Every human has a different energy requirement depending on their physical constitution (e.g., gender, age, height) and their physical activities (e.g., sleeping, standing, running). If this requirement is not met, weight loss occurs, and if it is exceeded, weight gain occurs. The essential components in food for this purpose are called macronutrients, which can be divided into carbohydrates, fats, and proteins.
Carbohydrates
During digestion, complex carbohydrates (e.g., starch) are first broken down into disaccharides (e.g., lactose, sucrose), and then disaccharides are broken down into monosaccharides (e.g., glucose, fructose). Monosaccharides are primarily used as direct energy sources. If there is no immediate need for energy, only glucose is stored as glycogen in the liver, kidneys, brain, and muscles. If these glycogen stores are full, excess glucose is stored in the form of fat.
Fats
During digestion, fats are broken down into fatty acids. These fatty acids can be classified into saturated fatty acids, monounsaturated fatty acids, and polyunsaturated fatty acids (Omega-3, Omega-6). Primarily, they are burned directly for energy within cells. Alternatively, as the name suggests, they are stored as fat in adipose tissue. Fats provide twice as much energy as carbohydrates and proteins.
Proteins
During digestion, proteins are broken down into their constituent amino acids. These amino acids are primarily used by the body to build and repair cells. While carbohydrates and fats also play supporting roles in this process, their contribution is secondary. If the body’s immediate need for protein is met, any excess amino acids are broken down and, if glycogen stores are full, stored as fat for future energy use. This is necessary because there is no storage mechanism for proteins in the body.
In addition to macronutrients, foods also contain micronutrients (e.g., vitamins, minerals). They do not provide energy, but they are still necessary for important functions in the body.