The Primary Sources of Energy in the Body

There are many sources of energy that keep humans functioning correctly.
There are many sources of energy that keep humans functioning correctly. (Image: Goodshoot/Goodshoot/Getty Images)

The human body, like all other living things, requires energy for survival. For humans, energy comes in the form of food. When food enters the digestive system through the mouth, it is broken down by acids and enzymes into smaller parts that can then be converted into energy by the digestive system. Food is composed of several different types of nutrients, most of which can be used as fuel for the body.


Sugars, specifically glucose molecules, are the most basic form of energy for the human body. Glucose is required at the start of the cellular respiration cycle, which converts food into energy for the body. While sugar is technically classified as a carbohydrate, all carbohydrates need to be broken down into glucose in order to be used in the body, and it is the glucose that is crucial for survival. Other examples of simple sugars include fructose, found in fruit, and lactose, found in milk.


Carbohydrates are the main source of fuel for the body. They can be simple, such as corn syrup, or complex like cereal or bread. Both types of carbohydrates are converted into energy in the human body, though complex carbohydrates provide other nutrients, including vitamins, minerals and fiber, so they are considered better for overall good health.


Protein is an essential nutrient for all living organisms. In addition to being the building blocks of muscle, protein is responsible for immune regulation, tissue growth, hormone regulation and a wealth of other functions. Excess protein in the diet is broken into individual amino acids, which can enter the cellular respiration cycle. It is important to note that while amino acids can replace glucose in cellular respiration, glucose is much more efficient at producing energy.


Newscasters and Web sites are constantly touting the dangers of trans and saturated fats, but the truth is fat is necessary for survival, although unsaturated fats are better than trans and saturated fats. Besides providing insulation, protection and structural support, one of fat’s main purposes is to store energy in the body. In fact, it is the most concentrated form of energy in the body. In the absence of sugars, carbohydrates and protein in the diet, the liver will convert fat to glycerol and fatty acids. In a process known as gluconeogenesis, the liver then converts the glycerol into glucose.

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