Total energy is the fundamental measure of the capacity to do work, bridging the universal laws of physics with the processes of a living organism. It represents the sum of all energy forms within a system, governed by physical principles that dictate how all matter functions. In human biology, total energy refers to the chemical energy captured from food and the ways this energy is expended to sustain life and physical activity.
The Universal Scientific Definition of Energy
Energy is formally defined as the ability to cause change or perform work. The forms most relevant to biology include chemical potential energy, stored in the bonds of molecules like glucose and fat, and kinetic energy, the energy of motion, such as muscle contraction. Thermal energy, or heat, results from the random motion of atoms and molecules.
The flow of energy is governed by the laws of thermodynamics, primarily the First Law, or the Law of Conservation of Energy. This law states that energy cannot be created or destroyed; it can only be transformed from one form to another, such as chemical energy transforming into mechanical energy. Biological organisms are open systems, exchanging both energy and matter with their surroundings, but they must adhere to this conservation principle.
Biological Energy Capture and Cellular Currency
Living systems acquire the energy they require by consuming chemical energy stored in organic molecules. Humans derive this chemical energy from the macronutrients in food, which are broken down through cellular respiration. This metabolic process extracts potential energy from molecular bonds and converts it into a usable form. The immediate energy carrier for nearly all cellular tasks is Adenosine Triphosphate (ATP), often called the “cellular currency.” Energy is rapidly released when ATP is broken down to Adenosine Diphosphate (ADP) and a phosphate group, powering cellular work like muscle contraction or nerve signaling.
The conversion of chemical energy into ATP is not perfectly efficient, a reality explained by the Second Law of Thermodynamics. This law states that every energy transfer increases entropy, meaning some usable energy is inevitably converted into a non-usable form, typically heat. For example, during the oxidation of glucose, approximately 40% of the energy is captured in ATP bonds, while the remaining 60% is dissipated as heat. This constant heat loss requires the body to continually take in energy to maintain its organized state and internal temperature.
Measuring Total Energy Flow in the Organism
The total energy flow in a living system is quantified using the kilocalorie (kcal). One kilocalorie represents the amount of heat energy required to raise the temperature of one kilogram of water by one degree Celsius. The total energy utilized by the human body over a day is defined as the Total Daily Energy Expenditure (TDEE). Scientists often use indirect calorimetry, which measures oxygen consumption and carbon dioxide production, to accurately determine TDEE.
TDEE is the sum of three main components that account for all energy-consuming processes.
Basal Metabolic Rate (BMR)
BMR is the largest component, accounting for 60% to 75% of the total energy burned daily. This is the energy required to sustain life at rest, powering fundamental involuntary functions such as breathing, heart circulation, and organ maintenance.
Thermic Effect of Food (TEF)
TEF is the energy used to digest, absorb, and metabolize the food consumed. This typically accounts for about 10% of TDEE.
Activity Energy Expenditure (AEE)
AEE covers all energy used for physical movement. This includes structured exercise and non-exercise activities like fidgeting and standing.
The Principles of Energy Storage and Management
Chemical energy from food that is not immediately used or dissipated as heat must be managed and stored. The body maintains two primary reservoirs for this excess energy. Glycogen, a complex chain of glucose molecules, serves as the short-term energy reserve, stored mainly in the liver and skeletal muscle tissue. When glycogen stores are full, excess energy is converted into triglycerides. Triglycerides, a type of fat molecule, form the long-term, high-density storage reservoir located predominantly in adipose tissue.
The flow between energy use and storage is continuously regulated by hormones, such as insulin and glucagon. Insulin promotes storage by signaling cells to take in glucose and convert it into glycogen or triglycerides when energy is abundant. Glucagon promotes release by signaling the breakdown of stored glycogen or fat when the body requires energy. This hormonal management results in the energy balance equation, where changes in body mass reflect the equilibrium between energy input and total energy expenditure.