Excerpted from The Athlete’s Gut: The Inside Science of Digestion, Nutrition, and Stomach Distress by Dr. Patrick Wilson.
Most athletes know ingesting carbohydrate can improve performance under the right circumstances. It’s no coincidence that carbohydrate-containing drinks and supplements are a ubiquitous presence on the sidelines of sporting events and at endurance-race aid stations. Decades’ worth of well-controlled studies support the notion that consuming carbohydrate during intense, prolonged exercise—whether it’s from a “scientifically formulated” sports beverage or a handful of candy—can help delay the onset of fatigue.1
At the same time, ingesting food during intense exercise (even in the form of easily digested carbohydrates) represents a challenge to your gut. Many a seasoned athlete has had an important race or contest ruined by an aggressive or poorly planned nutrition strategy. In some of these cases, overingestion of carbohydrate—or eating the wrong type of carbohydrate—is the root cause. Clearly, there’s a delicate balance when it comes to selecting the optimal amount of carbohydrate to ingest during exercise, a happy medium that exists somewhere between the extremes of consuming too little, leading to a bonk, and consuming too much, sending you straight to the roadside privy. Before we discuss finding this delicate balance, it’s worth briefly reviewing the reasons carbohydrate is often touted as the champion of fuels during exercise.
Fuels Burned During Exercise
When it comes to supplying the ATP needed to sustain exercise, two fuels reign supreme: carbohydrate and fat. During a mile race, an Ironman, a soccer match, or any other event that lasts more than a minute, you’re continuously breathing in oxygen and transporting it throughout your body to convert that avocado, fancy sports beverage, or plate of pasta into ATP. It’s this conversion of foodstuffs into mechanically useful energy that allows you to power your limbs, so you can run, bike, paddle, or swim, not to mention keep your vital organs functioning. Carbohydrate and fat burning are by far the most important sources of ATP production for the majority of athletic endeavors lasting longer than a couple of minutes.
Of these two fuels, the amount of carbohydrate stored in your body—known as glycogen—is much more limited. To put it into perspective, even super-lean athletes usually have a few days’ worth of fat stores to burn through before they risk running dry during exercise. Think of your capacity to store carbohydrate as a gas tank on a small car that holds 10–15 gallons of fuel and your capacity to store fat as gas tanks on a big rig that hold 150–300 gallons of fuel. The actual amount of glycogen a person stores varies with body size and diet, but an average person (150 to 175 pounds) might store 400–500 grams (1,600–2,000 kcal) of glycogen, mostly in their muscles and liver. In contrast, an athlete weighing 150 pounds (68 kilograms) with 10 percent body fat stores roughly 6,800 grams of fat (about 61,000 kcal). I’ll say it again. That’s 400 grams of carbohydrate versus 6,800 grams of fat. To take this illustration even further, a person weighing 300 pounds (136 kilograms) with a body fat of 50 percent has 68,000 grams of fat (over 600,000 kcal) stored away!
Given the body’s capacity to store massive quantities of fat, why don’t we exclusively rely on it to power all forms of exercise? The short answer is that fat burning is limited by one or more steps in the chain of events responsible for converting a fatty acid molecule into mechanical energy. This relative inability to burn fat becomes more pronounced as exercise intensity increases. Research by many independent scientists has shown that, on average, fat burning supplies half of the energy needed to power exercise at about 60 percent of VO2max (see Figure 4.1). As exercise intensity hits 80–90 percent of VO2max, fat burning typically contributes to 10 percent or less of energy production.2, 3 Eating a high-fat diet would allow an athlete to burn more fat at these higher intensities, but it would by no means completely abolish the need for carbohydrate burning during intense exercise.
Although scientists continue to debate what’s behind this fat-burning limit, the balance of evidence points to the transport of fatty acids into the power plants of our cells: the mitochondria. These incredible organelles are where fatty acids are oxidized for energy. But before this can happen, they must first be transported deep inside the mitochondria, which happens with the help of the molecule carnitine and several enzymes. High-intensity exercise (e.g., 200-meter dash, 60-second hockey shift) is believed to directly slow the mitochondrial transport of fatty acids, possibly through a reduction in free carnitine availability.4 Regardless of the true cause, it’s clear that burning fat alone cannot provide ATP fast enough to supply all the energy needed for high-intensity exercise.
Given the limits to carbohydrate storage—and its well-documented importance for fueling high-intensity exercise—it’s no surprise that most dietitians tell serious athletes to pay close attention to the amount of carbohydrate they eat. Athletes training at high volumes require somewhere between 2 grams and 5 grams of carbohydrate per pound of body weight for the whole day to maintain muscle glycogen stores.1 (This carbohydrate range roughly corresponds to one to four hours of moderate-to-intense exercise in a day.) Table 4.1 provides an example of the amounts and sources of carbohydrate that a 150-pound athlete would need to eat to achieve an intake of 4 grams of carbohydrate per pound of body weight, which is an appropriate amount to fuel a training day that involves two to three hours of moderate-to-high-intensity training.
Even though this menu represents a hefty amount of carbohydrate in comparison to what the average person eats daily, athletes competing in extreme events are known to consume even more. As an example, four-time Tour de France champion Chris Froome reportedly consumed 1.3 kilograms of carbohydrate on the day of stage 19 of the 2018 Giro d’Italia,5 and based on a reported weight of roughly 152 pounds, that calculates out to about 8.6 grams of carbohydrate for every pound of Chris Froome.
1. D. T. Thomas, K. A. Erdman, and L. M. Burke, “American College of Sports Medicine Joint Position Statement: Nutrition and Athletic Performance,” Medicine and Science in Sports and Exercise 48, no. 3 (2016): 543–568.
2. M. C. Venables, J. Achten, and A. E. Jeukendrup, “Determinants of Fat Oxidation During Exercise in Healthy Men and Women: A Cross-Sectional Study,” Journal of Applied Physiology 98, no. 1 (2005): 160–167.
3. T. Purdom et al., “Understanding the Factors That Effect Maximal Fat Oxidation,” Journal of the International Society of Sports Nutrition 15 (2018): 3, https://doi.org/10.1186/s12970-018-0207-1.
4. J. Jeppesen and B. Kiens, “Regulation and Limitations to Fatty Acid Oxidation During Exercise,” Journal of Physiology 590, no. 5 (2012): 1059–1068.
5. T. Fordyce, “Chris Froome: Team Sky’s Unprecedented Release of Data Reveals How British Rider Won Giro d’Italia,” BBC Sport, July 4, 2018, https://www.bbc.com/sport/cycling/44694122.
The Athlete’s Gut: The Inside Science of Digestion, Nutrition, and Stomach Distress is an in-depth look at the GI system that offers a much-needed resource for troubleshooting GI problems.