Your Guide to Fiber Intake for Runners
May 3, 2024
Back a few years ago, low carb intake while running was all the rage. Runners took fasted training to extremes. Many would (and still do) complete extreme two to three hour-long runs without any carbohydrates during or before. Now, at the time of this publication in 2022, some running coaches still mistakenly recommend it for athletes, even without the guidance of a sports dietitian. However, low carbohydrate availability in training may not enhance performance for long-distance runners, especially in the manner which many recreational runners practice it. Additionally, inadequate carbohydrate restriction (particularly outside of a periodized, monitored plan) may be detrimental to athlete health – and therefore performance.
The low carb training hypothesis emerged when researchers observed enhanced cellular level adaptations in response to limited carbohydrate availability. These observations were made initially in studies using untrained individuals. Reduced muscle glycogen increases cellular signaling to activate proteins such as AMPK, which are responsible for metabolic adaptations. As a result, there may be increased mitochondrial biogenesis and improved fat oxidation.
Given the improved cellular level adaptations seen in untrained athletes, researchers then began to apply low carb training to running and cycling in trained athletes. These approaches included fasted training, two-a-day training with a short time and minimal carbohydrate (CHO) replenishment between sessions, and sleep low approaches.
Notably, carbohydrate-restricted training sessions in the research were typically not performed in long runs (training sessions lasting 2-3 hours). Most laboratory train-low studies used endurance training sessions lasting 60-120 minutes. When implemented in case studies, fasted training sessions occurred early in marathon training, not during the specific preparation phase or taper phase (see Stellingwerff’s 2012 case study in the International Journal of Sport Nutrition and Exercise Metabolism).
As the studies progressed over the years, the results were at best equivocal for the efficacy of train low approaches. A 2009 study in Acta Physiologica found changes in muscle cell adaptations, but no changes in cardiorespiratory fitness. After examining eleven studies, a 2018 article in Sports Medicine found that only 37% of results indicated a small amount of performance improvement. The researchers concluded that “such muscle adaptations do not always translate to improved exercise performance.”
A 2021 meta-analysis published in the Journal of the International Society of Sports Nutrition examined the results of thirteen studies on periodic carbohydrate restriction. The researchers concluded that the present body of research does not support training with low carb running sessions. They stated: “the physiological stimuli prompted by undertaking an acute exercise bout with low CHO availability…does not translate into clear measurable enhancements of performance in already adapted endurance-trained athletes compared to training with high CHO availability” (Geji et al., 2021).
Let that sink in: high carb availability, not carb restriction during exercise, promotes performance. Let’s take a look more into the mechanisms behind that and why sports nutrition is leaving behind train low methods. If anything, the field is moving in the direction of investigating increasing carb dependence for optimal performance in long-distance running.
One common argument for glycogen depletion runs is increased fat oxidation. However, the present body of literature suggests that increased fat oxidation without improved carb oxidation does not improve performance. (Read more in this fantastic 2019 article in the International Journal of Sport Nutrition and Exercise Metabolism from the field’s top researchers.) Particularly, increased fat oxidation from carbohydrate restriction often comes with a decrease in running economy. Given that running economy is a known predictor of long-distance running performance, increased fat oxidation from carb restriction is thus associated with a decline in performance (or no improvements at best).
In a wealth of studies on nutrition periodization, train low methods such as glycogen depletion have fallen out of favor for “fueling for the work required” approaches. In this framework, athletes adjust their carbohydrate intake based on the demands of a training session. Carb intake before and during meets the needs of training sessions with high energy demands. For running, the eccentric loading and weight-bearing nature naturally increase energy demands compared to modalities such as cycling. This framework excludes glycogen depletion long runs, due to the high carbohydrate demands of long runs exceeding two hours in duration. Any train-low sessions (if any) occur during low-carbohydrate demand sessions such as recovery runs. This is only if the athlete has high-energy availability.
Training with adequately available carbs improves training quality and carbohydrate oxidation and oxygen-dependent glycolysis. In real-world terms, improved training quality means that can handle more training and adapt appropriately. Improved carbohydrate oxidation means that your body can better absorb and transport carbohydrates for ATP production.
Inadequate carb availability can impair bone turnover (Stellingwerff, 2019). This inhibition of bone turnover increases the risks of bone stress injuries. Needless to say, the six to eight weeks off of training due to a bone stress injury are detrimental to performance.
Additionally, a 2020 study in Nutrients found that carbohydrate restriction during exercise may disrupt your body’s iron homeostasis. A single session will not have much effect on iron. However, the researchers found that, over time, carb-restricted training increased hepcidin levels, which in turn blocked iron absorption. Iron is vital for athletes, as it forms hemoglobin and myoglobin, both of which are responsible for oxygen delivery.
