Calories burned vs. calories consumed

How many calories should you consume in a marathon swim?

According to an article on the “Nutrition Demands of Open Water Endurance Swimming,” swimming burns 2.93 calories per mile, per pound. The author, Don Macdonald, did the math and figured that he burns approximately 15,000 calories during a 24-mile swim. Later in the article, Macdonald goes on to say:

As you can imagine, it is difficult to eat 15,000 calories over a 13-hour period without training the stomach to handle this input.

Leaving aside the reasons (discussed previously) that the above formula is probably bogus, let’s think about this: eating 15,000 calories in 13 hours. That’s 1,154 calories per hour. Burning this many calories is one thing. You might be able to do it for an hour; probably not for 13 hours straight. But consuming that many calories is something else entirely.

Can you guess what would happen if you tried to consume 15,000 calories during a 24-mile swim? That’s right – you’d get sick and would not finish the swim. It’s not a matter of – as Don Macdonald says – “training the stomach to handle this input.” Nobody can train their stomach to process 1,150 calories/hour for 13 hours, while simultaneously swimming 24 miles.

Some basic facts about nutrition in endurance sports:

  • During an endurance event you’ll burn somewhere between 500 and 800 calories per hour, depending on effort, body weight, and other factors.
  • Your gut, however, can only process about 240-280 cal/hr (perhaps a touch more if you consume multiple carbohydrate sources). Any more than that and you find out what is meant by “gastric distress.”
    • For reference, the standard/recommended Maxim feed is 230 cal/hr. Hammer Perpetuem recommends 100-270 cal/hr, depending on body weight.
  • This produces a deficit of 250-500 cal/hr. How do you make up the difference – or do you just “run out” of energy? Two ways:
  • Stored glycogen in your liver. You have about 1,600-2,000 calories of this when you begin a swim. Typically, you will deplete this store within 2-3 hours (“hitting the wall”).
  • Body fat. This is a much richer (albeit less readily accessible) source of energy. At 160 pounds about 16% body fat, I have about 25 pounds of body fat – equivalent to 87,500 calories. I could swim all week off that!
  • (Technically, muscle protein is also a source of energy, but this shouldn’t be an issue if your carb intake is adequate.)
  • At any given time during a long swim, your body is using both glycogen and body fat – in addition to the carb infusions from your regular feedings – to provide fuel to your muscles. Your goal is not to replace all the calories you’re burning – just some of them. Your body takes care of the rest.

Lessons learned? Calories consumed ≠ calories burned. Do not attempt to consume 15,000 calories on a marathon swim – unless you’re Penny Palfrey and planning a 50+ hour swim. For a 13-hour swim, you shouldn’t need more than about 3,500 calories. For my longest swim of the year (9 hours in Tampa) I was fine with ~2,800.

And don’t believe anything you read on

(For more, I recommend this book by nutrition scientist Asker Jeukendrup, and also the knowledge base at Hammer Nutrition – e.g., this and this.)

Part 3 in a 3-part series. See Part 1 and Part 2.

8 thoughts on “Calories burned vs. calories consumed”

  1. I believe there’s an open question around the maximum processing per hour. Scientific testing of absorption rates occurred at ambient or higher temperatures. I don’t know (or at least haven’t come across) any long duration testing in cold. I genuinely don’t know if cold improves the bodies ability to process more, less or the same, if the water temperature is say 13C.

    Also ketosis, the transition from glycogen consumption to fat consumption, is difficult but is easier with experience. Experienced cyclists are used to the “knock” or “bonk” happening really quickly, because the energy demands are higher. Mike Oram tells crews to look for the swimmer to go grey at about 6 hours, as ketosis happens. For better trained swimmers, who have trained through this transition, it is often not quite so noticeable nor does it happen as early. I’ve found it can happen as early as 4 hours for me, i might have a difficult 30 minutes then feel ok again.

    1. Very informative comment – thanks Donal!

      Re: effect of water temp on max processing per hour. It may be an open question, but most likely we’re talking about matters of degree, not quantum leaps. If it’s like most relationships in nature, the effect size will be small (if still statistically significant). Perhaps a 10 or 15% increase? So instead of a max rate of 300 cal/hr, perhaps 15C water bumps it up to 330 cal/hr. The author’s suggestion of 1150 cal/hr would be a 380% increase. Not likely.

  2. I had been wondering what I would do with my spare time now that OW race season was done. The answer: read all your wonderfully informative posts! I’m learning tons. Thank you, and keep ’em coming!

  3. I think anyone who is planning a marathon swim SHOULD attempt to consume 300, 400, 500 calories per hour… but do it during a training swim. The lesson learned will be a valuable one, and you will recognize the symptoms if an overload happens during an event and hopefully be able to take corrective actions. Purge, dilute and start over.

    I have met more than a few swimmers whose feed plan included over 500 cals/hr. All I could say is good luck with that!

    1. A very good idea because I’m sure the inter-individual differences on max consumption are bigger than one would expect. Better to use experience to zero in on that range than taking the textbooks’ word for it. I actually was 320 cal/hr on Ederle…. quite a bit more than what the textbooks would say for someone my size.

  4. This whole issue is very interesting to me, and I’ve had a number of conversations with Donal about it. The reason that I find it interesting is that I have talked to lots of marathon swimmers since my swim about their feeding, and I have found that I seem to take on more fluid and more calories than most people. In fact, if the body can only process 240-280 calories per hour, then I should have been sick for practically my whole EC swim! Seeing as I knew that I was going to be out there for a very long time, calorie deficit was one of my big worries. My plan was to take on 500ml of double strength Maxim every half an hour (after the first two or three hours). So that’s 1 litre of double strength Maxim an hour. The serving size is 750ml and regular strength contains 233 calories, so double that (466 calories) and bring it up to a litre, that’s 621 calories per hour. Too much? Perhaps. But it gets better! I changed my feed interval on the way back because my shoulder was seizing up time I stopped so I wanted to stop less often. I changed to 45 minute feeds and asked my crew to increase the amount on each feed since I would be losing one feed every 90 minutes by doing this. From then on, I had 1 litre feeds (I drink quickly!). Of at least double strength Maxim (I know that they threw in an extra scoop pretty regularly!), every 45 minutes. That’s 1.3 litres an hour, bringing my calorie intake up to (at least) 828 calories/hour if my calculations are right. So over the 35 hours I probably took on over 25,000 calories. But I never once got sick, the only time I felt queasy was when I had boat fumes in my face for too long. And while my body may not have been able to process all those calories…something worked! I never felt like I ran out of energy and I never felt particularly tired (although I would have done anything for a new shoulder out there!). And I certainly won’t be changing it for my next big swim.

    In general I’m all about the science and the research. But when it comes to marathon swimming, I’m about learning what works for me. I don’t trust that studies done in other sports necessarily carry over to open water swimming, because we face a very different set of challenges to most other sports (cold, water conditions, lack of rest). I’m not by any means dismissing that research or saying that it’s useless to us, I just think that if something works for you, then you should go ahead and do it regardless of what the research says. We can use the research to guide us, but at the end of the day (and I think we are all in agreement on this), there’s no substitute for trying it out and seeing what works for you and your body (in training preferably!).

    1. A few thoughts:

      1. That’s amazing.
      2. There’s a lot we don’t know.
      3. “We can use the research to guide us, but at the end of the day (and I think we are all in agreement on this), there’s no substitute for trying it out and seeing what works for you and your body (in training preferably!).” — Could not agree more!
      4. I wonder… is max calorie consumption affected by liquid intake?

      Thanks so much for commenting.

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