Fueling an UltraSwim: How Much Do You Actually Need?

PART 1

One of the questions I hear most often around long-distance swimming is:

How much should I eat during the swim?

There are really two questions hidden inside that one. First, how much carbohydrate do you actually need? Second, how do you get it while swimming?

The first is a physiology question. It depends on your fitness, relative intensity, duration, temperature and the fuel you already have stored. The second is a practical UltraSwim problem: feed stations are positioned by distance rather than time, some stages have no feed at all, and carrying your own nutrition in the water is not particularly convenient.

So I have split this into two parts. Part 1 looks at how much you need and why. Part 2 takes those principles into the water and works out how to make them practical. My race at UltraSwim Switzerland provides the case study for both.

It is tempting to think the first part should have a simple answer: pick a carbohydrate target, multiply it by the number of hours you expect to swim and count the gels. Unfortunately, physiology is a little more interesting than that.

Current Precision Fuel & Hydration guidance for prolonged open-water swimming sits broadly in the 60–90 g of carbohydrate per hour range, adjusted for duration and intensity. For this discussion I am going to use 60 g/h as a practical planning anchor, not as a ceiling and certainly not as a prescription for everybody.

Fueling follows effort, not speed. Pace mainly tells you when you get to the next stop.

The same pace can have a very different cost

Before talking about grams of carbohydrate per hour, we need to talk about intensity.

Two swimmers can both be swimming at 2:00/100 m and experiencing completely different physiological demands. For one, 2:00 might be genuinely easy. They are aerobically fit, technically efficient and sitting comfortably in Z1 or low Z2. For another, exactly the same pace might represent upper Z2 or even Z3 and be much closer to their physiological ceiling.

As exercise intensity rises, carbohydrate contributes an increasingly large proportion of energy production. Muscle glycogen is used faster and fat contributes proportionally less.

The relationship isn't perfectly linear either. A useful coaching model is the shape of a lactate curve. At lower intensities, pace can increase with relatively modest changes in metabolic stress. Eventually you reach the part of the curve that starts bending upwards, where another small increase in speed becomes increasingly expensive.

Lactate itself is not a direct measure of carbohydrate use, but the curve gives us a useful picture. A well-trained swimmer may be able to increase speed substantially while remaining aerobically comfortable, while another swimmer may already be close to the steep part of their curve.

Pace alone does not tell us how much carbohydrate you need.

What happened in Switzerland

Most of my swimming during UltraSwim Switzerland was a little under 2:00/100 m. More importantly, my data showed that I was generally in Z1 to low Z2. I wasn't trying to race every stage; the effort was deliberately controlled.

My basic fueling pattern was fairly simple. I took a caffeinated gel immediately before the start, then used the official feed stations for my main in-water fueling, usually taking two gels, around 60 g of carbohydrate, at each stop. On the long crossing, because I was swimming more slowly with a group and therefore the time between official stations was longer, I supplemented that with an extra feed between stations rather than relying only on the boats.

I then started replacing carbohydrate again almost immediately after finishing.

I know from running and cycling that I can tolerate much higher carbohydrate intakes, including 120 g/h and above. So 60 g/h wasn't a physiological ceiling for me. It was an amount that seemed appropriate for the relatively low intensity of these swims and fitted reasonably well with the opportunities to feed.

In practice, the overall strategy was simple. I arrived at each start having fueled properly the previous day, ate normally in the morning, took a gel immediately before the swim and used the official feeds as the main source of carbohydrate in the water. Around 60 g/h was a useful working level for my predominantly easy Z1/low-Z2 swimming.

If the gap between feeds became longer, as it did on the crossing from France to Switzerland, I adjusted rather than blindly sticking to two gels per stop. I also carried only a small emergency reserve in the tow float rather than trying to carry my entire nutrition strategy with me. Once the swim finished, I started replacing carbohydrate straight away and continued eating through the afternoon and evening.

That is the basic model. Part 2 will get into the execution details, but you should not need Part 2 to understand the overall strategy.

You don't start with an empty tank

You are not trying to fuel an UltraSwim entirely from whatever you consume while you are in the water.