Low-energy availability can have deleterious effects for athletes, as described in this 2020 review published in Nutrients. For many recreational athletes, meeting energy needs without guidance is difficult enough. Glycogen depletion runs and other train-low approaches would make it even more difficult. Given that most female runners struggle to meet adequate CHO demands, train-low approaches can be dangerous. Male runners should be careful also, as chronically low CHO is associated with low testosterone levels.
As the field of sports nutrition progresses, researchers are connecting relative energy deficiency in sports (RED-S) and overtraining syndrome. A 2021 review in Sports Medicine theorized that undereating significantly contributes to overtraining syndrome. The two share similar pathways and manifest with similar symptoms. The researchers concluded that even small within-day energy deficits (300-400 kcal), when multiplied over months and years, can become clinically significant. This chronic energy and carb deficiency can inhibit an athlete’s ability to recover from and handle their training load. The key takeaway: eating enough – especially enough carbohydrates – and avoiding within-day energy deficits is vital for long-term growth as an athlete.
I’d argue also that repeated practice of glycogen depletion runs may be a red flag. It may be about presumed training adaptations or it may be about restricting calories. Disordered eating can be a slippery slope into amenorrhea, poor bone health, and clinical eating disorders (which carry an alarming mortality rate). Glycogen depletion runs should be particularly discouraged amongst athletes who exhibit disordered eating patterns.
When elites practice periodized nutrition, knowledge professionals guide them. These professionals closely monitor and individualize their training and nutritional practices. The research is presently equivocal on modest train-low approaches. However, low-carb running sessions, particularly those of longer duration and/or higher intensity, may lower the quality of training and increase injury risk. The question for each runner becomes: do the risks outweigh the (unproven and possibly unlikely) rewards?
Meanwhile, the research is trending in the direction of high carbohydrate availability in training. Until we know more, the research so far indicates that eating enough and fueling your body well in exercise allows you to handle greater training loads with less injury risk.
Disclaimer: I am working on my Masters of Science in Applied Exercise Science with a concentration in Sports Nutrition. However, I am not a registered dietitian.
Burke, L. M., Jeukendrup, A. E., Jones, A. M., & Mooses, M. (2019). Contemporary nutrition strategies to optimize performance in distance runners and race walkers. International Journal of Sport Nutrition and Exercise Metabolism, 29(2), 117–129. https://doi.org/10.1123/ijsnem.2019-0004
Gejl, K. D., & Nybo, L. (2021). Performance effects of periodized carbohydrate restriction in endurance trained athletes: A systematic review and meta-analysis. Journal of the International Society of Sports Nutrition, 18(1), 37. https://doi.org/10.1186/s12970-021-00435-3
Impey, Samuel G et al. (2018). “Fuel for the work required: A theoretical framework for carbohydrate periodization and the glycogen threshold hypothesis.” Sports Medicine 48 (5) (1031-1048. doi:10.1007/s40279-018-0867-7
McKay, A., Pyne, D. B., Burke, L. M., & Peeling, P. (2020). Iron metabolism: Interactions with energy and carbohydrate availability. Nutrients, 12(12), 3692. https://doi.org/10.3390/nu12123692
Nybo, L., Pedersen, K., Christensen, B., Aagaard, P., Brandt, N., & Kiens, B. (2009). Impact of carbohydrate supplementation during endurance training on glycogen storage and performance. Acta Physiologica, 197(2), 117–127. https://doi.org/10.1111/j.1748-1716.2009.01996.
Rothschild, J. A., Kilding, A. E., & Plews, D. J. (2020). What Should I Eat before Exercise? Pre-Exercise Nutrition and the Response to Endurance Exercise: Current Prospective and Future Directions. Nutrients, 12(11), 3473. https://doi.org/10.3390/nu12113473
Stellingwerf T. (2012). Case study: Nutrition and training periodization in three elite marathon runners. International Journal of Sport Nutrition and Exercise Metabolism, 22(5), 392–400. https://doi.org/10.1123/ijsnem.22.5.392
Stellingwerff, T., Heikura, I. A., Meeusen, R., Bermon, S., Seiler, S., Mountjoy, M. L., & Burke, L. M. (2021). Overtraining syndrome (OTS) and relative energy deficiency in sport (RED-S): Shared pathways, symptoms and complexities. Sports Medicine, 51(11), 2251–2280. https://doi.org/10.1007/s40279-021-01491-0
Stellingwerff, T., Morton, J. P., & Burke, L. M. (2019). A framework for periodized nutrition for athletics. International Journal of Sport Nutrition and Exercise Metabolism, 29(2), 141–151. https://doi.org/10.1123/ijsnem.2018-0305
