Why is Energy Needed for Humans? The Science Behind Life's Fuel

December 25, 2025 •

2 min read

According to the Food and Agriculture Organization, basal metabolism alone accounts for 45–70% of a person’s daily energy expenditure. Understanding why is energy needed for humans is key to appreciating how our bodies function, as this constant energy supply fuels every physiological process, from the beating of our heart to the firing of a thought.

Quick Summary

The human body requires a continuous supply of energy to power every function, including baseline metabolic processes and all physical activity. This energy, derived from macronutrients in food, is converted into ATP to fuel cells, enable growth, and maintain body temperature. An understanding of the body's energy use promotes better health and dietary decisions.

Key Points

Basal Metabolic Function: The body requires a significant amount of energy (45-70% of total daily use) just to power vital, involuntary functions like breathing, circulation, and brain activity while at rest.
Physical Activity: All forms of movement, from blinking to intense exercise, demand energy, with different energy systems (phosphagen, glycolytic, oxidative) being used depending on the intensity and duration.
Macronutrients as Fuel: Carbohydrates, fats, and proteins from food are the body's primary energy sources, with each having a specific energy density and role in fuel metabolism.
Growth and Repair: Energy is critical for the synthesis of new tissues during growth stages like infancy and adolescence, as well as for ongoing cellular and tissue repair throughout life.
Homeostasis: The body expends energy to regulate its internal environment, including maintaining a stable body temperature through thermoregulation.
ATP is the Energy Currency: At the cellular level, the chemical energy from food is converted into ATP (adenosine triphosphate), which powers nearly all cellular activities.
Energy Balance is Key to Health: A healthy weight and overall well-being depend on balancing the energy consumed from food with the energy expended by the body.

The Core of Human Function: Energy for Survival

Energy is fundamentally required for human survival, powering every cell to sustain life. This process begins with the food we consume, which serves as the fuel source for the body's complex biological systems.

Powering the Basal Metabolic Rate

Even at rest, the body uses energy for vital functions, a process known as the Basal Metabolic Rate (BMR). This includes:

Brain function, a high-energy consumer, maintaining electrical potential in nerve cells.
Organ function, with the liver, kidneys, and spleen requiring substantial energy.
Circulation and respiration, powered by the continuous work of the heart and lungs.
Cellular activity, including repair and synthesis, demanding energy for homeostasis.

Fueling Physical Activity

Energy for physical movement is a major part of daily expenditure. Different energy systems are utilized based on activity intensity and duration:

Phosphagen System: For short, high-intensity efforts.
Glycolytic System: For moderate-duration, high-intensity activities.
Oxidative System: For long-duration, lower-intensity activities, using oxygen, carbohydrates, and fats.

The Role of Macronutrients and Energy Balance

Macronutrients from food (carbohydrates, fats, and proteins) are the primary energy sources. Maintaining a healthy weight involves balancing energy intake with expenditure.


Macronutrient	Energy Value (kcal/g)	Primary Function as Fuel	Preferred Use Case	Efficiency	Storage Capacity
Carbohydrates	4 kcal/g	Rapid energy for immediate use	High-intensity exercise	High efficiency	Limited (as glycogen)
Fats	9 kcal/g	Concentrated, slow-burning fuel	Rest and low-intensity activity	High energy density	Unlimited (as adipose tissue)
Protein	4 kcal/g	Building and repairing tissues	Energy source in depleted states	Least efficient	Not stored for energy

Growth, Repair, and Development

Energy is essential for building and maintaining the body, particularly during growth periods like infancy, childhood, and adolescence, for new tissue synthesis. It is also vital for ongoing cellular and tissue repair, and in stages like pregnancy and lactation.

Thermoregulation and Homeostasis

Maintaining internal body temperature is energy-intensive, with the body expending energy to produce heat or cool down through sweating. This thermoregulation is key to homeostasis.

The Cellular Engine: ATP Production

All energy needs are ultimately met by adenosine triphosphate (ATP), produced through metabolic pathways like cellular respiration, converting energy from food into a usable form that powers cellular work.

Conclusion: Energy is the Essence of Being Human

Energy, derived from food, is fundamental for all human functions, from involuntary organ activity to physical feats. Maintaining a healthy energy balance is crucial for overall well-being. Energy is the defining force of human existence. Discover more about how your body uses macronutrients for energy here.

Frequently Asked Questions

The primary source of energy for the human body comes from the food we consume. The macronutrients—carbohydrates, fats, and proteins—are broken down through digestion and metabolism to provide the necessary fuel.

The Basal Metabolic Rate (BMR) is the minimum amount of energy the body needs to perform essential functions at rest, such as breathing and circulation. Physical activity energy is the additional energy required for any voluntary movement, from standing up to strenuous exercise, and is the most variable part of daily energy use.

The body converts the chemical energy stored in food into a usable form called adenosine triphosphate (ATP) through a process called cellular respiration. This process breaks down glucose, fatty acids, and amino acids to produce ATP, which powers cellular functions.

The brain is one of the most energy-intensive organs in the body. It constantly requires energy to maintain the electrical potentials in its nerve cells, which are necessary for all cognitive functions, including thinking, memory, and sensory processing.

A negative energy balance, where more energy is expended than consumed, leads to weight loss as the body draws on its stored energy reserves, primarily fat. Prolonged negative energy balance can result in malnutrition and the breakdown of muscle mass.

The three main energy systems are the phosphagen system for immediate, high-intensity bursts; the glycolytic system for short-to-moderate high-intensity activities; and the oxidative system for long-duration, low-to-moderate intensity exercise.

Yes, your energy requirements change based on your age, sex, body size, and activity level. Periods of growth, pregnancy, and lactation require additional energy, while a person's needs may decrease with age due to lower BMR and activity levels.