What is energy?

Energy (measured in either calories or joules) is what causes our body to move. Although we cannot directly see energy, we can see and feel its effects through heat and physical work. When we perform any form of physical activity a number of things happen inside our body such as:

· the muscles contract more forcefully;

· the heart beats faster so that more blood is pumped around the body quickly;

· the breathing rate increase and the lunges work harder to allow a higher intake of oxygen;

· the body regulates body temperature by releasing sweat to cool it off.

All these changes require higher amounts of energy. But what exactly is energy? To really understand what energy is, we must first explore some physiological concepts.

Inside every cell of the body, we find small molecules called Adenosine Triphosphate (ATP), made up of an adenosine molecule bonded with three phosphate groups (Figure 1). This ATP is able to store energy inside the bonds joining the phosphate groups with the adenosine. When the body needs energy, it splits one of these bonds, releasing the energy stored (Figure 2). Since the ATP now has only two phosphate molecules, it is now called adenosine diphosphate (ADP). Only around 25% of the energy is used to carry out actual work (such as muscle contractions), with the remainder being lost as heat energy. This is why the body warms up during physical activity. ADP is then converted back to ATP by adding another phosphate group and this cycle is repeated as another phosphate breaks down, releasing more energy (Figure 3).

Figure 1 ATP Structure

Figure 2 ATP releases energy by splitting from one of the phosphates groups.

Figure 3 ADP is then converted back to ATP by adding another phosphate group

The problem with ATP is that it is able to store just enough energy to keep the body going during rest. During exercise, ATP is used within a few seconds. However, the body is able to produce more ATP from the nutrients found in foods and drinks. The body breaks down carbohydrates, protein and fat into their basic form, as they pass through the digestive system, so that they can be absorbed into the bloodstream. Carbohydrates are broken down into glucose, fructose and galactose; fats are broken down into fatty acids and proteins are broken down into amino acids.

Although each of these nutrients have other specific functions, most of these nutrients are used as fuel to generate ATP. Carbohydrates can be used quickly for energy and are used mainly during quick and short bursts of physical activity such as sprinting and jumping exercises. Fats take longer to convert and are used in more endurance-based physical activities such as longer distance running and cycling. Proteins have important uses other than energy such as repairing damaged or building new tissues including muscles, bones and skin. Thus, protein is used only for emergencies (for example when there is a lack of carbohydrate or fat supply), or when they cannot be used anymore by the body. Sooner or later, all nutrients are used as energy.

Scientific Fact:

Alcohol is also another source of energy. It is used much quicker than other nutrients as it does not need to be broken down and mostly is absorbed immediately into the bloodstream after intake. However, unlike the other nutrients, alcohol is not an essential nutrient and the body can survive without it. In fact, the various effects of excessive drinking are well recorded including poor balance, delayed reflexes, loss of consciousness and weight gain.


Traversy, G., & Chaput, J. P. (2015). Alcohol consumption and obesity: an update. Current obesity reports4 (1), 122-130.

Thanks for reading, and as always stay fit!

Coach DB

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