Fundamentals of Human Nutrition/Protein quality
Foods of plant and animal origin contain different nutrients like proteins, but differ in the type and proportion of nutrients in them. Protein quality refers to the presence of all essential amino acids in the appropriate quantity in the proteins of food. A food protein that contains all the eight essential amino acids (nine for children) in the right proportion is termed a complete protein. On the other hand, a food protein that lack one or more of the essential amino acid is called an incomplete protein.
Hence, this section examines the concept of protein quality and a survey of plant food combination diets of some selected group of people in Abuja (Nigeria).
Essential amino acids: Proteins are generally essential in that they cannot be made from fat or carbohydrate, as a result of their component nitrogen that is not present in either fat or carbohydrate molecules. About 20 naturally-occurring amino acids make up the various proteins required by the human body1. Among these, some are essential while the rest are not. There are eight amino acids (nine amino acids, in the case of children) that the human body cannot synthesize. These have to be supplied through diet, and so, are called essential amino acids. They are methionine, valine, leucine, iso-leucine, threonine, lysine, tryptophan and phenylalanine. For children, the additional amino acid is histidine2. Newborn babies may also need a particular amino acid called taurine)3. Most foods of animal origin contain all the essential amino acids in the right amount, in contrast to foods of plant origin that usually lack one or more adequate quantity of the essential amino acids.
Limiting amino acids: Plant foods are cheaper sources of nutrients for man, especially in the developing world. However, foods from plant are low in some specific essential amino acids such as lysine, methionine and tryptophan. Such amino acids that are present in low proportion limit the functioning of the proteins, and are so called limiting amino acids of those particular proteins. Generally proteins in cereal like corn are limited in lysine, while pulses like beans are limited in methionine. Hence, the practice of various food combinations is necessary to ensure that those who depend largely on plant-based foods get complete protein diets. This practice of combining different food proteins is termed complementation; the resulting proteins are complementary proteins.
Bio-availability of Protein: Just as not all rays that strike a solution are all absorbed by the solution in Physics, similarly, not all nutrient intakes are utilized by the body. Some dietary intakes pass through the body unchanged and go out with as waste matter; others are digested but may not get as far as the circulatory system (bloodstream) while others get digested and taken up into the blood for utilization in the various tissues that require them. Bioavailability of protein is a measure of how easily the body (under normal conditions) can absorb the protein in food for its various functions in cells and tissues. Bioavailability is not just only a measure of essential amino acids, but it is also determined by the structure of the protein4. This refers to the ease of being broken down into its component amino acids and dipeptides. This amenability differs among different proteins, depending on the complexicity of their structure (folding). Thus, proteins that are easily unfolded and broken down are more available to the body than complex proteins that are not readily completely digested. A third factor that influences protein bioavailability is the presence of other molecules (such as carbohydrates and lipids) that usually accompany proteins in nature. Protein bioavailability is usually calculated by using different measures, each giving an incomplete contribution from different angle. According to a website4, these measures include Biological value, Protein Efficiency Ratio, Net Protein Utilization, Kjeldahl method and Protein Digestibility Corrected Amino Acid Score (this is the mostly used measure).
Survey on protein quality: A survey was conducted to determine the cultural food habits, with respect to complementary proteins consumption of some groups of people in Abuja, Nigeria. A group of randomly selected men and women from a few communities were administered a questionnaire to gather information on their tribes, cultural food combinations and frequency of consumption of their listed plant-based complementary protein diets. The sample was composed of people from tribal/ethnic groups like Hausa, Igbo, Yoruba, Ebira, Edo, Tiv, Boki and Eggon. The result of the survey indicated that frequently consumed food combinations of these people include corn and pea, millet gruel and beans pudding, tuwo (cereal meal) and soup, rice and beans, wheat bread and cocoa beverage, beans and corn, wheat and corn, plantain and beans, etc.
Hence, it was concluded that normal cultural food habits of the respondents involve regular consumption of complementary proteins foods.
1. Murano, P.S (2003). Understanding Food Science and Technology. Australia: Thomson learning Inc.
2. Ihekoronye A.I. and P.O. Ngoddy (1985). Integrated Food Science and Technology for the Tropics. London: Macmillan Publishers Ltd.
3. King F.S and A. Burgess (2000). Nutrition for Developing Countries. Great Britain: Oxford University Press.
4. Baseline Nutritionals (2008). Retrieved from www.nutribobyprotein.com on 11th March, 2013.