Metabolism and energy exchange

Proteins are broken down to amino acids in the intestine and absorbed; cells synthesise new proteins (building material, enzymes, some hormones). The body cannot make some amino acids – the essential amino acids – and they must come from food; meat, fish, eggs and dairy products contain all of them, and plant foods (legumes, grains) complement each other when eaten together. Carbohydrates are broken down to glucose and absorbed; glucose is the main energy source and the excess is stored as glycogen in the liver and muscles. Insulin and glucagon keep blood sugar normal; if they fail, diabetes may occur, which only a doctor identifies and treats. Fats are broken down to glycerol and fatty acids; they are an energy source (1 g fat about 9 kcal, 1 g protein or carbohydrate about 4 kcal), building material of cell membranes, keep heat and protect organs; they are stored under the skin. Certain fatty acids come only from food, so plant oil and fish are also needed. Substances can convert into each other, but a shortage of protein cannot be made up by other substances.