Carbohydrates, lipids and proteins make up the three macronutrients. Their dietary requirements are high relative to the micronutrients, also known as vitamins and minerals. All macronutrients are organic compounds composed of carbon, hydrogen, oxygen and sometimes other elements. As potential calorie sources, they can be oxidized to produce ATP, the body's energy currency. Mostly, though, the macronutrients differ with regard to individual functions and properties. For example, proteins and carbohydrates are water-soluble, but most lipids are not, so lipids require some extra work for the body to process.
Carbohydrates for Energy
Sugars and starches are the main digestible carbohydrates, and they provide glucose for energy production. The body stores extra glucose as glycogen, and excessive intake is converted to body fat. Glucose -- the only macronutrient that can provide energy without oxygen -- can fuel intense, short bursts of activity. Glucose is also necessary for brain function. The body will convert protein into glucose when glucose is depleted. Fiber is another type of carbohydrate that is indigestible and mainly supports the functioning of the digestive system.
Role of Lipids
The two main dietary lipids are fats/oils and cholesterol. Most cells use a fuel mix of fat and glucose when oxygen is available. Without adequate glucose, fat is incompletely metabolized and forms molecules called ketones, which are mostly excreted. Certain fats known as omega-3 and omega-6 are used to synthesize regulatory hormonelike chemicals. A certain amount of body fat is needed for energy reserves and protection of internal organs. Cholesterol does not supply calories, but it is a building block of very important chemicals such as vitamin D. Lipids are also the major component of cell membranes.
Proteins for Muscles and Bones
Protein is composed of molecular units called amino acids. It is the only macronutrient that contains nitrogen. Its primary function is to build and maintain body structures, such as muscle, bones and internal organs, and to synthesize important molecules such as antibodies, enzymes, neurotransmitters and various blood proteins. Protein can be used for energy, but that is not the body's preference. In addition, the body can convert protein to glucose, but neither carbohydrate nor lipid can be converted to protein. Amino acids are not stored, so a daily intake is required. Excess intake can be stored as fat, however.
Working as a Team
Even though all of the macronutrients can supply energy, a balanced diet providing all three will enable them to perform their specialized and varied functions in the body. And foods that provide macronutrients are also sources of important micronutrients as well. The dietary standard called Acceptable Macronutrient Distribution Ranges issued by the Institute of Medicine recommends that 45 percent to 65 percent of total calorie intake come from carbohydrate, 20 percent to 35 percent from fat and 10 percent to 35 percent from protein. Though flexible, the ranges help ensure an adequate daily intake of each macronutrient.