When it comes to energy, carbohydrates are often said to be the most important fuel to power your body. But carbohydrates are not necessarily the chief source of energy for your body at any given time. Fat, protein and carbohydrates are all used to produce human energy. The percentage of each substrate used depends on many variables, such as the type and duration of the activity, intensity level and what your current diet provides.
"Energy" is the capacity to do work. In the human body, calories are the fuel providing energy. The main nutrients that provide energy are protein, fat and carbohydrates. Many people think energy is only used during strenuous physical activity, but your body is constantly using high levels of energy for temperature control, respiration and all the other activities that support life.
The Brain Craves Carbs
When you eat carbohydrates -- such as breads, pasta, rice, fruits and vegetables -- they are converted to blood glucose to provide immediate energy and then converted to glycogen for later use. If you eat more calories than you expend, carbohydrates are also converted to fat to be stored for later use. In adequately nourished people, the central nervous system -- including the brain -- relies almost completely on glucose for energy, and the brain uses quite a bit of glucose. In the course of an overnight fast, nearly all reserves of glucose and glycogen are depleted. In addition to the brain, red and white blood cells use only glucose for energy.
Carbs Are Energy Efficient
There is a saying that "nature is lazy." That would explain why carbohydrates tend to be a chief source of energy; the body loses 5 percent of its energy when it has to store glucose as glycogen, and it loses 28 percent of its energy when it has to be converted to fatty acids for storage, rather than being used immediately for energy. Another important factor is that carbohydrate is required to metabolize fat, meaning muscle glycogen and blood glucose are limiting factors in your performance of any type of activity.
Different Activities/Different Fuels
Both glucose and fatty acids provide fuel for rest, as well as physical activity. The proportion of each depends on intensity and duration of the activity. You might be surprised to learn that during rest and normal daily activities, fats provide 80 to 90 percent of your energy. Carbohydrates provide between 5 and18 percent, and protein 2 to 5 percent. During physical activity, your needs for energy increase. Moderate-intensity exercise -- such as hiking, jogging, aerobics and cycling -- get about half the energy from glycogen stored in muscles and half from circulating blood glucose and fatty acids. The longer you exercise, the greater the percentage of fatty acids utilized. During strenuous bursts of activity, such as sprinting or heavy lifting, the body relies solely on glucose and glycogen for fuel. As the intensity becomes greater, so does the use of glucose and glycogen. If you are training for an endurance sport, a high-fat, low-carbohydrate diet lowers glycogen stores and decreases performance.
The Bottom Line
Your body has the ability to produce energy from fat, carbohydrates and protein. The Food and Nutrition Board of the Institute of Medicine gives the following ranges for daily nutrients: 45 to 65 percent carbohydrate, 10 to 35 percent protein and 20 to 35 percent fat. A diet that follows these guidelines provides a balanced amount of energy-producing nutrients. Eating the right amount of calories in the right balance each day will keep you energized and fueled optimally. If you are an athlete in training, adequate amounts and timing of additional carbohydrates are essential for optimal performance.
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- "Sports Nutrition: A Practice Manual for Professionals 4th Edition"; Dunford, Marie; 2006
- The National Academies Press: Using the Acceptable Macronutrient Distribution Range