Carbohydrate loading

What is carbohydrate loading and why do marathon runners do this before races? I asked Google Gemini to explain and share my own experience with it and other aerobic training strategies.

Energy production.

🕵️‍♂️ Carbohydrate loading, often called “carb-loading,” is a strategy used by endurance athletes, like marathon runners, to maximize the storage of glycogen (a form of carbohydrate) in their muscles and liver.

Here’s how it generally works and why they do it:

• Glycogen as Fuel: During prolonged exercise, the body primarily uses glycogen stored in the muscles and liver as its main source of energy. When these stores are depleted, athletes experience fatigue, often referred to as “hitting the wall” or “bonking.”

• Maximizing Stores: Carb-loading aims to supercompensate these glycogen stores beyond their normal levels. This means the body will have a larger reserve of readily available energy to draw upon during a long race.

• The Process:

1. Depletion Phase (Optional, but traditionally included): Historically, carb-loading regimens would start with a period of reduced carbohydrate intake and intense exercise to deplete existing glycogen stores. The idea was that this “shock” would then make the body more efficient at storing carbohydrates when they were reintroduced. However, many modern approaches skip or modify this phase, as it can be very taxing and lead to fatigue or injury.

2. Loading Phase: In the 3-6 days leading up to a marathon, the athlete significantly increases their carbohydrate intake while simultaneously reducing their training intensity (tapering). Carbohydrates typically make up 70-85% of their total caloric intake during this phase. Examples of carb-rich foods include pasta, rice, bread, potatoes, oats, fruits, and starchy vegetables.

• Why Marathon Runners Do It:

• Delayed Fatigue: By starting the race with maximal glycogen stores, runners can maintain a faster pace for longer before fatigue sets in. It helps prevent or delay the dreaded “wall.”

• Improved Performance: Having more available energy means the muscles can work more efficiently and powerfully throughout the race, potentially leading to better finishing times.

• Mental Boost: Knowing they have adequately fueled their body can also provide a psychological advantage.

Here’s a visual representation of how carb-loading helps fuel a marathon runner:

🔴 The primary source of energy in endurance events is free fatty acid processed in the mitochondria to spit a phosphate molecule from adenosine triphosphate (ATP) releasing energy for muscle contraction. Carbohydrates such as muscle and liver glycogen play a secondary role, likely for feeding the neurological systems of the body. Why do most people believe that glycogen is the main fuel source for running a marathon?

🕵️‍♂️ It’s interesting that you bring up the role of free fatty acids (FFAs) and ATP in endurance metabolism, as there’s a common misconception in your statement that needs clarification. Let’s break down why most people, and indeed sports science, consider glycogen (from carbohydrates) to be the primary fuel source for high-intensity endurance events like a marathon, and where FFAs fit in.

First, let’s address the fundamental process you described: “free fatty acid processed in the mitochondria to spit a phosphate molecule from adenosine triphosphate (ATP) releasing energy for muscle contraction.”

• ATP is the direct energy currency: You are absolutely correct that ATP is the immediate energy source for muscle contraction. When ATP is hydrolyzed (a phosphate group is “spit” off), energy is released.

• FFAs and Glucose regenerate ATP: Free fatty acids (from fats) and glucose (from carbohydrates, stored as glycogen) are not directly used for muscle contraction. Instead, they are metabolized through various pathways (like beta-oxidation for fats and glycolysis for glucose) to regenerate ATP from ADP (adenosine diphosphate) + Pi (inorganic phosphate). Both fats and carbohydrates contribute to the continuous production of ATP.

Now, let’s tackle why glycogen is considered the primary fuel for a marathon and why the idea of FFAs being primary in endurance events, and glycogen only for neurological systems, is incorrect in the context of typical marathon performance.

1. Energy Yield and Rate of ATP Production:

• Carbohydrates (Glycogen): While fats provide more ATP per gram, carbohydrates can be metabolized much faster to produce ATP. This rapid ATP production is crucial for sustaining the higher intensities typical of marathon running (where runners are often working at 60-85% of their VO2 max or even higher).

