My thoughts on intensity and metabolism

Endurance sports require a combination of physical and mental endurance. One essential factor in an athlete's performance is their fuel source, primarily determined by their recent food intake. Carbohydrates are the preferred fuel for high-intensity exercise, and sufficient glycogen stores are crucial for optimal performance. As exercise progresses, the body increasingly uses plasma fatty acids released from adipose tissue as fuel.

Additionally, an athlete's fitness and training status influence their body's ability to exercise, increasing their fat-burning capacity and endurance.

This article will explore how nutrition, fitness, and cadence affect an athlete's fuel source and performance during endurance sports. The chart below represents the fuel sources used by an athlete’s body during endurance exercise at various intensities relative to their aerobic capacity.

Fatigue

An athlete’s fatigue level is highly influenced by the availability of carbohydrate fuel, which tends to limit their capacity for high-intensity exercise. The brain continually monitors factors such as blood glucose levels, muscle glycogen stores, body and environmental temperature, and fatigue sensations, primarily based on this information. When carbohydrate fuel is scarce, the body reduces its maximal sustainable power outputs, shifting fuel utilization to the left on the graph concerning perceived effort. This is a preventive measure to avoid completely depleting carbohydrate stores.

For instance, a fit cyclist who can sustain 350 watts for 20 minutes when well-fueled may ride at 320 watts with a similar or higher perceived effort at the end of a 5-hour ride. But because the muscle glycogen is significantly depleted, the brain signals to conserve energy, thus making it harder to sustain the effort level for an extended period. However, carbohydrates can increase blood glucose, reduce fatigue, and boost work capacity. Additionally, riding at 320 watts for the first hour of the ride can deplete more glycogen than hour 5 of the same ride due to more available glycogen.

As exercise progresses, the body increasingly uses plasma fatty acids released from adipose tissue as fuel. Muscle triglyceride utilization decreases during medium to long bouts of exercise, like a 5-hour ride.

Also, riding at 320 watts for the first hour of the ride can use more glycogen than hour 5 of the same ride because more is available. At hour 5, there isn’t as much glycogen and blood glucose left. Your body must burn proportionally more fat at the same power output.

This fat will increasingly be in the form of plasma fatty acids released from adipose tissue as your bout of exercise progresses. Muscle triglyceride utilization decreases throughout a medium to a long bout of exercise, like a 5-hour ride.

Fitness and Training Status

As your fitness level improves, your body undergoes several changes that enhance your ability to exercise. These changes include an increase in aerobic capacity, the growth of your muscles' mitochondria, an increase in enzymes for burning fat and carbohydrates, improved mechanical efficiency, and possibly better regulation of exercise intensity and fuel utilization by the brain. These improvements are the very essence of "improved fitness." Ultimately, you will perform better at your chosen activity, and one defining characteristic of improved fitness is the ability to burn more fat at a given intensity.

Suppose you go on a 4-hour ride in February after not having ridden for more than 90 to 120 minutes in the last few months. You may find that your legs feel fatigued at the end of the ride unless you take it very easy. However, suppose you have been doing 4-hour rides every week or two for several months. In that case, you will likely be able to complete a 4, 5, or even 6-hour ride with the same or even less carbohydrate consumption per hour than your first 4-hour ride, without feeling more fatigued afterward, even with the same or higher average power output. Longer rides and high sub-threshold power levels can increase your fat-burning ability, especially if you include tempo riding at around 70-80% of your maximal aerobic power output. It requires the most fat-burning activity per unit of time.

It is possible that your maximal aerobic capacity in February for that first 4-hour ride could be similar to that in June or July for the 5-6 hour rides you might be doing, especially if you've been doing hard efforts in your training during the winter months. However, your muscle glycogen stores will be greater, and your fat-burning capacity will be increased in the summer due to months of longer training sessions. Most athletes would not train like this, as ideally, you would increase your aerobic fitness throughout the season. However, some athletes may follow this type of training pattern due to their competitive focus, such as a cyclocross racer who races from October through January but also races mountain bikes and roads in the summer. Despite having the same aerobic capacity in December as in July, this rider's endurance fitness may not be as high due to the lack of endurance training required for cross races.

Nutrition

Generally, an athlete's fuel source during exercise is primarily determined by their recent food intake. Consuming tea or coffee before exercise may result in burning more fat and/or extended exercise duration. Conversely, consuming carbohydrates before endurance exercise may lead to burning more carbohydrates, which is potentially problematic. If insufficient carbohydrates are consumed since the last glycogen depletion, burning more fat is likely. Still, the athlete may not be able to perform as hard or as long due to the brain's awareness of low carbohydrate availability. Carbohydrates are the preferred fuel for high-intensity exercise.

Endurance sports nutrition is a complex subject, but to perform at one's best, sufficient glycogen stores are crucial for fueling the body and preparing the brain for optimal performance. Refueling immediately after intense exercise is ideal, while overconsumption of carbohydrates immediately before exercise should be avoided. Easily digestible carbohydrates, protein, and fat are best consumed one to two hours before exercise, such as oatmeal with nuts or fried eggs on toast.

On the other hand, consuming a large number of carbohydrates, such as a bag of candy or a stack of pancakes with syrup, before exercise may result in a solid bonk, even if the body is well-fueled. A sudden rise in blood sugar and insulin production from a high-carbohydrate meal can benefit recovery. Still, insulin levels decrease during exercise, and glucagon levels increase to help break down glycogen for energy. Overindulging in pre-workout snacks or meals is therefore discouraged.

Cadence

The most efficient way to use fat as a fuel source during cycling is to maintain an optimal cadence, which utilizes more slow-twitch fibers to generate force. This helps delay fatigue by prolonging the use of fat as fuel rather than relying on glycogen and glucose. An optimal cadence typically falls in the 80-90 rpm range, where you are likely self-selecting the most fuel-efficient cadence for your intensity level. Increasing your cadence to 120 or 130 rpm makes you less efficient and requires more energy to maintain the same work output.

Conversely, if you decrease your cadence to 50 or 60 rpm, you need to increase the force of your pedal stroke by about 50% and recruit more fast-twitch muscle fibers, which use up more glycogen and produce more lactate. For optimal in-the-saddle cadences, the ideal range increases with power output. For example, at 150-200 watts, the optimal cadence is typically 75-85 rpm, whereas at 400+ watts, 100 rpm or more is likely best. It's important not to mash or spin too fast, which can lead to glycogen depletion or decreased efficiency.