The Heat Penalty — Why Your Carb Strategy Has to Change When It's Hot

It starts somewhere between the third and fourth hour of a hot Saturday ride. Pace creeps down. Power feels heavier than the number on the head unit suggests. The bottle that worked fine in May suddenly feels like it's not doing the same job. Most riders reach for more electrolytes. The research suggests the more interesting variable is carbohydrate.

When core temperature climbs, the body does two things at once: it burns through muscle glycogen faster, and it absorbs less of the fuel coming in through the bottle. That gap — between what's being used and what's being replaced — is the heat penalty. It shapes whether the back half of a long, hot ride looks like a finish or a fade.

What the Heat Actually Does Inside the Engine

Heat shifts the substrate mix. In a controlled trial at 35.4°C compared to 16.4°C, cyclists oxidised significantly more muscle glycogen and significantly less of the carbohydrate they ingested during the same 90-minute session (Jentjens, Wagenmakers & Jeukendrup, 2002). Same workload. Same drink. Different temperature. The body pulled harder on internal stores and used less of what came in.

A 2025 systematic review and meta-analysis confirmed the pattern across multiple studies: heat stress consistently elevates carbohydrate use compared with cooler conditions, accelerating the rate at which glycogen runs out (Mougin et al., 2025). The implication is uncomfortable for anyone who fuels the same way in July as they do in April: a strategy that holds at 18°C may not hold at 32°C.

Why "Drink More Electrolytes" Misses the Mechanism

Electrolytes matter in the heat. Sodium losses climb with sweat rate, and cramping conversations are a real fixture of August group rides. But sodium does not replace glycogen. It does not change how much glucose the small intestine can move into the bloodstream per hour. And it does not change the rate at which an overheating body converts ingested carbs into ATP.

When fatigue arrives in hot conditions and the fuelling plan is built around hydration tablets alone, the rider has solved one variable and ignored the bigger one. The carbohydrate side of the equation does the load-bearing work as heat strain deepens — a pattern covered in more detail in our piece on why electrolytes alone won't fuel a long ride.

The 2:1 Ratio in Hot Conditions

There is a useful finding buried in the heat literature. When cyclists rode for two hours at roughly 32°C and were given either glucose alone or a glucose-plus-fructose mix at a 2:1 ratio, the glucose-plus-fructose group oxidised exogenous carbohydrate at 76 g/h — a 45% increase over the glucose-only group at a matched dose (Jentjens et al., 2006).

This matters because the heat-driven reduction in carbohydrate oxidation can be partially offset by using two transporter pathways instead of one. Glucose moves through SGLT1; fructose moves through GLUT5. Running both at once lifts the ceiling on how much fuel actually gets used. In the heat, where every gram counts more, the formula a drink mix uses stops being a marketing line and becomes a measurable variable. The same principle is the foundation of the carbohydrate ratio your sports drink was designed for — the heat case is where it shows up most clearly.

What the Heat-Stress Research Recommends

A 2026 systematic review by Salame, Brown, Oueijan and McCullough examined nine randomised trials on carbohydrate supplementation in the heat and offered a practical framework (Salame et al., 2026). The summary is closer to "fuel with discipline" than "fuel harder."

Start the ride well-fuelled — the heat penalty is bigger when starting glycogen is already low. Use a carbohydrate intake rate that has already been practised in similar temperatures; aggressive intake on an untrained gut is more likely to fail when core temperature is rising. Favour a glucose-plus-fructose mix over single-source glucose. Be cautious with very concentrated drinks — gastric emptying slows when the body is prioritising blood flow to the skin.

For everyday Canadian athletes facing a hot June gran fondo or a humid summer Hyrox event, the practical version is simpler. Train the gut at high-carb intake during the spring shoulder weeks; the eight-week gut training protocol holds in heat as well as cool weather. On the day, choose a 2:1 formula at a concentration that has been tested in training. Build the fuelling cadence around the first three hours rather than waiting for the moment the legs feel heavy.

The Quiet Math of a Hot Saturday

The day a rider holds pace in the heat usually isn't the day they discovered something new. It is usually the day their carbohydrate strategy did not collapse under temperature. Glycogen lasted because intake matched oxidation. Intake matched oxidation because the formula was built for both transporters and the dose was rehearsed. The electrolyte plan handled cramping; the carbohydrate plan handled the engine.

Performance Drink Mix was designed for that case — a 2:1 maltodextrin-to-fructose ratio at a concentration that holds up across cool and hot conditions, built for everyday athletes who fuel the same on a hot July ride as they do on a temperate May one. Consistency is what changes the back half of a long, hot day. The research is on the side of riders who treat the heat as a fuelling problem first and a hydration problem second.