In part 1 of this blog, we discussed the body’s response and adaptations to exercising in the heat. In part 2 of this blog, we interviewed Chris Lowe (MS, BS) to provide you tangible take-aways and steps to take with your fueling and hydration while exercising in the heat. Chris is a sport nutritionist who has extensive experience working with elite endurance athletes and previously contributed to our blog on fueling during the offseason.
What are common misconceptions and mistakes that people make when it comes to fueling and hydrating in the heat, and what should individuals be cautious of when fueling and hydrating in the heat?
Lowe: Gastrointestinal issues can arise when training or racing hard in the heat. If an athlete's fluid intake isn’t aligned with their needs, the fluid deficit will increase. Therefore, the dehydrated athlete will have reduced blood flow to the gut, which can impair carbohydrate absorption.
For example, if an athlete is familiar with fuelling 100g of carbs per hour under temperate conditions and then competes away from home in a warmer environment, their trialled and tested fuelling plan may not work in the new environment, especially if fluid intake isn’t properly matched to sweat loss.
When temperature and humidity increase, it’s essential to offset fluid losses to minimise the fluid deficit to 2%. This will not only ensure that performance is protected but also support splanchnic (liver) blood flow and reduce the likelihood of ischemia, which is the dysfunction of the gut barrier.
From there, gut function is maintained, and normal fuelling amounts can be followed. With this in mind, when the temperature increases or you travel to a warmer environment, I’d recommend starting at the lower end of the fuelling range and testing gut tolerability. Then build up to your usual fuelling amounts if you have the green light.
TLDR: Training or racing in heat increases dehydration risk. This can lead to blood flow reduction to the gut making it challenging to absorb the carbohydrates you consume. Thus, fluid intake should be matched to sweat loss to reduce risk of GI distress. Keeping dehydration under ~2% will protect exercise performance and gut function in the heat.
How does intra-workout fueling and hydration strategies (e.g. amount of carbs and sodium, timing, etc.) change in the heat?
Lowe: There’s generally an increase in carbohydrate oxidation during exercise in the heat. Meaning that muscle glycogen may deplete slightly quicker than normal. As noted above, an athlete can experience reduced carbohydrate uptake in the gut when dehydrated. Therefore, this mismatch can deplete muscle glycogen more quickly, ultimately impairing performance.
As muscle glycogen is used at a higher rate, many will jump to the conclusion that they need to fuel more. Which, in part, is correct. But only if your gut can tolerate the higher amounts. Therefore, the first objective is to assess fluid needs.
This can be done by performing basic hydration testing:
Step 1: Weigh yourself right before a training session.
Step 2: Weigh yourself immediately after the training session. Ideally, not in your training kit as it’ll most likely be soaked in sweat (and will not reflect true weight loss during the session).
Step 3: Note how much fluid you consumed during the session. Note, 1 litre = 1kg. You can take this a step further and measure urine loss during a session. But for now, to keep it simple, let’s focus only on fluid intake.
Step 4: Then, we do some quick maths. For example, if you weighed 80kg before a 3-hour session and weighed 78kg immediately after, you’d have lost 2kg, or 2 litres of sweat. However, if you consumed 1 litre of fluid during the session, your actual sweat loss would total 3 litres. In this example, your sweat rate would be 1L per hour.
I’d recommend doing this exercise for different types of sessions and conditions to build a database. This can then be used to predict fluid needs for future sessions of varying intensities and climates.
Note: There’s zero need to replace 100% of the fluid you lose. In other words, if you lose 3 litres in a 3-hour session, you don’t need to consume 3 litres in total. The aim is to minimise fluid losses, so bodyweight loss doesn’t exceed 2% (Approx 1.6kg total for an 80kg athlete).
Use this data to better inform your hydration needs for your next session of similar demand and conditions (and as we’ll find below, the dangers of overhydrating and gaining weight during exercise).
There’s also evidence to suggest that heat acclimation can lower carbohydrate oxidation. So, this may be a natural adaptation to living in a warm, humid environment. But if not, this is where heat training could be beneficial from a fuelling perspective.
Practically speaking, if we know that gut sensitivity is higher during exercise in the heat, choosing the right fuelling products is essential. As a rule, when gut function is reduced, whether that be through hard exercise or heat (or both), fuelling products such as gels and powders often work best. This is where the athlete needs to trial different fuelling products in advance of racing or events to ensure they can hit their hourly fuelling amounts without experiencing gastrointestinal issues.
