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:

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.

(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 2.

Posted 30 October 2011 in: nutrition , tips Tags: science