Faulty Engines (The False Debate on Glucose Metabolism)

Imagine you own a hybrid car that runs on two types of fuel: Its primary fuel source is gasoline, which is the default fuel source for the engine, transmission, onboard computer system, and many other parts of the car. Its secondary fuel source is electricity, which it uses in concert with gasoline to fuel the rest of the car.

When the gas tank gets low, the car automatically begins using more and more electricity to fuel the engine, transmission, onboard computer system, etc. And when the gas tank is empty, the car undergoes an internal transition process whereby it switches over to running entirely on electricity.

But now, your engine is starting to have problems — it’s sputtering and creaking.

Your mechanic tells you your car is having problems processing fuel properly.

What are you more concerned about?

  1. Your car’s ability to use gasoline as fuel
  2. Your car’s ability to use electricity as fuel

Everyone, I imagine, will choose A. The choice is obvious: Your hybrid car’s overall ‘health’ is mainly predicted — and effected — by its ability to effectively and efficiently use gasoline as its primary fuel source. But when it comes to the human body, an admittedly far-more complex organism than a car (which is not an organism at all), otherwise sane people’s logic falters. Because, while this car-fuel analogy is obviously, and necessarily, over-simplified, it serves as a corollary for this so-called ‘debate’: Which primary fuel source is ‘healthier’ for humans, carbs or fat?

Now, all scientists agree that our brains, red blood cells, and active muscles are primarily and, by default, fueled by glucose. (These are our inexact corollaries to the “engine,” “transmission,” and “onboard computer system” above.)

Other bodily systems, such as the heart, are fueled primarily by fat. And fat can be converted by the body into “ketones” in order to fuel the brain when its glucose supply is low (just as our hybrid car could use electricity to fuel its engine, assuming its gas tank was low). So our bodies do not technically need to consume glucose, just as our hybrid car does not need any gasoline. But the take-away here, if it is not obvious, is non-debatable: major human systems are fueled, by default, primarily by glucose.

So, now, let’s consider a dilemma:

Your brain and body are having problems processing fuel properly.

What are you more concerned about?

  1. Your brain and body’s ability to use glucose as fuel
  2. Your brain and body’s ability to use fat as fuel

If the answer is A, which I imagine it is, the next question is, logically:

What is your next step?

  1. Repairing your brain and body’s ability to use glucose as fuel
  2. Increasing your brain and body’s ability to use fat as fuel

Both seem like decent options, and ideally we could answer “both,” but before we delve further down the decision tree, consider: We have already seen that glucose is the primary and default fuel source for the brain and other parts of the body. This is a biological fact, debated by no one. So, then, doesn’t it stand to reason that the greatest predictor and determinant of the “health” of a human brain and body is its ability to use glucose as fuel?

In the car analogy above, no one would say that the first thing the car owner should do is simply give up on using gasoline, ditch the gas tank entirely, and just run the car entirely on electricity. And yet, this is what otherwise sane people say we should do with the human body when it is having trouble using glucose as fuel (e.g. with Prediabetes): give up on glucose entirely and just run the brain and body on fat. This is called a “ketogenic diet,” and it is more and more popular.

But isn’t this approach a little extreme, a little “last resort”? Why would you just skip the entire stage of trying to fix your brain’s and body’s ability to run on glucose? Why jump right into the desperate, no-options-left approach, trying to get your brain and body (or your car) to run on its non-primary, non-default fuel source?

The only logical explanations for choosing a ketogenic diet are (not mutually exclusive):

  1. You don’t believe that your brain’s and body’s ability to use glucose as fuel can be improved, and therefore you have no other choice but to switch over to running exclusively on fat.
  2. You believe that eliminating glucose will somehow improve your brain’s and body’s ability to use glucose as fuel, whereby you will then re-introduce glucose.
  3. You believe that running your brain and body exclusively and indefinitely on fat — a non-default, non-primary fuel source — is inherently better for you than running it on glucose — its default, primary fuel source.

If A: Why have you given up so easily? Have you tried other ways to improve your brain’s and body’s ability to use glucose as fuel? Exercise/losing weight? Eating lower glycemic carbohydrates? Eating less fat overall, especially saturated fat? Eating more green vegetables and high-antioxidant foods? Eating a whole foods, plant-based diet? Fixing your digestion? Taking probiotics? Repairing your liver? Etc.

But if you feel you have tried everything — and your body’s ability to burn glucose for fuel has not improved — then I can see why you would want to move on to step

B: But, when you eliminate glucose from your diet, you must eat a higher-fat diet — or else risk starvation. Calories have to come from somewhere. But it has been shown repeatedly that saturated fat consumption reduces the body’s ability to effectively use glucose as fuel. In other words, if you are eating fewer carbs and more saturated fat to improve your glucose metabolism, it will likely drastically backfire, and you will never be able to re-introduce glucose.

Perhaps a ketogenic diet, based primarily on monounsaturated and polyunsaturated fats (think: avocados, nuts, and fish) which have not been shown to harm glucose metabolism, could, over time, improve glucose metabolism — but what would be the mechanism for this, other than possible weight loss?


C: If you are switching over to a ketogenic diet because you believe that it is inherently “healthier” to consume no glucose than to consume glucose — where is that belief coming from? How do you reconcile that belief with the fact that human brains and bodies use glucose as their default and primary fuel source? Is it possible that our default and primary fuel source is sub-optimal for long-term health?

This is entirely plausible: Our evolutionary process favored procreation and survival, not long, disease-free lives, and perhaps glucose metabolism is a short-term win (more sex and fight-or-flight energy) for a long-term loss (more disease, cellular waste, etc.). Perhaps a long, healthy, modern life is more compatible with a low or no-carb diet than a moderate or high-carb one. But how is it possible, then, that the longest-lived people on earth all eat a “high-carb” diet? (See: The Blue Zones.) Is it because they are exponentially more physically active than the average city-sweller? (This could be the reason.)

Perhaps we will see a generation of ketogenic people living easily to 100, 110, 120, and beyond… But I find it hard to ignore the evidence:

  1. The brain and other parts of the body run primarily and by default on glucose. (Fact)
  2. Glucose metabolism is the most important predictor and determinant of overall health. (My Inference)
  3. The longest-lived people on earth all eat a high-glucose diet. (Fact)

If your mechanic showed you that list above, and replaced the words referring to people and glucose with references to cars and gasoline, and then told you he believed that you should ditch the gas tank entirely and just run the car exclusively on electricity — wouldn’t you find another mechanic?

I wish you the best, whatever you decide.



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