Protein digestion starts in the stomach through an enzyme called pepsin and then continues in the duodenum and the next bit of gut called the jejunum, through enzymes secreted by the pancreas namely trypsin and chymotrypsin.
The proteins are broken down initially into polypeptides and then into their basic units namely amino acids. Amino acids are absorbed through the wall of the small intestine into the bloodstream. In the bloodstream, the amino acids are transported into the liver and other cells where they are utilised for important cellular functions.
The body requires a certain amount of protein for these functions and beyond this it is surplus to requirements. Protein cannot be stored in the body and therefore any excess that is taken in beyond this requirement is either broken down in the liver or excreted via the urine or it is converted to fat indirectly by first being converted into glucose via the gluconeogenic pathway.
Glucose which cannot be immediately utilised by the cells for energy or converted to glycogen in the liver is converted to fat and stored as excess adipose tissue. So, it is important to appreciate that a high protein diet runs a significant risk of promoting obesity.
The recommended daily intake of protein is approximately 0.8g per kilogram. Therefore, for an averaged sized 70kg male this equates to approximately 60g a day.
Proteins are found in both animals and plant foods. The amino acid profile of animal protein is closer to that of humans but all the necessary amino acids can be provided in the amounts required from plant sources. From a practical perspective, we get most of our protein in the diet from meat, poultry and fish (approximately 50%), cereals and cereal based food (approximate 25%) and dairy foods (approximately 15%).