Marathon swimming and low-carbohydrate diets

by Evan Morrison. 17 December 2011.

In marathon swimming, “nutrition” typically refers to the stuff consumed during a swim to provide energy. But in his recent webinar (previously discussed here) was more interested in what people eat when they’re not swimming - i.e., diet.

If I could summarize his point, it would be this: Endurance athletes are asking the wrong question. Sure, Maxim is probably better than Gatorade during a swim. But the more important issue is: How best to train our metabolism _through diet _so it will most efficiently convert fuel into energy. According to Peter (who now has a blog), the ideal solution is a ketogenic diet.

The ketogenic diet is a type of low-carbohydrate diet that restricts carb intake so severely (less than 60 grams per day - equivalent to a small-ish bowl of pasta) that the body is forced to burn fat for energy instead of the “easy” glucose offered by carbohydrates. I won’t get into the theory and biochemical justification for the diet here, but if you’re interested you might consider checking out (in order of sophistication):

The low-carb/paleolithic/ketogenic diet has been around a while - some might even argue, for several million years. (Note: There are subtle differences among the terms low-carb, paleolithic, and ketogenic, but for our current purposes we’ll ignore them.) But what’s intriguing about Peter’s argument is that he’s promoting this diet as an endurance athlete. Even Mark Sisson is quick to note that he’s a former marathon runner.


I actually read The Primal Blueprint a couple years ago - and found it quite compelling. But in the end, I decided against “going primal” because it seemed totally impractical to train for marathon swims without eating lots of carbs. Sounds like bonk city, right? According to Dr. Attia, however, not only can you train for endurance events on a low-carb diet, but you actually have an advantage over your carb-addicted competitors.

The key is being keto-adapted - being able to burn ketones (a byproduct of fat metabolism) for energy instead of glucose. While most people’s bodies are reluctant to transition from ketosis (the scientific term for “hitting the wall”), people who are keto-adapted do it seamlessly. This has important implications for endurance athletes because eventually, all endurance athletes have to burn fat for energy. After 2-3 hours, glycogen stores are exhausted. But if you’ve already trained your body to readily metabolize fat, your energy levels should be steadier through a long swim. At least in theory.

So, will I be hopping on the low-carb bandwagon? Meh…

I’m of several minds about this. On the one hand, I do find the science behind low-carb diets - the benefits for general health and sustainable weight loss - to be compelling. And I do think “primal foods” are delicious. Fresh, organic veggies… free-range eggs… grass-fed beef… dark chocolate… bacon…. What’s not to love?

On the other hand, I really like carbs, too. I’m not sure I want to give up my heaping plate of pesto pasta; an oven-fresh loaf of French bread; tabbouleh salad, hummus, and pita bread; pizza! I’ve also never had any trouble digesting grains - which is a common reason people turn to low-carb diets. And on a practical level, it’s difficult to avoid carbs in most modern societies.

Ultimately, diet is a very personal choice. I wouldn’t advise quitting carbs just for its potential benefits in endurance sports. Personally, I’m not convinced on that point. I used my custom maltodextrin formula on four swims this year, totaling almost 31 hours and more than 90 miles of swimming - and my energy levels stayed consistently strong (shoulder pain was a different issue).

Jonas Colting

Peter presented some interesting data on his own performance benefits from keto-adaptation - but as he freely admits, that’s just one data-point. Ultraman champion Jonas Colting is another interesting data-point. But as far as I know, there have been zero scientifically rigorous, large-sample studies of low-carb diets among endurance athletes.

It’s certainly worthy of further study.

If you do decide to follow in Attia and Colting’s footsteps, you may find the following articles at Mark’s Daily Apple helpful:

One final issue, regarding the weight-loss benefits of low-carb diets. There’s a funny thing about marathon swimmers… many of them actually don’t want to burn off all their fat. Some of them are (the horror!) desperate to gain weight! A little bioprene goes a long way in a cold-water channel. Apparently, Peter Attia has chiseled himself down to 7.5% body fat. Very impressive in most contexts, but probably not ideal for cold water.

Has anyone out there tried low-carb diets? What about during heavy endurance training? What have been your experiences?

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