Obligatory disclaimer: I am not a medical doctor, and the content of this website was created for informational purposes only. Such content is not intended as a substitute for medical advice, treatment or diagnosis.
In previous posts we’ve talked about the starvation response (aka starvation mode), metabolic adaptation (aka metabolic damage), the thermic effect of food, and thermogenesis. We’re currently running a series on macronutrients, and in last week’s post we paid special attention to carbohydrates, focusing on their classification, how they are digested, and their food sources. In this week’s post we’ll do the same thing for proteins. Just like before, given the sort of content that we’ll be dealing with in this post, rather than rely on scientific papers, we’ll use the following references:
- Gropper, S. S., & Smith, J. L. (2012). Advanced nutrition and human metabolism. Cengage Learning.
- Wildman, R. E., & Medeiros, D. M. (2014). Advanced human nutrition. CRC press.
- Hankey, C., & Whelan, K. (Eds.). (2018). Advanced Nutrition and Dietetics in Obesity. John Wiley & Sons.
What are proteins?
You know how we’ve been calling things like the starvation response, and metabolic adaptation by their I-need-to-sell-you-something-so-I’ll-give-you-something-to-fear-names (ie. starvation mode and metabolic damage)? Well, it’s a shame that protein never fell victim to that, because if it did, it would’ve been called Pro-tein, Brotein, Too-cool-for-you-tein, or something like that.
Protein is probably the closest thing we have to unobtanium, a material with extraordinary, almost magical, properties. See, protein can be found in most of our tissues, like our muscles, organs, skin, and blood. Protein’s importance is due to it being made up of amino acids (organic molecules made up of an acid group, an amine group, and a specific side chain group), which our body requires to manufacture its own proteins and other molecules that support life. We’ll follow the classification of proteins provided by Gropper and Smith, which is based on their biological functions.
Biological functions of proteins
- As catalysts. A catalyst is any substance that increases the rate at which a chemical reaction occurs, and enzymes are proteins that act as catalysts. In our body, such reactions are necessary to sustain life, and they take place within our cells. Examples of the processes in which enzymes take part are digestion, energy production, neuromuscular contraction and blood coagulation. Sometimes, in order for the reaction to take place, a cofactor (also known as coenzime) is needed, minerals like copper, iron, and zinc are examples of cofactors.
- As messengers. Hormones are our organism’s chemical messengers, and some proteins are hormones. In our case, hormones are made in endocrine glands and released into the bloodstream, which then transports them to their target organs. Hormones tend to be regulators of metabolic processes, either by modifying enzymatic activity or the synthesis of enzymes themselves.
- As structural elements. This is the function that you probably already know. As expected, proteins can be found in cardiac, skeletal (voluntary), and smooth (involuntary) muscles. However, there are other proteins, like collagen, elastin, and keratin that can be found in other tissues, like hair, bone, teeth, skin, tendons, cartilage, blood vessels and nails.
- As immunoprotectors. Part of our organism’s immune system uses immunoproteins (also known as immunoglobulins or antibodies), and the way they work is by sticking to antigens (foreign substances, like viruses or bacteria) and inactivating them. What follows is that the immunoglobulin and antigen structure is identified and destroyed through chemical reactions involving other proteins or cytokines.
- As transporters. Proteins that act as transporters combine with the substances they transport (like nutrients, vitamins and minerals) and then provides a mean of transportation within a given cell, into or out of cells or by carrying the substances in the bloodstream. There are several examples of transport proteins, but I think the clearest one is hemoglobin, which helps transport oxygen and carbon dioxide in red blood cells.
- As buffers. As we covered before, proteins contain an acid group in their structure, and so they can help in maintaining acid-base balance as buffers. Buffers are substances that reduce the impact of a pH change due to the addition of an alkali (basic) or an acid into a given solution. Different tissues and organs of our body must be kept within different pH values, and the presence of proteins in those tissues helps maintain the proper pH balance within them.
- As fluid balancers. The idea is that the presence of proteins in the bloodstream or within cells helps to attract water and maintain the correct osmotic pressure (so that our cells don’t shrink or swell). Because of this, the proper concentration of proteins must be kept in blood and in cells.
Now that we are aware of the importance of protein in sustaining life, let’s take a look at how proteins are digested by our bodies.
Unlike what we saw with carbohydrates, there is no appreciable digestion of proteins either in the mouth or in the esophagus, and real protein digestion does not begin until reaching the stomach. Due to the presence of hydrochloric acid in the stomach, proteins are broken down, leaving polypeptides (a long chain formed by more than 20 amino acids), oligopeptides (2 to 20 amino acids) and free amino acids as products. The compounds are then transported to the small intestine for their further digestion.
Once in the small intestine, the release of regulatory hormones and peptides signals the secretion of digestive proenzymes which will aid during digestion. Those proenzymes are secreted by the pancreas and once they reach the small intestine they are converted into their respective enzymes. The different enzymes produced are responsible for the hydrolization of polypeptides into oligopeptides and tripeptides, as well as the release of individual amino acids from the polypeptides. As this process goes on, the end result of protein digestion is the production of peptides (mostly dipeptides and tripeptides) and free amino acids. Most of these substances are then absorbed across the intestinal wall in order for the organism to use them. Of those amino acids that are not absorbed, they are used by intestinal cells to synthesize proteins and other compounds.
Just as we will cover carbohydrate metabolism in later posts, we’ll do the same for proteins, because there is a huge amount of ground to cover.
Sources of protein
Our body requires twenty different amino acids, and nine of those cannot be synthesized by our organism, so they must come from our diet and we call them essential amino acids. The remaining eleven amino acids can be synthesized by our body with compounds it already possesses, and so they’re called nonessential. In addition, proteins can be complete or incomplete, and such classification is dependent on whether those proteins contain all essential amino acids or not. Proteins from animal sources tend to be complete while those originating from plants tend to be incomplete.
Among animal sources of protein we can rely on meat, poultry, seafood, eggs, and dairy. On the other hand, plant sources of protein are legumes (like beans lentils and peas), and soy products like tofu and tempeh. Proteins can also be found in nuts, seeds, edamame, and grains. Of course, chances are that you’ve also heard of protein bars and powders.
So now we know a little more about one of the macronutrients: proteins. We have an idea of what they are, how they are digested, and where we can find them. Next week we’ll deal with fats / lipids. Hope you enjoyed this week’s post, and see you next week!
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