Why I take electrolytes differently during cold vs. hot races

For years, my race-day electrolyte plan was basically a scoop of powder in a bottle, repeated whenever I remembered. It worked well enough on mild runs, which is exactly why it stayed untested. The plan had no relationship to the weather, my sweat rate, or the amount of fluid I actually took in.

Testing turned hydration into a feedback process rather than a fixed recipe. A hot start outside Melbourne and a humid Brisbane course demanded very different behaviour from a freezing Canberra dawn. This is why my electrolyte approach changes between cold and hot races: I replace the fluid and sodium I am likely to lose while protecting gut comfort and avoiding the urge to overdrink.

Heat changes the size of the problem

In heat, skin blood flow rises and sweating accelerates. Blood moving to the skin helps release core heat, while evaporation carries heat away, but both processes put extra demand on the cardiovascular system. If fluid loss outpaces replacement, plasma volume can fall, heart rate climbs and pace becomes harder to hold.

Perspiration is less predictable than it looks. Sweat volume changes with temperature, humidity, clothing and effort, while sodium concentration can differ greatly between runners. A standard sports drink may suit one athlete and feel too weak to another, so I look for patterns in my own sweat rather than treating a generic bottle as a prescription.

On a hot course, I set a fluid target, drink regularly and use sodium to support the volume I am taking. I still respond to thirst instead of forcing every drop on a timer. Pushing large volumes of plain water can upset the stomach and contribute to exercise-associated hyponatremia, where blood sodium becomes dangerously low.

Cold hides the fluid loss

Cold sends a different signal. Vasoconstriction reduces heat loss at the skin, so sweat may be obvious and thirst may be low. Shivering restores heat but raises metabolic demand. The misleading part is the layer of clothing: an insulated jacket can trap enough perspiration to create a real fluid deficit.

That means a cold race usually needs less fluid than a hot race, yet it does not automatically need zero electrolytes. If I am still sweating heavily, wearing a salty jersey or using a salty drink, sodium losses continue. The useful comparison is still sweat volume and drink intake, not the thermometer alone.

At a freezing Canberra start, I keep the bottle easy to drink and avoid loading it with a strong concentrate. I may choose a warm fluid when race rules allow, but I do not increase intake simply to prove that I am prepared. Cold makes comfort and palatability decisive; a technically ideal mix that I cannot tolerate is not useful.

I match electrolytes to actual losses

I estimate sweat rate on familiar training routes. After accounting for fluid consumed, the change in body mass from start to finish gives a practical approximation of sweat loss, since one kilogram roughly equals one litre. I repeat the test in different weather and use the range, not one perfect number.

Sodium concentration is the next variable. I focus on sodium because it is the electrolyte most directly linked to sweat replacement; potassium, magnesium and trace amounts can usually come from normal food and a balanced drink. A useful mix comes from trial: drink it during training, check the stomach and compare how it feels with a more diluted option.

The same trial-and-error applies to equipment settings. This guide to string tension preferences shows why a recommended setting still has to suit the athlete, conditions and task rather than be copied blindly. I apply that logic to electrolyte strength: one controlled change per session reveals more than three new variables introduced together.

My race-day setup in two lists

The pre-start list protects me from solving a problem that has not happened yet. I keep it deliberately short:

• Record body mass, weather, planned fluid and sodium on familiar routes.
• Choose bottle volume around available aid stations and refill points.
• Test every new sachet, tablet or bottle during training.
• Keep a backup soft flask when the route or forecast is uncertain.

On course, the list shifts from preparation to observation. I track what I drink and how the stomach responds instead of waiting for dehydration symptoms:

• Drink to thirst and the planned range, adjusting for heat and effort.
• Use electrolyte-rich fluid when sweat loss is high or sodium intake is low.
• Pair carbohydrate with fluid without making one drink the sole fuel source.
• Refill at reliable aid stations and protect bottles from heat or freezing.

These lists are short because they need to survive fatigue. A more elaborate protocol would work in a spreadsheet, not on a technical trail at hour six.

Weather changes the execution plan

Hot-race bottle management is mostly thermal. I keep bottles shaded, use a vest that holds them against my back and select a drink I can tolerate without refrigeration. At a cold start, I use an insulated bottle, allow room for expansion if fluid freezes and keep the concentration moderate.

Australian runners have plenty of choice: sports drinks line supermarket shelves, chemists stock electrolyte powders and tablets, and imported mixes are easy to order online. I compare the nutrition panel rather than trusting the word electrolyte on the front. Australian food-labelling rules require ingredients to be listed and sodium to appear in the nutrition information panel; products making therapeutic claims may also fall under the Therapeutic Goods Administration’s oversight.

A 500 mL bottle and a tablet can therefore deliver very different amounts of sodium, while a sachet may be concentrated differently from the drink I use at home. Since Australian ultras often depend on refill stations and bring-your-own bottles, I confirm what each aid station supplies and carry a collapsible cup in case cups are not provided.

The forecast sets the starting volume, while observations on course adjust it. That is more reliable than changing the whole plan because a weather app did.

What the bottle taught me

The contrast became obvious on two different race days. Outside Melbourne, my pace faded as the temperature rose, my bottle emptied faster and plain water left me wanting more salt. A few weeks later at a cold Canberra event, thirst was muted and fluid needs were much lower, but forcing the same intake felt uncomfortable.

The lesson was not that heat always requires salt and cold never does. Each race needs its own baseline. Heat usually raises fluid loss and often increases the value of sodium in what I drink; cold usually reduces volume, while the need for electrolytes depends on residual sweat and the length of the effort.

The bottle is not the symbol of good fuelling; it is the measurement of it. What I want to remember is simple: match the fluid to the losses, match the sodium to the sweat, and let comfort, thirst and the conditions keep the plan honest.