Why carbohydrates matter in sport
Muscle and liver glycogen is the fuel of choice at moderate to high intensities. When the store runs out, performance drops sharply, the phenomenon known as "hitting the wall" among marathoners. The muscle cannot sustain power without available glycogen.
At low intensities (below 65% VO2max), fat is the predominant substrate. That is what makes aerobic base training in Z1-Z2 so efficient: you train the fat engine without depleting glycogen. Once you cross the anaerobic threshold, carb dependence increases exponentially.
How many carbs do you need?
Guidelines from ACSM and ISSN (reviewed by Louise Burke of the Australian Institute of Sport) recommend intake proportional to training load:
Moderate training (TRIMP 80 to 150): 5 to 7 g/kg
Heavy training (TRIMP above 150): 7 to 10 g/kg
ACSM · ISSN · Burke et al., "Carbohydrates for Training and Competition" (2011)
Timing: when you eat makes a difference
Pre-workout (1 to 3h before): 1 to 4 g/kg of low to moderate glycemic index carbohydrate. Test your individual tolerance to avoid gastrointestinal discomfort.
During long workouts (over 60 to 90 min): 30 to 60 g of carbs per hour, reaching up to 90 g/h in ultra-distance events by combining glucose and fructose. The limitation is the intestinal absorption rate, not gastric capacity.
Post-workout (first 30 to 60 min): 1 to 1.2 g/kg of fast carbs combined with 0.3 to 0.4 g/kg of protein. The anabolic window is real, but it does not need to be in the literal first 5 minutes.
Carbohydrate periodization
An advanced strategy is "train low": doing low-intensity sessions with reduced glycogen availability to stimulate adaptations in fat metabolism. Louise Burke has published studies showing improved fat oxidation with no negative impact on performance when periodization is done well.
Carb loading before races
For races longer than 90 minutes, glycogen supercompensation in the 2 to 3 days before maximizes stores. The modern protocol, validated by Bergström and Hultman since 1967 and refined afterwards, is simple: reduce training volume and increase carb intake to 8 to 10 g/kg for 2 to 3 days.
The classic limit of fueling was a single one: the intestine absorbs glucose through a single transporter (SGLT1), which saturates near 60 g per hour. The turning point came when Jentjens and Jeukendrup (2004) showed that adding fructose, absorbed through a different door (the GLUT5 transporter), raised the exogenous carbohydrate oxidation rate from about 1.0 g/min with glucose alone to 1.26 g/min with the mixture. At very high glucose and fructose intakes, Jentjens and Jeukendrup (2005) recorded as much as 1.75 g/min, the known practical ceiling of this dual transport pathway.
This gain is not confined to the lab. Currell and Jeukendrup (2008) compared glucose alone against glucose plus fructose in a 2:1 ratio, both delivering carbohydrate at 1.8 g/min, and measured 8% more power in the final time trial with the mixture. It is the same principle that sustains modern cycling: in the hardest stages of the Tour de France, teams report Tadej Pogacar and Jonas Vingegaard ingesting close to 120 g of carbohydrate per hour, something that requires precisely dual transport to be oxidized without gastrointestinal collapse.
The takeaway
Carbohydrate in sport is not a matter of eating more or less, it is a matter of following the load of the day: an easy Z2 workout asks for half the carbs of a heavy session, and that curve is the difference between the runner who hits the wall at 32 km and the one who reaches the finish line whole. The science of dual transport, glucose plus fructose, rewrote the ceiling of the possible: the same principle that sustains Pogacar and Vingegaard at 120 g of carbohydrate per hour in the hardest stages of the Tour de France is available in any sports gel on the shelf. The right dose is a conversation for a dietitian, but the central lesson fits in one sentence: treat carbohydrate as load fuel, not as the fixed villain of the plate.
