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Proteins are large chemical molecules composed of long chains of smaller molecules called amino acids. They play a crucial role in building and supporting the structure of the human body, as well as in the production of vitamins, hormones, various enzymes, and immune system cells. Proteins exhibit diversity in their structure, with four distinct levels of structure, namely primary, secondary, tertiary, and quaternary. To elucidate the role of proteins as a source of nutrition for humans, it should be noted that, for instance, cooking or heating food may leave the primary and secondary structures of proteins unaffected but can alter their tertiary and quaternary structures. In other words, cooked proteins, including vitamins, do not have the same nutritional value as raw ones, and this should be taken into serious consideration when planning a balanced diet.
The initial interaction of proteins with digestive enzymes occurs in the stomach, thanks to pepsins, which are proteolytic enzymes present in gastric fluid. In the stomach, a mixture of hydrochloric acid and pepsin enables the breakdown (dissolution) of proteins with long chains of amino acids into smaller fragments known as polypeptides. Digestion continues in the duodenum with pancreatic trypsin, another proteolytic enzyme, which further breaks down polypeptides into even smaller fragments called peptides. This process continues until the original structure of amino acids is achieved, regardless of whether they are of plant or animal origin. At this point, they are absorbed through the small intestine and transported through the portal vein to the liver for an initial assessment before being delivered to the tissues of interest through the bloodstream.
THE TWENTY AMINO ACIDS OF LIFE
Chromatography of protein extracts has demonstrated that the amino acids present in the natural proteins of all organisms, whether plant or animal, total twenty. In other words, life speaks only one language, which consists of thousands of words (proteins), constructed from just twenty letters (amino acids). Just as in a language, we have short and long words, these twenty amino acids can form a variety of protein chains of different lengths, which can contain a few tens or even hundreds of these twenty amino acids. There are proteins with nearly identical amino acid sequences that perform entirely different functions thanks to the discerning ability of the central nervous system, the endocrine system, and the immune system, which can distinguish even the slightest differences in their structures, down to the tertiary structure.
Among the twenty amino acids that compose protein chains, ten are considered non-essential, not because they are not necessary in the organism, but because we do not need to obtain them directly from our diet. This is because our bodies can synthesize them from other substances. On the other hand, eight amino acids cannot be synthesized by our bodies and must be acquired through our diet; they are therefore referred to as essential or basic amino acids. Lastly, two amino acids, histidine and arginine, are semi-essential because they are necessary in neonates during growth but not in adults. Rich sources of histidine, arginine, and the other eight essential amino acids include grains, legumes, nuts, and oilseeds.
There is no difference between amino acids of plant or animal origin in biochemistry. For example, lysine found in animal proteins is exactly the same as lysine found in plant proteins. Lysine is lysine, period. This fact debunks the myth that animal proteins are superior to plant proteins and discredits the notion of "noble" proteins. It also puts an end to the fallacy that babies must necessarily consume meat (of animal origin) or dairy products!
Indeed, the theory that we require meat to prevent protein deficiency is a blatant and intentional inaccuracy. Millions of vegetarians and vegans have been found to have better health than the general population!
Our bodies are essentially protein factories. The DNA and RNA in our cells encode and translate proteins for the survival of our organisms. The source of these proteins is completely irrelevant. I would like to ask if you prefer proteins from a steak (with around 20% protein content but also high in saturated fats, which clog our arteries) or a serving of legumes (with a protein content of about 23% and minimal fat content)?
For instance, beef contains approximately 20% protein because the cow developed its muscle tissue by consuming plant proteins (and, in fact, when it was once fed with animal proteins, it contracted bovine spongiform encephalopathy or mad cow disease). This clearly demonstrates that the plant world can and does provide us with the necessary proteins and amino acids!
Carnivorous animals feed on herbivores, while fish and marine mammals feed on other fish, which, in turn, feed on algae and plankton, namely, marine plants rich in proteins. Once again, it is confirmed that life on our planet relies on plants. Let's consider that we are not lions or tigers (carnivores), even though we might like to be, and we are not pigs (omnivores), although we increasingly resemble them!
**A special thanks to the Italian scientist Michele Riefoli, whose work inspired this endeavor.
Gerassimos Tsiolis, PhD in Biochemistry
University of Bologna, Italy