Primer on Protein
Proteins are simply chains of amino acids that are linked together in different ways. Amino acids are divided into two general groups: those essential in our diet (must be obtained from food), and those that are non-essential in our diet (can be made in our bodies by different amino acids). The essential amino acids include leucine, isoleucine, valine, lysine, threonine, tryptophan, methionine, penylalanine, and histidine. The non-essential amino acids include arginine, alaninie, asparagine, aspartic acid, cysteine, glutamine, glutamic acid, glycine, prolene, serine, and tyrosine. Complete proteins contain all of the essential amino acids in balanced proportion. Most animal and certain vegetable proteins are complete.
Protein function is typically dependent upon its shape. The ‘tertiary structure’ refers to how amino acid sequences are folded upon each other to give each protein its unique shape, and therefore function.
Proteins serve as the major structural component of all the cells in our bodies, and are second only to water in abundance. We need proteins for proper growth and maintenance, and for many routine body functions including the production of hemoglobin (our blood cells that carry oxygen), formation of antibodies to fight foreign threats, and supply the nitrogen needed to make DNA and RNA.
(Hormones and enzymes are also formed from amino acids; they help regulate metabolism, support the immune system and other body functions.)
There are many different sources of protein ranging from whole protein foods (such as milk, meat, fish, egg, and vegetables) to a variety of protein powders (such as casein, whey, egg, rice, and soy).
|Food||Amount of Protein (grams)|
|Spirulina 1 cup||64|
|Tempeh 1 cup||41|
|Dried Parsley 1 cup||31|
|Lentils, cooked 1 cup||18|
|Black Beans, cooked 1 cup||15|
|Tofu, firm 4 oz||11|
|Quinoa, cooked 1 cup||9|
|Peanut Butter 2 tbsp||8|
|Almonds 1/4 cup||8|
|Sun-dried Tomato 1 cup||8|
|Brown rice, cooked 1 cup||5|
|Broccoli, cooked 1 cup||4|
|Potato 1 med.||4|
|Lambs quarters 1 cup||4|
Digestion of protein begins in our stomach. The acid activates an enzyme called pepsin, and further digestion occurs by enzymes called trypsin and chymotrypsin. After these enzymes cleave the protein into amino acids, the intestines absorb them.
So, how much protein do we need? You will hear considerable debate upon this issue. Simply put, our protein needs mirror our overall energy needs and expenditure. Our body weight and composition, growth rate, physical activity, and presence of illness or injury determine our protein needs to a great extent. According to the Unites States and Canada Dietary Reference Intake Guidelines, the average sedentary female aged 19 to 70 requires 46 grams of protein per day. Equivalent males require 56 grams of protein per day. This amount of protein intake will average 0.8 g/kg/d. Athletes, extremely physically active people, and those with inflammation or infection will require 1.6 to 1.8 g/kg/d. Maximal daily protein intake may reach 25% of total energy requirements (for example, in severely burned patients), and may climb to 2 to 2.5 g/kg/d.
Our ability to metabolize protein depends a great deal upon our kidney function. After protein is digested into amino acids, they enter our bloodstream, and are converted to functional molecules in the liver. The liver also converts nitrogen in amino acids to ammonia, and then urea (a waste product). Our kidneys are responsible for excreting urea; and, if kidney disease is present, protein restriction may be needed.
Our body will utilize as much protein as possible for energy (see Dietary Energy for more details). If, however, we take in far more protein than our body needs, it will convert to glycogen or fat cells, or occasionally combine with calcium to increase the risk for kidney stone formation.
Food allergies are typically due to different protein sources (casein – milk, gluten – wheat and grains, peanut proteins, and those found in shellfish and other sea foods).
Proteins were first described by the Dutch chemist Gerardus Johannes Mulder, and named by the Swedish chemist Jöns Jacob Berzelius in 1838. In 1958, Frederick Sanger sequenced the first protein in insulin, and Max Perutz with Sir John Cowdery Kendrew solved the first protein structure in hemoglobin. Using X-ray diffraction analysis, they determined the three-dimensional structures of both proteins in 1962, and shared the Nobel Prize for Chemistry.