Carbohydrates are a cornerstone of athletic nutrition, yet they are frequently misunderstood. Some athletes embrace carbs as a primary fuel source, while others view them as an unnecessary contributor to weight gain or metabolic dysfunction. This polarisation is fuelled by popular dietary trends such as the ketogenic diet and low-carb high-fat (LCHF) regimens.
However, when it comes to optimising performance, recovery, and long-term health, a nuanced understanding of carbohydrates is essential. This article explores the science behind carbohydrates in athletic performance and health, debunks common myths, and offers practical guidance grounded in peer-reviewed research.
What Are Carbohydrates?
Carbohydrates are macronutrients found in a wide variety of foods including grains, fruits, vegetables, legumes, and dairy products. Chemically, they consist of carbon, hydrogen, and oxygen, and can be classified into three main categories: sugars (monosaccharides and disaccharides), starches (polysaccharides), and fibre (non-digestible carbohydrates). Once ingested, carbohydrates are broken down into glucose, which is used by the body for energy or stored in the liver and muscles as glycogen.
Carbohydrates and Energy Metabolism
Glucose is the preferred fuel source for high-intensity exercise due to its ability to be rapidly metabolised. During exercise, muscle glycogen is the primary substrate for ATP production, particularly in activities lasting longer than a few seconds and involving moderate to high intensity. Research by Cermak and van Loon (2013) demonstrates that carbohydrate ingestion before and during prolonged endurance events enhances performance by maintaining blood glucose levels and delaying fatigue. The rate at which glycogen stores are depleted is a limiting factor in prolonged performance, a concept often referred to as “hitting the wall” in endurance sports.
Glycogen: The Critical Energy Reserve
The body stores approximately 300-500 grams of glycogen, depending on training status and diet. Glycogen is hydrophilic, meaning it is stored with water, approximately 3 grams of water per gram of glycogen. This has implications for weight fluctuation and hydration strategies. According to Burke et al. (2011), replenishing glycogen stores after training is critical for recovery and repeated performance, especially in athletes training multiple times per day. Inadequate glycogen repletion can lead to overtraining symptoms and increased injury risk.
Low-Carb Diets and Athletic Performance
Low-carbohydrate diets have gained popularity among athletes aiming for body composition changes or metabolic flexibility. While ketogenic diets can enhance fat oxidation and have shown promise in ultra-endurance contexts, the literature suggests a trade-off in high-intensity performance. A study by Burke et al. (2017) found that elite race walkers following a ketogenic diet exhibited increased fat oxidation but impaired economy and performance compared to their high-carbohydrate counterparts. This suggests that while fat adaptation may benefit low-intensity endurance efforts, it may compromise efficiency in events requiring bursts of speed or sustained high output.
Carbohydrate Periodisation
A more tailored approach, carbohydrate periodisation, involves strategically varying carbohydrate intake based on training demands. This concept is supported by Impey et al. (2018), who argue that training with low glycogen availability can enhance mitochondrial biogenesis, while high-carbohydrate intake is essential for performance and recovery in key sessions. This dual approach allows athletes to gain metabolic adaptations without compromising performance in important training or competition.
Carbohydrates and Recovery
Post-exercise carbohydrate intake accelerates glycogen resynthesis, particularly when consumed within the first hour after exercise. Ivy et al. (1988) demonstrated that immediate carbohydrate ingestion post-exercise results in significantly higher glycogen resynthesis rates than delayed intake. Co-ingestion with protein (at a ratio of approximately 3:1) can further enhance recovery, as shown by Zawadzki et al. (1992), which is relevant for athletes with limited time between sessions.
Immune Function and Carbohydrates
Intensive training can suppress immune function, increasing the risk of illness. Carbohydrate intake before and during prolonged exercise may attenuate this effect. Nieman et al. (1998) found that consuming carbohydrates during prolonged exercise reduces the release of stress hormones and inflammatory cytokines, thereby supporting immune health. Athletes undergoing heavy training loads may benefit from regular carbohydrate ingestion to mitigate immune suppression.
Carbohydrates and Brain Function
Glucose is the brain’s primary energy source, and maintaining adequate levels is essential for cognitive function, decision-making, and motor skills during training and competition. Research by Lieberman (2007) has shown that hypoglycaemia during prolonged exercise can impair mental performance, a critical consideration in sports that require tactical thinking or technical execution under fatigue.
Body Composition and Carbohydrates
The belief that carbohydrates inherently lead to fat gain is not supported by scientific evidence. Weight gain results from sustained energy surplus, not carbohydrate intake per se. A meta-analysis by Hall and Guo (2017) found that, when calories and protein were matched, the ratio of carbohydrates to fat had minimal impact on fat loss. Therefore, athletes can include carbohydrates as part of a calorically appropriate diet without compromising body composition goals.
