Role of Proteins
We need protein to grow and for repairs. Proteins are used to make our body cells. Specifically, they are used in the formation of new protoplasm. Antibodies, enzymes and hormones are also made of proteins. Finally, proteins can provide us with energy (although the body only breaks down protein when all the carbohydrate and fat is gone: in other words when you are starving).
Protein comes from the Greek word proteios, meaning "primary" or "holding the first place." A Dutch chemist Gerard Johann Mulder, coined the word protein in 1838.
Proteins are made from chains of amino acids. There are twenty different amino acids, so the cell requires a lot of information to put a protein together (which out of the 20 is the first amino acid in the chain, which is the second, which is the third etc). This information ultimately comes from DNA. A protein often consists of hundreds of amino acids linked together.
Food Sources of Proteins
Lean meat, fish, eggs, milk and cheese are important sources of animal protein. All plants contain some protein, but beans, nuts or cereals are the best plant sources.
Unlike carbohydrates or fats which can provide us with energy, proteins typically are used to build parts of the cell. In other words they are some of the raw materials the cell needs to make cells and tissues. When an excess of protein is eaten, the extra protein can be broken down into energy-yielding compounds. Because protein is far scarcer than carbohydrates and yields the same 4 calories per gram, the eating of meat beyond the tissue-building demands of the body becomes an inefficient way to produce energy.
Complete proteins are foods that contain all the necessary amino acids. Most animal foods, such as meat, poultry, fish, eggs, cheese and milk, are complete proteins. Some plant proteins are complete, too. Soybean products, such as tofu, are also complete proteins. Eggs are a good source of complete proteins.
Incomplete proteins are proteins containing small amounts of one or more essential amino acids. Most plant foods are incomplete, such as legumes (beans and peas), nuts, seeds, grains, and vegetables.
Although plant proteins are incomplete, it is still possible to get all the essential amino acids by eating a combination of plant proteins. For example, peanut butter is low in the amino acid methionine. Bread has a lot of methionine, but it lacks lysine and isoleucine. So a peanut butter sandwich becomes a complete protein.
Foods from animal sources contain complete proteins because they include all the essential amino acids. In most diets, a combination of plant and animal protein is recommended: 0.8 grams per kilogram of body weight is considered a safe daily allowance for normal adults.
Too much protein in the diet can be dangerous. The extra
protein contains nitrogen, which is changed in the liver into waste called urea.
The kidneys get rid of this nitrogen waste in urine. Too much protein can put
stress on the liver and kidneys. When extra urine has to be formed to remove
the excess waste, the body can be dehydrated. Too much protein can also make
one overweight, as the excess proteins are changed into fats in the liver which
is stored in the body.
Lack of proteins will lead to a weak body, unable to fight against diseases. Dieting can result in the body not getting enough nutrients. You may get enough calories for your energy need, but you do not have all the essential amino acids.
A lack of protein is virtually unknown in the diet in this country (so companies that promote protein drinks or amino acids supplements are wasting your money).
The deficiency of protein results in the disease kwashiorkor, which is typically seen in countries where starvation is a problem.
One of the symptoms of kwashiorkor is a swollen stomach, which ironically makes a child look well fed at first glance.