Suits, skins and body temperature

Getting ready! (Credit: Simon Willis — Sunart Media)

If there’s a subject that sparks intense debate among open water swimmers is whether or not to wear a wetsuit. Most of the swimmers I know wear one, but it’s certainly not a unanimous vote. Many believe — usually those who turn up in a swimming costume and a towel tucked under their arm — that “skins” is the way to go. Personal preferences aside, does your body know whether or not you’re wearing a wetsuit? How does your core temperature change in each situation?


Swimming in skins involves an important period of acclimatisation to cold water, with increasingly challenging swims and longer periods at lower temperatures. It’s not an easy process, but for those that do it, well worth it! This is not a “one size fits all” and there’s some evidence to indicate that acclimatisation to cold water depends on temperatures experienced on a regular basis. In other words, if swimmers are used to waters at 15°C, if they wish to move to colder conditions at 5°C, they need to undergo acclimatisation again to the new temperature.

For swimmers not used to cold water, starting off with a difficult swim is a reckless approach and it can be dangerous. Not only does our body lose heat much faster in water than in air — about 25–40 times faster in fact — but it’s also pretty poor at fighting this heat loss. We really were not designed to stay in cold water for long.

If you’re not used to the cold, “thermoneutral” temperature — where you can stay still in the water and maintain deep body temperature without shivering — sits within a very narrow range. If water temperature deviates much from body temperature (about 37°C), you start to feel cold if you stay still in the water for a long time. Obviously, the further away from 37°C, the faster your body starts to lose heat.

For experienced “skins” is a different story, and water temperatures can drop below 20°C and your body can still cope as long as you keep swimming. You may need to swim somewhat vigorously to ensure the heat produced by your body is enough to offset heat loss. Studies have shown that just mild exercise may actually accentuate temperature drop as increased blood circulation to muscles takes heat away from your core.

By the time the thermometer drops to 10°C-15°C, however, the risk of hypothermia increases significantly. The big problem in this situations is that, when core temperature goes past a critical level (different for every person), both metabolism and shivering slow down, which accelerates heat loss even further. It’s important to note that water temperature is not the only factor involved: There have been reports of hypothermia at 22°C but, at the same time, trained swimmers do ice-miles on a regular basis and don’t develop this condition.

Even if you’re acclimatised to cold water and your body is able to cope with the initial shock, your skin still twitches as it gets colder. Most swimmers learn to stop the shivering but, within a few minutes, nerves and muscles, particularly in the upper limbs, are affected and each stroke becomes more difficult. As body temperature continues to drop, blood flow is restricted, which in turn delays transport of oxygen. Muscles are forced to revert to anaerobic metabolism, resulting in an earlier production of the dreaded lactate and energy stores soon run out. You can also blame your tiredness on a higher water viscosity at 0°C compared with 25°C, which leads to a greater use of energy in cold conditions.

Finally, when you’re out of the water, it’s important to continue monitoring how you feel. For a period after the swim, your body core temperature may continue to drop before you start to warm up. This is believed to put a strain on the heart as the cold blood returns from the periphery. Following “skin” swimmers after the 2000 New Year’s Day Alcatraz Swim, researchers noticed this after drop in temperature in 10 of 11 swimmers assessed. Numbers are low, but this hints at a common occurrence and swimmers may need to be monitored even in recreational swimming.

On the other side of the spectrum, there’s anticipatory thermogenesis, where it’s possible to increase core body temperature within a few minutes of getting wet. One of the world’s best-known cold water swimmers, Lewis Pugh, swam across a glacial lake on Everest and the North Pole relying on this ability. Scientists are still struggling to explain how this works exactly, but a research group from Zurich University is keen to find out. Shadowing a swimmer during his attempt to perform three ice-swims in 15 hours at the harbour of Paderborn at the Lake Lippe, Germany, researchers noted a 1–2°C increase in body core temperature within 5–15 min after immersion. They suggest this is a physiological response in some trained athletes, but further research is needed.


