Protein is a macronutrient along with fats and carbohydrates.
Protein forms the building blocks of life, is essential for life and inadequate and poor protein will result in less then optimum functioning of body systems and disease.
Protein is the most abundant substance in the body besides water and is of primary importance for growth and development of all body components. Protein is comprised in all bodily tissues (blood, muscles, skin, hair, nails, organs), bones and nerves. Protein is also used to manufacture the chemical messengers that regulate our endocrine, immune and digestive system, hormones such as thyroid and insulin, antibodies to fight pathogens such as bacteria and viruses, and enzymes which are catalysts to initiate chemical reactions for everything. There are transport proteins that move fat-soluble and water-soluable vitamins and minerals throughout the body so that our cells can do their thing, function. Protein is essential.
Molecules of protein are made up of amino acids, there are 22 amino acids which are required in specific patterns, to be present at the same time and in correct proportions to manufacture a human protein which may then be utilised for all the functions as outlined above.
9 of these amino acids are called essential as they cannot be made in the body and must be supplied by diet. A nonessential amino acid may become essential through illness or conditions of trauma (extreme stress).
A complete protein is a food which contains all essential amino acids. Meats and animal products are complete protein foods. Animal proteins contain saturated fats which in moderation provide all the necessary nutrients, however a diet high in saturated may be associated with heart disease and cancer. Most plant foods are not complete proteins (some exceptions include soy and chia seeds) and must be combined in certain ways to ensure that all essential amino acids are available to synthesise complete proteins for correct bodily functioning. Nuts and seeds with grains, grains with beans and legumes, and nuts and seeds with beans and legumes. A vegetarian diet can provide ample protein requirements as long as correct combining is followed.
A high quality protein is one that can be digested easily and of good quality, eggs for example. Daily protein requirements as recommended by World Health Organisation is 0.8-1.2 g per kilogram of your body weight. A guide often used to determine the protein requirement at each meal is the size and thickness of your palm. This is a guide and is entirely dependent on your activity levels and your individual metabolic requirements.
It is important to understand this when working out your protein requirements. When you buy meat for example you may purchase 500 g of beef mince, this is not 500 g of protein, the meat also contains fats and water. Unless you had each cut of meat biochemically analysised exact quantities of protein cannot be determined. The protein content is also dependent of each individual animal, what the animal was reared on, where it came from etc. So realistically I use the palm method as explained above and look at approximately 150 - 200 g of weight of meat purchased per person.