Why Fuel Timing Matters
Ask most athletes what makes a great training block and they will talk about mileage, intensity, and recovery. What is far less commonly discussed and yet arguably just as important is the fuel that makes all of it possible. Carbohydrate intake before, during, and after training is not a finishing touch on an athlete's nutrition plan. It is foundational architecture. And when it is consistently inadequate, the consequences reach far beyond feeling sluggish in a session. They penetrate the body's hormonal, metabolic, and cellular systems in ways that can silently undermine performance and long-term health.
This article, by Dr. Sam Impey of Hexis, examines the evidence for why carbohydrates are central to sustained metabolic health in athletes, what happens physiologically when they are chronically restricted, and how to translate the science into practical fueling decisions.
Carbohydrates Are More Than a Fuel Source
The traditional framing of carbohydrates as little more than an energy currency for exercising muscle is a significant understatement of their role. Dietary carbohydrates function as primary signalling molecules for the neuroendocrine system. Muscle and liver glycogen depletion, alongside low circulating blood glucose, signals a state of physiological stress directly to the hypothalamus the brain's central energy sensor, triggering adaptive responses that extend well beyond the immediate training session [3].
This is why carbohydrate availability has been formally recognised by the 2023 International Olympic Committee (IOC) Consensus Statement on Relative Energy Deficiency in Sport (REDs) as having an additive and independent impact on metabolic health, and importantly, separate from total caloric intake [3]. In other words, even when an athlete consumes sufficient total calories, insufficient carbohydrate can independently drive hormonal disruption, impair iron regulation, suppress immune function, and accelerate the onset of endocrine dysfunction [4].
Muscle glycogen has also been reframed in contemporary sports science literature. Rather than a passive fuel reservoir, it is now understood to act as a 'fuel sensor' one that actively regulates the molecular adaptations your muscles make to exercise [4]. Get this wrong repeatedly, and you compromise the very adaptations your training is designed to produce.
The Metabolic Cost of Under-Fueling
The concept of Energy Availability (EA) which is defined as the dietary energy remaining to support basic physiological functions after subtracting the energy cost of exercise provides the framework for understanding how the body responds when fueling is insufficient [5]. When EA drops below approximately 30 kcal per kilogram of fat-free mass per day, the hypothalamus begins to down-regulate systems it perceives as non-essential, preserving resources for survival over performance [5].
The thyroid axis is among the first to respond. High training volumes and intensity combined with sustained periods of low carbohydrate availability has been shown to independently suppress the hypothalamic-pituitary-thyroid axis, with studies consistently demonstrating reductions in circulating triiodothyronine (T3) a pattern that appears to be driven by carbohydrate deprivation specifically, rather than caloric restriction alone [1,2]. T3 is a primary regulator of metabolic rate, and its suppression results in lethargy, cold intolerance, impaired glycogen storage, and weight loss resistance, a cluster of symptoms clinically referred to as Low T3 Syndrome [6].
Critically, T3 and insulin do not respond to energy restriction with a simple threshold effect. Research by Loucks and colleagues demonstrated that these hormones decrease in a linear, dose-dependent fashion as soon as energy availability begins to fall, even at levels considered subclinical [5]. This means metabolic rate is being eroded earlier, and more subtly, than most athletes would expect.
When carbohydrate is unavailable as fuel, the body is forced to upregulate gluconeogenesis manufacturing glucose from non-carbohydrate sources to maintain blood glucose supply to the brain. The substrates for this process are amino acids stripped from skeletal muscle and collagen-rich connective tissues [4]. The athlete's body begins consuming itself. Simultaneously, cortisol rises and insulin-like growth factor 1 (IGF-1) falls, creating a catabolic hormonal environment that actively opposes the muscle growth and tissue repair that training demands [6]. What that means for anyone exercising is a reduced ability to complete training, slower recovery and greater risk of injury.
Performance Consequences
The performance ramifications of chronic under-fueling are substantial and measurable. Severe energy restriction decreases muscle protein synthesis even when dietary protein intake is adequate and training stimuli are maintained [6]. This means an athlete can be hitting their protein targets, completing their sessions, and still losing muscle — because without adequate carbohydrate, the anabolic environment needed to support repair simply does not exist.
The practical impact on performance is starkly illustrated in case study data. An elite female Muaythai fighter monitored across a seven-week period that included a chronic weight loss phase and a period of rapid weight loss saw her peak cycling power output drop by 27% during the most energy-restricted phase of her camp [9]. Her resting metabolic rate depressed by 253 kcal per day, thyroid hormones declined, and renal strain markers elevated, all consequences of the body entering an energy conservation state. Performance and physiology degraded together!
A less visible but equally damaging phenomenon is within-day energy deficiency. An athlete can technically achieve an adequate 24-hour EA total while still spending prolonged periods during the day in acute energy deficit. Research shows that athletes who spend more than 11 to 21 consecutive hours in a daily energy deficit exceeding 400 kcal exhibit suppression of resting metabolic rate and altered endocrine profiles identical to those experiencing chronic under-fueling [7]. The timing of carbohydrate intake, not just the total, matters.
How Much Carbohydrate Do Athletes Actually Need?
Carbohydrate requirements vary significantly based on training volume, intensity, and the adaptation being targeted. During periods of high-intensity or high-volume training, intakes of 4 to 10 g per kilogram of body weight per day are generally recommended to preserve glycogen stores, maintain blood glucose, and protect the neuroendocrine milieu from perceiving a state of famine [3,4]. The lower end of this range is appropriate for moderate training days or recovery days; the upper end is reserved for heavy training blocks and competition preparation.
