Marathon Swimming Nutrition: Osmolality and why it matters

On a sunny late morning in Chicago last summer, I told Ted Erikson about the nutrition plan I’d recently used for Tampa and MIMS. Ted EriksonMy plan called for an hourly cycle of two Maxim feeds and one Perpetuem feed. Ted sort of chuckled, and then said something I’ll never forget: “You know, Evan… all you really need is glucose.”

And he’s right: Glucose is the basic unit of energy. Whether you feed on Gatorade or Maxim, it all ends up as glucose anyway. I mention this story because it’s worth remembering as you read what follows. When I said in the previous post that “some carbs are better than others,” I don’t mean that maltodextrin is the be-all-end-all, magical elixir of marathon swimming. It’s not. Many swimmers – including some of the best – have used “simple sugars” to fuel a marathon swim. You can, too!

However, it’s my view (based on both research and experience) that the basic recommendation to an aspiring marathon swimmer – in the absence of strong preferences otherwise – should be a maltodextrin-based fuel. 

One reason is taste – simple sugars are much sweeter than maltodextrin. The neutral-to-slightly sweet flavor of maltodextrin provides much greater control over the final taste of your beverage. However, this is (quite literally) “a matter of taste” and not generalizable. 

Another reason is a bit more obscure. It has to do with how carbohydrates are metabolized in your gut. One important difference between maltodextrin-based sports drinks and sucrose/HFCS-based drinks is their osmolality. I could attempt to explain what this means, but I thought it’d be more fun to get someone who actually knows what he’s talking about.

Brandon SullivanSo, allow me to introduce Brandon Sullivan. Sully is a former teammate of mine on the Columbus Sharks Masters. He is also a certified marathon swimmer, having completed the 2010 USMS 10K Championship in Noblesville. More relevantly, he has a PhD in Biochemistry from (the) Ohio State University!

Sully has generously agreed to explain what osmolality is, and why it matters to endurance athletes. Thanks dude!

* For the record, Ted Erikson’s nutrition plan for his legendary 1967 Farallon Islands swim consisted of “glucose plus anything to flavor and pour, e.g. peaches, pea soup, etc.”

[Ed. Note — emphases added.] I was recently asked to compare and contrast nutritional strategies from a biochemical perspective. It is an interesting question as the data is sparse and controversial. In fact, I was severely misinformed before writing this article! In a recent post (Choose your glucose polymers) I scribbled a comment that reflects some of the most common misconceptions in carbohydrate nutrition. I am glad Evan has given me the opportunity to write a guest post and set the record straight – well at least straighter.

Let us start the discussion by explaining why carbohydrates are important to endurance athletes. Muscle contractions require ATP – the body’s energy currency. Therefore, it is imperative to constantly provide the muscles with ATP to sustain desired performance. The production of ATP is primarily driven by the metabolism of carbohydrates. Quite simply: eat carbs, make ATP, have energy for swimming.

types of carbohydrates in energy drinksThe major carbohydrates found in energy drinks are: glucose (aka dextrose), fructose, sucrose, maltose, high-fructose corn syrup and maltodextrin (see figure). Glucose and fructose are both single units (monomers); sucrose and maltose link two sugars forming a disaccharide; and maltodextrin links several glucose units to form a polysaccharide. High-fructose corn syrup (HFCS) is a commercial preparation of glucose and fructose monomers. Regardless of the chemical composition, all of these molecules are broken down and converted to glucose before entering the blood stream. From the perspective of the muscles involved in swimming, the carbohydrate source is irrelevant.

So, are there meaningful differences in our choice of carbohydrate? Absolutely.

The primary difference between energy drinks is the carbohydrate composition. Gatorade and Powerade provide carbohydrates from HFCS, or mixes of glucose, fructose and sucrose. Designer energy drinks like Infinit and Perpetuem prefer to market products with maltodextrin.

One of the greatest misconceptions is that simple sugars like glucose are absorbed into the blood too quickly, making it hard to balance energy needs for the duration of a marathon swim. Many athletes believe that since maltodextrin requires chemical modification before entering the blood it must be slower than glucose and provide more consistent long-term energy. This is actually not true.

gastric emptying rates
Gastric emptying rates of glucose vs. maltodextrin (Vist & Maughan, 1995)

I will ignore the marketing data provided by several companies and discuss the results of a Journal of Physiology article published in 1995 (Vist and Maughan). Here, the authors measured the rate at which food is emptied from the stomach to the intestines for a series of four liquid meals, all with the same volume (~20oz):

  1. 24g glucose feed;
  2. 24g maltodextrin feed;
  3. 113g glucose feed;
  4. 113 g maltodextrin feed.

