As It Gets Colder, Your Fueling Strategy Needs to Change—Here’s How to Master It.

The transition from the sweltering, humid days of August to the crisp, leaf-strewn paths of late September and October represents more than just a change in wardrobe for the dedicated runner. It marks a physiological shift in how the body processes energy, handles thermal stress, and responds to the demands of endurance training. For many athletes, the summer months are a period of survival, where the primary goal is simply to finish a workout without succumbing to heat exhaustion. However, as the mercury drops, a unique window of opportunity opens. Cooler temperatures allow for higher intensities and longer durations, but they also demand a complete overhaul of your nutritional approach.

As a sports dietitian and lifelong runner, I have observed a recurring pattern among my clients: the "summer fueling deficit." During the peak of heat, many runners find it nearly impossible to maintain the gold-standard fueling protocols of 60 to 90 grams of carbohydrates per hour. The oppressive humidity and rising core temperatures often lead to a "hard no" from the digestive system. But as we move into the autumn of 2026, the easing weather provides the perfect environment to "train the gut" and optimize performance. To understand how to make this transition effectively, I spoke with fellow registered dietitians Kristy Baumann and Allison Knott to break down the science of seasonal fueling.

The Science of Heat and Digestive Shutdown

To understand why fueling becomes easier in the fall, we must first look at why it is so difficult in the summer. The human body is a master of prioritization. When you run in high temperatures, your primary physiological objective is thermoregulation—keeping your core temperature from reaching dangerous levels. To achieve this, the body initiates a process known as splanchnic hypoperfusion.

"When you run in the heat, your body is focused on cooling itself down," explains Kristy Baumann, RD, founder of Marathon Nutritionist. "Blood moves toward the skin to help you sweat and cool your core body temperature, which means there’s less blood available to your digestive tract." This shunting of blood away from the stomach and intestines can reduce blood flow to the gut by as much as 80 percent. Without adequate blood flow, the mechanical and chemical processes of digestion slow to a crawl.

This creates a "bottleneck" in the stomach. Any carbohydrate gels, chews, or sports drinks consumed sit in the digestive tract longer than intended. The result is a litany of GI issues: nausea, bloating, cramping, and the dreaded "runners’ trots." Furthermore, the high sweetness and viscous consistency of many endurance products become sensory "turn-offs" when the body is under thermal stress. In the cooler air of autumn, however, this blood-flow diversion is significantly reduced. With more blood remaining in the gastric region, the body can more efficiently transport glucose and fructose across the intestinal wall and into the bloodstream, where it can be used by working muscles.

Establishing the Pre-Run Foundation

The cooler weather often encourages runners to push their mileage higher as they prepare for late-fall marathons or turkey trots. This increased volume requires a more disciplined approach to pre-run fueling. The industry standard for endurance athletes is to consume between 1 and 4 grams of carbohydrates per kilogram of body weight in the one to four hours leading up to a session.

For a 150-pound (approximately 68-kilogram) runner, the math translates to a significant amount of energy. If you are four hours out from a long afternoon run, your target is roughly 272 grams of carbohydrates. This is not the time for a light snack; it requires a substantial meal. A hearty grain bowl featuring quinoa or farro, a large sandwich with a side of fruit, or a bowl of pasta can help top off glycogen stores that may have been depleted during the workday.

As you get closer to the run—specifically the one-hour mark—the goal shifts to 1 gram of carbohydrate per kilogram of body weight. For that same 150-pound runner, 68 grams of easily digestible carbs is the target. This could be a fruit smoothie, a large banana with a piece of toast, or a yogurt parfait with granola. The objective here is to provide a final boost of blood glucose without overtaxing the stomach just as the workout begins.

The Mid-Run Carb Hierarchy

Once you are on the road, the goal of fueling is twofold: to maintain blood glucose levels for immediate energy and to spare muscle glycogen stores. Sparing glycogen is critical because once those stores are depleted, the body begins to increase muscle protein breakdown to create energy, which significantly extends recovery time and increases the risk of injury.