During exercise itself, for sessions exceeding 60 to 70 minutes at moderate-to-high intensity, ingesting 30 to 90 grams of carbohydrate per hour can maintain blood glucose levels, spare glycogen, and delay fatigue [4,]. For sessions exceeding two hours, intakes toward the upper end of this range ideally combining multiple carbohydrate sources such as glucose and fructose to maximise intestinal absorption are recommended.
Post-exercise, rapid glycogen resynthesis is a critical but frequently neglected component of recovery. Consuming 1.2 grams of carbohydrate per kilogram of body weight immediately post-exercise, and again at hourly intervals for the first four hours after finishing, produces the most rapid rate of glycogen replenishment [4]. Combining this with 20 to 30g of protein supporting glycogen restoration and muscle repair.
The clinical evidence reinforces just how powerful even modest carbohydrate additions can be. In a six-month intervention trial by Cialdella-Kam and colleagues, collegiate female endurance runners with exercise-related menstrual dysfunction were prescribed a daily supplement providing an additional 360 kcal including 54 grams of carbohydrate and 20 grams of protein without any reduction in training load [8]. By increasing EA through targeted carbohydrate supplementation alone, the athletes restored menstrual function in an average of 2.6 months, and mood state depression scores improved by 8% [8]. The hormonal system, given the right fuel, recovered.
Practical Fueling Principles
Translating the evidence into daily practice comes down to a small number of consistent behaviours. First, carbohydrate intake should be periodised to training load higher carbohydrate intake on heavy training days, and lower, but never zero, carbohydrate intake on true rest days. Treating every day the same regardless of demand is one of the most common causes of inadvertent under-fueling in athletes who are otherwise engaged with their nutrition.
Second, high-intensity sessions should, generally, not be trained in a fasted state. While low-carbohydrate training has a place in some periodised training models, consistently performing quality, high-intensity work without available glycogen impairs both the training session itself and the adaptive signalling that follows. The hypothalamus cannot distinguish intent from outcome it responds to the fuel environment it detects.
Third, do not delay post-exercise carbohydrate. Athletes who train in the morning and wait until lunch to eat properly are accumulating hours of within-day energy deficit. A targeted post-training snack even something straightforward like a banana and nut butter on toast/bagel can meaningfully shift the metabolic signal the body receives.
Finally, watch for the early signs that carbohydrate intake may be insufficient: persistent fatigue that does not resolve with rest, poor session quality, frequent illness, mood disturbances, unexplained weight loss, and cold intolerance. These are not signs of overtraining in isolation they are often signs of under-fueling presenting through the same physiological pathway.
Conclusion
Carbohydrates are not a dietary luxury for athletes, nor are they a variable to be minimised in pursuit of leanness. They are the primary substrate through which the body sustains the hormonal, metabolic, and cellular conditions required for adaptation and health. When chronically restricted, they trigger a cascade of consequences from suppressed thyroid function and elevated cortisol to impaired protein synthesis and reduction in performance that no amount of training volume or protein intake can offset.
Fueling adequately is not separate from training. It is part of training. The athlete who eats to perform, and eats consistently enough to maintain metabolic health across a season, is the athlete who adapts, recovers, and competes at their best".
References
[1] Mountjoy M, Sundgot-Borgen J, Burke L, et al. IOC consensus statement on relative energy deficiency in sport (REDs): 2018 update. Br J Sports Med. 2018;. 52(11): 687 – 697
[2] Papatriantafyllou E, Siopi A, Mougios V, et al. Low-glycemic load diets and thyroid function: a narrative review and future perspectives. Nutrients. 2024;16(3):347.
[3] Mountjoy M, Sundgot-Borgen JK, Burke LM, et al. 2023 International Olympic Committee's (IOC) consensus statement on Relative Energy Deficiency in Sport (REDs). British Journal of Sports Medicine. 2023;57(17):1073–1097.
[4] Lodge MT, Ward-Ritacco CL, Melanson KJ. Considerations of Low Carbohydrate Availability (LCA) to Relative Energy Deficiency in Sport (RED-S) in Female Endurance Athletes: A Narrative Review. Nutrients. 2023;15(20):4457.
[5] Loucks AB, Verdun M, Heath EM. Low energy availability, not stress of exercise, alters LH pulsatility in exercising women. Journal of Applied Physiology. 1998;84(1):37–46.
[6] Jeppesen JS, Hellsten Y, Melin AK, Hansen M. Short-Term Severe Low Energy Availability in Athletes: Molecular Mechanisms, Endocrine Responses, and Performance Outcomes — A Narrative Review. PMC. 2025. Available from: https://pmc.ncbi.nlm.nih.gov/articles/PMC12180388/
[7] Logue DM, Madigan SM, Melin A, et al. Low Energy Availability in Athletes 2020: An Updated Narrative Review of Prevalence, Risk, Within-Day Energy Balance, Knowledge, and Impact on Sports Performance. Nutrients. 2020;12(3):835.
[8] Cialdella-Kam L, Guebels CP, Maddalozzo GF, Manore MM. Dietary Intervention Restored Menses in Female Athletes with Exercise-Associated Menstrual Dysfunction with Limited Impact on Bone and Muscle Health. Nutrients. 2014;6(8):3018–3039.
[9]Bulinova V, Wanger A, Kumstat M. Weight cycling and relative energy deficiency in sport syndrome in an elite female muaythai athlete: a case study. Frontiers in Sports and Active Living. 2025.