How to fuel a long open water swim

How do you fuel a long open water swim? For anything under about an hour, you don't need to eat in the water at all: a proper breakfast covers it. Once you're swimming for more than an hour or so, you need carbohydrate on the move, the same as a runner or a cyclist, and that means planned feeds every 20 to 30 minutes from a bottle, a feeding pontoon or a support kayak.

Swimming gets left out of most fuelling advice because you can't eat with your face in the water. That's exactly why swimmers get it wrong. The classic mistake is treating a long sea swim like a pool session and only drinking at the end.

Why swimmers under-fuel

Three things make swimming different.

You can't graze. On a bike you can nibble every ten minutes without thinking. In the water every feed costs you time, rhythm and sometimes position in a pack. Most swimmers respond by feeding as little as possible.

You don't feel the sweat. You do sweat in the water, especially in a wetsuit or warm water, you just never see it. So thirst and fluid loss creep up on you.

The events are long. A review of open water nutrition points out that most open water races run from one to six hours, which puts real pressure on muscle fuel stores, hydration and temperature control. A 10km race is a two-hour-plus effort for most age-groupers, and a 25km race or a channel crossing takes many hours.

How many carbs per hour

The general sports nutrition guidance from the American College of Sports Medicine works for swimming too, as long as you think in time, not distance.

What do the best actually take? One published case study followed an elite female open water swimmer through seven Grand Prix races of 15km to 88km, lasting three to twelve hours. She averaged about 83g of carbohydrate and 12g of protein an hour, with sodium and caffeine in every race. That is a professional with a support boat and years of practice, so don't start there. It does show that swimmers can take on cyclist-sized amounts once they've trained for it.

What the 10km research shows

This is where I want to be honest, because the evidence for the classic 10km race is mixed.

In one study, eight elite open water swimmers swam three 30-minute blocks at their 10km race pace, then swam to exhaustion. Taking 45g of carbs in 550ml of water during each of the two breaks let them last noticeably longer in the final test than a placebo drink did, about 169 seconds against 102. The authors said carbs should help in the final part of a 10km race.

In another, ten elite swimmers did a 10km session as 20 repeats of 500m, taking either water or a carb drink at 60g an hour. The carbs made no difference to their times or how hard it felt.

My read: around the two-hour mark, carbs help most when the end of the race is hard and you've been pushing all the way. For age-groupers, who take longer over the same distance, the case for feeding gets stronger, not weaker, because you're in the water longer. And past three hours there is no debate. You need to eat.

How to feed without losing the race

Whatever you take, it has to go down in a few seconds while treading water.

• Liquid first. A carb drink gives you fuel and fluid in one swallow. It is the easiest thing to get down while floating. Most long swimmers build their plan around it.

• Gels as a top-up. Tuck them in a wetsuit sleeve or swim cap, and take them with a mouthful of drink, not seawater.

• Soft food on very long swims. On swims of many hours, swimmers often use banana, tinned peaches or similar soft food from a support boat. Anything you have to chew for long is hard work.

• Feed on a schedule. Every 20 to 30 minutes is the usual rhythm for long swims. Waiting until you feel empty is too late.

• Practise the actual feed. Rolling onto your back, drinking, and getting going again is a skill. Practise it in training, including how you'll get the bottle back to your crew.

• Watch what you swallow. Gulping drink quickly makes it easy to swallow air too, which leaves you bloated. Small, steady swallows go down better.

In races of 10km or longer you'll often have feeding pontoons, where your coach hands you a drink on a pole. In shorter races and most training swims you carry your own, or you don't feed at all.

Breakfast still matters most

Even on long swims, the meal before matters most, because the first hour is fuelled entirely by what you ate before. The review of open water nutrition puts going into the race with full glycogen stores and good hydration at the top of the list.

Eat a familiar, high-carb, low-fibre breakfast two to three hours before, the same one you've used before long training swims. Porridge with honey, toast and jam, a bagel, a banana. And no, you won't drown if you swim soon after eating. US Masters Swimming calls the old "wait 30 minutes" rule plain wrong for swimmers who need to fuel.

Temperature changes the plan

Cold water and warm water push you in different directions.

In cold water, the review of open water nutrition notes that changing the temperature of what you drink is one way to help with body temperature in extreme conditions. A warm carb drink from a flask on a support boat is the obvious version.

In warm water or a wetsuit you still sweat, you just can't see it. Plan fluid and some sodium into each feed, especially if you dry with white salt marks on your skin and cap. Our salty sweater guide covers how to tell.

Pool sessions, triathlon and swimrun

Long pool sessions follow the same rule: a bottle on the end of the lane for anything much over an hour. Plenty of swimmers never bother, and a long main set is where it shows.

For triathletes, the swim is usually fuelled by breakfast, then you eat on the bike. For an Ironman, you'll be in the water for an hour or more before the bike, so starting topped up matters. Our triathlon fuelling guide covers the full race. Swimrun is closer to a long run with swims in it: carry gels in a pocket and eat on the run legs, where it's easier.

FAQ

How many carbs per hour for a 10km swim? For most age-groupers a 10km swim takes well over two hours, so 30 to 60g an hour is a sensible start, rising towards 90g on longer swims once your gut is used to it.

How often should I feed on a long swim? Every 20 to 30 minutes is the common rhythm. Shorter gaps cost too much time, longer ones leave you running on empty.

Do I need electrolytes in the water? On long or warm swims, yes, some sodium in your drink makes sense. Don't count on swallowed seawater to cover it.

Can I eat solid food while swimming? On very long swims, soft foods from a boat work. In races and most training swims, drinks and gels are much easier.

Sources

• Shaw G, Koivisto A, Gerrard D, Burke LM (2014). Nutrition considerations for open-water swimming. International Journal of Sport Nutrition and Exercise Metabolism 24(4):373-381.

• Kumstát M, Rybárová S, Thomas A, Novotný J (2016). Case study: competition nutrition intakes during the open water swimming Grand Prix races in elite female swimmer. International Journal of Sport Nutrition and Exercise Metabolism 26:370-376.

• Baldassarre R et al. (2022). Carbohydrate supplementation during a simulated 10-km open water swimming race: effects on physiological, perceptual parameters and performance. European Journal of Sport Science.

• Baldassarre R et al. (2018). Carbohydrate supplementation does not improve 10 km swimming intermittent training. Sports 6(4).

• Thomas DT, Erdman KA, Burke LM (2016). American College of Sports Medicine joint position statement: Nutrition and athletic performance. Medicine and Science in Sports and Exercise 48(3):543-568.

• US Masters Swimming. Open water 101: nutrition on long swims.

In MOVE you can plan carbs per hour on any swim, pool or open water, and on the swim leg of a brick. The app never stops you planning a bottle on a swim, so the feeds you practise in training are the feeds you'll take on race day.