The word protein comes from the Greek word "protos" meaning first. Proteins are essential for growth and repair, the functioning and structure of all living cells. Hormones such as insulin, control the level of blood sugar, enzymes such as amylases, lipases and proteases, are crucial for the digestion of food, the antibodies help fight infections, muscle proteins allow contraction, etc. Therefore, the proteins are indeed essential to life.
Amino acids, parts of a whole
The proteins are composed of amino acids, building blocks, which are linked together. A normal protein may contain 300 or more amino acids. Each protein has a number and a specific sequence of amino acids. The shape of the molecule is important because it usually determines the function of the protein. There are about twenty different amino acids that are commonly found in plants and animals.
Amino acids are classified as essential (essential amino acids that the body does not produce and must be obtained through diet) or non-essential (non-essential amino acids that the body can produce endogenously from other proteins). There are eight amino acids considered essential for adults: leucine, isoleucine, valine, threonine, methionine, phenylalanine, tryptophan and lysine, and nine for children (the above plus histidine).
When a protein contains essential amino acids in the amount necessary for human beings, is said to have a high biological value. When one or more essential amino acids are present, but in insufficient quantities, it is said that this protein has a low biological value. The amino acid that is present in fewer than necessary is called "limiting amino acid."
Cycle of proteins
Our body continually produces and removes protein. After eating, the proteins are broken down during digestion into amino acids, which are absorbed and used to produce other proteins in the body. A daily intake of adequate protein and energy ensures that this cycle continuing.
The proteins are found in various foods. The animal sources of protein such as meat, fish, eggs, milk, cheese and yogurt, provide proteins of high biological value. Plants such as legumes, cereals, nuts, seeds and vegetables provide little biological value protein. However, as is usually the limiting amino acid in different proteins from different plants, the combination of different plant sources in the same food (vegetables and cereals) is often a mixture of higher biological value. These combinations are often found in traditional recipes from every continent (for example, beans with rice, pasta and cassava, chick peas with bread and potatoes with lentils, etc.).
Omnivorous diets (with foods derived from animals and plants) in the developed world provide adequate amounts of protein. However, subgroups of the population who consume no animal products may have difficulty meeting their protein needs.
Vegetarian diets and intake of proteins
Vegetarian diets are based on cereals, vegetables, fruits, legumes, seeds and nuts, and the removal of meat and fish. Within vegetarian diets, there are variations, as some include dairy products and eggs, other dairy products but does not include eggs, whereas stricter vegetarian diets exclude all animal products. Subsistence crops, in particular, have no major source of protein of high biological value, so those who follow such diets may have difficulty meeting their protein needs, especially in case of additional requirements due to growth (children and pregnant). Therefore, in the case of plants, the combination of proteins from different plant sources and a balanced selection of foods are very important to ensure the necessary levels of essential amino acids.
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To maintain a normal renewal of the proteins needed for proper growth and repair of body tissues, 10-15% of our total intake of energy should come from protein.
1. Dietary Reference Intakes for Energy, Carbohydrates, Fiber, Fat, Protein and Amino Acids (Macronutrients) 2002. Chapter 10: Protein and Amino AcidsNational Academy of Sciences http://www.nap.edu
2. Reeds, P. Dispensable and Indispensable Amino Acids for Humans. J. Nutr. 130: 1835S – 1840S, 2000.
3. WHO, Diet, Nutrition and Prevention of Chronic Diseases, 2003
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