Cold-water swimming: How to acclimatise for an extreme triathlon or late-season swimming

Want to race - or train - in cold water? Here’s the science behind what happens to your body when you're immersed in cold(er) water — and how the right kit can help you cope with the extreme conditions

The cold Norseman swim in Eidfjord by Jose´ Luis Hourcade

Go to any triathlon and alongside the registration tent, you’ll see a board announcing the day’s water temperature and, depending on the figure, cheers or groans from the racers. But how much do we, as age-group athletes, really understand the impact of cold(er) water on swimmers and the different circumstances that can impact that?


As the summer drifts by and you enter the cooler months (or if you’re contemplating a race with a chillier swim like Norseman) then it becomes key to know just how your body will react to the water. Then you need to plan what kit and training adaptations you can make to help keep you warm and in the water for that bit longer.

To understand more, we talked to Jørgen Melau, the chief of the medical crew for the notoriously chilly Norseman Xtreme Triathlon in Norway. Melau is doing a PhD at the University of Oslo and has made research into cold-water swimming his area of interest, so he’s a good man to have on hand if your race asks triathletes to swim 3.8km in a fjord that rarely tops 14°C.

“The Norseman research project started in 2015,” Melau explains. “A few days before the race, we measured a water temperature of 10°C in Eidfjord and we really didn’t know what to do. So we tested the swim ourselves, measuring our core temperature with ingestible temperature pills [these transmit core temperature to a receiver via Bluetooth]. We swam the whole 3.8km in the fjord, wearing a normal wetsuit. No hoods, balaclavas or vests. From the results, we decided to shorten the race swim to 1,900m.”

That initial piece of work led to a bigger research project around cold-water swimming, including studies on core temperature and biomarkers each year at Norseman, as well as other famously cold races, including ÖtillÖ swimruns and the Oslo triathlon. So where do the risks lie?

Cold-water physiology

“One of the most immediate dangers with cold-water swimming is called ‘autonomic conflict’,” Melau says. “The rapid change in environment from being on shore (or on a ferry) and then suddenly jumping into cold water is very stressful for the heart. Two very powerful responses are activated, namely cold shock and the diving response. These have opposite effects on the heart and can be very dangerous. That is one of the reasons why, at Norseman, we spray the athletes with cold water on the ferry before they jump into the fjord.”

“Secondly, there’s hypothermia,” Melau continues. “This is when the athletes are in danger of becoming too cold. The longer the athletes are in the water, and the colder the water, the colder they become. One of the aims of our studies is to understand when the water temperature is too cold to train or race.”

Based on previous research projects, the ITU have a stringent set of guidelines regarding the recommended time in the water based on race distance, water temperature and air temperature [see ITU rules online at]. But the rules don’t offer advice on how you can extend your open-water season once the temperatures drop…

How to adapt to the cold water

One man that knows a lot about swimming in cold water is fitness expert and strongman swimmer Ross Edgley. In 2018 he swam some 2,000km around the British Isles, racking up 10-12hrs of swimming per day and not finishing until early November. We asked him how he adapted.

“Cold water can have a profound impact on your performance,” Edgley told us. “It’ll affect your swim technique in the water, your breathing, whether you’re tapping into that lactic threshold because you’re panicking in the water. For me, the only way I’ve found to get better at it is what we call a ‘healthy hardship’, in that you’ve just got to keep on getting in cold water.

“With experience, instead of going into shock when you enter cold water, your respiratory system will go, ‘Right, we’ve been here many times before, we know what to expect’, so your capillaries will vasoconstrict to send blood to your core, and once your core is warm enough they’ll open up again and send the warmth back to your arms. The only way to really learn and understand it is to get out and do it. But build up gradually, the same way you would build up the mileage for a run.”

This is something that Melau’s research agrees with. “The human body is good at adapting to different environments. The good news is that, just as we can acclimatise to altitude, we can acclimatise to cold. A number of scientific studies conducted among Inuit and Arctic fishermen have shown that individuals who live and work in cold surroundings tolerate cold better.

“To prepare for a cold swim and become accustomed to your body’s response to the cold, you should practise swimming in cold waters under safe circumstances. Such acclimatisation could considerably reduce your risk of cold-water shock. In the beginning, limit your exposure
to cold water to short periods in shallow waters. Then gradually increase duration to longer swims close to the shore and ideally escorted by a boat.”

What is the after-drop?

It’s also worth bearing in mind the ‘after-drop’ effect. This is the phenomenon of your core temperature continuing to drop for 30-45mins once exiting the water, as your body sends blood back away from your core to your limbs. It’s important to warm up gradually to limit this. Although it might be tempting to head for a steaming hot shower, this can actually exacerbate the effect. So instead, aim to slip out of any wet clothes, wrap yourself up and sip some warm liquid to make the process more gradual. Edgley has some experience of this. “For the first month I naturally allowed my body to warm itself up after being in the sea as we didn’t always have hot water or radiators on the boat. My body was still able to cope. DryRobes help, but ultimately you need to trust that your body is more powerful than you think.

“These days we don’t like being cold. We’ll turn up the heating or reach for an extra jumper. Our ancestors didn’t have that luxury, but the biology is the same.”

So acclimatisation is key – as are careful strategies around warming the body after swimming. But what else can we do to make the cold more bearable? Adding body fat is one route to go, but triathletes are notoriously lean and adding weight won’t help bike or run times. So what about kit?

What cold water gear options are there out there?

There are thermal wetsuits available from brands including Blueseventy and Huub that include a furry lining to keep you warmer for longer and, anecdotally, the 220 team have found these helpful in colder swims. But how about hats, gloves and booties? Do they really work when core temperature is the key to staying in the water for longer?

Melau is also conducting research into this area. “We believe that balaclavas, gloves and booties help to keep core temperature as normal as possible while swimming in colder water. The balaclava should cover your neck, since there are large blood vessels there. The carotid arteries are located close to the skin, and exposure of the carotid arteries to cold water is a major source of heat loss. Be aware that the balaclava shouldn’t be too tight, as you don’t want to compress the blood vessels. We’re not sure about vests and we’re planning research into them, as some previous studies suggest this should be looked into more.”

Anyone who watched the Great British Swim will know that Edgley is a man who likes to experiment with his swim kit. “Gloves and booties help, but there’s no substitute for knowing how your body is feeling before there’s a problem.”

So where does this leave us? Primarily,  your body will adapt to the cold, but it’ll take time. Keep your cold-water swims shorter than you think you can handle at first and don’t set yourself overly-ambitious ‘time goals’.  Focus on how your body is feeling, stay close to shore and make sure you have a buddy with you at all times. Also understand how to manage and spot cold-water shock (on water entry) and hypothermia (which sets in after time in the water). Getting the right gear will also aid comfort and help you deal with the cold.

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