size and mass, even with a slower metabolic rate per pound.
Higher Relative Metabolism: Children have a significantly faster metabolic rate per kilogram of body weight compared to adults, particularly infants and toddlers.
Growth is Energy-Intensive: A major reason for children's high relative energy needs is the substantial caloric demand required to fuel rapid growth and tissue synthesis.
The Brain's Role: The developing brain of a young child is a huge energy consumer, demanding a much higher percentage of daily calories than an adult's brain.
Body Composition Changes: As adults, a larger proportion of lean body mass is muscle, which has a lower resting metabolic rate than the vital organs that make up a greater proportion of a child's lean mass.
Activity Level Variation: While highly variable, children often have a higher general activity level and are less metabolically efficient during movement, contributing to their energy expenditure.
Metabolic Decline with Age: A person's metabolic rate gradually slows down after reaching adulthood, further reducing their relative energy needs.

Total Energy Needs vs. Relative Metabolic Rates

When most people ask, "Why do adults need more energy than children?" they are referring to the total number of calories required to sustain the body each day. From this perspective, the statement is correct; an average adult requires a higher total daily caloric intake than an average child. However, this is largely due to the adult's much greater body size. When you analyze energy expenditure on a per-kilogram basis, the story is quite different. The metabolic rate of an infant or a child is significantly higher per unit of body weight than that of an adult. Children are essentially 'burning superfast' to power their rapid development.

The Energetic Demands of Growth

One of the most significant factors driving a child's high metabolic rate is the process of growth and development. The energy consumed is not just for maintenance and daily activities but also for synthesizing new tissues, organs, and bones. Infants, in particular, double their body weight in just a few months, and this rapid expansion is incredibly energy-intensive. As children grow, the proportion of energy dedicated to growth decreases, but it remains a substantial factor throughout childhood and adolescence.

Basal Metabolic Rate (BMR) and Body Composition

Basal metabolic rate (BMR) is the energy expended by the body at rest to maintain essential functions like breathing, circulation, and cell production. A child's BMR per kilogram of body weight is notably higher than an adult's. This difference can be attributed to several aspects of body composition:

Brain Power: A young child's brain is an incredible energy consumer. Studies have shown that the brains of toddlers can use up to 43% of all the energy the body uses in a day. This is a much higher percentage than in adults and is crucial for fueling cognitive development.
Higher Organ-to-Muscle Ratio: The lean body mass of infants and young children contains a higher proportion of metabolically active organs relative to total body weight compared to adults. In adults, a larger portion of lean body mass is composed of skeletal muscle, which has a lower resting metabolic rate than organs.
Lean Body Mass Decline: As individuals enter early adulthood, their lean body mass gradually declines, which is a major contributor to the age-related reduction in BMR.

Physical Activity Levels (PAL)

Physical activity level (PAL) is the most variable component of daily energy expenditure, and it also plays a role in the comparison between adults and children. While an adult's work and exercise may consume many calories in total, children are often in a near-constant state of motion, with an inherently higher overall activity level.

A simple breakdown of how physical activity affects energy expenditure:

Non-Exercise Activity Thermogenesis (NEAT): Children exhibit high levels of NEAT, which includes fidgeting, maintaining posture, and other unconscious movements. This constant low-level activity adds up to a significant energy burn.
Play and Exploration: Children's play is often physically demanding and exploratory, fueling their development and creativity. This naturally high level of physical engagement further contributes to their elevated energy expenditure.
Efficiency: Children are also less metabolically efficient than adults during certain activities like walking or running, which means they expend more calories to perform the same task.

How Energy Requirements Differ: A Comparison


Feature	Children (relative to body size)	Adults (relative to body size)
Basal Metabolic Rate	Higher due to rapid growth and metabolically active organs.	Lower as growth ceases and lean mass composition changes.
Energy for Growth	Requires significant energy for synthesizing new tissue.	Requires no additional energy for growth after maturation.
Brain Energy Use	Extremely high, up to 43% of total energy in toddlers.	Decreases significantly as the brain matures.
Body Composition	Higher proportion of metabolically active organs relative to muscle.	Higher proportion of skeletal muscle, which has a lower metabolic rate.
Physical Activity Level	Often higher and less metabolically efficient during movement.	Typically lower or more controlled, although highly variable.
Absolute Caloric Needs	Lower overall caloric needs due to smaller body size.	Higher overall caloric needs due to larger body size.

Implications for Nutrition and Health

Understanding the nuanced differences in energy needs has important implications for nutrition and public health. For children, ensuring adequate energy and nutrient intake is critical for proper growth and development. Under-nutrition during these formative years can lead to serious health issues, including stunted growth and delayed puberty. The high metabolic demands of children also mean they may metabolize medications faster, requiring careful dosage management.

For adults, maintaining a healthy energy balance is about matching intake to the body's more stable, maintenance-focused requirements. With a slower metabolic rate and often more sedentary lifestyles than children, adults must be mindful of their total caloric intake to prevent weight gain and related health problems. This shift in energy dynamics with age highlights the need for different nutritional strategies throughout life.

Conclusion

In summary, the notion that adults need more energy than children requires a clear distinction between absolute and relative energy requirements. While adults consume a greater total amount of energy due to their larger size, a child's metabolism is far more intense on a per-kilogram basis, driven by the immense energetic demands of growth and development. The peak metabolic rate seen in infants and toddlers gradually slows down over time, stabilizing in adulthood. This understanding explains why children require high-quality, nutrient-dense foods to fuel their energetic growth, while adults must focus on balancing their intake with their slower, maintenance-level energy needs.

For further reading on the energetic demands of aging, you may find this resource helpful: Aging and Energy Intakes in Humans, Dogs and Cats.

Frequently Asked Questions

Yes, children do have more energy than adults on a per-kilogram basis. While an adult's total caloric needs are higher, a child's metabolism is much faster for their size, primarily to support growth and development.

Toddlers are known for their high energy levels because their bodies are burning calories at an incredibly fast rate to fuel growth and a highly active brain. This, combined with their naturally high physical activity, gives the perception of boundless energy.

Yes, metabolism slows down as we age. A person's metabolic rate decreases after early adulthood, influenced by a decline in lean body mass and often a reduction in physical activity.

Children and adults need the same basic nutrients, but in different amounts. Children require high nutrient density to fuel their growth, while adults need fewer calories overall and should focus on maintaining a healthy weight as their metabolism slows.

The brain is a massive energy consumer, especially during childhood. The developing brain of a toddler can use up to 43% of all daily energy, a proportion that decreases significantly as the brain matures.

Basal Metabolic Rate (BMR) is the energy the body uses at rest. It is highest per kilogram during childhood due to growth and metabolically active organs. After age 20, BMR decreases by about 2% per decade.

An adult can have a higher total metabolic rate than a child due to their larger body size. However, it is rare for an adult to have a higher relative metabolic rate (per kilogram) than a growing child, even if the adult is very active.