The precise ionic calibrations that facilitate cell life have not changed substantially since the beginning of life itself. Even now, our bodies retain salt in times of scarcity and excrete excess salt when we don’t need it. This ability to regulate the amount of salt in our bodies and to seek it out in times of need has allowed us to survive and thrive in almost every type of geographical region in the world-but, in essence, our blood still reflects the ancient ocean where life began and from which it evolved.
Compared to the dramatic changes in the form, structure, and function of organs that occurred during vertebrate evolution, the fact that the electrolyte makeup of the extracellular fluid has generally remained constant suggests that salt balance is an evolutionary ad-aptation. This adaptation remains tightly regulated for sustaining life for all vertebrates, including marine and freshwater fish and turtles, reptiles, birds, amphibians, and, yes, mammals. That fact is foundational to the theory that all animals-including humans are thought to have evolved from creatures that originated in the ocean.’
Once sea invertebrates developed a closed circulatory system, they would have needed to evolve organs called kidneys to help them re-absorb and excrete salt and water (among other things). Until then, the salty ocean would have been integrated into the invertebrate it-self. From an evolutionary perspective, then, the kidneys likely first evolved in the sea and would therefore consider salt a friend, not an enemy. This fact seems to be lost in our current debate about optimal salt intake.
An organism’s ability to retain and excrete salt is critical in order to provide the proper cell function and hydration that sustains life. There is no better example of this than fish that are able to live in both freshwater and saltwater. Most of these fish can actively reab-sorb or excrete sodium via their gills, allowing for drastic environmental change in saltiness.”° The gills of these fish serve much like the kidneys of a human, reabsorbing or excreting sodium depending on whether they have too much or too little salt in their body, thereby helping to maintain normal electrolyte and water balance. Another evolutionary adaptation to maintain salt and water homeostasis is the heavy armor-plating seen in freshwater reptiles. This adaptation allows maintenance of normal electrolyte and fluid balance, as the shell counters the drastic difference in osmotic stress of living in a freshwater environment-where the concentration of salt is much less than that of blood.”
Despite significant changes in the saltiness of the animals’ environments, their organs continued to evolve in order to maintain normal salt concentrations, and hence water balance, in the blood, no matter where their travels took them-even as they took those first critical slithers onto land.
Tetrapods, the first four-limbed vertebrates, are thought to be the last common ancestor of amphibians, reptiles, and mammals. These animals were first able to leave the seas by swallowing air into their gut.” Once these creatures were on land, their kidneys had to adapt from living in the salty environment of the sea to one that was relatively salt-scarce.
While there are many theories about the origin of land-based animals and the rise of vertebrates from invertebrates, our kidneys and our salt cravings are big clues that we more likely evolved from marine animals rather than freshwater animals. 3 If we did come from the sea, the evolutionary ability to retain sodium would have been a requirement, one that allowed the maintenance of blood pressure and circulation of blood through the tissues once on land. These animals, once bathed in salt water, were now faced with the relative salt scarcity of the desert, rain forest, mountains, and other non marine environments. Thus, not only was it important to retain salt, but a “hunger” for salt would have evolved in these animals to ensure that their needs were met. This “hunger” would provide a physiological signal—an appetite-to seek out salt whenever a deficit was on the horizon. Their brand-new closed circulatory systems would give them an enhanced ability to maintain sodium and water homeostasis, mostly due to the evolution of the kidneys, bladder, skin, intestines, and other endocrine glands not present in ancient marine invertebrates.
In the animal kingdom, there are no dietary guidelines, of course—no medical directives to create a conscious effort to restrict salt intake. Indeed, many animals (especially those hunting in the sea ingest large amounts of salt simply as a matter of course during their daily lives. Take, for example, reptiles, birds, and marine mam-mals, such as the sea lion, sea otter, seal, walrus, and polar bear, that hunt prey living in the ocean. These animals take in large amounts of salt, both from the animal itself and from salt water, during a kill, particularly if they eat oceanic invertebrates, which have the same salt concentration as the ocean.. For these marine mammals, the salt content of their blood is not very different from that of terrestrial mammals?-and since they are ingesting sea water, which is four to five times as salty as their blood, that salt must be excreted via their kidneys.
This basic physiology of the kidneys is the same in humans. In fact, research has shown that patients with normal blood pressure and kidney function can easily excrete ten times as much salt as we normally consume in a day. & The reason why humans cannot solely live on seawater is not that our kidneys cannot handle excreting the high salt content—it’s that in order to do so, water must leave with it, which would eventually cause dehydration (and eventual death!).
But if we had enough access to freshwater to replace what is lost during the excretion of that salt, humans would absolutely be able to drink seawater. Almost without exception, salt and water regulation is a well adapted survival mechanism for nearly all animals-and this includes all primates, including humans.
Source : The Salt Fix: Why the Experts Got It All Wrong–and How Eating More Might Save Your Life by James DiNicolantonio
Goodreads : https://www.goodreads.com/book/show/30555572-the-salt-fix
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