In contrast, there are many who opt to go into the “dark side” by wearing a wetsuit. Not just a wetsuit, but if temperatures plummet to 5°C (which they often do in the UK), hats, gloves, and booties often also come out.

In contrast with skins, wetsuits don’t need such an intense acclimatisation process as neoprene protects body core temperature (within certain limits!) This means longer in the water even at low temperatures. For example, studies show that Korean woman divers — known as Haenyeo — can fish for hours in the cold waters around the island of Jeju with only a minimal drop in core temperature if they wear a wetsuit. However, without protection, their core temperature drops fast even in waters up to 20°C.

Wetsuits are essentially a collection of different layers to help trap body heat. Of all the layers present, neoprene is the most important. Chemically speaking, this synthetic rubber is a carbon-based chemical formed by repeating blocks of chloroprene (2-chloro-1,3-butadiene). Chemistry aside, neoprene is an ingenious piece of technology and all the bubbles of nitrogen trapped inside the rubber make it an extremely powerful insulator.

Wetsuits work by allowing a small layer of water to trickle in between the suit and skin, getting trapped there. It may sting in the face and hands at first, but your body quickly warms up this water. After a few minutes, the initial shock disappears and you start to feel comfortable. Incredibly, a layer of neoprene and a small amount of warm water are enough to keep you cosy. The trick for any wetsuit to work is to make sure the water doesn’t flush. If this happens, warm water keeps getting replaced with cold water and you might as well not wear a suit at all.

Not only keep you warm, but wetsuits can also improve your time by about 3% to 7%, which may not sound much, but it adds up to a 50- to 100-metre difference in a 1500m swim. The explanation for this is simple: nitrogen bubbles combined with a smooth surface give “suits” extra buoyancy and less drag. As swimmers find it easier to hold their body in a more horizontal position, it means less energy to avoid sinking and more energy for propulsion… Studies show that over 800m, the total number of strokes was 7% lower with a wetsuit, which means the distance covered per stroke is greater. The result: fewer strokes to cover the same distance means less energy used, resulting in lower oxygen uptake and lower heart rate.

The impact depends on your swim experience and body frame. The improved buoyancy favours mostly newbies, who tend to let their legs sink while swimming. Top-notch swimmers might not see any changes since they’re likely to be more streamlined in the water even without a wetsuit. On the other hand, if you’re carrying a few extra pounds around the waist, you already have a built-in “keep warm & buoyancy system” (can I call it bouncy blubber?) and it’s unlikely the wetsuit will have a great influence. In contrast, without the protection of fat, slimmer swimmers are more likely to feel the warm embrace of a wetsuit.

Deciding on colour and style of the wetsuit is a personal choice, but there is one aspect that may have serious consequences: picking the wrong size. Sadly, there have been some cases of sudden deaths during swimming events, mostly during competitions. It’s impossible to name a single factor responsible for these events, but recent research shows that wetsuits — particularly a tight-fitting one — may trigger sudden and potentially fatal arrhythmia or even acute pulmonary oedema. A serious point here — having problems breathing or chest pain is a sure sign it’s time to get out of the water (and out of the suit). According to the researchers, those with a history of hypertension or other cardiac problems may need to consider buying a suit on the larger side of the recommended range.

Swimming across Loch Sunart (Credit: Simon Willis — Sunart Media)

The bottom line?

From a physiological point of view, your body does know whether you’re swimming in skins or suits. Whichever option you go for, your first swims will be stressful and short with a big impact on core temperature, but your body will eventually learn to cope with the shock as you get acclimatised to the cold.

Despite the potentially higher impact on body core temperature, “skins” defend their practice as a way to a greater sense of freedom without the constraints and limitations of a wetsuit. On the other hand, for less skilled swimmers like me, the safety net of neoprene is the perfect way to improve swimming skills and feel better for it! More importantly, suits have allowed many people to take up this sport who possibly wouldn’t otherwise.