Guest Blog: Nutrition in Long Distance Swimming


Dr Nick Murch

There is little less in endurance sport that divides opinion as much as the subject of nutrition. So I thought I would put my feelings down to enable others to benefit from what I have learned in my open water swimming and medical career to date. I have done three recognised extreme marathon swims and worked as medical support in the High Dependency Unit at the end of the London Marathon. Some of this may be controversial or against accepted thought, but then I’m all about a debate and there is no ‘one size fits all’ plan.

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Basically the take home message is that there is no right answer, so you don’t need to read on. If you do I’ll use some science to make the point.

Most people use a carbohydrate gel or solution to feed whilst swimming long distances. These are usually composed of small molecules that provide rapid release energy but can cause the stomach to distend (causing nausea) and may have, in my opinion, laxative effects. Many distance athletes get “runners’ trots” (think a certain female marathon runner) and this may be due to excessive pure water consumption, the laxative effect of feeding agents and diversion of blood away from the bowels to the muscles. It is not dirty water that (usually) causes swimmers to develop gastrointestinal upset.

Some maths:

The body’s main stores for aerobic exercise are approximately (80kg man) (note carbohydrate and protein both store 4 calories / gram  and fat 9 calories / gram)

Glycogen (carbohydrate) in liver: 120g   = 480 calories

Glycogen in muscle: 350g = 1400 calories

Fat stored throughout body: 16kg (20 percent body weight) fat: 144,000 calories

Protein (can make up to 5 percent of energy requirements but not ideal to break down when exercising, so we will ignore)

Therefore there is only 1880 calories worth of carbohydrate stored in his body.

In extreme prolonged sport it is possible to use 700 to 800 calories per hour, but the maximum carbohydrate replacement intake is suggested at approx.: 

Max<1gm/kg/hr– 60 to 90gms per hour so 240 to 360 calories per hour – let’s say 320 calories for an 80kg male.

Many people feed hourly or half hourly in long distance swimming. There is clearly a mismatch in intake versus exhausted calories:

Hour 1: 1880 cals to start, 800 cals exhausted, 320 cals taken in = 1400 calories

Hour 2: 1400 cals to start, 800 cals exhausted, 320 cals taken in = 920 calories

Hour 3: 920 cals to start, 800 cals exhausted, 320 cals taken in = 440 calories

Hour 4: 440 cals to start, 800 cals exhausted means 360 cals in deficit with max intake 320 calories… and the body must switch to an alternative fuel source.

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This is where fat metabolism comes in to play. The body must break down fat stores to create energy, which may promote the development of molecules called ketones which can make the blood slightly acidic in some individuals. This can be uncomfortable if unanticipated and leads to the stage of an endurance event where one ‘hits the wall’ or ‘bonks’.

Simple carbohydrate ingestion may cause spikes in blood sugar levels and a reflex release of insulin, which blocks the body from breaking down fat, so those jelly babies or energy drinks may actually be hampering physical performance in the long term. Ways to minimise the ‘glycaemic rollercoaster’ may be to feed regularly small amounts or to have a more sustained released carbohydrate mixture.

I’m a big believer than we should train as we mean to fight – and so I spent several weeks on a low carbohydrate, high protein, high fat diet while preparing for my English Channel crossing (21 miles, 11 hours) and Lake Geneva traverse (69km, 32 hours). I would also train for up to 7 hours with just a coffee beforehand and the odd sugary treat every hour – clearly relying on my ‘bioprene’ to provide the majority of my energy whilst swimming.

Whilst on the subject of coffee, many people are either for or against caffeine during exercise. I, for one, have at least 5 coffees a day so if I didn’t have a caffeinated drink during a long swim I would likely get withdrawal symptoms. If naïve to caffeine then I would strongly advise against taking it during exercise.

One nutrient we cannot do without is water – the body needs 25 to 30 mls / kg/day maintenance hydration (i.e. when not exercising) so 2L to 2.4L for the 80kg athlete plus insensible losses (sweating, heat, water lost on extra deep breathing etc.). Hydration is key for prime athletic performance. East African marathon runners traditionally used tea as their means of hydration, and ran at approximately 2 to 3 percent dehydration as optimal offset of performance versus weight. Anything more than 5 percent dehydration causes a massive drop off in performance. Swimmers are less worried about weight due to relative buoyancy.

Pure water may cause problems too though, as salt (sodium) is also lost in sweat and replacement with pure water may cause a dilution of the blood which can cause problems with both physical performance and health. I personally do not subscribe to the argument that being in salt water will replace salt lost in sweat so I take an electrolyte supplement containing Potassium, chloride and magnesium (amongst others). There are some basic electrolytes in some of the branded carbohydrate drinks so read the label!

The main reason for a periodic feed is, in my mind, two fold. One is for a psychological lift to have some human contact, and to look forward to that placebo effect Jelly baby. One truly swims to the next feed in times of hardship. The other is for the crew to assess how you are performing and whether you are hypothermic, hallucinating etc.

The take home message is, to train as you mean to fight – train on empty and consider  running ketogenic at times (not if you are diabetic – this can be dangerous). Keep feeding regimes, and hence your metabolism, versatile and do what works for you.

