Marathon swimming and low-carbohydrate diets

Following up my previous post on channel swimmer/physician Peter Attia’s webinar about “Nutrition for Open Water Swimming”…

Peter Attia

As you may have heard, the topic of the webinar was broader than the title indicates. In marathon swimming, “nutrition” typically refers to the stuff consumed during a swim to provide energy. But Dr. Attia 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 glycolysis to 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?

15 thoughts on “Marathon swimming and low-carbohydrate diets”

  1. Very interesting. He/they may well be on to something.

    I haven’t yet become an endurance athlete, so I can’t comment on that. But one thing I wonder about is whether Dr. Attia is lumping together all endurance athletes, regardless of what kind of diet suits them on a day-to-day basis.

    I don’t eat added sugars (sugar, honey, corn syrup, the lot) or refined grains, and I eat very little fruit because my body overreacts to sugars. I get a rush then a crash. It takes a toll on my body and screws up my emotions.

    Complex carbs such as beans, lentils, and sweet potatoes are foundational to my diet (grains are in there, too, but I do best with non-grain sources). When I limit these carbs, depression takes hold. I won’t go into what I understand of the biochemistry involved, but I’ll just say that I think your body had better be able to maintain high levels of beta-endorphin & dopamine (among other neurotransmitters) if you plan to adopt a very low-carb diet. Some bodies can, some don’t seem to be able to.

    1. This is a great comment – thanks Bubbles! Beans, lentils, and other complex carbs are foundational to the diet of many others, too – probably hundreds of millions. And they live what by most standards would be considered perfectly healthy lives. Maybe they’re on to something too!

      From what I understand of people who have transitioned to low-carb diets, there’s often a period of low energy (even mild depression) at the beginning. Apparently, after a couple weeks the body adapts and energy levels return to normal (or better than normal, because the energy is steadier).

      At the same time, I’m certain there must be individual differences in people’s ability to adapt to such a diet – both physiologically and psychologically.

  2. Everything is better in moderation. I never trust diets that live in the extremes. Ketosis is actually harmful to the body. Diabetics test themselves to make sure they are not in ketosis. I find it sad and hysterical that a decade ago millions of Atkins dieters were out buying pee sticks hoping for a negative result!

    1. “Everything is better in moderation” – truer words were never spoken.

      Re: ketosis being dangerous, I wonder if you’re confusing it with ketoacidosis? Dr. Attia and Prof. Phinney (of the Art of Science of Low-Carb Living) seem pretty adamant that the latter isn’t dangerous.

  3. i thought training in a glycogen depleted state would ‘trigger’ adaptations for fat burning, as in Sports Nutrtion: lab to table

    1. I think we’re talking about the same thing – with what you’re describing being a “half-measure” compared to full keto-adaptation. The disadvantage I see with training in a glycogen-depleted state (without being keto-adapted) is that glycogen depletion makes for very unpleasant workouts (i.e., poor performance).

  4. Im sorry I can’t come up with anything more sensible here, but I’m transfixed by that picture of Jonas Colting. His stomach looks like a smiling face! It’s very disturbing.

  5. Hi, interesting article. In response to your question, we’ve had one of our athletes who was a very ‘high carb burner’ (determined through physiology tests) even at relatively low intensities. Under the direction of a sports nutritionist he started having low carb, high fat/protein breakfasts before long rides, and felt loads better on the rides – no ‘burning’ legs early on, and able to keep going for longer. However if the intensity of the ride went up at all he didn’t have the power to respond. He has this breakfast occasionally to train his body to use fats better at low intensities, but not before important sessions that are higher in intensity, and only as a one-off meal. Still need carbs to refuel post training to help aid recovery and adaptation to training.

  6. Looking back through old blog entries of yours. I was lightly LCHF (100 grams or less) back in the states, but it is too hard to do here. Our next assignment will take us to a big city where I may be able to do LCHF again (I felt great on it and several issues I had in the realm of health disappeared when I was on it). I’ve got a big swim planned in 2017 (ok, big for me), and I’d like to do it keto-adapted.

    My question, which I’m still researching, is what happens to the body if someone who is lightly LCHF then pounds down the carbs for 7-8 hours of a swim? One of my biggest fears in life is becoming diabetic, and I am afraid that a shock to the body of 270 calories per hour of almost all carbs will have some deleterious effect. ideas?

    1. Hey Mike. I’ve heard people on LCHF diets have gotten good results using Generation UCAN on long swims, which has a more gradually metabolized carbohydrate. @DanSimonelli on the Forum is one…

      1. Funny you mention UCAN. That’s what I used during Issyk Kul. I practiced with it best I could (2+ hour swims on my tethers in my pool). I cannot pinpoint that the UCAN was why I got nauseous during Issyk Kul; cudda been the waves just as much.

        This year I’m going to go up to the lake (2+ hour drive one-way) a few times to practice with the kayaker and will try the UCAN again. I never felt any stomach issues in my pool, but perhaps the UCAN plus waves don’t mix?

        I have talked privately with Dan about UCAN.

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