Carbohydrate is the body's preferred fuel. It is also the most efficient one. Per litre of oxygen consumed, carbohydrate oxidation produces roughly 7 to 8 percent more energy than fat oxidation. At race intensity, where your oxygen budget is fixed, that difference is not trivial. Every gram of carbohydrate your body can access and burn is a gram that produces more mechanical power for the same metabolic cost.
This is the argument that the Norwegian triathlon programme has built their race nutrition around. Not just that carbohydrate is useful. That it is superior. And that the only thing limiting how much of it you can use during a race is how much your gut can absorb.
Why Carbohydrate Is More Efficient
The biochemistry is straightforward. Oxidising carbohydrate yields approximately 5.05 kilocalories per litre of oxygen. Oxidising fat yields approximately 4.69. The difference comes from stoichiometry: carbohydrate molecules contain more oxygen within their structure, so they require less external oxygen to combust fully.
During a race, you are working at a fixed fraction of your VO2max. You have a ceiling on how much oxygen you can consume. If more of your fuel mix comes from carbohydrate, you extract more energy from each litre of oxygen you breathe. That means more watts on the bike. More pace on the run. Not a lot more per gram, but across five or eight hours the effect compounds substantially.
This is why fat, protein, and any other non-propulsive substrate cost you speed. They require more oxygen per unit of energy produced. At race intensity, oxygen is the scarce resource. You want every litre doing the most work possible, and carbohydrate delivers on that better than anything else.
The Bottleneck Is Your Gut, Not Your Muscles
Research using isotope tracers and muscle biopsies has confirmed that muscle carbohydrate uptake capacity far exceeds what the gastrointestinal system can deliver. Your muscles can use more carbohydrate than you can absorb. The historical ceiling of 60 to 90 grams per hour was never a muscular limitation. It was a GI limitation.
The Norwegian athletes have pushed intake to 160 grams per hour during Ironman racing, delivered as a glucose-fructose drink mix that uses two independent gut transporters in parallel. Peak measured carbohydrate utilisation in those athletes exceeds 240 grams per hour when you include endogenous glycogen. The gap between what they take in and what their muscles burn tells you how much headroom remains. The muscles are hungry for more. The gut is the constraint.
This reframes the entire conversation. The question is not whether you should fuel with carbohydrate. The question is how high you can push your gut's absorption rate without causing distress.
Gut Absorption Is Trainable
This is the critical insight. Gut absorption capacity responds to progressive overload the same way your cardiovascular system does. It has to be trained systematically, with repeated exposure at progressively higher rates, over weeks and months. You would not expect to hold a new threshold power without building toward it. Gut tolerance works the same way.
Athletes who train their gut consistently across long sessions raise their absorption ceiling. Athletes who attempt race-day fuelling rates they have never rehearsed discover their ceiling through cramping, nausea, or worse. The gut is a physiological system. Treat it like one.
The Norwegian programme periodises carbohydrate intake during training. Some weeks target race-level rates. Other weeks pull back to minimum levels needed to complete sessions. The gut adaptation is maintained through regular reinforcement, not constant exposure. It is structured the same way you would structure any other training variable.
Why This Is the Biggest Performance Revolution in Endurance Sport
The argument from the Norwegian programme is that advances in race nutrition, not advances in training methodology, are the primary reason modern endurance performances have surpassed the doping era. The training principles have not changed dramatically. Sub-threshold volume, intensity control, and progressive development are not new ideas. What changed is how much fuel athletes can deliver to working muscles during competition.
When you combine a well-developed aerobic engine with high-rate carbohydrate delivery, the effects stack. A fitter athlete races at higher absolute power outputs where carbohydrate is the dominant fuel source regardless. The fitter you are, the more carbohydrate you burn, and the more you benefit from being able to replace it in real time. High-carb fuelling is not something an aerobic-dominant athlete grows out of. It becomes more valuable as fitness improves, because there is more power for it to support.
Elite athletes in the Norwegian programme achieve fat oxidation rates exceeding one gram per minute while consuming a high-carbohydrate diet. This demonstrates that aerobic development and high carbohydrate availability are not competing strategies. They work together. The aerobic engine handles the baseline metabolic demand. Exogenous carbohydrate keeps the fuel mix tilted toward the substrate that produces the most power per litre of oxygen.
What This Means for Age-Group Athletes
You do not need to hit 160 grams per hour to benefit from this approach. But the principle applies at every level. Push your carbohydrate intake as high as your gut will tolerate without distress. Train the gut progressively across training blocks. Use glucose-fructose combinations to access both absorption pathways. And do not under-fuel out of caution on race day. Coming in below the rate you have trained for is one of the most common mistakes in long-course racing.
Build the aerobic engine through consistent sub-threshold work. Train the gut with the same discipline you bring to your run or bike sessions. When both are in place, you are racing on the most efficient fuel your body can use, absorbed at the highest rate your physiology can sustain. That is where the time comes from in the back half of a long race. Not from willpower. From better energy delivery to muscles that know exactly what to do with it.
Carbohydrate is the most efficient fuel you have. The only limit is how much your gut can deliver. Train the gut like you train everything else.