Why swimmers stop to eat, and when it's worth it
You can't eat on the move in open water the way you can on a bike or on a run: you have to stop, or slow down substantially. In most races the simplest method is also the most common — a gel tucked into your swimsuit, opened on the go when you need it. In longer, non-loop crossings, an escort boat or kayak comes into play, handing off a bottle or a gel; the fixed feeding pontoon, the classic image from FINA races, is actually the exception rather than the rule for anyone racing point-to-point rather than on a loop. Under 90 minutes of racing, the body is still running on muscle glycogen, and stopping for a gel costs more time than it's worth. Past that threshold, stores really start to run low, and a regular feed — however it gets to you — holds the pace that would otherwise drop in the closing stretch.
How many carbs, and where the number comes from
Open-water swimming guidelines (Shaw et al., 2014) put the target at 60-90 grams of carbohydrate per hour for races longer than 90 minutes, from sources that get absorbed through different pathways in the gut — usually glucose and fructose together, not double the dose of the same sugar. It's the same logic behind mixed sports drinks: one sugar type saturates its absorption channel, two types together open a second one. Anyone aiming for the higher end is well served by having practiced it in training — the gut adapts to absorbing carbs fast, it doesn't start out ready.
What actually goes into a gel
The gram count doesn't tell the whole story — the sugar combination matters too. Up to 60 g/h, a single carbohydrate is enough: glucose or maltodextrin, which the body treats the same way. Past that threshold you need a second sugar that follows a different absorption pathway in the gut: fructose, usually at a 2:1 ratio with glucose — the combination with which Jentjens and colleagues (2004) measured the highest oxidation rates ever recorded in a lab. On the label it usually shows up as "2:1" or "multiple transportable carbs".
In longer races (past 4-5 hours) some swimmers also add a protein source — a whey protein drink, or BCAA capsules — to limit the muscle breakdown that comes with prolonged exercise. It isn't essential the way carbohydrate and fluid are: in the Kumstát and colleagues (2016) case study the swimmer took in around 12 g/h, in doses of 8 to 38 grams, 3-4 times per race, but it stays an optional add-on rather than a rule.
For the solid option the table above refers to, what shows up most often in real feeding logs is simple: a banana (the only solid food in the Kumstát case study), an energy bar, plain biscuits, high-sugar jelly sweets. None needs preparation, and all of them chew down in a few seconds.
The energy math doesn't add up, and that's normal
Swimming for hours burns more energy than can come in through the mouth: a rough estimate (Kumstát and colleagues, 2016, citing McArdle et al., 2010) puts the expected cost at around 10 kcal per kilogram of body weight per hour — for a 70 kg swimmer, roughly 700 kcal/h in a long race. But the gut, during exercise, can only absorb about 240-360 kcal an hour — the same 60-90 g/h of carbohydrate from above, just in a different unit. The gap between what you burn and what you can actually replace isn't a problem to fix — it's the physiology of any ultra-endurance sport. What actually helps you keep up the pace of absorption is changing flavor and texture at each stop — sweet, savory, liquid, solid — because palate fatigue, after hours of identical gels, makes people stop eating before their body asks them to.
That 10 kcal/kg/h alone isn't the whole story: Shaw et al. (2014) cite another study (Zamparo et al., 2005) specifically to say that stroke efficiency is "a key determinant" of energy expenditure in open-water swimming. That study (read in full on 22/9) actually measured it: in men the cost rose from 0.89 to 1.31 kJ/m for just a 9% increase in speed (1.35 to 1.47 m/s), in women from 0.69 to 0.95 kJ/m for a 6% increase — not because of technique in that range, but because water resistance grows with the square of speed. Technique shows up instead in a different experiment in the same study: after a 2 km trial at race pace, at the exact same speed as before, energy cost was 21% higher on average, together with a shorter distance per stroke.
The expected cost above uses both effects — it rises with the square of your pace, and rises further with distance up to a 21% ceiling on longer races — but it isn't Zamparo's formula: their study covers only 1.29-1.50 m/s (world-class swimmers, one of them a world champion) and a single before/after on a near-maximal 2 km effort, not hours of endurance swimming at amateur paces. How we extend those two real findings to slower paces and longer races is our own editorial choice — the same treatment this site gives wetsuits and salt water — not a number Zamparo guarantees for your specific case.
A wetsuit also weighs on the total, but in the opposite direction: it insulates from the cold, so the body spends less energy keeping its core temperature — a saving we estimate here at up to 8% below 18°C, scaled to how cold the water is, fading out above 26°C where there's no heat to conserve. This is also our own editorial number, not a measurement from a study.
For women the expected cost is 2% lower: Zamparo and colleagues measured a lower cost per metre in female swimmers (0.69→0.95 kJ/m against 0.89→1.31 in men), but at different speeds from the men, so we don't derive a coefficient from it. The 2% is our own editorial choice, not a number from the study.
Fluids and salt: how much, and in what drink
Fluid loss while swimming is close to half a litre an hour in most races — the same "non-sweat-visible" loss expected in water (Cox et al., 2002) — but it isn't a fixed number: Shaw et al. (2014) explicitly state that these losses increase with intensity and with water temperature (Cade et al., 1991), up to 1.2-1.6 L/h in the most intense efforts. That's why the fluid target here rises toward 700 mL/h above 26°C and drops toward 500 mL/h below 18°C. In the Kumstát and colleagues (2016) case study the athlete drank an average of 662 mL/h, all as a single drink with electrolytes and gels dissolved into it: not a separate bottle of plain water and a sports drink, but one liquid doing both jobs.
