Updated: Apr 6
We hear it often, people complain about consuming too many calories. Many people try to reduce their calorie intake to lose weight, whilst others try to eat more calories to gain weight. But what exactly are calories? Why are so many people obsessed about it?
There is a good reason for that! Calories are simply defined as energy (Liu et al., 2014). Our body needs this energy to survive. Even in a state of rest, our body is still working hard to maintain life. Our digestive system is still working, the heart is still beating, and the lungs are taking in new oxygen. To maintain all these body systems working, the body needs energy. The word metabolism is used to sum all the energy expended by the body during rest.
When we do any form of physical movement, our body starts using more energy. Our body systems increase their work. The muscles contract harder to produce more power, breathing increases so the lungs can take in more oxygen and eliminate more carbon dioxide, and the heart beats faster to pump more blood. Although we cannot see this energy, we can see and feel its effects as when we use energy our body starts to heat up (yes that’s why our body releases disgusting sweat during intense physical activity to cool down the body) and move. But what exactly is energy and where does it come from?
Energy is measured in calories or joules. One calorie is the amount of heat it requires to raise the temperature of 1 gram (equivalent to 1 ml) of water set at 14.5 °C by 1 °C. One joule is the amount required to exert a force of one Newton for a distance of one meter. However, since the calorie and the joules represent very small amounts, they are usually measured in kilocalories (kcal) or kilojoules (kj). A kilocalorie represents 1000 calories and a kilojoule represents 1000 joules. One kilocalorie is enough to raise the temperature of 1 kg (equivalent to 1 L) of water set at 14.5 °C by 1 °C. 1 kcal is equal to 42 kj.
In our body, we find a chemical substance called Adenosine Triphosphate (ATP in short). They are found in every cell of the body. They are built up from the breakdown of carbohydrates, protein, fat, and even alcohol. ATP is made up of an adenosine compound and three phosphate compounds. These phosphate compounds are connected to adenine by bonds in which energy is stored. When our body needs energy, ATP breaks down one of these bonds to release the energy trapped within these bonds. When a phosphate group breaks down, ATP transforms to Adenosine diphosphate (ADP), meaning that the Adenine compound is bonded only with two phosphate compounds. Only some of the energy released goes to physical work (for example muscle contraction). Most of the energy (around 80%) is wasted as heat, making humans inefficient with using energy stores. ADP is then reconverted back to ATP as a phosphate group is added to the structure and the process can continue in a cycle.
The problem with ATP is that the body can store only a limited amount of ATP at any one time. This small amount is enough to keep the body going during a state of rest. However, this amount depletes in just a few seconds when we perform different forms of physical activity. In this case, the body must quickly re-generate ATP using the compounds found in the food we eat.
Different foods and drinks contain different nutrients that can be used for energy including carbohydrates, fats, and protein. When we consume foods or drinks with these nutrients, they are broken down via the digestive system into their basic forms and absorbed into the bloodstream. Carbohydrates are broken down into a single sugar (glucose, fructose, or galactose), fats are broken down into fatty acids, whilst proteins are broken down into amino acids. Apart from their role as energy producers, these nutrients also have other important roles for the body. Alcohol is also a nutrient that provides us with energy. However, unlike other nutrients, it is not broken down but absorbed rather quickly into the bloodstream. Also, alcohol does not have any other nutritious benefit, thus it is considered as a non-essential nutrient. Consuming large amounts of alcohol will have detrimental effects on your health.
Carbohydrates, fats, proteins, and alcohol release different amounts of energy. For every 1 g, carbohydrates and protein provide 4 kcal of energy, fat provides 9 kcal, whilst alcohol provides 7 kcal. This shows that fat gives out more than twice as energy as either carbohydrates and fat. However, this does not mean that fat is the best source of energy. The body can break down and use carbohydrates quicker, and it is used mainly when we require short but quick bursts of energy like sprinting and jumping. In contrast, the body takes longer to use fat for energy. Thus, it is used mainly during prolonged but less intense movements like walking, jogging, and cycling. Due to its importance in tissue growth and maintenance, protein is used for energy only as the last resort (when carbohydrates and fats are in short supply).
All foods contain a different mix of nutrients. For example, 100 g of Salmon has 12 g of fat, 22 g of protein, and 0 g of carbs. In contrast, 100 g of wholemeal bread yields 3.4 g of fat, 13 g of protein, 41 g of carbs.
Having an understanding of the nutrients component of foods and the energy values is the start for understanding the importance of nutrition. Still, this is just the start!
Liu, P. J., Bettman, J. R., Uhalde, A. R., & Ubel, P. A. (2015). ‘How many calories are in my burrito?’Improving consumers’ understanding of energy (calorie) range information. Public health nutrition, 18(1), 15-24.