Our bodies are finely tuned machines and like most machines our bodies need fuel. All the energy that our bodies need comes from the foods that we eat. So, if you deprive your body of any of the three sources of energy (proteins, fats and carbohydrates), it won't run efficiently. That is why a well balanced diet is so important. According to University of Illinois At Urbana Campaign, some cells in our bodies use carbohydrates exclusively for energy, so our bodies have devised a complex way to turn proteins and fats into energy too. Meaning we need all three of these "macronutrients" to survive. Our bodies also have a way of prioritizing: burning carbohydrates first, then fats and finally proteins.
Carbohydrates provide our bodies with its main source of energy. There are two types of carbohydrates: simple and complex. Vision Learning explains that simple carbohydrates are either monosaccharides or disaccharides, more commonly known as fructose and glucose. Complex carbohydrates contain simple sugars, except that they are in the form of long chains known as polysaccharides. Most polysaccharides contain starch (pasta and potatoes) and take longer for the body to break down. Thus, the quickest amount of energy comes from simple carbohydrates such as those found in fruit.
Fat is another source of energy that is essential to our bodies. Not only does fat help us better utilize other nutrients, fat actually helps to transport some vitamins to the necessary areas of our bodies. Some fats even contain vitamins such as vitamin A, vitamin D, Vitamin E and vitamin K. Fats have as much as two times the calorie content as carbohydrates and when eaten in excess can cause weight gain, but fats that are eaten as part of a balanced diet provide energy for heating and fueling the body; what is not used is stored away for later.
Protein is the third primary source of energy that our bodies need. Protein is extremely important to the growth of our cells and all of the metabolic and biological processes that go on inside our bodies. According to Merck, protein is vital to the growth of muscles, skin and connective tissue. In fact, protein is so important to our bodies that it is usually only burned when there are no remaining carbohydrates or fats.
Believe it or not, alcohol does offer our body's energy to burn, but alcohol energy is far from the healthiest source of energy. In the book "Alcohol in Health and Disease," author Dharam P. Agarwal explains that the energy content of alcohol is actually quite high but cannot be used as a main source of energy because it also has a high toxicity level. In large amounts, alcohol can be lethal, especially if there is more alcohol consumed then food. Alcohol also disrupts the metabolic process. Alcohol cannot be absorbed by the body and used as a fuel source later, so it needs to be burned first. Instead of proteins, carbohydrates and fats being burned in succession, alcohol becomes priority.
Our bodies have yet another source of energy that is better described as the "alternative energy" produced by lactic acid. Lactic acid is produced by the muscles and can cause fatigue if not used wisely. University of California explains that athletes who make efficient use of lactic acid will actually be creating an additional form of energy to power their bodies. In fact, working though the burn during exercise can actually train your muscles to reuse the lactic acid that it puts out as a backup energy source.