Food must be supplied regularly to meet the energy needs of the body.
Although the energy value of each nutrient utilised by the body is known, very few foods are actually made up of a single nutrient. Most foods are a combination of the metabolically active ingredients that constitute protein, carbohydrate (fibre, starch and sugar) and fat. These so-called "macronutrients" are also interwoven with "micronutrients" – vitamins and minerals. The energy value of the micronutrients is, however, insignificant and too small to take into consideration when calories are counted.
Breakdown of glucose
All the macronutrients, except fibre, are sources of energy. In the body, fats are broken down into fatty acids, carbohydrates (sugar and starch) are converted to glucose, and protein is broken down into the basic building blocks, the amino acids.
During the digestion process, glucose gets converted to glycogen – molecules that can be stored in the liver and muscles for instant use as energy. Although the liver can convert glycogen back to glucose, the muscles can't. Muscles can, however, use glycogen directly or release it into the bloodstream.
Ingested glucose and sucrose (table sugar) gets absorbed and released into the bloodstream almost immediately. Generally, an amount of 5 grams of glucose are available for direct use in the bloodstream. The rest of the glucose gets stored as glycogen. These stores are short-term and glucose can be released into the bloodstream as the need arises.
Breakdown of protein and fat
Proteins are broken down to amino acids upon ingestion. In the liver, these amino acids are partly reassembled for use in other bodily functions and partly released into the bloodstream, from where its potential energy can be used. About 100 grams of amino acids are stored in the bloodstream for instant use.
Fat, which is converted to fatty acids during the digestion process, can be stored in almost all body cells. These packets of energy, stored in so-called adipose tissue (connective tissue packed with masses of fat cells), are the body's energy reserves, to be used when all other energy sources are depleted.
The process of actual release of energy is complicated. In simplified terms, the glucose, amino acids and fatty acids are converted into ATP (adenosine triphosphate) by all cells that contain mitochondria through an intricate cycle of processes. The potential energy of the metabolically active substances is released when the ATP is broken down into smaller units.
Due to a slight inefficiency in the digestion process, the body can't use all the energy available in the food we eat.