When the American chemist Wilbur Atwater first explored the magic of the calorie in the early 1890s, no one gave much thought to the concept of dieting. In fact, at the turn of the previous century it was a lot more fashionable for women to be voluptuously rounded.
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In the last few decades, however, energy intake has become all the rage as weight loss facilitators capitalised on what could be dubbed the Century of the Calorie. But what are calories and why should they be given a second thought?
The power of fuel
Like all living organisms, the human body needs fuel to function. And, like in all mechanical machines, this fuel needs to be burned in the presence of oxygen to produce the energy that gives the body the ability to perform tasks.
However, the living organism differs from machines in the sense that it also needs fuel when it's resting – just to keep it running. For this reason, fuel needs to be supplied on a regular basis.
Earth's great energy source
All energy on earth comes from the sun, is used directly by plants, algae and cyanobacteria through the process of photosynthesis, and is used indirectly by humans, animals and other organisms. Energy cannot be created or destroyed. It is merely transformed from one form to another or from one "host" to another.
Plants store energy in the chemical bonds of glucose, which mostly encompasses starch – the human's primary source of energy. When you eat a potato, you are indirectly ingesting the sun's energy. But because the sun's energy has been changed in the potato, it needs to be converted back to its original form to be used by the body. This transformation is a simple example of the never-ending processes of energy conversion that keeps the universe intact.
Metabolism
To convert the potato's potential energy, several metabolic processes need to take place in the body. These involve intricate chemical changes in the cells in which the ingested fuels go through the so-called Krebs cycle. A compound called adenosine triphosphate (ATP) is produced and used to supply energy for all of life's processes. An example is muscle contraction. When muscles contract the chemical energy of ATP undergoes another transformation resulting in the kinetic energy of muscle motion.
Human fuel
Carbohydrate, fat and protein are the three most important fuels in human nutrition. All of them are metabolised and stored in different ways. These fuels also fulfil other needs in the body. According to specific needs at specific times, the energy sources are allocated to different processes.
As an example, protein's most important function is to build and maintain the body's tissues. In other words, protein makes children grow taller and repairs cells in the adult body. This is the body's number one priority. Only when all the growth and repair work is done, protein is used as energy source. But if you are starving, the protein you eat will be used exclusively to provide energy and some of your body's other functions will deteriorate accordingly.
Energy in vs. energy out
To keep the body intact, energy input (or energy "ingested") needs to be balanced with energy output (or energy "expended"). When more energy is ingested than expended, weight will be gained and vice versa. The body can only use a certain amount of energy at a time. Excess energy gets stored in the form of fat.
The potential energy value of food products can actually be measured. By keeping track of energy input and energy output, one can maintain a healthy weight. Both energy expenditure and energy potential are measured in terms of calories or kilojoules, depending on the system you use (metric=kilojoules, imperial=kilocalories). South African labels usually specify both.
Calorie counters can be used to calculate the calorie values of the food you eat. A rough estimation of energy expenditure can also be made, although this is less accurate.
If you don't have the time or energy (!) to count your calorie input and output, a few simple dietary guidelines might actually do the trick, without making weight management an obsession. - (Carine van Rooyen, Health24)
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