Fueling the Heat: The Science of Gut Training for the Summer of Gravel

The summer of gravel is here. If you have signed up for a race like Unbound Gravel, or any other summer events, you know the heat can often be the limiting factor on event day. As the temperature climbs, success on race day is determined by your ability to keep your engine fueled when your body core temperature is rising. The problem? Extreme heat and high-carb fueling are naturally at odds. When you get hot, your body’s priorities shift and smooth digestion becomes hindered. However, through a process called “gut training”, you can teach your body to handle fuel even when the mercury rises.

What Happens Inside When the Temp Goes Up?

When you’re pushing big watts in the heat, your body enters a state of triage. It has to choose between sending blood to your muscles to keep you moving, sending it to your skin to cool you down, or sending it to your gut to process fuel.

Blood Flow Diversion: To keep you cool, your body diverts up to 80% of blood flow away from your stomach and toward your skin. This effectively “shuts the door" to your digestive tract, making it much harder to absorb that gel or drink mix.
The Sloshy Stomach: As you sweat, you lose blood plasma. Your blood becomes thicker and harder to pump. This dehydration slows down your stomach’s emptying rate, which is why fuel starts sitting like a brick and leads to that heavy, bloated feeling.
Heart Rate Drift: You might notice your heart rate rising even if your power stays the same. This is cardiovascular drift. It is a sign your body is working overtime to manage heat and hydration, leaving very little energy left for digestion.

How to Bulletproof Your Gut for Race Day

Here are five science-backed ways to optimize how your body handles fuel in the heat:

1. Train Your Gut Like a Muscle

The Science: Practicing high-carb fueling actually increases the number of transporters (proteins that carry sugar from your gut to your blood) in your digestive tract.
The Goal: For 4 to 6 weeks before your race, use your full race-day fueling plan during your long rides. This teaches your gut to stay active even when you are working hard.

2. Use the Two “Door" Approach

Your body absorbs different sugars through different pathways. Glucose uses one "door," and Fructose uses another. The Fix: If you only use glucose, the door gets jammed at about 60g per hour. By using a mix like Neversecond’s 2:1 ratio, you open both doors. This allows you to absorb 90g or even 120g of carbs per hour without a traffic jam in your stomach.

3. Watch Your Mix Concentration

In the winter, you can get away with "syrupy" bottles. In the heat, that is a recipe for disaster.

Isotonic is Key: If your drink is too concentrated, it actually pulls water out of your blood and into your gut to dilute it. This causes bloating and diarrhea.
The Sweet Spot: Aim for a 6% to 8% concentration (ex. 60-80g carbohydrates per 1L of fluid). This allows the fluid to pass through your stomach almost as fast as plain water.

4. Sodium: The Secret Key to Fueling

Sodium is not just about preventing cramps. It is actually the key that unlocks the door for glucose. The transporters in your gut need sodium to physically pull sugar into the bloodstream. Including extra sodium, such as the Neversecond S200, helps you absorb both water and carbs faster. Note: Consuming excessive sodium beyond your sweat losses can cause gastointestinal distress or contribute to dehydration, so it is important to test your specific needs during training.

5. Mechanical Delivery and Form

The Volume Effect: A larger volume of fluid in the stomach actually stimulates faster emptying. Taking regular, substantial gulps every 15 to 20 minutes is often more effective than frequent micro-sipping because the weight of the fluid helps push fuel into the small intestine and empties the stomach more quickly.
Liquids vs. Solids: Solids require more blood flow to break down. In high-heat environments, rely on liquids and gels to minimize the energy cost of digestion.

The Gut Training Protocol: 6 to 10 Weeks Out

Progressive Overload: Treat your gut like a muscle. Start at a baseline such as 40g/h and increase by 10-15g/h per week during your long endurance rides.
Daily Substrate Availability: A high-carbohydrate daily diet with up to 70% of total calories keeps your digestive enzymes and transporters primed for race day.
Volume Tolerance: Practice drinking high fluid volumes that mimic your projected sweat rate. Current literature suggests aiming to replace 60-80% of hourly fluid losses to prevent excessive cardiovascular drift.

My Summer of Gravel Fueling Protocol

When the weather is mild, I usually aim for one bottle of Neversecond C90 (90g carbs) per hour. But when the heat turns up, I change my strategy to keep my stomach happy:

In My Bottle: I switch to a "thinner" mix. I will put two scoops of C30 into a large 750ml bottle. This gives me 60g of carbs and a good dose of sodium, but in a much more diluted, easy-to-digest form.
In My Pockets: To get my total carbs back up to that 90g to 120g range, I use C30 Gels as needed.
Why This Works: It allows me to drink more fluid with an overall lower concentration of mix, making it a lot easier on the digestive system to handle.

Summary

Winning in the heat is not just about who has the highest threshold. It is about who has the best-trained gut. By gradually increasing your carb intake in training, using a multi-sugar blend, and keeping your bottles "thin" and salty, you can avoid the dreaded gut shut down. Don't leave your fueling to chance. Treat your digestive system as a core part of your training. On race day, you will be the one moving forward while others are held back by the heat and a failing gut.

References

Gisolfi, C. V. (2000). Is the GI System Built For Exercise? Physiology, 15(3).
Jeukendrup, A. E. (2014). A Step Towards Personalized Sports Nutrition. Sports Medicine, 44.
Jeukendrup, A. E. (2017). Training the Gut for Athletes. Sports Medicine, 47.
Leiper, J. B. (2015). Factors affecting gastric emptying and intestinal absorption. Nutrition Reviews, 73.