On Peter Attia’s nutrition webinar

Yesterday Open Water Source hosted a fascinating web-presentation by Peter Attia, a physician and Catalina Channel solo swimmer. The topic: Nutrition for Open-Water Swimming. Right up my alley, to say the least! There’s good news and bad news.

Bad news first: The webinar was oversubscribed so, despite pre-registering a week ahead of time, I got locked out. The good news: I was able to obtain the audio and slides, and “listen in” after the fact. (Friendly suggestion to the good folks at Open Water Source: Please don’t overbook your webinars. I realize they’re free, but still…)

The even-better news: The webinar was excellent. Though, somewhat different than I expected. A few weeks ago a friend sent me a whitepaper authored by Dr. Attia, entitled “Swimming in the Intensive Care Unit.” The gist of the paper is that a marathon swim is enormously stressful on the body, producing physiological symptoms not unlike those of a patient in the ICU with a traumatic injury. Therefore, proper nutrition is critically important to the success of such an endeavor. His recommendations boiled down, interestingly, to almost exactly what I had discovered on my own:

  • The purpose of feeding during a swim is to supplement your body’s other energy sources (glycogen and fat), not to replace every single calorie you burn.
  • Liquid feeds are better than solid feeds, because solids are difficult to chew and digest while swimming.
  • Feed often – every 15 or 20 minutes – to minimize blood sugar fluctuations.
  • An 8-10% carbohydrate solution (equivalent to, at most, 270 calories per hour) is best.
  • Maltodextrin is a better carb source than dextrose and/or fructose – its lower osmolality is less likely to produce gastric distress.
  • Fluid intake should be enough to require urination at least every hour.
  • Augment the carb drink with protein (or preferably, free-form amino acids) to mitigate muscle breakdown.
  • Do not supplement electrolytes in a saltwater swim (at most, perhaps a small amount of potassium).

Indeed, Dr. Attia’s specific product recommendations corresponded exactly to the products that, independently, I had already found to work best: Maxim and Hammer Perpetuem. So – good for me. Aren’t I smart.

What I didn’t realize until yesterday is that the “Swimming in the ICU” paper is actually outdated! Dr. Attia wrote it circa 2009, but in the past two years has completely changed his thinking and approach to nutrition.

How so? Stay tuned for Part 2

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 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 1 and Part 2.

Swim efficiency and energy consumption

In the last post I bemoaned the lack of credible science about marathon swimming. One is reminded of the William Goldman quote about the movie industry: Nobody knows anything.

Here’s a good example. A few days ago a Facebook friend linked to an intriguing-looking article. Published on a science-y looking website (“Your one-stop resource for longevity, health, exercise, nutrition, and scientific articles all to help you live a longer, fuller life”), the article is authored by marathon swimmer Don Macdonald.

One section seemed of particular interest: “Nutritional Demands of Open Water Endurance Swimming.” An excerpt:

Nutritional endurance demands biochemical changes of your body. The basic calculation for the amount of calories burned while swimming is 2.93 calories per mile, per pound. I weigh 207 pounds, and therefore burn 14,556 calories in a 24-mile swim, (2.93 calories x 24 miles x 207 pounds = 14,556 calories). You must also add 10-15 percent of your burnt calorie total for the energy it takes your body to keep itself warm. In this case, adding another 1,500 calories.

2.93 calories per mile, per pound. Really? How do you figure?

Does it seem likely that calorie burn depends only on distance, and not the time taken to complete the distance? If I swim 10 miles in 4 hours, does a slower swimmer who takes 7 hours burn the same calories as me, despite spending 3 more hours in the water (assuming equal body weight)? Do I burn the same calories in a 1500m warm-up as during a 1500m race?

Actually, there is a school of thought (with some scientific basis) that calorie burn is independent of speed/time in “animal locomotion” generally. As Wikipedia (referencing a 1973 Science paper) explains:

The most common metric of energy use during locomotion is net cost of transport, defined as the calories needed above baseline metabolism to move a given distance, per unit body mass. For aerobic locomotion, most animals have a nearly constant cost of transport – moving a given distance requires the same caloric expenditure, regardless of speed. This constancy is usually accomplished by changes in gait.

The idea is, calories are a measure of work – the work required to move a given body mass a given distance. Hence the common rule-of-thumb in running: 1 calorie (technically, kilocalorie) per kilometer, per kilogram. Running at higher speeds burns more calories, but this is counterbalanced by the reduced time taken to complete the distance. More recent evidence has complicated this view – showing differences in calorie burn between walking and running a given distance. For what it’s worth, though, many runners seem to think the rule-of-thumb comes pretty close.

