The amount of protein needed in one's diet requires consideration. This is because it's not so much the protein we require, but the amino acids that the protein source in question contains. Since different proteins contain different percentages of the 20 biologically active amino acids, logic would tell us that depending on the types of food we eat, we will get varying percentages of amino acids. What's the significance here? Protein can be categorized according to its biological value. The higher the biological value of a protein, the higher the amount of essential amino acids it contains. Of course, all proteins contain amino acids, but certain proteins contain more of the types of amino acids we need. Meats and eggs are primary examples. If more meats are included in the diet, the less the requirement for protein. This is because meats and eggs contain the essential amino acids in similar proportion for synthesis of human tissues.

Plant proteins, on average, have a lower biological value in comparison to animal (meat) proteins. Since a vegetarian diet excludes meat products, one can compensate by combining plant products of different biological values to supply the body with appropriate amounts of essential amino acids. For example, kidney beans are rich in the amino acid lysine, but low in percentage of the amino acid methionine. Maze contains a high percentage of methionine, but a low percentage of lysine. We can see that by combining these two plant products completes part of the diet in regards to protein.

Now that we have some insight into the composition of proteins, how much protein do we need? It is safe to say that the average individual requires approximately just under a gram of protein per kilgram of body weight. This means that a woman weighing in at about 50kg (roughly 111 lbs.) requires between 40-50 grams of protein per day. The protein required should be of mixed biological value. This is why a balanced diet is important (since different foods contain different percentages of amino acids). Individuals with more physical demands on the body requure more protein. A person who exercises vigorously obviously requires more protein, since they are damaging tissues while exercising. To build muscle as well, one needs to have a positive nitrogen balance. Nitrogen balance takes place when the amount of nitrogen consumed equals the amount passed in the urine, faeces, and sweat. Note that nitrogen is a breakdown product of protein, so this is a good indicator if an individual is getting enough in the diet. Positive nitrogen balance is when nitrogen intake is greater than excretion. We can see this when children grow, when we build muscle in the gym, in pregnancy, and recovery from illness, surgery, etc. Negative nitrogen balance is when nitrogen excretion exceeds intake. We can witness this during times of stress, starvation, injury, illness, etc.

Protein requirement also changes depending on the amount of carbohydrates we include in the diet. Carbohydrates are required as a fuel source. When deficient of adequate carbohydrates, amino acids are used for the production of glucose. This is why diets suggesting low carbs are to be considered carefully. Glucose is the primary fuel of the brain and without it, signicant damage and even death can occur. As we can see, there are many factors to consider when dieting, or attempting to alter any aspect of the metabolism.