Why do we need proteins?
Proteins are the major structural component in our body; therefore they are needed for growth but also for maintenance and repair. They have many other functions as enzymes, transporters, hormones and precursors of many more functional molecules. Therefore proteins should be eaten every day. Their needs are calculated on the basis of nitrogen balance studies, that is how much is used by the body. This amount varies with age and condition: more proteins are needed for growth, pregnancy, in elderly people and in some illnesses and injuries.
How much do we need?
The needs for adults are of 0.83 g/kg body weight/day of good quality protein, which amounts to 58 g per day for a person weighing 70 kg.
Did you know that in a 150 g beef steak, there is 31 g protein?
There are also requirements for individual essential amino acids.
Dietary proteins are mainly used to replace those structural and functional proteins, only the surplus is used for energy (4 kilocalories per gram). Also in the body, very few proteins are available for immediate energy (1%), the others are used only in case of caloric deprivation such as fasting, famine or some long term illnesses. In case of lack of protein in the diet, amino acids can be almost totally recycled.
Protein and amino acid requirements in human nutrition. Report of a joint FAO/WHO/UNU expert consultation, WHO Technical Report 935, Geneva, 2007
Essentials of Human Nutrition, J. Mann and A. S. Truswell Editors. Oxford University Press, 2012