In cooler weather, you can finally hit the targets that felt impossible in July. The fueling plan should be scaled based on the duration of the effort:

1. 30 to 75 Minutes: For shorter runs, your body typically has enough stored glycogen to power through. However, research into "carb mouth rinsing"—where you swish a sports drink in your mouth for 5–10 seconds and then spit it out—has shown that the brain’s oral receptors can signal the central nervous system to reduce perceived exertion, providing a performance boost even without swallowing the calories.
2. 1 to 2 Hours: Aim for 30 grams of carbohydrates per hour. This is the equivalent of one standard energy gel or roughly 16 ounces of a typical sports drink.
3. 2 to 3 Hours: Increase the target to 60 grams per hour. This is the "sweet spot" for most marathoners. It can be achieved by taking a gel every 30 minutes or combining a gel with a carbohydrate-electrolyte drink.
4. 2.5 Hours or More: For ultra-endurance efforts, the goal is 90 grams per hour. To reach this level without GI distress, athletes often need products that use a multi-transportable carbohydrate blend (usually a 2:1 ratio of glucose to fructose) to utilize different absorption pathways in the gut.

Training the Gut: A Seasonal Project

One of the most common mistakes runners make is assuming they can simply "do" their race-day fueling without practice. Allison Knott, MS, RDN, CSSD, emphasizes that the gut is a highly adaptable organ that must be trained just like your quadriceps or lungs.

"This means taking fuel on long runs, rides, or other training sessions that last more than an hour," Knott says. "This allows time to practice fueling and make modifications as needed." As the weather cools, use your weekday runs to experiment with textures. Some runners prefer the simplicity of liquid calories, while others find that "real food" options like dried apricots, honey sandwiches, or pretzels work better in the fall when the stomach is less sensitive.

Kristy Baumann suggests a "low and slow" approach to increasing carb intake. If you struggled to consume 30 grams an hour during the summer, don’t jump immediately to 90 grams. Build up by 10 or 15 grams each week. By the time you are four to eight weeks away from your goal race, your long runs should serve as a "dress rehearsal," where you use the exact products and timing you plan to use on race day.

The Sweat Rate Variable and the "Indian Summer" Risk

While the focus in autumn shifts to carbohydrates, hydration remains a critical pillar. The danger of cooler weather is that the "thirst cue" often diminishes, leading runners to under-hydrate. To combat this, athletes should perform a sweat rate test. This is a simple calculation that determines how much fluid you lose per hour of exercise in specific conditions.

The formula is: [(Pre-run weight − Post-run weight) × 16 + fluid consumed] / hours = sweat rate (ounces / hour).

For example, if you weigh 150 lbs before a one-hour run and 148 lbs after, while drinking 20 ounces of water during the run, you have lost 2 lbs (32 oz) plus the 20 oz you drank, totaling a sweat rate of 52 ounces per hour. Knowing this number allows you to tailor your fluid intake to prevent more than a 2% loss in body mass, which is where performance typically begins to decline.

Finally, one must always be prepared for the volatility of autumn weather. "Indian Summers" can bring unexpected heat waves on race morning. If you find yourself facing a 75-degree race day after training in 50-degree weather, do not abandon your fueling plan, but do pivot your hydration.

"The biggest adjustment you’ll need to make is your fluid and electrolyte intake," Baumann notes. "When it’s warm and you’re racing at a higher intensity, you’re sweating more… be ready to dial it up in the days leading up to race day." This includes "salt loading" or increasing sodium intake to help the body retain the fluids you are drinking.

By treating autumn as a season of nutritional refinement rather than just a break from the heat, runners can enter their winter training or fall races with a more resilient digestive system and a more powerful engine. The crunch of leaves underfoot is a reminder that the time for survival running is over; the time for performance fueling has arrived.