Fundamentals of Human Nutrition/Protein quality
5.2 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.
5.2.1 Essential amino acids
Essential amino acids are amino acids that the body is incapable of creating given the body metabolism (Nicoteri, 2013). Therefore, the source of such amino acids is the diet. Foremost, Histidine is an essential amino acid that supports the development and maintenance of tissues in many body areas, especially the myelin sheath. Some of the plant sources of histidine include rye, wheat, and rice. another amino acid is valine, which is important for maximum repair and muscle growth by ensuring that the muscles endure during the body development processes (Shike, 2009). Sources of valine are spinach, beans, and hemp needs. Tryptophan, also given the name relaxing amino acid, is another essential amino acid. It is a neurotransmitter and also aids in keeping the nervous system and brain healthy. Some of the sources of tryptophan include seaweed, chia seeds, and spinach. Threonine is other crucial, essential amino acid. It helps in the body's wellbeing by galvanizing a healthy central nervous system, heart, liver, and immune system. For instance, it aids in the digestion of fatty acids to protect liver failure and buildup of fatty acids. Good sources of Threonine are figs, wheat, and avocados. Phenylalanine exists in three forms L-phenalynaline, DL phenalynaline, and D-phenalynaline. Phenylalanine converts into tyrosine after ingestion, which is additional amino acid for making brain chemicals, including the thyroid hormones. Its sources include almonds, avocado, and seeds. Methionine is an essential amino acid and it contains sulfur mineral, which supports bone cartilage production (Ballentine, 2010). It is equally important in muscle growth and creatine formation, which are required for maximum cellular energy. Its source includes chia seeds, oats, and legumes. Lysine maintains proper carnitine and hence lowers the level of cholesterol. Some of the sources of lysine are hemp seeds, chia seeds, and soy protein. Isoleucine is a unique form of leucine, which enables the body to produce hemoglobin and energy (Insel, 2013). Further, it propels the growth of nitrogen in the muscle cells. Its primary sources include quinoa, kiwis, and apples. Leucine is important for muscle growth and strength. Additionally, it regulates the blood sugar through insulin moderation after and before exercise. Some sources of Leucine include seaweed, peas, and pumpkin.
5.2.2 Limiting amino acids
Protein quality is determined by the essential amino acid profile of a certain protein. The more complete the protein is, the higher its quality. Limiting amino acids are the amino acids whose lack thereof inhibits proper protein synthesis and thus lowers the protein quality of a certain source. For example, lysine (an essential amino acid) is consistently found in much lower concentration in plant-protein foods than in animal foods and it is most likely to be the limiting amino acid in a diet based mostly on cereals (Young & Pellet, 1994). Modest amounts of legumes or animal proteins can offset lysine imbalance in such diets. Within plant proteins, soy protein is considered of better quality that corn protein. (Whitney & Rolfes, 2013). Corn is 9.4% protein and has a lysine score of 49 while soybean is 36.5% protein and has a lysine score of 115 (Young & Pellet, 1994). In general it is helpful to try consuming high quality proteins over low quality proteins. Higher quality can be described as a source of protein that has several, if not all, of the essential amino acids. The essential amino acids include Histidine, Isoleucine, Leucine, Lysine, Methionine, Phenylalanine, Threonine, Tryptophan, and Valine. Limitation of amino acids is also based on the quality of the protein one is consuming, such as digestibility and its ability to support growth. This can be linked to bioavailability, as some forms of protein may be more bioavailable than others. This is especially important for vegetarians or vegans. Although it is possible for vegans and vegetarians to meet all their protein needs it may come with the act of needing to take a wide variety of plant based proteins in order to consume all the essential amino acids. Complementary proteins, eating several food sources that contain different amino acids, are a great way to prevent limiting amino acids (Whitney & Rolfes).
