The Problem With Heat

TLDR — Key Takeaways:

If you’re training or racing in the heat:

• It can actually be quite dangerous!

• Heat reduces blood flow to the gut, making fuelling harder.  Liquid carbohydrates are better tolerated than solid foods.

• Sweat rates increase, and fully replacing losses is usually unrealistic.  Some dehydration is inevitable, so the goal is to manage, not eliminate it.

• Sodium must be paired with sufficient fluid to be effective and safe.

• Management strategies include external cooling, cold fluids, heat adaptation, and reduced intensity.

Hot weather adds a layer of complexity to endurance training that many athletes underestimate. It’s not just that you feel hotter or sweat more — heat changes the physiology of performance in ways that directly affect hydration, digestion, and your ability to fuel properly.

Understanding what your body is doing in the heat and why your usual strategies don’t work in the heat is key to maintaining good performance and avoiding ending up in hospital with heat stroke or worse. 

Why Heat Makes Hydration So Difficult

When you train in warm conditions, your body shifts blood flow toward the skin’s surface to help release heat. This is an essential cooling mechanism, but it comes at a cost: less blood is available for your digestive system.

With reduced blood flow to the gut, solid foods become harder to digest and much slower to empty from the stomach. This is why athletes often feel bloated, nauseous, or “full but starving” during hot long runs or races — the gut simply can’t keep up.

At the same time, your sweat rate increases dramatically. Many endurance athletes lose over a litre of fluid per hour in warm weather. Some lose significantly more. And while it’s important to rehydrate, most people cannot drink enough to match their sweat rate.  Imagine trying to drink a full litre of fluid every hour during a marathon, triathlon, or long race. For most athletes, that’s neither realistic nor comfortable.

This is why some degree of dehydration in the heat is almost inevitable — but it’s manageable if you understand what your body is doing.

The problem is, you can’t replace everything you lose

Because drinking to match sweat rate isn’t usually achievable, hot-weather performance becomes a balancing act:

• minimise sweat rate

• protect gut function

• slow the rate of dehydration

• maintain enough fluid and sodium for performance.

The goal is not to finish perfectly hydrated — it’s to stay as close as practical while supporting digestion and preventing the steep performance decline that comes from excessive fluid and sodium loss.

Strategies for Managing Hydration in the Heat

You can’t eliminate the effects of heat, but you can reduce them. 

1. Use external cooling to reduce heat stress

Cooling the body externally can significantly reduce core temperature and slow sweat production. Useful tools include:

• ice bandanas

• ice in your hat or sports bra

• cold sponges

• cold water poured over the neck, arms, and head

• jumping in an ocean, river or lake mid-run if you have to!  The payoff is worth it!

2. Drink cold fluids when possible

Cold drinks help reduce core temperature and are absorbed more comfortably in the heat. They can also help maintain gut tolerance by easing the thermal load on digestion.

3. Prioritise liquid carbohydrates over solids

Because digestion is slower in the heat, rely more on simple carbohydrates:

• sports drinks

• gels

• liquid carbohydrates e.g. juice, ginger beer.

These require less blood flow to the gut and are far easier to absorb when core temperature rises.

4. Include sodium with your fluids

Both water and electrolytes (predominantly sodium) are lost in sweat, so both need to be replaced in order to rehydrate, and at the same ratio as they are lost.  If you only drink water, your body won’t hold onto it because it dilutes the remaining sodium in your body, which can lead to hyponatraemia.  

Sports drinks, electrolyte tabs, and salt capsules are all valid options for electrolytes/sodium, but many sports drinks contain far less sodium than you’d assume, so you may need additional sodium e.g. from salt tablets.

Many people don’t have the correct understanding of how to use salt tabs correctly or safely.  It’s extremely important to balance salt intake with sufficient water, otherwise it can lead to stressed kidneys, increased dehydration, hypernatremia, impaired performance, and in extreme cases contribute to acute kidney injury (blood in urine). The danger is not the sodium, but sodium without water.  This is one of the most important areas of sports nutrition, and engaging a nutritionist can be a game changer and a life saver.  

5. Heat adapt in training

Heat adaptation reduces sweat rate, improves plasma volume, and increases your ability to tolerate warm conditions. Adaptation doesn’t make you immune to dehydration, but it meaningfully reduces the strain.

6. Dial back the intensity

One of the most effective heat management strategies is to reduce intensity.

Higher intensity = more metabolic heat = higher sweat rates and greater strain on the gut and cardiovascular system.

Heat makes the same workload more physiologically demanding, even if pace or power looks slower.

Dialling back intensity helps:

• reduce heat production

• reduce sweat rate

• preserve blood flow to the gut

• improve fuelling and hydration tolerance

• manage fatigue more effectively.

In summary:

Hot-weather performance isn’t just “harder” — it’s physiologically different.

• Blood flow is diverted away from the gut, making digestion harder

• Sweat rates increase, and full fluid replacement is unrealistic

• Intensity that feels manageable in cool conditions may be unsustainable in the heat.

Managing heat successfully means:

• accepting some dehydration

• reducing intensity

• using cooling strategies

• using liquid carbohydrates

• pairing sodium with adequate fluid

• adapting gradually through heat exposure

When you understand what heat is doing to your body and adjust fuelling, hydration, and effort accordingly, you can still perform well even on the hottest days, AND avoid ending up in hospital.