Glucose, fructose, glycerol and amino acids are all components of the foods you eat. These molecules can serve as energy sources to fuel your activities: glucose and fructose from carbohydrates, glycerol from fats and amino acids from proteins. With the exception of fructose, these molecules can also serve in a structural capacity in your body. Consuming a well-balanced diet ensures you have adequate carbohydrates, fats and proteins to meet your body's needs.
Glucose
Glucose is a monosaccharide, the most basic unit of carbohydrate. Glucose can exist as a single molecule or it can bind with fructose to create a disaccharide called sucrose. It can also bind with galactose to create lactose, another disaccharide. Glucose comprises the starch molecules you eat as dietary carbohydrates and is the basic unit of glycogen, the form of starch your body stores in your liver and muscles. Glucose is your body's prime source of fuel, and it supplies four calories for every gram you consume. Food sources of glucose include grains, fruits and vegetables.
Fructose
Fructose, also known as fruit sugar, is a monosaccharide similar in structure to glucose. Comprising half of a sucrose, or table sugar, molecule, fructose serves as a fuel source when it is plentiful in your diet. Your body metabolizes fructose in a different way than it does glucose, and you metabolize it differently in your liver than you do in your muscle tissue. Certain metabolic disorders may result in an inability to properly metabolize fructose and may be fatal if you consume fructose or sucrose-containing foods. Fructose, as with glucose, provides four calories of energy per gram, and is abundant in fruits and any foods rich in sucrose.
Glycerol
Glycerol, a short chain of carbon and hydrogen atoms, provides the backbone to a triglyceride molecule. Three fatty acids attach to the glycerol to create the triglyceride, or lipid. Food sources include fats and oils. The triglycerides in your diet provide calorie-dense energy, supplying nine calories for each gram of fat you eat. Triglycerides can also serve as your body's energy reserves by storing as fat tissue. When your body metabolizes a triglyceride, the glycerol portion of the molecule enters your liver where it converts to glucose and is burned as a carbohydrate.
Amino Acids
A protein molecule consists of a chain of amino acids linked together and folded into a globular structure. Your body synthesizes proteins from the amino acids in your diet to create blood cells, build muscle, manufacture enzymes, synthesize hormones and support your immune system. Your body may also burn amino acids for fuel when other dietary energy sources are scarce. Amino acids supply four calories of energy per gram you consume. They are found in meats, milk products, fish, eggs, grains and legumes.
References
- The Medical Biochemistry Page; Carbohydrates; February 2011
- The Medical Biochemistry Page; Non-Glucose Sugar Metabolism; February 2011
- The Medical Biochemistry Page; Lipids; February 2011
- The Medical Biochemistry Page; Amino Acid Metabolism; July 2011
- University of Illinois McKinley Health Center; Macronutrients: the Importance of Carbohydrate, Protein, and Fat; March 2008
