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 1vigor.com.
Part 3 in a 3-part series. See Part 2.