The French word 'protéine', coined in 1838, comes from the Greek word 'proteios', meaning 'the first quality'. Proteins are essential to life, just not in the hu-moo-ngous quantities once believed (*check the end of this post for a list of the good, bad and ugly of protein sources).
Proteins furnish the body with energy (1 gram of protein = 4 kcal of energy). But they do so much more. When broken down into amino acids, by our digestive system, they repair cells and make new ones. They are vital for growth and development in pregnancy, childhood, and adolescence.
A protein's basic structure is a chain of amino acids. Strung together in different configurations, amino acids create myriad proteins in the body. Our DNA determines those configurations. So, some genes might call for short chains of amino acids to make a tissue or substance, while other genes call for longer chains or 3D shapes. Protein is a major component of muscle, bone, skin, hair, organs, glands and bodily fluids (except urine and bile). It also forms haemoglobin, which carries oxygen in our blood; and enzymes, which spark biochemical reactions. Without protein, we'd be stuffed (toys).
Call the AA! Just 21 amino acids provide raw materials for the 10,000+ proteins that make us what we are and furnish us with spare parts. Unlike carbohydrates and fats, we cannot store amino acids. Excess protein is turned into glucose or fat; it is not used to repair tissue. We must eat protein daily. So, we drink split-pea soup; our digestive system breaks down the proteins into amino acids; and our body calls on those amino acids for repair and maintenance.
Amino acids are also needed to break down proteins, however! So we must eat enough protein, while bearing in mind that too much - especially animal-source - can be detrimental to health. Amino acids are classified into three groups: 'essential', 'non-essential' and 'conditional'. For a full list of amino acids, see here. 'Essential' amino acids cannot be made by the body and must be supplied by food. 'Non-essential' amino acids are made by the body from essential amino acids, or via the normal breakdown of proteins. 'Conditional' amino acids tend not to be 'essential', except in times of illness or stress.
In the past, specific protein foods were classed as 'complete' or 'incomplete', defined by how many essential amino acids they contained. Animal proteins were considered 'complete', while plant proteins were labelled 'incomplete' because it was thought necessary to mix two or more types together at every meal (e.g. beans and grains) to furnish the body with the nine essential amino acids contained in a juicy T-bone. Today, scientists believe that getting a balance of amino acids over the course of the day is more important than sucking them all up at one sitting, so proteins are no longer classed as 'complete' or 'incomplete'. Eating a variety of protein foods is important to get all the essential amino acids, though, especially if you are vegan.
Moo-ve over, Cowslip! You're (almost, but not quite) a has-been... Essential amino acids are found in plant and animal foods (see the Goodies lists above). The package protein comes into matters too. Plant proteins are accompanied by carbohydrates, fibre and micro-nutrients. Pulses, especially, contain a triad of protein, fibre and resistant starch that keeps blood sugar stable and aids weight control. Animal foods sport vital nutrients hard to find on a plant-based diet (e.g. vitamin B12, EPA and DHA), so modest amounts of high-quality animal foods can be helpful. However, the animal protein package contains saturated fat and minimal fibre - not to mention pollutants, antibiotics and other industrial delicacies. Small, wild fish, such as anchovies and herring, caught in clean waters (if that's not an oxymoron), provide some EPA and DHA. Small fish contain fewer pollutants than larger fish. Organic, grass-fed meat contains B12. However, animal foods are best served as an accent to plant-based meals, if at all. EPA, DHA and vitamin B12 can be taken as supplements, perhaps more reliably so.
All is not well... Studies have linked a high-animal protein diet to gout, kidney damage, diabetes, heart disease, osteoporosis, colon cancer and many other health issues. Dairy products are problematic, too. Cancer survivor Prof. Jane Plant, author of Your Life In Your Hands, witnessed her tumours shrinking when she gave up dairy. Her book cites studies linking cancer to hormones and proteins in dairy products and some meats.
This question has not been definitively answered. It depends on height, weight, gender, state of health, etc. See here for age/ gender recommendations from the Centers for Disease Control and Prevention in the United States. We need adequate, but not too much, protein. Protein malnutrition is rare in the West. Whereas, problems of animal protein excess are common.
Most people (vegan to omnivore) can meet their needs by eating a wide variety of natural foods. Raw vegans and convalescents might find protein supplements helpful (e.g. Sunwarrior protein products, or Pulsin hemp protein). Pop into The Hopsack for more advice on what type of protein might suit you best.
Finally, plant protein is cheaper for us and cheaper for the planet. Compare the retail price of organic black beans to organic, grass-fed mince... Compare, too, the environmental price on one point alone (there are many). The methane cows donate to the atmosphere is a rampant greenhouse gas. Whereas, beans fix nitrogen (another greenhouse gas) into the soil, removing it from the air to nourish the earth. If you fear that beans + humans = greenhouse gas emissions, fret not.
Check out this guide to cooking beans (most of the practical stuff is near the end), paying special attention to soaking, rinsing, water pH, cooking methods and adding kombu. Once you get the hang of preparing and cooking pulses, you'll never again be in danger of turning a frat boy into an anagram of himself.