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Food Protein Nourishes our Body as Amino Acids and Peptides ...

NEXT > Food Protein and Amino Acids
Are Essential Components of Our Diet

What foods contain protein?

Because protein is vital to life, all life forms will contain protein; however, the protein content will vary. In general, foods of animal origin will have greater protein content than plants and plant-derived foods (see Protein in Foods). Among the foods that have the highest protein content (percent of calories) are water-packed tuna and egg whites. Being an animal, tuna (and other fish) contain skeletal muscle for locomotion. Thus, eating finned or shellfish provides protein sources that are fairly similar to human skeletal muscle proteins. Meanwhile, the predominant protein in egg whites is albumin (e.g., ovalbumin and conalbumin) and ovomucoid, globulins and lysozymes. Another popular protein sources with this group, because of their protein density, is milk. The principal proteins in milk are caseins and whey, which are actually families of related proteins. Cereal grains produce a vast array of proteins (including albumins); however, the most interesting proteins may be gliadin and glutenin. When these proteins are mixed with water, such as when we make dough, gluten is formed. Gluten provides the structural basis for the network that traps gases produced by yeast when dough rises. Soy lacks these proteins, and ingredients need to be added to soy flour to make it rise to a light bread. Gluten continues to be a topic of interest as many people either experience an allergy or alleged intolerances to foods that contain it.


What are some foods with the highest protein content?

Egg whites, fish, lean meats, low fat milk are popular with people seeking concentrated protein sources such as athletes, bodybuilders and others weight trainers. For instance, water-packed tuna such as Albacore can have 80% of it calories from protein or 20 grams per 3 oz. serving. One 3-ounce steak of Yellowfin Tuna also has about 20 grams of protein, which is about 87% of the calories. Meanwhile, egg whites and many egg white products such as Egg Beaters are largely protein as well. Protein supplements also provide a concentrated protein source and typically deliver isolated protein sources or blends of sources. By and large these sources are whey, casein, and milk, soy and egg protein isolates and concentrates. Meanwhile additional plant-based and non-animal proteins are becoming more popular and include pea, rice, potato, hemp and spirulina. Protein foods and supplements are used for general diet enhancement as well as for muscle development and sport performance, weight management as well as for conditions and disease management.


How are proteins digested?

The goal of protein digestion is to disassemble proteins to their constituent amino acids and smaller peptides that can be absorbed. Protein digestion begins in our stomach as swallowed food is bathed in the acidic juice. In fact, the presence of protein/amino acids along with distension of the stomach causes stomach juice to ooze from glands in the wall of the stomach. The acid serves to straighten out the complex three-dimensional design characteristic of many proteins. Scientists refer to this as denaturing the protein or changing its natural 3-dimensional design. This will make it easier for protein-digesting enzymes in the stomach and small intestine to do their job. This is analogous to straightening out a ball of yawn so that you can cut small lengths. An enzyme called pepsin is found in stomach juice and begins to break the bonds between amino acids. The impact of pepsin is significant yet incomplete, as most of the bulk of protein digestion takes place further along in the small intestine. As partially digested proteins make their way into the small intestine, a battery of protein-digesting enzymes attack and break down protein into very small amino acid links and individual amino acids. Most of these enzymes come from the pancreas and include trypsin, chymotrypsin, carboxypeptidase A and B, elastase, and collagenase. These enzymes are made, packaged, and released by our pancreas in an inactive form. It is not till they reach the small intestine that these enzymes are activated by another enzyme produced by the small intestine called enterokinase (enteropeptidase). The reason for this complex system is to protect the pancreas and the duct that connect to the stomach from the protein-digesting activity of these enzymes.


How are amino acids absorbed?