But what about swimming? Is the “net cost of transport” constant, regardless of speed? Does 2.93 calories per mile, per pound make any sense, even as a rule-of-thumb? I’m inclined to say… no. The reason: Efficiency. Humans are very efficient walkers and runners – it’s what we’re evolved to do. An elite runner converts 90% of energy expended into forward motion – but even a recreational runner is about 80% efficient. (I assume Terry Laughlin got these numbers from science, but I’m not going to hunt for it.)

An elite swimmer, however, is only about 9% efficient. And a novice swimmer is astoundingly inefficient – T.L. estimates 3%. Humans are pretty terrible swimmers, all considered.

It makes sense that the “net cost of transport” would be fairly constant on land – because humans efficiently convert additional effort into additional speed. In the water, however, most of our efforts are wasted. Water is both dense (compared to air) and unstable (compared to the ground). Even large increases in effort produce relatively small changes in speed. It would seem to follow, then, that the “net cost of transport” in swimming depends very much on speed! Moreover, skilled swimmers are much more efficient than unskilled swimmers – compared to the relatively small differences among runners. Sun Yang’s net cost of transport is less than mine, and my net cost of transport is far less than the average triathlete.

What else is wrong with 2.93 calories per mile, per pound? Let’s plug in some numbers. In the above quote from Don Macdonald, he uses a 24-mile swim as an example. That happens to be the same distance as the Tampa Bay Marathon Swim. Earlier this year I completed this swim in 8 hours, 59 minutes. Flavia Zappa, the last swimmer to finish, came in at 15 hours, 10 minutes (results link). Assume for the moment that we weigh the same.

If calorie burn is only a function of distance, that means Flavia and I each burned 11,251 calories (2.93 * 24 miles * 160 pounds). In Flavia’s case, that produces a not-unreasonable-sounding (but still high) rate of 742 calories per hour. But for me, 11,251 calories equates to 1,252 calories per hour. Not likely.

Isn’t it obvious that an efficient swimmer will burn fewer calories per mile than an inefficient swimmer? To believe a rule-of-thumb like 2.93 calories per mile, per pound, you essentially have to believe that there is no such thing as efficiency in swimming. Anybody who knows anything about swimming, of course, knows that swim speed is mostly about efficiency.

UPDATE 10/31: Karen raises a great point: Energy expenditure during a marathon swim will also depend on conditions (not just water temperature, as Don Macdonald mentions). Swimming through big swells, chop, and whitecaps will burn more calories than swimming across a glassy lake.

The second in a three-part series. See Part 1 and Part 3.

On science in marathon swimming

In marathon swimming, there’s very little in the way of credible science – that is, methodologically rigorous, experimentally controlled, peer-reviewed science. It’s not hard to understand why: Open-water swimming, especially the marathon variety, is a tiny market compared to land-based endurance sports. Market size is related to the potential for making money, and the potential for making money is, in turn, related to funding and motivation for scientific research. Even in triathlon (an enormous, lucrative market), swimming is often seen merely as a warm-up to the bike and run, so there’s little effort to understand it.

As a result, marathon swimmers are left with approximately four strategies for acquiring knowledge about their sport – specifically, the physiological demands of long-distance swimming, and the nutrition required to fulfill those demands:

  1. Figuring out what is known, scientifically, about land-based endurance activities, and applying it to swimming.
  2. Figuring out what is known, scientifically, about pool swimming (in which races last anywhere between 20 seconds and 15 minutes), and applying it to marathon swimming (in which a race or solo event may last 10 or 15 hours).
  3. Word of mouth – finding out what works for other marathon swimmers. This is how most people discover Maxim – because that’s what they use in the English Channel.
  4. Individual trial-and-error. Penny Palfrey likes watered-down porridge and chocolate ice cream. Who knew?

Most successful marathon swimmers use each of these strategies at some point. The problem with the hybrid approach, however, is that it neglects one very important thing: actual, science-based knowledge about marathon swimming. As science has continually shown since at least Galileo, there’s a lot we don’t know – and much of what we think we know might actually be totally false.

A few rhetorical questions, off the top of my head:

  • How are nutritional needs affected by the environment in which the activity occurs? E.g., how is running a marathon in 60-degree air different from swimming a marathon in 60-degree water?
  • By corollary, are products designed for land-based endurance activities sub-optimal for water-based endurance activities?
  • Is digestion during an endurance event affected by physical orientation? E.g., swimming horizontally vs. running vertically?
  • How does electrolyte loss differ between running and swimming? Are supplemental electrolytes necessary while swimming in a saltwater (i.e., electrolyte-rich) environment?
  • How should fluid consumption be adjusted for cold-water swims vs. warm-water swims?

Any others?

The first in a three-part series. See Part 2 and Part 3. Also see this post by Donal Buckley on choline supplementation.