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People have asked me what I have used during a swim. So I have decided to share my nutrition plan for both split swims like the Champion of Champions and Geneva relay (hourish long swims, few hours rest) and the North Channel – likely 12 hours plus. I will also, in future, share my training log for my English channel crossing as this is another common question that Channel aspirants ask. My training this year has been more, um, minimal.

I have an interest in the science behind open water swimming; trying to demystify some of the old wives’ tales. An area of particular expertise is nutrition for marathon swims.  I’m well known to be built for comfort and not for speed and have an ability to eat that more than matches my ability to swim.

Some background for those who don’t know me:

  • Windermere one way (10 miles) 2015, 4h36

  • English Channel crossing (21 miles) 2016,  11h40

  • Lake Geneva Signature 70 solo (42 miles, fifth person to complete it officially) 2016, 32h46 with membership of the ’24 hour club’.

For Windermere, I had very little thought about nutrition as I was naive and inexperienced. For the other two, I used ‘traditional’ feeds of maltodextrin (CNP, Maxim, etc.) but with gastrointestinal side effects.

I also, very surprisingly, managed to win the relative sprint of the 2018 British Long Distance Swimming Association Champion of Champions (South) event (5 then 3 then 1 miles) which was good practice for relay preparation and refueling.

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In the run up to the North Channel, I did my 6 hour qualifying swim on tea and warm blackcurrant juice with an ice cream at 5 hours for my salt water tongue – bliss!  Proof that high carbohydrate energy drinks are not required for everyone. As I’ve stated before, channel swimmers go to fat burning early in a swim and sugary drinks can cause insulin spikes which in turn reduce the ability for the body to use fat as a fuel source.

People may point to fact that nutrition info released regarding Chris Froome (and presumably Bradley Wiggins and soon to be Sir Geraint Thomas) uses a maltodextrin / fructose mix which clearly works for him, but cyclists can take small amounts often and so avoid the insulin spikes. Top channel swimmers do this too – small amount every 5 to 7 minutes.

Those of us who feed for a psychological boost and for our team to assess us during a longer swim, usually feed hourly which is clearly not the same.

I am plesased to have had continued success in 2018 which has brought me my North channel swim (Northern Ireland to Scotland, 21.5 miles), 11h48 in waters at 10 to 12 degrees (‘Number 53’) which was one of the coldest crossings on record as well as the Lake Geneva Signature 70 relay (four person, 42 miles) 2018, 22h05

A split swim like champion of champions or a relay of 2 hours each or less is a different prospect to a longer swim, as there is carbohydrate available as glycogen in the body.  So routine advice about carbo-preloading and reloading is correct in my opinion. After each of my legs for C of C and the Geneva relay,I used a recovery drink with protein, electrolytes and carbohydrate and ate pasta, snickers and bananas – if Science in Sport, Mars or Fyffes read this, please send supplies!

Some confounding factors for the attached feeding plan – adapted from one sent from the excellent Infinity channel swimming – as requested by my North Channel pilot. It’s a bit like a school maths puzzle to solve pre-swim.

  • Dispersible paracetamol can not go in tea or coffee so use blackcurrant juice

  • Electrolyte with caffeine tastes of cola so no mixer needed

  • Ucan (sustained release carbohydrate that may avoid insulin spikes) was chocolate flavoured but does not in my experience have enough liquid so needs a water chaser

  • Maltodextrin only for a boost towards the end of the swim if really need a sugar rush, don’t want a sugar crash after!

  • Tea and coffee – models own 

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I also let my team surprise me at times…

I hardly touched my solids during the swim as I didn’t want them.

I vomited twice on the boat on the way back from Scotland to Northern Ireland,  not sea sickness! The maltodextrin was sitting high and I suspect I had delayed gastric emptying. I’d also been fed ‘early’ which meant I had too much volume.

Looking forward I’m keen to explore opportunities to research nutrition in marathon swimming and this I hope is a real prospect if we can find some funding.

I hope this feeding plan is a useful point of reference for marathon swimmers.  And remember: Train as you mean to fight, and do what works for you.

About Nick

Nick is a Consultant in Acute Medicine and Medical Education including Simulation at The Royal Free Hospital in London, gaining his first consultant post in 2010. Having initially trained at UWCM, Cardiff, he completed junior medical jobs in South Wales before travelling up the M4 for the North Thames Acute Medicine Training Programme. 

Nick is a keen swimmer. His background is one of pool swimming and beach lifeguarding as a teenager; this was followed by 15 years playing water polo up to an International standard. More recently he has taken up more open water swimming, building on his tendency to natural bioprene, to enable a Windermere one-way swim in 2015 and an English Channel crossing and Lake Geneva solo in 2016. In 2018 he added the North Channel to his long list of accomplishments.

He has an interest in helping others with their goals and volunteered on the beach in 2017 and can be found in the water in Dover when training for his own events.

Nick has an interest in the science behind open water swimming; trying to demystify some of the old wives’ tales. He has twice helped run the High Dependency Unit at the Finish of the London Marathon and also provided medical support to the World Tae Kwon Do championships. He is a proud member of #teamselkie and well known to be built for comfort and not for speed.