Paradoxically, if you experience gastrointestinal issues during exercise, the recommendation is to continue fuelling to promote blood flow to the gut. Athletes will often stop drinking fluid or consuming carbs, which can further worsen the issue. Therefore, my advice would be to reduce the pace and slowly ‘drip feed’ carbs and fluid to restore gut function. Easier said than done…
Sodium is a hotly debated topic, and recent research on sodium modelling has challenged the norm, suggesting that day-to-day sodium requirements aren’t increased in athletes because sodium is regulated by the kidneys and sweat glands to maintain stores of approximately 1g/kg body weight, which are mostly stored in the extracellular fluid.
Before, during exercise, and after exercise, it’s the relationship between sodium and water that influences performance and health. The major concern is overconsuming fluids beyond our needs, which can cause weight gain and lead to hyponatremia, a dilution of blood sodium.
The following framework by McCubbin can be used to determine whether sodium is required during your sessions:
Figure 1: Figure from McCubbin, 2025.
TLDR: In the heat, the body utilizes more carbs and has reduced gut carb absorption. To minimize the impacts of the heat on carbohydrate absorption, ensure you have a hydration strategy as well. Use easily digestible carbs (gels/drinks), test tolerance, and adjust your fueling accordingly. If gut issues arise, don’t stop fueling! Slow down and “drip feed” carbs and fluids. Despite the current hype around sodium, understand that your body can effectively regulate sodium content and that sodium replacement should be strategic and personalized.
Should pre-exercise fueling and hydration strategies be altered for exercise in the heat? If so, how?
Lowe: The aim for any athlete wanting to optimise performance is to start the session or race with the tanks topped up and hydrated. This doesn’t change regardless of the climate or the level of competition.
Pre-exercise fuelling can remain the same unless there’s a heightened need for carbohydrates due to poor recovery from the previous day's session or race in the heat. In other words, your recovery is your fuelling, and your fuelling is your recovery.
Similarly, if an athlete has a fluid deficit from the previous day or if baseline sweat rates are higher, then there will be a need to increase fluid intake.
Tip: To maximise fluid retention, consume fluids with meals and aim to drip-feed throughout the day. Be proactive with your hydration, not reactive. If you go hours with minimal fluid, then try to play catch-up, you may be visiting the toilet a bit too frequently because you’re not retaining the fluid you’ve consumed. Adding sodium to drinks can promote thirst and increase fluid intake in some, which can be a helpful strategy if athletes forget to drink little and often.
TLDR: Pre-exercise fueling is not significantly altered with the heat. Always start well-fueled and hydrate. Only increase carb intake if your recovery from a previous session was poor and increase fluids if you’re dehydrated. Hydrate steadily throughout the day rather than playing catch-up (not all at once), ideally with meals, and consider adding sodium to help retain fluids and encourage drinking.
Should post-exercise fueling and hydration strategies be altered after exercise in the heat? If so, how?
Lowe: The principles of replenishing and rehydrating remain consistent between hot and temperate environments. But the magnitude of their needs depends on the degree of dehydration and depletion, as well as the recovery window. Essentially, how quickly does the athlete need to get to baseline ahead of the next session or race? If it’s within 24 hours, the urgency increases. Here’s what we focus on:
Replenishment: Aim to consume 1-1.2g/kg of carbohydrate every hour for 3-4 hours following depleting exercise. This will take advantage of the higher replenishment rates observed during this period. Beyond this, replenishment rates typically drop by half. So, if glycogen replenishment is urgent, then timing is, too!
Rehydration: Aim to replace 150% of fluid losses. This is where hydration testing becomes helpful, so you have an indication of how much fluid you lose during a session or race and how much you need to replace. For every 1kg (1litre) lost, it’s recommended to consume 1.5 litres to ensure adequate rehydration. My advice would be to consume this fluid in the hours following alongside your recovery meal to maximise fluid retention.
Ultimately, to restore performance, proper rehydration and replenishment are needed. My advice is to plan your recovery snacks and beverages before the session, so there’s minimal delay afterwards, and you can initiate the recovery process immediately. Otherwise, the recovery window closes, and the next day, performance will be compromised.
Take-home message: How you recover today will dictate how well you perform tomorrow.
TLDR: The basics of recovery do not change given the heat. However, depending on how quickly you need to return to optimal performance should dictate how regimented you should be with your refuelling and rehydration. For example, If the recovery window is small (<24 hours), prioritize rapid carb intake (1–1.2 g/kg/hour for 3–4 hours) and rehydrate with ~150% of fluid lost. In any case, plan ahead so you can refuel and rehydrate immediately to minimize the delay that can hurt next-day performance.