Your body already carries substantial energy stores. Carbohydrate is stored in the muscles as muscle glycogen, where it is available to the working muscles. Carbohydrate is also stored in the liver, where liver glycogen plays an important role in maintaining blood glucose. Beyond that, even a lean endurance athlete carries a very large reserve of energy as fat.

The carbohydrate you consume during the swim is exogenous carbohydrate, meaning carbohydrate coming from outside the body. It supplements the endogenous carbohydrate you already carried into the water.

This is why calorie expenditure and carbohydrate intake are not the same calculation. If your watch tells you that you are burning 700 calories an hour, it does not follow that you need to eat 700 calories an hour. Some energy comes from fat, some from stored glycogen and some from the carbohydrate you consume while exercising.

The objective is not to replace every calorie while you are burning it. It is to provide enough additional carbohydrate to support the effort, maintain carbohydrate availability and avoid unnecessarily running down the stores you started with.

The water itself has a cost

Swimming has another complication compared with land-based endurance sport: you are immersed in water.

Water can remove heat from the body very effectively. In cooler conditions, some of your metabolic energy is therefore being used not only to move you forwards but also to help defend body temperature.

That does not give us a sensible rule such as “add another 20 g/h below 18°C.” Water temperature, body composition, wetsuit use, swim intensity, exposure time and individual cold tolerance all affect the thermal balance.

There is also an interesting trade-off. A racer produces more metabolic heat because they are working harder, but they may finish sooner. A slower swimmer may be working at a lower absolute intensity while remaining exposed to the water considerably longer.

Switzerland was warm-water swimming. The Waterbook expected approximately 24–26°C, so cold stress was unlikely to be a major driver of my fueling requirement. The same four-hour swim in much colder water would be a different problem.

Yesterday helps fuel today's swim

This becomes particularly important in a stage event.

When you finish a long swim, some muscle glycogen has been used. If sufficient carbohydrate is available afterwards, those stores can be rebuilt. The glycogen you put back into your muscles during the afternoon and evening is largely still there the following morning.

Muscle glycogen is not substantially drained simply because you go to sleep. The liver has a different job. During the overnight fast, liver glycogen helps maintain blood glucose, so liver stores fall more significantly overnight.

That changes how I think about the traditional enormous endurance-sport breakfast.

If you finish today's stage, start refueling immediately, continue eating carbohydrate through the afternoon and then have a carbohydrate-rich dinner, much of tomorrow's muscle-fuel preparation has already taken place before you go to bed.

Breakfast still matters, but it is more of a top-up than an emergency reconstruction job. Restore some liver carbohydrate, hydrate, eat enough that you feel comfortable and get some readily available carbohydrate into circulation.

Tomorrow's swim starts being fueled when today's swim ends.

So why eat during the swim?

If you have muscle glycogen, liver glycogen and a large fat reserve, why bother eating while swimming?

Partly because exogenous carbohydrate gives the working muscles another fuel source and helps maintain carbohydrate availability as exercise continues. But I think there is another reason that is particularly important in open water.

Swimming is highly focus-dependent. You are constantly sighting, holding a line, staying on feet, managing contact, maintaining rhythm, staying relaxed when the water gets difficult, judging effort and making small decisions about pace and position.

You can still be physically capable of swimming while becoming a noticeably worse swimmer. Your line starts wandering, you sight too frequently, your stroke shortens, rhythm disappears, small irritations become big ones and the whole swim suddenly feels harder.

The effects of carbohydrate during exercise are not purely muscular. Research using carbohydrate mouth rinsing has shown that merely detecting carbohydrate in the mouth can influence brain regions associated with reward and motor control, even before that carbohydrate has been absorbed.

That doesn't mean every loss of concentration during a swim is caused by low carbohydrate availability. But it supports the broader point that fueling and endurance fatigue involve the central nervous system as well as the muscles.

For me, feeding during a long swim is therefore partly about protecting the quality of the swimmer: staying alert, positive, technically sound and capable of making good decisions.

Hunger may not be the first warning sign. Sometimes the warning sign is simply that the swimming has started to become complicated.

