Open water swimming places nutritional demands that are quite different from pool swimming. Swims are longer — often lasting several hours. Water temperatures in the UK are typically between 10°C and 18°C for much of the season, meaning your body burns extra calories just maintaining core temperature. And unlike running or cycling, you cannot eat during the activity itself (at least not easily), which makes what you eat before and between swims critical.

This guide covers the practical nutrition principles for open water swimmers in the UK, from pre-swim fuelling and post-swim recovery to planning food for multi-event days and long-distance challenge swims.

Why Open Water Swimming Burns More Calories Than You Expect

Most swimmers underestimate how many calories they burn during an open water swim. In cold water — which covers the majority of the UK outdoor swimming season — your body expends significant energy on thermoregulation in addition to the muscular effort of swimming. A swimmer covering 3–4km in 14°C water may burn 700–1,000 calories or more, depending on body composition, stroke efficiency, and how long they are immersed.

For longer events — 5km, 10km, or marathon distance swims — calorie expenditure can reach 3,000–5,000 calories across the duration of the swim. Even with feeding stops on pontoons during longer races, most swimmers finish in a significant calorie deficit.

This means that pre-swim nutrition matters more for open water swimmers than for almost any other endurance discipline. You cannot easily top up during the swim, so your glycogen stores at the start — and what you eat during any breaks — determine how well you sustain your effort across the full distance.

Pre-Swim Nutrition

The goal of pre-swim eating is to maximise glycogen stores without creating digestive discomfort in the water. Stomach issues during an open water swim are not just unpleasant — they can affect your breathing pattern, your stroke, and your ability to sight effectively.

3–4 hours before the swim, eat a substantial meal rich in carbohydrates with moderate protein and low fat. Porridge with honey and banana, toast with nut butter, or rice with a simple sauce all work well. This gives your body enough time to digest before you enter the water.

60–90 minutes before the swim, have a smaller carbohydrate-rich snack. This tops up glycogen without sitting heavy in your stomach. A flapjack, a banana, a small bowl of cereal, or a calorie-dense bar works well here. The key is that it should be something you have tested before — race morning is not the time to try a new food.

15–30 minutes before the swim, some swimmers take a final small carbohydrate hit — a few bites of something easily digestible. Others prefer to enter the water with nothing recent in their stomach. This is highly individual and depends on your tolerance. Experiment during training swims to find what works for you.

For early-morning events — which most UK open water races are — the pre-swim meal window can be challenging. If your race starts at 7am, eating a full meal at 3am is unrealistic. This is where calorie-dense, easy-to-eat foods become particularly valuable: something you can consume at 5:30am that delivers substantial calories without requiring preparation or a large volume of food. For more on calorie-dense breakfast approaches, see our guide to calorie-dense foods.

Fuelling During Longer Swims

For swims under 60 minutes, most swimmers do not need to eat during the swim itself — pre-swim fuelling is sufficient.

For swims lasting 60–90 minutes, a carbohydrate drink in a bottle on a kayaker's boat or at a pontoon feeding station is usually enough. Sports drinks, diluted juice, or maltodextrin solutions are common choices.

For swims over 90 minutes, you will need both liquid and solid fuel. The challenge is consuming it quickly — most feeding stops last only 10–30 seconds, and you are treading water while eating. Food needs to be soft, easy to swallow with minimal chewing, and high in carbohydrates. Bananas, energy gels (though many experienced open water swimmers dislike their texture in this context), carbohydrate drinks, and soft flapjacks are common options.

The principle is the same as any endurance discipline: aim for 30–60g of carbohydrates per hour for moderate-duration swims, and up to 90g per hour for very long swims. But the delivery mechanism is harder than in running or cycling because you have no pockets, no pack, and very limited time to eat. Practise your feeding routine during training — the mechanics of treading water, eating, and resuming your stroke need to be rehearsed, not improvised on race day.

Post-Swim Recovery Nutrition

Recovery nutrition matters more in open water swimming than many swimmers realise, particularly because cold water immersion suppresses appetite. You may emerge from a long swim feeling nauseous, cold, and not remotely hungry — but your body urgently needs fuel to begin recovery and to restore core temperature.

The most effective approach is to eat within 30–60 minutes of finishing, focusing on carbohydrates to replenish glycogen and some protein to support muscle repair. A recovery smoothie, a warm bowl of porridge, a sandwich, or a calorie-dense bar with a hot drink all work well. Warm food and drink are particularly helpful after cold water swims because they assist with rewarming from the inside.

If appetite is low — which is common after cold water immersion — choose foods that are calorie-dense and easy to eat in small amounts. Something soft that can be picked at gradually is often more realistic than a full meal. For strategies on eating when appetite is suppressed, see our guide on what to eat when you have no appetite.

For swimmers competing in multi-event days or doing back-to-back training sessions, recovery nutrition between swims is critical. What you eat after your first swim directly affects your performance in the second. Prioritise fast-digesting carbohydrates, adequate fluid, and electrolytes — particularly sodium, which you lose through sweat even in cold water.