— Carbs per hour (g/h)
— Carb source recommendation
— Fluid per hour (ml/h)
— Sodium per hour (mg/h)
— Total carbs (g)
— Total fluid (L)
— Execution plan (per 20 min)
Methodology
This calculator applies a duration-tiered carbohydrate model grounded in current sports nutrition consensus. For events under 75 minutes, exogenous carbohydrate has minimal performance benefit; for events of 1–2.5 hours, 30–60 g/h from a single glucose source is recommended; for events beyond 2.5 hours, 60–90 g/h using a 2:1 glucose-to-fructose ratio maximises intestinal absorption and delays fatigue (Jeukendrup, 2014; IOC Consensus Statement, 2016). Gut training level adjusts the upper range downward for athletes new to race fueling to reduce gastrointestinal distress risk. Hydration is estimated at 400–800 ml/h, modulated by sport and intensity; running carries higher GI stress due to repetitive impact and is set conservatively. Sodium replacement targets 500–1,000 mg/h for events over 90 minutes to maintain plasma volume and prevent hyponatraemia. The execution plan divides totals into 20-minute intervals for practical in-race application.
FAQ
What is the 2:1 glucose-to-fructose ratio and why does it matter for long events?
The gut has two separate intestinal transporters for carbohydrates: SGLT1 absorbs glucose (and maltodextrin), while GLUT5 absorbs fructose. Each transporter has its own absorption ceiling — roughly 60 g/h for glucose alone. By combining glucose and fructose in a 2:1 ratio you recruit both pathways simultaneously and can absorb up to 90 g/h without increasing gut distress. For events under 2.5 hours this extra capacity is rarely needed, but beyond that threshold the oxidation advantage is well-documented (Jeukendrup, 2014). In practice this means combining glucose-based sources (maltodextrin gels, sports drink) with fructose-containing foods (fruit, sucrose-based gels) in the right proportion.
How do I train my gut to tolerate more carbs?
Gut training is an adaptive process. Begin with 30–40 g/h in your long sessions and increase by 5–10 g per hour each week as long as you remain comfortable. Use the same products you plan to race with — form matters as well as quantity. After 6–8 consistent sessions at a given intake level most athletes see meaningful improvements in absorption and a reduction in nausea. Practise fueling even on lower-intensity days to build the habit and expose the gut to regular intake. Arriving at race day having never practised with 90 g/h and expecting to use it is one of the most common nutrition mistakes in endurance sport.
Why does the calculator treat running differently from cycling?
Running creates repetitive vertical impact that mechanically agitates the gastrointestinal tract, which increases intestinal permeability and reduces blood flow to the gut. As a result, runners experience GI symptoms — nausea, bloating, cramping — at lower carbohydrate intakes than cyclists covering the same duration. The calculator applies a conservative fluid and carbohydrate ceiling for running and for the run leg of triathlon. Cycling allows higher intakes because the body is supported and impact forces are absorbed by the bike. HYROX includes both running and functional training stations, so an intermediate figure is used.
What counts as a 'trained gut' for this calculator?
Select 'Trained' if you have deliberately practised race nutrition in at least six long training sessions (90 minutes or more) at or near the carbohydrate and fluid amounts you intend to use on race day. This means using gels, drinks, or real food at planned intervals — not just drinking water and hoping for the best. If you have practised occasionally but not consistently, choose 'Some'. If this is among your first attempts at structured in-race fueling, choose 'None' — the calculator will recommend a lower, safer starting range.
Why doesn't the calculator recommend specific brands or products?
The calculator is intentionally brand-agnostic. The underlying science applies to carbohydrate amounts and ratios, not to specific commercial products. Any gel, bar, chew, or drink that delivers the right carbohydrate quantity and ratio will produce the same physiological effect. Recommending specific brands would also require disclosures around commercial relationships and would date the tool quickly as product formulations change. Use the gram targets this tool provides, then match them against the nutrition label of whatever products you have available and enjoy eating.
How do I read and adapt the execution plan?
The execution plan breaks your total carbohydrate and fluid targets into 20-minute blocks, which aligns with typical gel and bottle checkpoints on most race courses. Treat the numbers as a guide rather than a rigid prescription — conditions on the day (heat, effort level, stomach feel) should always take precedence. If you feel nauseous, reduce carbs at the next interval and increase fluid. If you feel strong and have been practising high intakes, you may push slightly above the plan. Never try to 'catch up' on missed calories by doubling a later interval; spread any shortfall across several subsequent windows instead.
Results are evidence-based estimates and do not substitute for individualised advice from a registered sports dietitian. Sweat rate, heat, humidity, and personal GI tolerance vary significantly. Always practise your fueling strategy in training before race day.
