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Protein Foods, Digestion, Stomach Acid (HCl), Digestive Enzymes, Absorption, Processing, Insulin, Glugagon

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What are some foods with the highest protein content?

Egg whites, fish, leaner 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 concentrate protein source and are extremely popular with athletes and fitness enthusiasts. Protein supplements provide isolated protein sources or blends of sources. By and large these sources are whey protein isolate and concentrate, casein isolates, milk protein isolates, soy protein isolate and egg white isolate (e.g egg albumin).


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 intestinal 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 will take place. Therefore, as a general rule, the absorbed form of protein will be individual amino acids.

Fragments of proteins can also be absorbed and are important in developing the immune system during infancy as well as are linked to many food allergies reactions.

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, much of which will be the branch-chain amino acids (BCAA), namely leucine, isoleucine, and valine. This is probably 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.


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

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.

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 while insulin would promote the formation of glycogen and muscle protein as well as promote the production and storage of some fat if enough protein is consumed. All of these efforts 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.


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 several of these amino acids can be used to make fat. Insulin promotes this process of making fatty acids from excessive amino acids. However, the conversion of excessive amino acids (like carbohydrate) to fat is not as efficient as was once thought and it turns out that more of the excessive amino acids will be used for immediate energy.


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