Amino acids are taken up by the cells that line the small intestine, then move out of the backside of those cells and enter the bloodstream. Meanwhile, small peptides, consisting of just a couple or a few amino acids linked together can also be brought into these cells where final digestion to amino acids can take place. As a general rule, the absorbed form of protein will be individual amino acids. However, some peptides created by strategic digestion can be absorbed and are important in developing the immune system and support other beneficial actions. On the contrary, during infancy peptides resulting from inefficient digestion of certain foods are linked to many food allergies. Moreover, predigesting protein (referred to as “hydrolyzed”) can yield easier digesting proteins as well as beneficial peptides. Amino acids are absorbed into circulation, more specifically the portal vein, which delivers the amino acids to our liver. The liver removes a lot of amino acids from circulation. In fact it is typical for only about one-fourth of the absorbed amino acids to circulate beyond the liver. However, much of which makes it past the liver in the “first pass” will be the branch-chain amino acids (BCAA), namely leucine, isoleucine, and valine. This is because these essential amino acids are needed by our skeletal muscle to replace what was used for energy during fasting or exercise. Additionally these amino acids play a role in maintaining and developing muscle mass, which is important for weight lifters as well as people losing weight. Leucine in particular seems to have a special relationship with muscle by activating the key signally hub in muscle cells, called “mTOR” (mTORC1), that increases the production of proteins.


How are amino acids from the diet processed in the body?

Amino acids are always found in circulation, however this tends to represent 1% or less of total amino acids in the body. Said differently, roughly 99% of amino acids are components of proteins within cells, part of cell membranes and within the fluids that immerse cells and in circulations (e.g. blood and lymphatics). That means that after amino acids are absorbed and the level of amino acids increases in circulation, they are taken up by cells and are 1) used to make proteins, 2) used to make other molecules or 3) used for energy purposes. The fate really depends on the amino acid, the type of cell and the nutritional/metabolic state of the body. For instance, red blood cells don’t make proteins nor will they use it for energy or convert to other molecules. Meanwhile, muscle will largely use them to make protein while using a small portion for energy while the liver will do it all.


Do amino acids increase the release of insulin and other hormones?

The amino acids that enter our blood from our digestive tract evoke a release of insulin from our pancreas. However, the ability of elevated blood amino acid concentrations to cause the release of insulin is nowhere near as potent as elevated glucose. Regardless, the increased presence of circulating insulin will promote the uptake of amino acids in certain tissue, primarily muscle, as well as promote the building of new protein in muscle and tissue throughout our body. And, as mentioned in the previous chapter, the increase in insulin will also lower glucose levels. Thus, amino acids can have a glycemic lowering effect. In addition, insulin can assist in amino acid uptake into cells including muscle. The increase in the level of circulating amino acids after a meal can slightly increase the level of glucagon as well. Considering this, aren’t the actions of insulin and glucagon opposite, thus making this scenario counterproductive? Consider the following scenario. What if our sole source of food was wild game for a period of time? Having the effects of insulin and glucagon would allow the conversion of some amino acids to glucose in the liver to be released into blood to fuel other cells. Meanwhile the insulin would promote the formation of glycogen in muscle and help build preserve muscle protein and body fat if enough protein is consumed. All these functions would leave that person in better shape for enduring an extended period of time before they ate again. This certainly may have been the case for our distant ancestors when enduring winters or prolonged dry seasons when vegetation might not have been available. However today food protein is largely consumed with carbohydrate at meals and snacks, therefore lessening this whole application.


What happens to excess amino acids absorbed from the diet?

Amino acids from diet protein in excess of the needs of cells are not stored as protein. So, unlike fat, we do not store excessive diet protein as body protein. Instead our liver breaks down amino acids in excess of our needs and they can be used for energy. In addition, several of these amino acids can be used to make fat however this does not seem to be a strong drive when protein is responsible for the excess calories and especially if the person exercises regularly. However, when excessive protein is consumed with a high amount of carbohydrates then insulin can increase the efficiency of this conversion. The general weight gain that many inactive people experience is largely attributable to overconsumption of calories in general whereby more of the dietary fat is transitioned to body fat and some of the carbohydrate and protein can also be converted too, but less efficiently. This leads to high blood sugar as well as higher triglyceride levels in the blood.

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