• Fats (FFAs): Fat metabolism is slower and requires more oxygen per unit of ATP produced compared to carbohydrate metabolism. While abundant, fats are primarily utilized as a fuel source during lower-intensity exercise or when carbohydrate stores are significantly depleted.

• The Crossover Concept: There’s a well-established “crossover concept” in exercise physiology. As exercise intensity increases, the body shifts from relying predominantly on fat for fuel to relying predominantly on carbohydrates. For marathon pace, carbohydrates are the dominant fuel.

2. Muscle and Liver Glycogen are Direct Muscle Fuel:

• Muscle Glycogen: This is stored directly within the muscle cells and is the most readily available and preferred fuel source for those specific muscles during high-intensity exercise. It does not primarily fuel “neurological systems” during exercise; that’s largely the role of blood glucose.

• Liver Glycogen: The liver stores glycogen to maintain blood glucose levels. When blood glucose starts to drop during exercise, the liver breaks down its glycogen and releases glucose into the bloodstream, which then can be taken up by active muscles and the brain.

• Brain Fuel: While the brain primarily relies on glucose, it gets this glucose from the bloodstream, which is maintained by liver glycogen stores and carbohydrate intake. It’s not that muscle glycogen itself fuels the brain.

3. The “Wall” and Glycogen Depletion:

• The experience of “hitting the wall” in a marathon is a classic example of severe muscle and liver glycogen depletion. When glycogen stores run out, the body is forced to rely almost entirely on fat metabolism. Because fat metabolism is slower, the intensity of exercise must significantly decrease, leading to a dramatic drop in pace and extreme fatigue. If FFAs were the primary fuel, this “wall” wouldn’t occur in the same way.

4. Research and Performance Benefits:

• Extensive research over decades consistently demonstrates that carbohydrate availability is a major determinant of endurance performance. Strategies like carbohydrate loading (as discussed earlier) and consuming carbohydrates during a race are highly effective because they maintain glycogen stores and blood glucose, thereby sustaining higher exercise intensities for longer.

• Athletes with higher pre-exercise muscle glycogen stores consistently perform better in endurance events compared to those with lower stores.

In summary:

The belief that glycogen is the main fuel source for running a marathon is supported by robust physiological evidence. While free fatty acids are crucial and provide a vast energy reserve, their rate of ATP production is insufficient to sustain typical marathon running intensities. Glycogen, both from muscles and the liver, provides the quick and abundant ATP needed for muscle contraction at these higher outputs. The “wall” is a direct consequence of running out of this crucial carbohydrate fuel.

Context matters

During the years that I ran marathons (1975 to 1980), I was a fan of LSD.

Long, slow distance (LSD) training made up 95% of all my runs (80 KMs per week). When training with my buddies, we ran a speeds that allowed conversations. Excepts when running up stairs or steep hills, we never reach hyperventilation or lactic acid build up in our muscles. Today, this level of training intensity is referred to as Zone 2. Now, at 74, I target my age-specific Zone 2 heart rate range (105 to 118) for all of my 60- to 90-minute Nordic Cross Skating outings.

Gemini’s answer assumed that the runner was racing in a marathon. My comments apply to everyday LSD training.

One goal of Zone 2 training

Building “fat-burning” aerobic capacity is key. It would be my base energy source if I were to begin training for a marathon today as it was in my youth.

In my last running marathon in Detroit, I set my PB (2:53:17) by adding two months of “quality” training to build upon my LSD fat-burning base. I did “track workouts” at the local school running multiple laps at “race pace” twice per week. This prepared me to run more efficiently at higher speeds and to adapt my body to make better use of carbohydrates for energy production.

In my first and last marathon on Nordic Cross Skates in 2017, I completed the 42 KM course in 2:45:16 at the age of 66. Participating in the roller-ski section of the North Shore In-line Marathon is one of my proudest accomplishments. I was the only person on Nordic Cross Skates and there were a lot of surprised faces and a few cheers as I crossed the finish line in downtown Duluth, MN that sunny Fall day.