When a diet is limited in even one amino acid the body’s entire process of protein synthesis is limited. Let’s say you were to build a wooden bucket to fill with water. If you ran out of wood and one side of the bucket was shorter than all the others, you can only fill the bucket as high as the lowest wall. The same concept is behind protein synthesis, explaining why limited amino acids can cause problems in several bodily processes (PROTEIN & AMINO ACIDS). Proteins (built by amino acids) play a role in transportation, acid-base regulation, hormonal activities, enzymatic processes, regulation of fluids and structure such as that of collagen in teeth or bone. To limit one or several essential amino acids can throw off an array of bodily processes (Whitney & Rolfes). Histidine, although produced in the body in small amounts, the quantities are so low that it is still considered an essential amino acid. Histidine is important in production of hemoglobin, tissue building, and strengthening of the immune system as a whole. Because of this, Histidine is especially important for someone healing from an injury or going through normal growth. Because of the amino acid’s link to immune function, lack thereof can lean to increased likelihood for infection and worsening of allergies. A lack of supply can be even more serious in the younger population, as children rapidly grow and Histidine aids the process. This can lead to stunts and disruptions in normal growth and development. Soybeans, chicken breasts, and beef are all excellent sources of this essential amino acid specifically (Amino Acid Studies).
Bioavailability refers to the digestibility of a protein. Proteins from animal sources have around 90-100% digestibility while plant proteins have 80-90% digestibility compared to the reference protein (egg) which has a perfect protein quality score (Young &Pellet, 1994). Animal proteins have a more complete amino acid profile along with higher digestibility which make them of higher quality, however, adequate intake can be achieved with strictly plant-protein sources.
5.2.4 Plant Proteins
Animal proteins are considered complete (containing all essential amino acids) while plant proteins are considered incomplete. Plant proteins have a more diverse amino acid content pattern but are usually missing one or more essential amino acids. Plant protein sources include legumes, grains, seeds, vegetables, and nuts. Those who follow a diet free of animal protein can get their amino acid needs by eating a wide variety of the above listed plant protein foods. A good strategy used all around the world is protein complementation, the pairing of two incomplete proteins to make a meal that contains a complete amino acid profile. A good example of protein complementation is a rice and beans meal, a staple of many Latin American countries where people may be less likely to be able to afford animal protein (Whitney & Rolfes, 2013). It is important to say that complementary proteins do not have to be present in the same meal or consumed at the same time as long as the average daily intake meets or exceeds the recommended amounts. Also, soy protein is considered of high nutritional value and can be used in combination with most cereal grains to improve the overall quality of the total protein intake (Young & Pellet, 1994).
Plant proteins, contrary to popular belief, are good sources for protein. The body does not require large amounts of protein in order to function. In fact, only 1 out of any 10 calories consumed needs to come from protein (Mangels, 2013). The body also does not need animal protein. Instead, the body requires twenty amino acids, nine of which are called essential because the body does not produce enough of them. They are histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. Our diet must provide these nine amino acids for our bodies. They are found in the protein we eat and serve as the building blocks for protein in the body. Without all twenty amino acids, a protein cannot be made (Whitney, Rolfes, 2002). Many, but not all plant proteins, provide all of the nine essential amino acids. The sources of food that produce all essential amino acids are considered high quality. Among them are quinoa, soybeans, and spinach. Other sources of plant proteins include black beans, chickpeas, peas, almonds, cashews, peanut butter, and broccoli. It is recommended that a person’s diet is varied, so that throughout the day, they are meeting all their nutrient needs, including protein. If they do this, they will most likely meet all of their protein needs without having to worry about missing out on a particular amino acid. This is important for those who only consume plant proteins, because if they consume a vegetable that only provides 7 of the 9 essential amino acids, then they will make up for it simply by snacking on cashews later on in the day. Furthermore, high protein diets are not advantageous for one’s health. Getting the right amount of protein is good enough. People who consume plant based diets typically get 10-12% of their kilocalories from protein. Those who are not on a plant based diet tend to consume around 14-18% (Mangels, 2013). Benefits of plant based proteins include the fact that they’re alkaline-forming. Our body’s pH is naturally in an alkaline state. Animal protein is acid-forming, and so when it is digested, the body must take minerals from the bones to sustain its alkaline pH. Meat consumption is also a source of saturated fat in the diet. High saturated fat intake is correlated with risk of heart disease. A plant based diet consists of more unsaturated fats and so lowers this risk. A final benefit of consuming plant proteins is that the environment isn’t used up in the production of plants as much as it is when raising animals. Animal meat requires the use of water, land, and energy to be produced. Plant production simply takes up less space and fuel (Brazier, 2013).
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