HIGH CARBOHYDRATE ATHLETIC FUELLING. A FAD METABOLIC DUMPSTER FIRE , PART 2

BY DR. BAYNE FRENCH

Link to Part 1

In Part 1, I outlined the details of the FASTER study so that the opposite of high-sugar fuelling could be understood. By understanding the extremes, perhaps a healthy and sustainably enjoyable fuelling method can be developed for your own endurance demands.

Biochemistry and Metabolism

I believe the best decisions are made from an informed position, free from bias. Becoming informed and casting off bias are truly transcendent human qualities, and therefore exceedingly rare.

I think it is critical to have a basic understanding of what happens chemically when your bloodstream is flooded with sugar. With this knowledge, you may choose to stick with the status quo because it is working for you, or you may choose to go in a completely different direction.

I have written about metabolism many times.A disordered metabolism leads to unwanted weight.I believe that most disease — most of what we will all eventually die from — is the result of distorted metabolism. i.e. diabetes, stroke, heart attack, Alzheimer’s disease and cancer. An efficient metabolism reduces disease, medication use, time wasted sitting in doctors’ waiting rooms, and enhances both healthspan and lifespan in a remarkable way.

My entire perspective on health and wellness, including athletic fuelling, is viewed through the lens of metabolism.

Metabolism involves the conversion of food into energy, and the storage of that food for future energy needs. In simple terms, it looks like this:

Food + Oxygen = Carbon dioxide + Water + Energy

Food contains many things, including fibre, vitamins, minerals, pigments and other nutrients. For metabolic purposes, whether it is protein, carbohydrate or fat, food consists mainly of carbon, hydrogen and oxygen. Protein also contains nitrogen. Just four basic elements.

This is not intended to be an exhaustive discussion of biochemical reactions. The purpose of this article is to discuss high-carbohydrate fuelling for exercise and the harm that may result from it. For that reason, the focus here will be on blood sugar.

Sugar is sucrose. It consists of one molecule of glucose and one molecule of fructose linked together. Most carbohydrate, whether from table sugar or a slice of bread, is broken down in the intestine into single molecules of glucose and fructose.

Glucose is absorbed into the bloodstream by its own transporter complex, called SGLT1. Fructose has its own transporter, called GLUT5. These are the transporters that Dr Jeukendrup, discussed earlier, showed could be increased in number and efficiency through “gut training”.

Enzymes are proteins that help chemical reactions move forward. There are enzymes that specifically metabolise glucose. When glucose builds up and is present in excess, these enzymes are shut down. This is called feedback inhibition.

So despite huge amounts of sugar being consumed, and improved sugar absorption from the gut through “gut training”, there are still bottlenecks in how much glucose can actually be converted into energy — ATP.

And this is what we are really trying to do: make lots of ATP, because ATP is what makes muscles move.

Insulin

Insulin is a hormone produced in the pancreas. Like all hormones, it travels through the bloodstream and exerts effects far away from the organ where it is made. There is no more powerful regulator of metabolism than insulin.

When blood sugar rises, insulin is released. All of our 40 trillion cells have a docking site on their surface for insulin. This allows sugar to move from the blood into the cell. Insulin is powerfully anabolic, meaning it builds things. It builds muscle, glycogen (the storage form of carbohydrate), and is particularly effective at building fat.

First of all, during heavy exertion it does not make much chemical sense for a hormone responsible for building things to spike, when what we really need is to break things down to provide fuel. However, insulin is very good at — and in fact essential for — allowing glucose to enter the cell. Once inside the cell, glucose undergoes a chemical process called glycolysis, and a small amount of energy is produced in the form of ATP. After glycolysis, the remnants of glucose enter the citric acid cycle and eventually oxidative phosphorylation. This final set of reactions produces the majority of ATP.

ATP is directly responsible for muscle movement. And that is what endurance athletes really care about: repetitive, prolonged muscle movement under load, preferably performed better than everyone else in the race. Some athletes want this so badly that metabolic consequences be damned.

As mentioned, insulin is the most powerful metabolic hormone. It is so powerful that three other hormones are required to counterbalance its effects: glucagon, epinephrine and cortisol. We have already discussed that insulin allows glucose — blood sugar — to enter the cell. Inside the cell is where glucose is converted into energy, ATP.

