What Is the Effect of a Low-Carb Diet for Cyclists and Triathletes?
Fat-adapted diets and low-carb diets have been gaining popularity, but what does this mean for an endurance athlete? Is it worth it to try these diets? The short answer is no–for most cyclists and triathletes in most situations, restricting carbohydrate intake is going to diminish performance potential.
But why is that? To answer this question, we will look at the energy systems of the body, how your body fuels exercise, and how macronutrients affect fatigue.
The Energy Systems
Ultimately, all human activity is fueled by Adenosine Triphosphate, or ATP. The human body can use carbohydrates, fat, and sometimes protein to create ATP and fuel exercise. Depending on the intensity of the activity, your body will use different pathways to fuel performance.
High Intensity Efforts of 5 to 15 Seconds—Phosphagen System: The phosphagen system allows for instant ATP production and instant energy.
High Intensity Efforts up to a Few Minutes—Anaerobic System: The Anaerobic system has limited ATP production. This system fuels high intensity efforts up to a few minutes. This system relies exclusively on carbohydrates.
Low Intensity Efforts from Several Minutes to Many Hours—Aerobic System: The aerobic system allows for unlimited ATP production and fuels lower intensity efforts from a few minutes to many hours. This system relies on carbohydrates and fat. The lower the intensity of the effort, the more the body can shift to using fat for fuel. The more fitness an athlete has, the faster they can move while fueling their activity aerobically.
It is important to understand that these energy systems are on a continuum, there are no set points where your body shifts entirely from one system to another.
How Your Body Uses Carbohydrates, Fat, and Protein During Exercise
Now that we understand the energy systems, let’s look at how your body uses the different macronutrients–carbohydrates, fat, and protein–to fuel exercise at these different intensities.
Carbohydrates (Anaerobic and Aerobic Activity)
Carbohydrates are quickly processed by the body to fuel moderate to high intensity efforts and they are the only macronutrient that can supply energy for anaerobic activities such as sprinting or steep climbs. Carbohydrate metabolism is fast, it can be completed with or without oxygen (aerobic or anaerobic), and carbohydrates are stored directly in the muscle itself, making them instantly available to provide energy. Carbohydrate stores in the body are limited, so it is important to ingest carbohydrates so they are available to use for training and racing.
Fat (Aerobic Activity Only)
During activities of moderate to low intensity, such as steady state easy cycling or walking, a majority of energy is derived from fatty acids in the blood. This is a slower process that requires more steps compared to metabolizing carbohydrates. In addition, fatty acids are not instantly available as most fats are stored in remote places in the body and must be delivered to the muscles via the bloodstream. Carbohydrates must also be present to metabolize fats at the rapid rates needed to support training and racing.
Protein
Protein is not typically used as an energy source for the body. However, when caloric expenditure is high, or carbohydrate intake is low, the body can convert protein to carbohydrate to supplement its energy needs through a process called gluconeogenesis. Protein is not normally a major source of energy, providing less than 10% of energy needs. However, most of the protein used in gluconeogenesis comes from breaking down muscle, so this is not a process an athlete would want to rely on.
How Macronutrients Affect Fatigue
As discussed earlier, your body uses the above macronutrients to produce Adenosine Triphosphate, or ATP, to fuel exercise. Muscle cells never run out of ATP, but when they begin to run low, fatigue sets in. This fatigue causes a decrease in level of activity and lower energy demands. This fatigue then gives the energy systems a chance to replenish ATP as you slow down. Since carbohydrate metabolism is faster, carbohydrates can replenish ATP more quickly if carbohydrates are available. If only fat is available, activity intensity will need to remain at a level low enough that ATP can be replenished by the slower process of fat metabolism.
How a Lack of Carbohydrate Limits Performance
By limiting carbohydrate intake, or by not consuming enough carbohydrates during exercise, you limit your body to rely on a more complex process to replenish ATP, and thereby reduce or eliminate your ability to perform higher intensity efforts. The more fit an athlete is, the faster they can move at a low to moderate pace to stay in a steady state aerobic fat burning zone. However, even ultra-endurance events include higher intensity efforts for aspects such as climbs, technical sections, or changes in pace to stay with a group or to attack.
If you think of the human body as having two separate fuel tanks–one for moderate to high intensity activity and one for moderate to low intensity activity–limiting carbohydrates is essentially not filling your second fuel tank. If your goal is to optimize performance, it makes sense to fill both tanks.
By ensuring adequate carbohydrate intake, you ensure your body can replenish energy needs efficiently to optimize performance. Adequate carbohydrate intake also helps ensure that valuable muscle protein will not be used for energy.
When a Low-Carb Approach Could Make Sense
There may be a few select instances when it is practical for an athlete to rely more heavily on fat for fuel such as ultrarunning, when carrying enough food is difficult and food is hard to digest, or for very long distance cyclists and bike packers traveling through remote terrain with limited access to re-supply food. These athletes should understand the following:
Their ability to move at a higher intensity will be limited
They risk using muscle protein to create carbohydrate
This approach works better for men than for women
The decision to take a low-carbohydrate approach should be balanced with event or training goals, the terrain an athlete needs to cover, personal preferences, and an understanding of the risks to long term performance.
Conclusion
If you know for sure you can maintain a purely aerobic pace for the entire duration of your event, you could get by with being fueled primarily by fat. However, if you want to be able to tap into some higher efforts and perform at your full potential, you will need to eat carbohydrates.
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Sources
Fink, Heather Hederick and Alan Mikesky., Practical Applications in Sports Nutrition. 6th ed.,, Burlington, MA, Jones & Bartlett Learning, 2021.
Photo by Trang Doan