Putting on weight for swimming: how would this affect your health?

The perceived wisdom is to deliberately put on weight to increase your natural insulation for marathon swimming. But how might this affect your health, asks Simon Griffiths.

Your ability to tolerate extended periods of time in cold water is, to a large extent, determined by your body size and shape. Large, rounded bodies cope better with the cold than skinny, lanky ones, other things being equal. If you have marathon swimming ambitions and don’t already possess the ideal body, the perceived wisdom is to deliberately put on weight to increase your natural insulation.

Additional weight makes a difference. Open water swimmer and runner Pip Bennett used to weigh around 90kg. In recent years, he’s increased the amount he runs and competes at a high level in aquathlon, and his weight has dropped to 78kg.

He says, “I used to revel in 18-degree water. Now I have to work hard in 20 degrees to stay warm enough.” The change is so marked that Pip raced the 3km at the National Masters Open Water Championships in 2024 rather than the 5km. He worried he would have struggled with the additional 2km in 19-degree water.

Despite the obvious cold water tolerance benefit, deliberately increasing your weight could have negative health consequences. Being overweight increases your risk for a range of illnesses. As many of us swim to support and improve our health, is this something we should do?

One option may be to temporarily change our weight. William Ellis crossed the English Channel this summer after increasing his weight from around 77 to 86kg in the two months leading up to his swim. At 183cm tall, this pushed his Body Mass Index (BMI) from what’s considered a healthy 23 to a slightly overweight 25.7. Having done something similar for his round Jersey swim last year, he’s confident he will lose the weight again.

Nevertheless, Will had some concerns about the weight gain process. On top of his regular healthy diet, he consumed lots of processed carbohydrate snacks, which are known to generate insulin spikes and negatively impact your metabolic health. He would be wary about pursuing this approach to gain weight long term. In fact, Will had shied away from swimming the Channel for many years, despite it being a long-term ambition, because of his concerns about putting on weight.

What changed for Will was the realisation that, for him, a relatively modest increase in weight would be sufficient. Acclimatisation to cold is multifaceted and putting on weight is not the only option.

“Living in Brighton, I’m lucky to be close to Sea Lanes, which is heated to around 18 degrees all year,” says Will. “I spent many hours training here through the winter before putting on weight and I’m convinced it made a huge difference.”

The other thing Will pointed out is that the fitter and faster you are, the less weight you will need to put on. If you’re fit, you can work at a higher rate and generate more internal heat. If you’re fast, you should be in the water for less time.

This is a complex field with lots of variation between individuals but it’s worth reflecting on the possible consequences to your long-term health to achieve your short term swimming ambitions and considering what you can do to mitigate potential harms. Some suggestions include:

1. Don’t assume you need to put on weight. Your current body shape might be perfect.
2. Acclimatise as much as you can through exposure before adding weight.
3. Work on your speed and fitness.
4. Maintain a healthy and balanced diet.
5. Feed frequently on your swim to help keep your energy levels high.