Insulin also has many other effects:

• It promotes the formation of glycogen, our storage form of carbohydrate.
• It prevents the breakdown of glycogen.
• It causes the conversion of carbohydrate into fat.
• It increases body fat percentage.
• It prevents the breakdown of fat.
• It helps convert protein into muscle tissue.
• It stimulates glycolysis, the first step of glucose metabolism inside the cell.
• It prevents gluconeogenesis, the formation of new glucose.
• It prevents the conversion of lactic acid into glucose.
• It promotes cellular proliferation, increasing the risk of heart disease and cancer.

Metabolic Consequences of Elevated Insulin

When blood sugar is high and insulin levels are elevated, you become a one-trick pony. You cannot use protein and fat to create new glucose in a process called gluconeogenesis.

More importantly, you cannot access fat for fuel. Fat tissue is broken down in a process called lipolysis, and insulin directly inhibits this. After fat is broken down, it is metabolised through a process called beta-oxidation. Insulin also inhibits this reaction. As a result, the enormous amount of energy — ATP — that fat can provide is completely wasted in a high-insulin environment.

Lactic acid is normally produced during glycolysis. Much more is produced during strenuous exercise. High-level athletes know this compound as something that causes soreness and is generally a nuisance. However, lactic acid can also serve as a source of energy. It can be taken up from the bloodstream by the liver and converted back into glucose. Insulin interferes with this process as well.

Heart muscle preferentially burns fat for fuel. Heart cells are highly efficient at beta-oxidation, breaking fat down into smaller pieces that then enter the citric acid cycle and eventually oxidative phosphorylation, producing a large amount of ATP.

Insulin directly interferes with the breakdown of fat and with the heart’s ability to produce energy. I may be wrong, but I would argue that most endurance activity requires a well-functioning heart, and high insulin levels caused by a bloodstream flooded with sugar directly impair the heart’s ability to burn fat.

Also mentioned above is the fact that insulin stimulates cellular proliferation, or growth. Fu et al. (Mol Metab. 2021) reported that insulin directly and indirectly affects arteries, “exacerbating the development of endothelial dysfunction, atherosclerosis, restenosis, poor wound healing and even myocardial dysfunction”. These abnormalities occur independently of blood sugar levels.

Patil et al. (Mo Med. 2012) detailed the many cardiovascular complications that can affect endurance athletes. These problems occur more frequently, and at a younger age, in endurance athletes. Insulin is a primary driver of this.

Leitner et al. (Biochem J. 2022), in a fascinating article entitled Insulin and Cancer: A Tangled Web, detailed the chemical mechanisms by which insulin may drive cancer formation. Thirteen cancer types are found at much higher rates in individuals with obesity. A major driver for the vast majority of obesity cases is chronic hyperinsulinaemia — prolonged, ongoing elevation of insulin levels in the blood.

Although Patil et al. reported on cardiovascular disease in endurance athletes, there is currently no compelling evidence that endurance athletes have a higher risk of cancer. There is, however, a plausible mechanism of increased risk associated with high insulin levels. Recommendations of 60–90 grams of carbohydrate per hour produce extremely high insulin output from the pancreas.

Muscle Types

You may have heard of white and red muscle fibres. It is as true in your Christmas turkey as it is in us.

White muscle is lighter in colour. It is considered a fast-twitch muscle fibre, capable of short, explosive contractions. Turkey breast contains this type of muscle. Sprinters and weightlifters also tend to have a disproportionate amount of white muscle. White muscle is most efficient at burning glucose to create ATP.

Red muscle is a slower-twitch muscle fibre, capable of prolonged, sustained and repetitive contraction. That sounds a lot like endurance movement. This is where high-carbohydrate fuelling recommendations become inconsistent with efficient movement. Red muscle is most efficient at burning fat as its primary fuel to form ATP. High-carbohydrate fuelling works directly against this. That makes the advice from Dr Jeukendrup and Andy Blow even more questionable.

A Bit More Biochemistry

I explain how sugar harms performance in an article titled Sugar: The Antithetic Performance Enhancer. Here are just a couple of highlights from that article, out of many:

Nitric oxide is a powerful chemical produced by the body that increases blood flow to tissues. It also helps glucose enter muscle tissue. It is a performance enhancer. Sugar directly interferes with the formation of nitric oxide.

Sugar also drives the production of antidiuretic hormone (ADH). This means you urinate less and retain more fluid, which further interferes with your power-to-weight ratio. In addition, ADH is a vasoconstrictor, meaning it narrows blood vessels to muscle. None of this is helpful for performance.

