A reader writes (or rather a commenter comments):
One question which has always bothered me is, given that humans evolved on a hunter gatherer diet, and human metabolism is suited to burning ketones/fats for energy. Then why is it that we need blood glucose as an essential resource, and as a consequence insulin and all the metabolic machinery related to it. Why are vital body organs so dependent on glucose? Why evolve such a mechanism?
I was just reading how quickly saliva can break down starches, and if you continue down the digestive tract a significant amount is dedicated to providing humans with the capability of dealing with carbohydrates in some fashion. Why evolve such a GI tract if carbs were only part of, if at all, of your nutritional intake? And why is blood glucose so critical to human survival, too low and you can die.
I don’t have a clear answer to these questions and welcome your thoughts.
This is indeed a wonderful question; let’s tackle it.
First, a little very basic evolutionary science…
In the early days of life on earth the source of energy for the most primitive organisms in existence was the sun. These organisms captured the sun’s energy and stored it as glucose and starch (starch is simply a long chain of glucose molecules; a storage form of glucose, if you will) through the process of photosynthesis. Photosynthesis is the process whereby light energy from the sun, carbon dioxide and water combine to produce glucose and/or starch and discharge oxygen in the process. Early on the vast oceans contained plenty of inorganic oxygen acceptors, but ultimately the stockpile of these became oxidized. The oxygen from photosynthesis then began to be discharged into the atmosphere.
Once a substantial level of oxygen was available in the atmosphere organisms developed that could use this oxygen as electron acceptors for their own respiration, and, in effect, reversed the photosynthetic process. These organisms – the forerunners of us – were able to consume the energy stored in the glucose and starch of the organisms that operated photosynthetically and combine it with oxygen in the atmosphere and produce carbon dioxide and water. Photosynthesis in reverse. But the original energy source was still the sun, since the sun provided the energy to make the glucose and starch in the first place. Today herbivores eat plants, which obtain their energy from the sun, and carnivores eat herbivores. Omnivores eat both plants and animals (usually herbivores) so omnivores, herbivores, and carnivores all get their energy either directly or indirectly from plants, which get their energy from the sun.
Which means that we all eat the sun.
Since the most primitive organisms not using photosynthesis to obtain energy used the stored glucose and starch from those that did, it’s no wonder that we evolved with a primitive basic need for glucose as a sort of primary fuel.
But the fact that we don’t need to consume glucose and/or its storage form, starch, directly despite our requiring it for life is strong evidence that we had a meat-eating past. Let me explain.
Natural selection is a harsh taskmaster, one that rigorously weeds and trims systems back that aren’t required for optimal performance of the creature in question. If, for example, a specific organism requires substance X for life and there is no substance X in the environment of this organism, then the organism will have developed the metabolic machinery to convert whatever is at hand to substance X. If it doesn’t and it has no substance X, then it dies, so it has to have a means to synthesis substance X. The processes that synthesize substance X require energy, but it’s worth the expenditure of this energy because if there is no substance X made, there is no life. The energy used to synthesize substance X is then not available for other uses and the organism budgets accordingly.
Now let’s say that our substance X-requiring organisms migrate to an area that is rich in substance X. They no longer have to make it–they can simply consume it. But all the machinery is there within them to make it, consuming energy. Sooner or later somewhere along the way one of these organisms is going to be born with a mutation in its substance X synthesizing machinery. In the days before the vast fields of substance X were available that all these organisms are now feeding on, this mutation would have been fatal. But now it provides a huge metabolic advantage.
While all the other organisms are feeding on substance X and spending energy to run their own substance X-synthesizing pathways, the organism with the mutation can use that energy elsewhere giving it a powerful edge in the reproductive sweepstakes. Over time all these organisms will evolve to the point to which none of them have to waste energy on the internal machinery to make substance X. When scientists study these organisms later on, they will call substance X vitamin X because it is essential to life for these creatures and they can’t make it themselves.
Getting back to us, we realize that we have all the internal biochemical machinery to make glucose out of protein and out of the glycerol from disassembled fat (triglycerides), but we have no machinery to make amino acids (the building blocks of protein) and no machinery to make the essential fats. These essential amino acids and these essential fats have to come from our diet.
And we can to a certain extent replace glucose with ketones, which come from the partial breakdown of fat in the liver.
So, we need glucose for many cellular processes simply because of our primitive systems dating back millions of years that evolved when glucose was really the only food available. But we’ve evolved ways to make glucose out of fat and protein and evolved a method to replace some of the glucose by ketones, which are a fat by product.
What this should tell us is that over the recent past of our evolutionary history protein and fat have been readily available and glucose may not have been. Where do we get protein and fat? The main source for both is meat.
Obviously in our Paleolithic past (and before) we had plenty of meat and not much starch, otherwise we would have evolved differently. If we had evolved in a situation in which we had plenty of starch and no meat, we would have evolved a way to make protein out of carbs (which we can’t) and essential fat out of carbs (which we can’t). The fact that we are structured the opposite tells us the real story. And should lead us to reckon that if we evolved eating primarily meat and not much, if any, plants that we are fine tuned metabolically to operate optimally on such a diet. Which is the reason a low-carb diet works so well to reverse the diseases caused by eating in the reverse of our evolutionary heritage.
Because of this evolutionary heritage we have, I’ve always found it amazing as well as a little ignorant that educated, intelligent people tell us that we need to load up on carbohydrates, which we have the ability to manufacture ourselves, at the expense of fat and protein, which we don’t. As some of my rednecked friends would say: it just don’t make no sense.
Indeed it doesn’t.
Isn’t this a better way to deal with comments than answering them individually? MD just came over and read this post as I was writing it and said: “What you’ve been doing is writing a dozen posts per day, but they’ve been going up as responses to comments whereas they could have been going up as real posts that everyone reads. What a moron!”