Carbohydrates for Strength and Power Athletes
While endurance athletes are typically more associated with high carbohydrate needs, strength and power athletes also benefit from sufficient carbohydrate intake. Carbohydrates support high training volumes, improve training quality, and facilitate recovery. Haff et al. (2000) noted that carbohydrate ingestion before and after resistance training improved performance and reduced muscle damage markers. This reinforces the importance of carbs beyond just endurance performance.
Practical Recommendations
Carbohydrate needs vary depending on the sport, training load, and individual tolerance. According to the American College of Sports Medicine (Thomas et al., 2016), general guidelines are:
- Light training (low-intensity or skill-based): 3-5 g/kg/day
- Moderate training (1 h/day): 5-7 g/kg/day
- High-volume endurance training (1-3 h/day): 6-10 g/kg/day
- Extreme training loads (4+ h/day): 8-12 g/kg/day Timing also matters. Consuming carbohydrates before, during, and after exercise optimises performance and recovery. Low-glycaemic carbs are beneficial before prolonged sessions, while high-glycaemic options are more effective post-exercise for rapid glycogen restoration.
Conclusion
Carbohydrates are neither inherently good nor bad. Their role in an athlete’s diet depends on the context of training, performance goals, and individual response. While low-carb strategies may have niche applications, the overwhelming consensus in the scientific literature supports carbohydrates as essential for athletic performance, recovery, cognitive function, and health. Rather than vilifying or blindly embracing them, athletes should approach carbohydrate intake strategically, using evidence-based practices to support their goals.
Key Takeaways
References
Burke, L.M., Hawley, J.A., Angus, D.J., Cox, G.R., Clark, S.A., Cummings, N.K., Desbrow, B. and Hargreaves, M., 2002. Adaptations to short-term high-fat diet persist during exercise despite high carbohydrate availability. Medicine and Science in Sports and Exercise, 34(1), pp.83-91.
Burke, L.M., Ross, M.L., Garvican-Lewis, L.A., Welvaert, M., Heikura, I.A., Forbes, S.G., Mirtschin, J.G., Cato, L.E., Strobel, N., Sharma, A.P. and Hawley, J.A., 2017. Low-carbohydrate, high-fat diet impairs exercise economy and negates the performance benefit from intensified training in elite race walkers. Journal of Physiology, 595(9), pp.2785-2807.
Cermak, N.M. and van Loon, L.J., 2013. The use of carbohydrates during exercise as an ergogenic aid. Sports Medicine, 43(11), pp.1139-1155.
Hall, K.D. and Guo, J., 2017. Obesity energetics: body weight regulation and the effects of diet composition. Gastroenterology, 152(7), pp.1718-1727.
Haff, G.G., Schroeder, C.A., Tesch, P.A. and Bowers, J.L., 2000. The effect of carbohydrate supplementation on multiple sessions and time to recovery in elite weightlifters. Journal of Strength and Conditioning Research, 14(2), pp.197-204.
Impey, S.G., Hearris, M.A., Hammond, K.M., Bartlett, J.D., Louis, J., Close, G.L. and Morton, J.P., 2018. Fuel for the work required: A theoretical framework for carbohydrate periodisation and the glycogen threshold hypothesis. Sports Medicine, 48(5), pp.1031-1048.
Ivy, J.L., Katz, A.L., Cutler, C.L., Sherman, W.M. and Coyle, E.F., 1988. Muscle glycogen synthesis after exercise: effect of time of carbohydrate ingestion. Journal of Applied Physiology, 64(4), pp.1480-1485.
Lieberman, H.R., 2007. Cognitive methods for assessing mental energy. Nutritional Neuroscience, 10(5-6), pp.229-242.
Nieman, D.C., Henson, D.A., Davis, J.M., Angela, G., Johnson, R.L., Angela, H., Murphy, E.A., Dumke, C.L. and Utter, A.C., 1998. Carbohydrate intake attenuates the post-exercise cytokine response. International Journal of Sports Nutrition and Exercise Metabolism, 8(4), pp.404-413.
Thomas, D.T., Erdman, K.A. and Burke, L.M., 2016. Position of the Academy of Nutrition and Dietetics, Dietitians of Canada, and the American College of Sports Medicine: Nutrition and athletic performance. Journal of the Academy of Nutrition and Dietetics, 116(3), pp.501-528.
Zawadzki, K.M., Yaspelkis III, B.B. and Ivy, J.L., 1992. Carbohydrate-protein complex increases the rate of muscle glycogen storage after exercise. Journal of Applied Physiology, 72(5), pp.1854-1859.
