The Science of Staying Fueled

The Science of Staying Fueled: Why Hydration & Carbs Matter More Than You Think

As we head into summer training, having a game plan for your nutrition and hydration becomes even more of a necessity. If you’ve ever bonked during a long run, suffered through heat and humidity without sufficient fluids, or felt completely depleted by the end of a workout, you’ll know how much of a difference it makes to have a fueling strategy.

This week, we feature insights from Tash Cooper-Smith, MSc and Sports Scientist for Precision Fuel & Hydration, to better understand the science and practical insights behind performance nutrition. Tash provides clear explanations on everything from training your gut to the dangers of over-drinking plain water, and why carbohydrates do a whole lot more than just provide energy.

For all the runners out there - whether you’re gearing up for your summer XC build or starting a marathon block - dive into the conversation below from an expert who knows what it really means to fuel performance.

Stay tuned until the end of the interview to enter our Summer Training Giveaway in collaboration with Precision Fuel & Hydration!

Q&A with Tash Cooper-Smith, MSc

For runners who have never intentionally fueled during training, what’s the most effective way to begin building that habit? What should they start with in terms of what products, dosage, and frequency to consume fuel while running?

“Start small and start early - those are the two biggest things. A lot of runners wait until they're already feeling fatigued before they take anything on board during their training, and by that point you're playing catch-up. The habit I always recommend building first is just taking something within the first 20-30 minutes of a run, before you feel like you need it.

Gels or chews are a great place to start. They are easy to carry, easy to practice with, and easy to digest when you're moving. As a general guide: for runs over 60 minutes, aim for 30-60g of carbohydrate per hour; for anything over 90 minutes to two hours, we'd recommend pushing towards 60-90g/h to meaningfully support your performance. One gel or a serving of chews every 30-45 minutes gets you building towards those targets without overwhelming your gut.

And it’s important to remember, your gut is trainable. The more consistently you practice fueling in training, the better it gets at absorbing carbohydrate on the move, and the more comfortable the process becomes. Don't save your fueling practice for race day. Whatever you plan to use on the start line, use it in training first. Every run or exercise session can be used as a dress rehearsal.”

When working with athletes preparing for the same event, how much variability do you see in their fueling strategies? Beyond total carbohydrate intake, what individual factors most influence how you tailor an athlete’s approach?

“Huge variability - even between athletes doing the exact same race. Two people can line up at the same start line with completely different fueling plans, and both be right for them.

Beyond total carbohydrate intake, the factors I look at most are: gut tolerance (some athletes can handle 90g/h no problem, others struggle at 60g/h), training status and how well they've been practicing fueling in training, pace and intensity (higher intensity usually means a greater reliance on carbohydrate, so pacing strategy really matters), aid station access on course, and - honestly - personal preference and perception. If someone hates the texture of a particular gel or nutrition product, they're not going to take it when they're deep into a race and suffering. That matters.

Experience level and goal of the event are also significant factors. An athlete who's completed 10 events and is aiming for a personal best has usually refined their strategy and knows exactly what works for them.”

Carbs are an energy source for runners, but why else are they important?

“This is one of my favorite topics, because carbohydrates really do get reduced to just "fuel", and they're so much more than that.

Carbohydrates play a major role in preserving muscle protein. When carbohydrate availability is low, the body will start breaking down muscle for energy, which is the last thing you want in endurance sports. They're also critical for immune function; athletes in heavy training blocks who are chronically low on carbohydrate tend to get sick more frequently. And there's a strong central nervous system component too. Your brain runs almost exclusively on glucose (carbohydrate), so when carbohydrate stores are depleted, whether through training and racing or insufficient energy intake, decision-making, mood, and perceived effort all suffer. That's why you can feel mentally “done” before your legs have technically given up.”

What’s one widely held belief about hydration that you think is misleading or oversimplified? How should runners be thinking about hydration more effectively?

“That drinking lots of fluid is the key to staying hydrated. It sounds logical, but it misses a really critical piece of the puzzle - and in some cases, acting on it can actually cause harm.

When exercising and training a lot, fluid (or water) alone isn't enough. Sodium is what allows your body to retain and effectively use the fluid you consume. Without adequate sodium, drinking large volumes of plain water - particularly before or during a race - can actually dilute your blood sodium levels, leading to hyponatremia. It's more common than people think, and the symptoms can range from feeling off and nauseous all the way to something much more serious.

We see this play out constantly through sweat testing at PF&H - sweat sodium concentration varies enormously between individuals, which means sodium needs are just as personal as fluid needs. The better framework is to think about hydration as fluid and electrolytes working together, not just how much you're drinking. Getting that balance right, for you as an individual, is what actually moves the needle on performance and safety.”

Are there key differences in fueling needs or strategies for female athletes that are often overlooked?

“Yes, to a certain extent, and it's an area where sports science still has a lot of catching up to do. Most research underpinning sports nutrition guidelines has been conducted on male athletes, so there are some important nuances worth highlighting.

Female athletes tend to rely more on fat oxidation and less on carbohydrate during exercise compared to males, and this can shift across the month due to hormonal fluctuations. That doesn't mean female athletes need less carbohydrate; fueling should always match the effort, but it's worth being aware of.

One practical implication of this is fasted training. Female athletes appear to be more sensitive to the cortisol response that comes with training in a fasted state, which can disrupt hormone balance and affect recovery and adaptation over time, so it’s not something we would recommend, whereas male athletes tend to tolerate it better.

Body size or absolute muscle mass is also worth mentioning. Female athletes are often smaller, meaning their absolute energy output per session is typically lower, but that's not a reason to fuel less during training. Fueling should remain proportionate to effort; it is overall daily energy intake where the difference is most relevant.”

Looking ahead, what advancements in sports science or nutrition are you most excited about?

“The individualization piece is a big one - we're getting much better at moving away from population averages and understanding what a specific individual actually needs. Tools like sweat sodium testing and assessing carbohydrate oxidation are pushing that forward in really meaningful ways.

I'm also excited about the growing body of research into female athlete physiology. It's long overdue, but there's real momentum behind it now.

And on a more practical level, I think there's significant room to improve how we communicate nutrition guidance to athletes and it’s something we work hard to do at PF&H through a range of different platforms and means. The science is advancing, but if athletes can't understand or apply it in the real world, it doesn't deliver.”

Enter Our Summer Training Giveaway ☀️

To celebrate the launch of summer training and help you keep your body primed for those hot miles ahead, we’re teaming up with Precision Fuel & Hydration for an ultimate training bundle giveaway!

What You Can Win:

• A Free HAX Training Plan: Complete access to choose from over 300 personalized training plans in our library. From the 800m up to the marathon, we have options built for runners of all abilities and experience levels.

• A $75 Precision Gift Card: Stock up with premium electrolytes, high-carb gels, chews, and hydration mixes to fuel all those summer miles.

Check out the details on this Instagram post to enter!

Take Your Training to the Next Level

If you want to map out your best summer of running yet, we’re here to support you in the journey:

Browse Our Training Plans: Looking for some structure? Grab a plan or unlock the Ultimate High School Running Training Plan Library for just $24.99/month.

Work 1:1 with a HAX Coach: Ready for fully individualized guidance? Partner with one of our elite coaches to take the guesswork out of your mileage, workouts, and overall training strategy. Spaces are limited for the summer block. [ Find Your Coach ]

Thanks for reading this issue of The Hammer!
If you have questions about training, finding the right coach, or picking a training plan that fits your summer goals, hit us up anytime at [email protected]. We're here to help you train with purpose.