The word protein comes from the Greek word “protos”, meaning first element. Proteins are essential elements for growth and repair, good functioning and structure of all living cells. Hormones, such as insulin, control blood sugar levels; enzymes, such as amylases, lipases, proteases are crucial for digestion of foods; antibodies help us fight infections; muscle proteins allow contraction, etc. So, indeed proteins are essential to life!
Amino acids, the building blocks
Proteins are made up of amino acids, the building blocks, linked together. A typical protein may contain 300 or more amino acids. Each protein has its own specific number and sequence of amino acids. The shape of the molecule is important as it often determines the function of the protein. There are about twenty different amino acids commonly found in plants and animals.
Amino acids can be classified as either essential (indispensable amino acids that cannot be produced during metabolism by the body and therefore must be provided by our diet) or non-essential (dispensable amino acids that can be produced endogenously in the body from other proteins). Eight amino acids (Leucine, Isoleucine, Valine, Threonine, Methionine, Phenylalanine, Tryptophan and Lysine) are considered essential for adults and nine (those mentioned above plus Histidine) for children.
When a protein contains the essential amino acids in the right proportion required by humans, we say that it has high biological value. When one or more essential amino acids are present in too short amount, the protein is said to have low biological value. The amino acid that is in shortest supply in relation to need is termed the limiting amino acid.
The protein cycle
Proteins in our body are constantly being built and disposed of. After we eat, proteins are broken down by digestion into amino acids. Amino acids are then absorbed and used to make other proteins in the body. Adequate protein and energy intake, on a daily basis ensures the cycle continues.
Proteins are found in different foods. Animal sources of protein, such as meat, poultry, fish, eggs, milk, cheese and yogurt, provide high biological value proteins. Plants, legumes, grains, nuts, seeds and vegetables provide low biological value proteins.
However, as the limiting amino acid tends to be different in different vegetable proteins, combination of vegetable sources of proteins in the same meal (e.g. legumes or pulses with cereals), results often in a mix of higher biological value. These combinations are generally found in traditional culinary recipes from the different continents (e.g. beans with rice, pasta or manioc, chick-peas with bread, lentils with potatoes, etc).
Omnivorous diets (containing foods derived from animals and plants) in the developed world provide adequate amounts of protein. However, subgroups of the population who avoid all foods of animal origin may have difficulties in meeting their protein requirements.
Vegetarian diets and dietary protein supply
Vegetarian diets are based on grains, vegetables, fruits, legumes, seeds and nuts, with the elimination of meat, fish or fowl from the diet. Variations exist within vegetarian diets, where some include dairy products and eggs (e.g. lacto- ovo vegetarian diets), others include only dairy products but not eggs (e.g. lacto-vegetarian diets), while the strictest vegetarian diets include no products produced by animals (e.g. vegan diets). Vegan diets in particular, may lack the main sources of high biological value proteins and people following these diets may have difficulty meeting their protein requirements especially to support extra needs due to growth (e.g. children and pregnant women). Therefore in the case of vegetarians, vegans in particular, the combination of proteins from different vegetable sources and a balanced food choice are very important to ensure that required levels of essential amino acids are attained.
In order to be able to maintain the normal protein turnover, necessary for proper growth and repair of body tissues, 10-15% of our total energy intake should come from proteins.
- 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
- Reeds, P. Dispensable and Indispensable Amino Acids for Humans. J.Nutr. 130: 1835S – 1840S, 2000
- WHO, Diet, Nutrition and Prevention of Chronic Diseases, 2003.