Carb Metabolism and How a High-Carb Diet Can Make you Suck in the Gym

by Sergio Rivera

Lance Armstrong was going to win this race. With about 25 km to go in Stage 16 of the 2000 Tour de France, Armstrong was hanging around in second place, allowing Richard Virenque to pace the race. Everyone watching the race saw the plan: Armstrong was saving himself for one last push to win the race.

Three kilometers later, on the final climb of the Tour, Armstrong shocked the entire world. He was in trouble.

Armstrong grimaced as he struggled to keep up with Virenque. Shoulders swaying and head bobbing, he was losing precious pace. By the middle of the climb, he was already a minute behind first place, Virenque.

One of the commentators spelled Armstrong’s destiny in one succinct statement: “I think in fact Armstrong has absolutely hit the wall.”

Virenque went on to win that stage of the Tour, with Armstrong closing the race at 8th, two minutes behind first place. He would go on to win Tour de France 2000 nonetheless, but this race showed a chink in his armor.

"The problem was a bonk; I didn't eat enough and on the last climb I had no energy…” Armstrong would later explain.

But how did he lose so much energy? Wasn’t he supposed to be on a high-carbohydrate diet that ensures long-term energy supply?

It’s the Carbs, Bonkers!

Bonking, also known as “hitting the wall”, happens when you run out of fuel supply in the middle of a high-intensity activity. This explains Armstrong’s abrupt loss of power on the final climb of Stage 16 of Tour de France 2000.

The process of bonking is internal, both physiological and psychological. First, you feel a sudden craving for food, and about 10 minutes later, you experience chills and shakes as adrenaline is released in your bloodstream.

If you push on without resupplying your carbohydrate levels, your brain goes haywire. You lose focus and fall into a depressive state. By this time, your energy supply in the form of glucose and glycogen has come to its bitter end.

That may seem counterintuitive. After all, Tour de France riders consume a pre-race mix of slow-release carbohydrates and eat simple carbohydrates during the race. In fact one of the highlights always include a cyclist having to maintain balance while munching on a deli sandwich, burrito, donut, or banana among many other sources of carbohydrates.

Nutrition before and during the race is intended to give the riders between 6,000 and 9,000 calories, enough supply to last them during the entire stage of the race. For reference, 9,000 calories is about 60 six-inch pancakes, or 10 pints of Ben and Jerry's Cookie Dough Ice Cream.

The average ride burns around 6,000 to 7,000 calories on a single race stage. And yet Armstrong, and many other bikers in the history of the Tour de France, bonked.

How the hell is that possible?

Here’s the answer: bonking happens when your body is used to burning carbohydrate alone for fuel.

In order for us to understand bonking, we have to first understand how our body breaks down, uses, and stores carbohydrates.

Carbohydrate Metabolism

Carbohydrates are basically sugar molecules that exist in different forms – from a single molecule unit called glucose, to a more complex string of 2 or more glucose units (disaccharides, oligosaccharides, and polysaccharides).

When carbohydrates – except fructose – are digested, the body reduces them to glucose. Glucose is our body’s currency for blood sugar.

In the fasted state, a healthy non-diabetic individual weighing 70 kg has 4 grams of glucose diluted in the blood stream. A large increase or decrease in blood sugar levels is considered by the body as dangerous.

Go below 4 grams and you’ll have hypoglycemia (as happens in Diabetes Type I).

And if your body is unable to pull down your blood sugar levels back to the normal range after a meal, then you’re suffering from hyperglycemia (as happens in Diabetes Type II).

For this reason, your body needs to clear the extra glucose in your bloodstream after a high-carbohydrate meal. When you drink that 12 oz. can of Coca-cola, which has 39 grams of carbohydrates, your pancreas releases insulin to drive the glucose elsewhere in your body.

Under normal circumstances, the muscles will take up the extra glycogen for “real time” burning or storage. To store glucose for future use, the glucose molecules are strung together to form glycogen.

Your liver is also interested in glucose for storage, since it keeps blood glucose levels within normal ranges when you are fasting, or during exercise.

And then there’s the brain, a furnace that burns glucose for fuel. The brain is not very good at storing glycogen, so it relies on the liver for much needed glucose.

Your brain will burn 30 grams of carbohydrate on any given day. So what happens when your carbohydrate intake goes below 30 grams a day? The brain tells the liver to free stored glycogen into glucose units, or break down fat into fatty acids and ketones.

Ketones are compounds that result from the breakdown of fat in a process called ketosis. Think of ketones as back-up fuel for when glucose is not available.

The Carbohydrate and Ketosis Love and Hate Affair

The average American male, who weighs about 88 kg (195 lbs), will burn 82 calories per hour at rest. 82 calories is equal to around 20 grams of carbohydrates. So at least half of that can of coke you just drank a while ago, which has 39 grams of carbs, should be tucked away in storage, preferably as glycogen.

