June 8

Why do we need glucose?

12  comments

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!”
I agree.


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  1. Excellent post again Dr. Mike. And it included one of my favorite topics, evolutionary biology. I am sure your explanation gets your point across but it somewhat misrepresents how evolution works. You write, “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.” A more accurate way to explain it would be to say that the species with the machinery to convert whatever is at hand to substance X survive and he others don’t (survival of the fittest). Those with out the machinery won’t develop it, they just die, those with it are predisposed to survive in an environment with a limited supply of substance X. They are the fittest and they survive.
    The opposite also holds true. We DON’T need our earlobes (or at least we have found no use for them) yet evolution has not driven them away. They persist, just hanging there begging to be pierced. Snakes have no legs, yet they have a pelvis. Appendix?
    I don’t know if I explained it any better and it took Darwin a million examples to describe it in his book but there is a subtle difference in the way evolution is explained. Your way certainly worked well here. So I am sorry to nit-pick but it is a pet peeve of mine (I hope nit-pickers aren’t a pet peeve of yours).
    Nope.  Feel free to nit-pick at will. 

  2. David LaCivita wrote: I don’t know if I explained it any better
    You didn’t. The Doc’s way was fine. But you did get in print.

  3. Dear Dr Mike:
    I need some help with the Protein Power LifePlan long term, and I guess a comment on this wonderful piece is as good a place as any to post it.
    Myself, my husband and our daughter have been on the lifeplan for three and a half years now. We are finding weight maintenance harder and harder. We have cut out dairy products– the high fat cheese, sour cream and coffee cream. We eat more chicken breast, fish and low fat pork and beef. We usually grill or steam the meats. We eat lots of vegetables. Our diet is organic, with grass fed beef. Our alcohol consumption is very modest, and has gotten to the point of a glass of dry red wine per day. Essentially, we eat at the Intervention Level most of the time. We have pretty much stopped “honey tree” days except for Christmas and birthdays. Honey tree days make us REALLY sick. we don’t eat “low carb” breads and the like. We all run and do slow burn weightlifting. Unless we are working hard, i.e. putting up firewood, doing home construction, we slowly but surely store fat. What do we need to do to curtail this? Obviously, ketosis, the efficiency or lack thereof, must be part of this problem. None of us is fat or over weight really, but the 5 pound creep happens. You and MD have been on this so long you must have dealt with this too. any help would be appreciated. robyn
    Hi Robyn–
    MD and I stay on a pretty much intervention level most of the time.  But the 5 pound creep happens, even to us.  When it does, we do one of the following depending upon our mood and circumstances: we turn to an all-meat diet, which is incredibly effective; we turn to a few shakes per day and one meal a la Thin So Fast; or we fast intermittently, eating a low-carb diet on the eating days.
    These regimens work well for us.  We lose the few pounds we’ve picked up and go back on our regular low-carb fare.
    Hope this helps.
    Cheers–
    MRE 

  4. Good post and thank you for your dedication. A little remark about glucose that should not be forgotten. Glucose metabolism (anaerobic) is much, much faster than fat metabolism and is therefore very important in an environment where a danger can appear anytime. The little glucose that is stored in glycogen is sufficient for a sprint here or there and increases the chance of survival of a being capable of it. It is also interesting to see that the body will never completely empty its glycogen reserves even in severe starvation (see Lutz).
    In my opinion, glucose is necessary for short intensive efforts, for any other case, free fatty acids and ketones are used. On oversupply of glucose, the body will try its best to get rid of it without waste, so it will use as much it can in aerobic glucose metabolism and will make stock of the rest.

  5. To nit-pick yet again…the first microorganisms on the earth almost exclusively did NOT engage in photosynthesis. This did not evolve until one to two BILLION years after primitive life got is start.
    When photosynthesis finally did evolve, the oxygen it polluted the sea, and eventually atmosphere, with wiped out most life on earth, including many of the photosynthetic organisms.
    I don’t believe that, even after small percentage of life able to survive in an atmosphere full of oxygen pollution had spread, the photosynthetic ones were ever “most” life on earth.

  6. I was interested in Robyn’s question – because I’ve also been addressing the five pound creep. I’ve asked the question on the discussion forum to see what other people have found. It’s in the “Staying Power” section for those who want to check it out – or join in the discussion.

  7. I ended the 5-lb creep by eating MORE fat (and 300 more calories), not less, while keeping to 30g ECC and avoiding grains & high casein dairy. This route has made it easy to maintain w/no cravings.

  8. Hi Mike,
    Thank you for a superb post. It has clarified my own construct of human metabolism. I have been low carbing for 5 years now and my own experience is that the even on a diet of 30-60 grams of slow carbs, I feel a little sluggish, but on a virtual zero carb diet I feel much better.
    I don’t know, but despite everything I have learned over the past 5 years, there remains an inherent fear or confusion, that surely I need some vegetables everyday, I can’t eat all this fat and protein, can’t be good for me.
    Reading the comment from deirdra, that if you eat more fat, you lose more weight and stop carb carvings is very true. The extra fat seems to signal the mitochondria to burn it more efficiently including stored fat. But we still have this societal conditioned fear that all this fat (and heaven forbid, saturated fat) can’t be good for you. I still struggle with this most days.
    The picture of your bougainvillea is absolutely beautiful. Once again thank you for all you do.
    Best regards,
    Tahir.
    Thank you for the great question that inspired the post.

    Cheers–
    MRE 

  9. Thank you for this terrific explanation – very clear and comprehensive!
    Also, I’m intrigued by the all-meat diet. I’m finding low carb not effective enough in terms of weight loss and am curious if I’ll finally see a difference with all meat, all the time … I guess I’ll have to experiment and see.
    Hi Gazelle–
    Please keep me posted on how you do if you decide to try the all-meat diet.
    Best–
    MRE 

  10. So is it still necessary to eat carbohydrates? I know we still need the necessary essential nutrients like vitamins/minerals, which would be from vegetables but don’t vegetables have carbohydrates? So would you be perfectly fine (unless you had a disease, etc.) if you ate a high-protein/high-fat/no-carb and vegetables?

  11. Here’s an answer for Kevin, based on my understanding… (feel free to correct/append)
    The non-starchy vegetables do provide phytochemicals and we may benefit from them. We do not need to eat carbohydrates as the liver is able to create the necessary carbs using protein (and partially fueled by ketones). Vegetables aren’t a necessity, it was observed in the early Inuit, although they ate organ meats which have a lot of vitamins and minerals.).
    Even in disease, this would typically be the way we subsisted (with few if any carbs, usually not enough to bypass the fat/ketone metabolism). Remember the body may go into repair mode when food is scarce (why intermittent fasting may be beneficial).
    Eating carbs when one has a disease may be more detrimental than beneficial due to glucose swings. Another example of detriment is in the case of cancer, where it is observed that tumor cells are fueled by glycolysis (a process that requires a lot more sugar than most cells). The hypothesis is that while a majority of cells adapt to lypolysis/ketones, the liver produces only sufficient glucose for the tissues that require it, and no more (starving off the cells that use glycolysis).

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