Sodium runs in the opposite direction from a plain liquid: commercial sports drinks often contain little of it, and swimmers in long races add it separately — 400-450 mg an hour is what Kumstát measured on the same swimmer, added precisely to make up for drinks too diluted in sodium. Here too the variable is temperature, not salinity: Shaw et al. (2014, Table 1) explicitly flag "sodium considerations for hot environments" for races of 25 km and up, and above 26°C the target here rises toward 500-550 mg/h — our own extension of that note, not a measured figure like the baseline. Neither Kumstát nor Shaw find a difference tied to fresh or salt water though, and seawater swallowed by accident isn't a sodium source worth counting on anyway — its concentration is higher than the kidneys can handle.
Matching the loss exactly by drinking the same amount still isn't the goal: in long races, drinking more than needed has caused real cases of exercise-associated hyponatremia — blood sodium diluted by excess water, not a lack of salt — and women are hit more often (Wagner et al., 2012; Weitkunat et al., 2012). The practical rule stays drinking to thirst and conditions rather than chasing a number, and in cold water favoring more concentrated carbohydrate sources (gels, dense drinks) over a larger fluid volume.
The days before, the pre-race meal, and caffeine
The carbohydrate count for a long race doesn't start on race day. In the 36-48 hours before, for events over 90 minutes, the useful load is higher than usual: 10-12 grams per kilogram of body weight per day (Burke et al., 2011), spread across regular meals — pasta, rice, bread, potatoes, fruit — not packed into one huge meal the night before.
The last chance to top up carbohydrate stores is the meal 1-4 hours before the start: 1-4 grams per kilogram of body weight, toward the higher end if your stomach tolerates it and the race is long. Total race caffeine starts at 3 mg per kilogram of body weight (the slider goes up to 6, the top of the effective range given by the IOC consensus statement): in 5-10 km races that finish with a hard closing surge it's concentrated mostly in the hour before the start, while in much longer races — measured by Kumstát and colleagues in an elite swimmer across seven Grand Prix races — the same total is spread in small doses alongside carbohydrate for the whole duration. On the upper limit, EFSA sees no safety concern for an adult up to 200 mg in a single dose (about 3 mg/kg) and up to 400 mg a day of habitual intake: in a race the dose is split into small amounts at the stops, through a caffeinated gel or Coca-Cola (about 9.6 mg per 100 mL), and anyone not used to caffeine should stay at the low end.
Why the suggestion changes with current, wetsuit and temperature
The table above doesn't come from an isolated pace — it uses the same current, wetsuit and saltwater correction as the Crossing Time Predictor, because a race against the current takes longer: more stops, not just more time. The starting suggestion for carbs per hour follows water temperature, not because of a specific research figure but as our own editorial choice: below 18°C it starts from the top of the range (90 g/h), because the body spends more to hold its temperature; above 26°C from the bottom (60 g/h), favoring hydration over a higher carbohydrate volume. It stays a starting point — the field can be changed by hand at any time, and swimmers who often go without a wetsuit in cold water already know, from direct experience, that those conditions call for eating more — solid food included — than a race in mild water.
Recovery after the finish
In the hours after a long race the body has to rebuild two things: the muscle glycogen stores, nearly emptied, and the muscle tissue itself, somewhat damaged by the prolonged effort. For glycogen, speed genuinely matters: starting on carbohydrate as soon as possible, right after getting out of the water, makes a real difference — waiting two hours or more can roughly halve the resynthesis rate compared with starting straight away (Betts & Williams, 2010, citing Ivy et al., 1988). If there's another race or session in the following hours, the reference range is 1.0-1.2 g of carbohydrate per kilogram of body weight per hour, for the first 4 hours (Burke et al., 2011) — the same amount that under normal circumstances can instead be spread across the whole day.
For protein, the popular idea of a 30-minute "anabolic window" you can't afford to miss doesn't hold up: a study that compared three ways of spreading the same protein dose over the 12 hours after exercise — eight small doses every 90 minutes, four medium doses every 3 hours, two large doses every 6 hours — found that roughly 20 g doses every 3 hours stimulated muscle protein synthesis best across the whole day, not the most immediate pattern (Areta et al., 2013). A single dose stays effective for about 5 hours, and protein synthesis stays elevated for up to 24 hours after exercise: what matters is continuing to supply protein over the following hours, not speed in the first few minutes. In practice: servings of roughly 20 g of high-quality protein, 3-4 times over the day after the race, each alongside a carbohydrate source (Shaw et al., 2014).
There's also a practical reason, not just a muscular one, to add protein: if you can't hit the carbohydrate target above — maybe because a real meal isn't available right then — adding protein still speeds up glycogen resynthesis, an effect that adds to carbohydrate rather than replacing it (Betts & Williams, 2010). Same as with BCAAs during the race, there's no need for a separate isolated product here either: a whole-protein source — yogurt, eggs, legumes, a protein drink — already contains it.