If, however, your glycogen stores are already full when those 39 grams of carbs hit your bloodstream, your body has no other choice but to convert the extra glucose to fat for storage – via a process called lipogenesis.

Once the glucose is turned to fat, it stays as fat. The carbohydrate metabolic pathway is a one way street that ends when carbohydrate is turned into fat.

It will take a different process to turn fat back into glucose. This means that – unless you let your body use fat as its primary source of fuel – you’ll never burn those fat stores for energy.

Lipogenesis is the metabolic endpoint of the carbohydrate you consume. If your body needs glucose later on, it will break down your glycogen stores for glucose.

And if you haven’t been eating a large amount of carbohydrate in days, which means your glycogen stores should have been totally used up, then it will break down non-carbohydrate stores, such as protein and fat, into glucose via a process called gluconeogenesis.

Again, it’s the Carbs, Bonkers!

How does carbohydrate metabolism explain Armstrong’s bonk?

By the time Armstrong bonked, he was already cycling for more or less five hours. He said he hadn’t eaten enough; on the final climb of Stage 16, all of his glucose supplies (free glucose and glycogen) were fast running out.

Sensing that glucose levels were too low for comfort, instincts took over. Armstrong’s brain had to prompt the body to lower performance or else his glucose reserves would empty up, bringing his brain into a lights-out hypoglycemic coma.

And why not just use ketones for energy?

Our bodies rely on patterns, and Armstrong’s pattern relied on high-carbohydrate intake. The thing about ketosis is that it does not kick in until daily carbohydrate intake is consistently at or under 50 grams per day for a number of days.

In other words, Lance Armstrong’s body, was not adapted to use ketones for energy.

Why Your High-Carb Diet is making you Suck at the Gym

Well, I’m no endurance athlete and there’s a good chance you aren’t either. So what’s in it for us?

Armstrong’s bonk shows us that long-term high-carbohydrate diets, aside from stimulating fat storage lipogenesis, limits our body’s flexibility when it comes to using other sources of energy. This carbohydrate intake pattern makes your brain dependent on carbohydrate as its primary source of energy, blocking out ketogenesis for as long as it can until there’s already an obvious shortage of carbohydrates.

I’m sure you’ve had days when you just didn’t feel like you have enough energy to work out. You might have felt lazy, preferring to just sit around and chat while your gym time was slowly wasting away.

On those days, you might have not eaten enough, or burned all of your carbohydrate stores before you finished your workout.

Sure you can say that the simple solution is to keep having a high carbohydrate diet before, during, and after workouts. But doing so also makes you prone to fat storage. Simply put, maintaining a high-carbohydrate diet to make sure that you don’t hit the wall during workouts is a step forward and a step backward all at the same time.

Low Carb Dieting For Better Lean Muscle Gains, Minimized Fat Storage, And Zero Risks of Bonking

Carbohydrate reserves pale in comparison to fat in terms of capacity. These carbohydrate reserves aren’t significantly changed unless you deplete them via intense exercise or go on days with extensive carbohydrate restriction.

That said, a pattern of high-carbohydrate eating coupled with a low-intensity exercise regimen (or no exercise at all) only leads to fat gain; a majority of your carbohydrate stores remain full and untouched, and a large fraction of the carbohydrate you consume is considered surplus by the body, and is turned to fat for storage.

For this reason, individuals who are looking to lose weight fast by committing to high intensity exercise, without changing their high-carbohydrate diet, often feel like they’ve hit a wall of sorts. They bonk. Their brain, having to deal with low-carbohydrate levels all of a sudden, tells the body to minimize effort. They feel tired, demoralized, and depressed. The temptation to eat grips them to the core.

When the body senses a pattern of low energy supply, your brain can stimulate muscle breakdown to cut energy expenditure and provide additional energy. This is the common case among endurance athletes. They look skinny because their body is adapted to the long grind: less muscle means less energy expenditure.

Compare this with a keto-adapted body forged by a pattern of low-carbohydrate diet, coupled with high intensity interval training.

Being keto-adapted puts your body in a position where it can use multiple sources of energy: glucose (although you’re not taking it in as much as you would have in a high-carb diet) and ketones.

High intensity interval training meanwhile, tells your brain that there’s no need to cut energy expenditure by taking away muscle. In fact, it’s the other way around. High intensity exercise tells your brain that it needs more muscle for power.

Sure the preference for power over endurance isn’t a guarantee that you’ll beat Lance Armstrong in one long race. But if you hang around enough and keep up with Armstrong until the final 100 meter of the race, I can guarantee you two things.

First: you’ll have enough power to out-cycle him in that 100 meter sprint.

Second: you’ll never bonk.

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