Why your calorie needs change on training days
Most people eat roughly the same thing every day, then wonder why a 20 km run leaves them flattened and a rest day leaves them restless and snacking. The energy cost of a training week is not spread evenly, so intake that never changes is either too low on the hard days or too high on the quiet ones.
What the guidance says
Carbohydrate recommendations are expressed per kilogram per day and scale with training load.
Sports nutrition guidance sets daily carbohydrate targets on a sliding scale — roughly 3–5 g/kg for light activity up to 6–10 g/kg for one to three hours of moderate-to-high intensity endurance work per day.
Burke et al. · J Sports Sci · 2011 · Peer-reviewed researchProtein needs are driven by body mass and training status, not by whether today is a training day.
Position-stand guidance places daily protein for training individuals at roughly 1.4–2.0 g/kg, distributed across meals, with no recommendation to lower intake on non-training days.
Jäger et al. · J Int Soc Sports Nutr · 2017 · Position standPersistently eating less than your training demands carries real physiological costs.
Consensus work on relative energy deficiency in sport links low energy availability to impairments in bone health, endocrine function, immunity and performance, in both women and men.
International Olympic Committee · Br J Sports Med · 2023 · Consensus statementWhat a week might look like
Protein stays at roughly 1.6–2.2 g/kg across all of those days . Fat absorbs most of the remaining difference, though it is normal for total energy to be several hundred calories higher on a long-run day.
Two reasonable approaches
Match intake to the day
Suits — Better session quality and less rest-day overeating
Costs — Requires knowing your week in advance and some tracking
Steady daily intake
Suits — Simple, habit-friendly, fine at moderate training volumes
Costs — Under-fuels the hardest days once volume gets high
Practical rules
- Set protein once, by body mass, and keep it the same every day.
- Add carbohydrate around the session rather than spreading it thinly — before, during if the session is over about 90 minutes, and after.
- On rest days, take the reduction mostly from starchy carbohydrate, not from protein or vegetables.
- Do not reduce intake the day before a long run or a key session.
- Judge the approach over a fortnight of session quality, not over the scale from one morning to the next.
How CALIBR handles it
Your Fuelling targets in CALIBR move with what is actually on your plan for that date. A long-run day raises the carbohydrate target and shifts more of it around the session; a rest day lowers it while holding your protein target steady. Swap a meal and the targets follow the swap, so the numbers you see match the day you are going to have.
Key takeaways
- Carbohydrate is the nutrient that should change day to day; protein should not.
- Guidance scales carbohydrate per kilogram of body mass with training load.
- Cut rest-day intake from starch, not from protein or vegetables.
- Never under-eat the day before a key session.
- Chronic under-fuelling has documented health and performance costs, not just slower progress.
Common questions
References
- [1]Carbohydrates for training and competitionBurke et al. · J Sports Sci · 2011 · Peer-reviewed research
- [2]International Society of Sports Nutrition position stand: protein and exerciseJäger et al. · J Int Soc Sports Nutr · 2017 · Position stand
- [3]2023 IOC consensus statement on Relative Energy Deficiency in Sport (REDs)International Olympic Committee · Br J Sports Med · 2023 · Consensus statement
- [4]Nutrition and athletic performance (joint position statement)ACSM, Academy of Nutrition and Dietetics & Dietitians of Canada · Med Sci Sports Exerc · 2016 · Position stand
- [5]Sports nutrition resources — fuelling, energy availability and supplementsAustralian Institute of Sport · 2025 · Professional guidance