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You eat every day. But have you ever stopped to think about why you eat and the effect that different types of food have on the internal workings of your body?

You eat when you get hungry. Simple enough. Hunger is your body’s way of telling you that the amount of energy it has available is getting low and that it needs more. For animals, including humans, energy is essential to sustaining life and supporting a variety of different functions. For instance, it allows cells to build molecules and transport substances across cell walls. And it makes it possible for your body to move, whether at the cellular level or in the form of whole-body motion, like running or walking.

Did you know? Your brain needs glucose to function properly. Your body will make sure your brain receives enough before providing glucose to other organs and tissues.Cells store energy as adenosine triphosphate molecules, or ATP. Your body releases this stored energy by breaking the bonds between the three phosphate groups in the adenosine triphosphate molecules.

Most ATP is created in an organelle (specialized part of a cell) called a mitochondrion. Individual cells can have many mitochondria, and these require oxygen to create ATP. When the energy demands of the cell exceed the amount of oxygen that your blood can deliver, your body switches to a process called lactic acid fermentation. This process is much less efficient and yields fewer ATP molecules than when oxygen is plentiful, but it still gets the job done!

The easy availability of energy from these sources at the beginning of a workout makes exercise easier. But when these supplies start to diminish and lactic acid begins to build up, you begin to feel a burning sensation in your working muscles. This feeling begins to go away again as your heart rate increases, providing more oxygen for your muscles to create ATP and carrying away the lactic acid in the blood.

Three different macronutrientscarbohydrates (sugars, including glucose), fat, and protein—can serve to replenish ATP. Granted, glucose is the major source of energy in animals. Furthermore, certain types of cells, like neurons and red blood cells, are incapable of converting fat or protein into useable forms of energy and rely solely on glucose. In fact, if they are threatened by a lack of energy, neurons and red blood cells will monopolize your body’s supply of glucose rather than share it with other tissues and cell types. Fat and protein serve as back-up energy sources for other kinds of tissues when glucose is in short supply, or when another energy source would be more efficient to use.

Did you know? As the length and intensity of physical activity increases, so does use of fat as an energy source. Highly-trained endurance athletes make the switch to burning fat faster than most people, a phenomenon known as the crossover effect.Fat is a very efficient way to store energy since it creates more ATP per gram than glucose or protein. Therefore, any food energy that your body doesn’t require to replenish its stores of glucose is stored as fat. Although fat is a better way to store energy, it takes longer to convert it into ATP. Therefore, it is only used when glucose is in short supply, or during activities where the length of time to create energy from fat is acceptable. This is why you will often see gym equipment refer to a “fat burning zone” that corresponds to a lower heart rate than the “cardio zone”, since a long, slow workout will burn a higher proportion of calories from fat. Although this can be an enticing reason to lower the intensity of your workouts, the most important factor in weight loss is calories burned, regardless of the source.

Even though protein can be broken down and used as a source of energy, it is not considered an energy source. This is because protein is an essential component of tissues, performing structural and functional roles throughout your body. For example, proteins function as catalysts for important chemical reactions in the body, they transport substances into and out of cells, and they provide structural support to cells and tissues. As a result, your body will only use protein as an energy source as a last resort, under extreme conditions of energy starvation. In these situations, the body literally starts consuming itself!

You need to eat to provide energy for all of the activities you perform during the day. Even when you are at rest, your body remains extremely active, using energy in the form of ATP to power all of the processes necessary to sustain life. So don’t just think of food as something for your entertainment and enjoyment. The truth is that your body craves food at a much deeper level than just satisfying your taste buds.


General information

Interesting Facts About Glucose (Laurel Brown, LiveStrong.com) Nutrition for Everyone: Carbohydrates (US Centers for Disease Control and Prevention Nutrition for optimal athletic performance (Dietitians of Canada) The Nutrition Source: Fats and Cholesterol (Harvard School of Public Health)

Scholarly publications and textbooks

Lehninger A, Nelson DL, Cox, MM. 2008. Lehninger Principles of Biochemistry & eBook. 5th ed. WH Freeman. Thompson J, Sheeshka JD, Manore M. 2010. Nutrition: a functional approach. Pearson Canada, Toronto, ON.

Nathan Michaels

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