Recall that half of sugar is fructose. Fructose is a powerful driver of uric acid, which is associated with multiple chronic diseases. Although uric acid is best known for causing gout, its metabolic effects are much broader. Here is an excerpt from another article I wrote, Uric Acid and Its Metabolic Effects:

Whether or not you suffer from gout, elevated uric acid promotes insulin resistance, raised blood sugar, type 2 diabetes, high blood pressure, distorted cholesterol levels, systemic inflammation and weight gain. It is directly linked to a large proportion of all-cause mortality, especially cardiovascular disease (Chen et al. Arthritis & Rheumatology, Feb 2009).

The evidence linking elevated uric acid with metabolic chaos is strong enough that it is considered an independent risk factor. That means elevated uric acid alone is associated with disease.

An average adult human has around 400 grams (14 ounces) of stored carbohydrate in the form of glycogen. This corresponds to less than 2,000 calories. On its own, that amount could provide enough ATP for around 90–120 minutes of vigorous exercise.

A lean adult human can have 10 kilograms (more than 20 pounds) of fat, providing upwards of 100,000 calories.

With excessive sugar fuelling, blood sugar levels spike. Glycogen stores may remain full, but a great deal of blood sugar is left over. It will inevitably spill over into fat formation. The stress hormone cortisol rises during heavy exertion. Cortisol tends to direct fat storage into the midsection, creating visceral fat. This type of fat is different from the fat stored under the skin, known as subcutaneous fat. Visceral fat releases hundreds of harmful chemicals that drive organ dysfunction over time.

High sugar intake → Elevated blood sugar → Elevated insulin → Insulin resistance

Further elevated insulin → Increased cardiovascular disease and cancer risk → Fat formation and inability to burn fat

Exercise, along with a relative reduction in calories — as seen with lower-carbohydrate endurance fuelling — triggers the formation of a protein called AMP-activated protein kinase (AMPK).

Stay with me here. AMPK does many good things. Two of the most important are these:

AMPK stimulates mitochondrial biogenesis. This is the formation of new mitochondria, and mitochondria are what produce ATP to fuel performance.

AMPK stimulates autophagy. This is the removal of old and damaged cellular components. It is essentially cellular housekeeping. Poor autophagy contributes to several diseases.

Let’s End This Thing

I have absolutely no use for high-carbohydrate fuelling during exercise, and no respect for those who recommend it. I view it as harmful to health on many levels. The many metabolic effects of sugar are not in dispute, and none of them are favourable. If you try to dispute them, then in my view you are either ignorant, trapped in dogma and the status quo, or on the payroll of some sugar-promoting entity.

There are so many metabolic flaws in the recommendation for high-carbohydrate fuelling during exercise. With the absurd practice of gorging on sugar, it is not biochemically possible to generate a huge amount of energy from fat, nor to effectively support mitochondrial biogenesis and autophagy. On top of that, high insulin levels result, driving visceral fat formation, cardiovascular disease and numerous cancers.

I have put myself through levels of physical suffering that I would not wish on people I dislike — and that is a lot of people. Effort and discomfort that sometimes lasted for days in pursuit of things with points and full curls. I am all for seeking an edge and investigating anything that might provide an advantage over other members of my own species — or others. Stuffing myself with a known toxin and a major disruptor of healthy, efficient metabolism is not an edge at all, regardless of the impressive CVs of the pontiffs recommending it.

In a way, it is a leap of faith to undergo a complete metabolic shift in how you generate fuel. It takes time — perhaps up to a year — to become truly fat-adapted, meaning able to extract vast amounts of energy from fat. It is rare for a high-level endurance athlete to change a behaviour that appears to be working well, especially when their peak performance window is relatively short. I am not writing for those people.

If you are not of that extreme performance persuasion, I would urge you to reject this metabolically flawed trend. Do not become a one-trick pony, entirely dependent on sugar to generate ATP. Lower carbohydrate intake per hour results in lower insulin levels. Fat stores then become available, which can provide an extraordinary amount of ATP for muscle contraction. Most importantly, lower insulin levels over time are associated with a much lower risk of disease.

Always be your own advocate. The willingness to follow deeply flawed and unhealthy guidelines often has a great deal to do with psychology. Seeing influential people do something successfully encourages others to copy them. Break free from that. Ask yourself difficult questions. Understanding the basic mechanisms gives you a foundation from which to pursue a different — and much healthier — exercise fuelling strategy. I hope this article helps provide that.