When you’re training for a triathlon, the mantra “fuel for the work required” couldn’t be more true. Carbohydrates (CHO) are the key fuel for moderate to high intensity work — but the how, when and how much matter just as much as the what. You want to avoid under-fueling (you’ll suffer performance, recovery and adaptation) and over-fueling (you’ll carry excess weight or blunt fat burning when desired). Let’s break it down.
1. Nutrition Prioritisation / Nutrition Periodisation
Often referred to in sports science as periodised nutrition, this is the strategic alignment of your nutrition (especially carbs) with training phases, session demands and adaptation goals. Biomedres+3NSCA+3worldathletics.org+3
In practice for a triathlete this means:
- On high-intensity or key sessions: you prioritise high carbohydrate availability (to support performance and stress adaptation).
- On lower-intensity or easy sessions: you might choose reduced carbohydrate availability (to promote metabolic flexibility, fat adaptation or recovery without excess CHO).
- Across days/weeks: you vary carbohydrate intake in line with training load (micro-, meso- and macro-cycles). worldathletics.org+1
Why do this? Because your body responds differently depending on fuel availability. Training with low glycogen or reduced CHO can trigger adaptations (e.g., increased mitochondrial signalling) but it may compromise performance if used at the wrong time. BioMed Central
So your goal: match your carb availability to the work required — neither all high, nor all low.
2. Exogenous vs Endogenous Carbohydrates
- Endogenous carbs = glycogen stored in your muscle and liver, plus circulating blood glucose. These are your internal carbohydrate stores.
- Exogenous carbs = the carbohydrates you consume during exercise (or immediately before) via drinks, gels, bars, etc.
Why make the distinction? Because during training (especially long or high intensity), you’ll tap into endogenous stores and you can supplement with exogenous carbs to maintain blood glucose, spare muscle/liver glycogen and delay fatigue. Recent work has shown that exogenous carb oxidation (how fast the carbs you ingest are used) varies considerably between athletes. BioMed Central+2MDPI+2
For example: a 2025 proof-of-concept study found that some athletes could achieve peak exogenous oxidation with ~65 g/h of glucose rather than 90 g/h, without impairing performance—suggesting more isn’t always better for everyone. BioMed Central
3. How Much Carbohydrate Can You Oxidise?
Many athletes consume 120 g+ per hour, but the evidence suggests oxidation peaks around 90 g/h (for mixed carbs). The literature supports this: guidelines for prolonged exercise (>2.5 h) often cite up to ~90 g CHO/h as a “ceiling” for maximal exogenous oxidation in many athletes. NSCA+2BioMed Central+2
What does “extra” carbohydrate do (i.e., above what you oxidise)? Some plausible benefits:
- It may support central nervous system (CNS) / brain fuel (glucose for brain activity, delaying central fatigue).
- It may signal to the body “fuel is available” which may permit you to sustain higher output or delay fatigue.
- It may help recovery post‐session by providing substrate for glycogen re-plenishment.
The evidence is still emerging: for instance a recent review described how carbohydrate supplementation influences glycogen storage, blood glucose and performance, and noted that the “extra” may not always yield proportional oxidation gains—it depends on individual capacity, gastrointestinal tolerance and session demands. MDPI
Therefore: consume enough carbs to match the work demand, train your gut to tolerate higher intakes if needed (e.g., for long rides/races), but recognise that blindly hitting super high numbers (e.g., 120 g/h) may not yield extra oxidation and may increase GI risk.
4. “Go Slow – Go Fast Fuel” Framework
A practical approach you use: match your type of fuel to session intensity and substrate-use.
- Go Slow (Zones 1 & 2 / easy endurance): These sessions rely largely on the oxidative system—fat and low-intensity carb use. Carbohydrate intake during these sessions can be relatively modest. Example: you aim for ~40 g CHO/h on a long endurance ride (16 weeks out from race) and you might use slower‐releasing sources (oat bars, bananas) rather than fast sugar hits.
- In Between (Zone 3 / moderate‐high tempo): This is a blend — carb requirement starts to increase, and you may shift from slow to a mix of slow + fast release carbs.
- Go Fast (Zone 4+ / high intensity, interval, race work): These sessions are highly glycogen-dependent. Fast releasing carbs per hour (gels, sports drink, lollies) are appropriate to support high exertion and maintain intensity.
Putting the example in context: If your goal is 40 g CHO/h on an easy ride and an oat bar has ~20 g CHO, you could eat one every 30 min. On a high intensity session you may target 60-90 g CHO/h (or whatever your personalised max is), via faster carbs.
5. Practical Tips & Athlete-Wise Adjustments
- Train your gut: If you plan to use high CHO/h in races, you must practise tolerating that during training (so your stomach and gut handle the load).
- Match carbs to session length + intensity: Short easy spin → minimal carb. Long or intense session → higher carb.
- Periodise across training blocks: In heavy load blocks, you might favour higher carbohydrate availability. In adaptation blocks (where you want metabolic stress) you might intentionally reduce CHO on selected sessions. But only if you can maintain training quality. BioMed Central
- Use individualisation: As noted above, oxidation capacity varies. Start with guidelines (~30-60 g/h for 1-2.5 h sessions; up to ~90 g/h for >2.5 h) and adjust based on your performance, gut comfort, and how you feel. BioMed Central+1
- Recovery matters: The carbs you take during and immediately after key sessions help replenish glycogen, repair, and adapt.
- Be aware of fat vs carb trade-offs: At higher intensities your reliance shifts more toward carbs (“crossover effect”). NSCA
6. Common Mis-conceptions & Pitfalls
- “More carbs = always better” — not true. If you overshoot your oxidisable capacity, you risk GI issues or unused carbs sitting in your gut or blood but not being used.
- “Easy days don’t need any carbs” — you still need some carbs for recovery, brain fuel and training quality.
- “Train low all the time to burn fat” — unless your goal is specific metabolic adaptation and you’re under coach supervision, constant CHO-restriction can negatively affect performance and adaptation. A meta-analysis found no overall performance benefit from chronic CHO restriction in endurance-trained athletes. BioMed Central
TRAIN SMOOTH 