The pre-start gel

I took a caffeinated gel immediately before each start. I wasn't trying to fill my muscle glycogen stores with one gel; that job had largely been done beforehand.

The gel meant that I started the swim with exogenous carbohydrate coming into the system rather than waiting until the first feed. For stages where the first official feeding opportunity might be an hour or more away, that is particularly convenient. It also matters on stages where there is no official in-water feed at all.

The finish line is part of the fueling plan

My fueling didn't stop when I touched the finish. Quite the opposite.

I generally started eating carbohydrate almost immediately. A typical first intake was around 140 g of carbohydrate, largely from sweets. That sounds like an enormous snack, but I didn't really think of it as a snack. It was the beginning of the next fueling cycle.

The immediate goal was simply to start replacing carbohydrate. I didn't need the perfect recovery meal while I was still wet; a proper meal could provide protein, carbohydrate, fat and micronutrients afterwards.

This became particularly important on the day with two stages. What I ate after Stage 1 was not merely “recovery food”. It was already part of fueling Stage 2.

Post-swim nutrition is therefore not separate from race fueling in a stage event. It is the beginning of the next stage.

What about hydration and sodium?

Carbohydrate is only part of the feeding problem. Fluid and sodium need their own strategy, and I am much more reluctant to give universal numbers here.

Fluid requirements vary with conditions, exposure time, swimmer characteristics and individual sweat losses. Sodium losses also vary considerably between athletes, so there is no sensible formula that should generate sodium intake from swim pace alone.

My Switzerland strategy gives a useful example. I used a 750 ml bottle containing three PH1000 electrolyte tablets and drank roughly half before the swim and the remainder afterwards. During the swim I was mindful to drink at the stations.

That was my practiced strategy, not a generic recommendation and not something I would suggest copying simply because it worked for me.

The more useful principle is that carbohydrate and hydration are related but separate calculations. You need to manage both across the swim and across the day, but they do not need to happen at exactly the same moment.

And caffeine?

Most gels I used in Switzerland were caffeinated. That suits me because I know I tolerate caffeine well. Caffeine can reduce perceived effort and support alertness, which interests me particularly in a sport where concentration and mood matter so much.

But caffeine is optional. Responses vary, and total dose matters when caffeinated products are used repeatedly. There is also an obvious stage-racing trade-off: a caffeine dose that makes an afternoon stage feel easier but interferes with that night's sleep may be a poor bargain when you need to swim again the following morning.

Carbohydrate is part of the fueling strategy; caffeine is an optional tool.

If you intend to use caffeinated products at UltraSwim, practice that strategy before the event too.

The 24-hour model

The more I think about Switzerland, the less useful it seems to reduce UltraSwim fueling to one grams-per-hour number.

I prefer to think about a continuous 24-hour cycle. During the afternoon and evening, replace carbohydrate used in today's swim and rebuild muscle glycogen. As we discussed earlier, overnight, muscle glycogen is largely preserved while liver glycogen supports blood glucose. Breakfast tops the system up, restores fluid and gets you ready to start comfortably.

A carbohydrate dose immediately before the swim means you don't have to wait until the first feed for exogenous carbohydrate. During the swim, carbohydrate supplements the fuel you carried into the water, with the amount influenced by duration and relative intensity. At the finish, the next cycle starts immediately.

So how much do you need? There isn't one number. Fitness, intensity, duration, efficiency, temperature, carbohydrate tolerance and the quality of your pre- and post-stage fueling all influence the answer.

The next problem is more practical. Even once you have chosen a sensible carbohydrate target, can you actually get that amount into yourself while you are swimming?

Feed stations are positioned by distance, not by time, some stages have no in-water feed at all, and the same two gels can represent a very different hourly intake for a 1:45/100m swimmer and a 2:45/100m swimmer.

That is what Part 2 is about: taking the physiology from Part 1 and turning it into a race plan. We will look at the actual UltraSwim feed structure, what one, two or three products at a stop really means at different swim speeds, how to deal with stages that have no feed at all, and some of the surprisingly basic fueling questions that come up again and again.

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