Fuel for the work required — carbohydrate periodization for runners
Carbohydrate is no longer a static daily number. It is a daily variable tied to the training load. Learn the sliding scale that elite coaches now use.
The 2016 joint position statement of the Academy of Nutrition and Dietetics, Dietitians of Canada and the American College of Sports Medicine established the global sliding scale for runner carbohydrate intake: 3–5 g/kg/day for low-intensity days, 5–7 g/kg/day for ~1 hour moderate training, 6–10 g/kg/day for 1–3 hour moderate-to-high training, and 8–12 g/kg/day for >4 hour commitment days. Under this framework, a 70 kg runner's daily carb target varies from 210 g on a rest day to 700+ g on a long-run or race day.
The operational translation — popularised by Trent Stellingwerff (IJSNEM, 2018) and Impey et al. (Sports Medicine, 2018) — is "fuel for the work required". Match carbohydrate intake to each session's demands rather than applying a single daily ratio. Easy aerobic runs can be completed with lower carbohydrate availability, amplifying mitochondrial and fat-oxidation adaptations. Threshold sessions and intervals require higher carbohydrate availability to protect workout quality. Long runs and races require full fuelling and race-nutrition rehearsal.
The "train-low, compete-high" family of protocols exploits this. Two-a-days with low-carb recovery between sessions, overnight fasted easy runs after a glycogen-depleting evening session, and Laurent Marquet's sleep-low protocol (MSSE, 2016) — evening high-intensity session, low-carb dinner, sleep, fasted easy run, then refuel — produced measurable performance gains in trained triathletes. The effect is not magical; it selectively stresses the cellular adaptations that shift toward greater fat oxidation capacity.
The counterweight is important. Louise Burke and collaborators have repeatedly warned against chronic train-low in high-volume athletes due to immune suppression, under-recovery, and RED-S risk. Periodization must be periodized. Use train-low strategically in base phase; use fuel-high in race-specific work and peak phases. Never combine low-carb training with energy restriction — that is the recipe for broken runners.
Practical template for a 70 kg marathoner in the build phase: 4 g/kg (280 g) on complete rest; 5–6 g/kg on easy runs 45–75 min; 7–8 g/kg on tempo or interval days; 8–10 g/kg on long runs ≥2 hours; 10 g/kg in the 36–48 hours before a race simulation or race. Carbohydrate shifts by the day, not by the week.