Energy Production & Metabolism

  • Metabolism is the sum of all chemical reactions that occur in living cells.
  • Energy metabolism can be defined as the physical and chemical processes in the body that convert chemical energy stored in molecules into energy that can be used for cellular processes.
  • The body breaks down 3 energy-yielding nutrients:
  1. Carbohydrates, which are used for energy production.
  2. Lipids, which are used for energy production.
  3. Protein, which are mostly used as building blocks but are also used for energy production.
  • These nutrients are broken down Into 4 basic units - glucose, glycerol, fatty acids and amino acids.
  • Anabolic reactions occur when small molecules are put together to build larger ones
  • Catabolic reactions consist of the breakdown of larger molecules to smaller ones to release energy.
  • The body uses ATP (adenosine triphosphate) produced in the mitochondria of cells to transfer energy released during catabolic reactions to power anabolic reactions. The body converts chemical energy of food to chemical energy of ATP and heat. Energy is lost again when the body uses ATP to do its work. ATP fuels the citric acid cycle.
  • The citric acid cycle (also known as the Kreb’s cycle) takes place in the mitochondria and is the process of energy production. The phase which comes after is called the electron transport chain, which requires oxygen, which is then reduced to water. 
  • Carbohydrate is broken down in a process called glycolysis and can occur either aerobically or anaerobically to provide pyruvate, which then breaks down to Acetyl CoA. Acetyl CoA can be used to synthesis fat to store energy or to generate ATP.
  • Fat is broken down into Acetyl CoA in a process called beta-oxidation. The process is called lipolysis.
  • Proteins are broken down when amino acids are deaminated into acetyl CoA, pyruvate or other compounds which can enter the citric acid cycle. The breakdown of amino acid causes the release of urea into the blood which is excreted via the kidneys. 
  • Other molecules involved in energy production include NAD+, NADH (from niacin), FAD and FADH2 (from riboflavin). 
  • Fat provides the most energy at 9kcal/g. 

Energy Balance: Factors which affect energy expenditure vs energy intake

  •  Metabolism is the conversion of food to energy. This energy is then used by the body to perform activities such as eating, moving, and sleeping. 

The first law of thermodynamics

  • Energy cannot be created or destroyed but merely transferred from one state or another.
  • Plants create chemical energy via photosynthesis to create nutrients. Animals obtain chemical energy from food by eating plants and other animals.
  • Chemical energy is then transferred to mechanical energy for movement, chemical energy to synthesize new macromolecules and electrical energy created when ions are actively transported across the cell membrane.  Energy can also be stored as fat. A small amount may be used for cell repair and growth. when food is used by the body for chemical, mechanical and electrical work, it is accompanied by a loss of energy in the form of heat.
  • Body temperature must be kept constant, and this is maintained by heat transfer to surroundings.
  • Heat energy which is no longer available for use is known as entropy
  • A calorie is the amount of heat required to raise the temperature of 1 gram of water by 1 degree Celsius. 
  • Kilocalories (kcal) are used to describe nutritional calories (1 kcal is 1000 cal). 
  • A joule is a measure of energy used when a mass of 1kg is moved through 1 metre by a force of 1 newton. Again, a nutritional kilojoule (kj) is 1000 joules.


 The second law of thermodynamics states that when energy is transferred or transformed, more and more of it is wasted. (i.e. an increase in entropy).


  •  Energy balance is achieved through regulation achieved via energy storage and dietary energy intake vs energy expenditure.
  • The control of the mechanism of energy balance is extremely precise. Just a small dietary energy intake or failure of energy regulation may eventually lead to obesity.
  • Failure of energy regulation may also lead to malnutrition or cachexia. 
  • If energy expenditure isn’t equal there will either be a negative energy balance, where the body will use its energy stores.
  • Energy in the body can be stored as fat, glycogen (carbohydrates and short-term energy reserve) and protein. Protein is rarely used by the body for energy unless there is a severe case of starvation or cachexia.
  • A positive energy balance where the body will increase its energy stores primarily as fat.
  • The body only stores or metabolises fat if energy intake changes for a period of several days. 


Energy expenditure depends on various factors:

  1. Basal metabolic rate – this is the rate at which food is converted into heat transfer and work done by the body when it is completely resting and performing basic physiological functions only to sustain life. It compensates for over and under eating as the body would rather not lose its own fat to replace a reduced food intake.  Weight loss will then be slower.
  2. Physical activity and movement – exercise assists with weight loss as it raises metabolic rate and produces heat transfer.
  3. Thermogenesis there is an increase in energy expenditure in response to the thermic effect of a meal (diet-induced thermogenesis), usually at 10% of the energy content as a meal.


Energy Requirements

  • Individual body requirements are dictated by body weight, resting metabolic weight and free fat mass.
  • The resting metabolic state is calculated with the Harris-Benedict Equation.
  • Energy requirements to maintain energy balance should be equal to total energy expenditure.
  • Energy expenditure is reduced in the elderly and in sedentary individuals and males tend to have a higher energy expenditure.