The authors concluded that the more dilute feeds (24g) were emptied faster, and that maltodextrin was emptied faster than glucose at both concentrations (see chart). What is the primary difference between the maltodextrin and glucose feeds? Osmolality.

Osmolality is defined as the number of molecules per kilogram of solution . It turns out that our body has a much easier time measuring concentrations than masses. Therefore, differences in concentrations (osmolality) have great influence on our physiology.

gastric emptying visualizationConsider a hypothetic example where you are 2h20m into a marathon swim and taking your seventh feeding of 5 ounces (see figure). If your took in 16 glucose molecules in 5 ounces, the concentration of glucose in your stomach would be 16/5 = 3.2 molecules/oz.

When concentrations are high your body responds in two ways. First, it empties the stomach sluggishly resulting in slower intestinal absorption and ATP production. Second, it floods the stomach with water to increase the volume. Note that if the volume increases from 5 to 10 oz, the concentration halves – 1.6 molecules/oz. This later phenomenon is known informally as bloating which in turn decreases appetite and comfort.

Now consider a second option, in which you feed on 2 molecules of maltodextrin. The concentration is considerably lower; 2/5 = 0.4 molecules/ oz. This means it will be emptied into the intestines faster and provide the same amount of theoretical energy (576 ATP). Once in the intestines, enzymes instantly break down maltodextrin to glucose, which are transported to the muscle for ATP production.

These lines of logic have led several companies to produce energy drinks with maltodextrin as the primary carbohydrate source. While many athletes have found these products to improve performance it is still unclear if these are solely osmolality effects or if other factors are in play (e.g., viscosity, solubility, taste). The answer to these questions will require larger sample sizes and likely interest outside our niche sport.

The presence of fructose has a dramatic effect on the quality of energy drinks, albeit not solely an osmolality issue. Fructose is easily converted by muscle cells into glucose, but it is absorbed from the intestines into the blood stream by a different process that requires transporters and is significantly slower. Once the transporters are fully occupied fructose accumulates in the intestines. The body responds by supplying the intestines with more water which leads to cramping and diarrhea. For these reasons many athletes avoid products like Powerade and Gatorade for endurance events lasting longer than a couple hours. Glucose and its polymers, on the hand, are efficiently transported from the intestines to the blood stream by an active process that requires sodium. This is one of many reasons to include a nutrition strategy that also supplies electrolytes.

13 thoughts on “Marathon Swimming Nutrition: Osmolality and why it matters”

  1. Wow, this is great. Thanks to both of you.

    If I understand correctly, there would be no reason for a backup simple carb. If you’re already in trouble, it sounds like those would only make it worse.

    How much fructose can the body handle? I’ve been using roughly 1 oz. of fruit juice for every 9 oz. of water to in my maltodextrin, primarily flavor, also for a small amount of electrolyte.

    1. 1 oz of standard fruit juice should be fine. There is actually research that showed mixes small amounts of fructose with maltodextrin had beneficial effects. Since they use two different absorption pathways they don’t interfere with each other. So you get all the maltodextrin + some fructose. I would recommend using enough fructose to provide flavor, but not enough that it ends up being a significant source of carbohydrate.

      1. Thanks, Sully! It’s great to have an in-house expert.

        Katie, some further info. A nutrition scientist in the UK, Asker Jeukendrup, has been the main guy doing research on malto+fructose combinations. Here are some links to a few scholarly articles: [1] [2] [3] [4]. He also has a good book.

    2. Katie, glad you liked it.

      I’d imagine individual differences in people’s ability to handle fructose. Some unfortunate folks have this problem, for example.

      My full-strength feeds (which I only use during actual races – i.e., extended high-intensity efforts) have more like a 1/3 ratio of fruit juice. I’ve never had any problems with it, but others’ mileage may vary.

      Another thing to consider is that fruit juice isn’t all fructose – there’s glucose, too (and sometimes sorbitol). And different fruit juices vary in their ratios of fructose to glucose. Here are some estimates of those ratios (remember, sucrose is 1.0):

      • apple juice: 2.67
      • orange juice: 2.2
      • pear juice: 5
      • grape juice: 1.05
      • prune juice: 0.61
      • honey: 1.23
      • agave nectar: 1.5+
      • high-fructose corn syrup: 1.22


  2. This is a good place to copy-and-paste some comments from Jen A. on Facebook yesterday, in response to my previous post:

    I don’t buy the whole “maltodextrin is better than simple sugars” thing in marathon swimming. I’ll be very interested in reading the follow up article.

    I have type 1 diabetes (“juvenile diabetes”), and I’ve spent the last 24 years of my life counting carbs, dosing carbs, watching carbs absorb, etc. I’ve tested my blood sugars up to 40+ times during 18h+ swims. I know that my strategy works perfectly well in terms of blood sugar management. I make a 6.7% solution of koolaid, and swap in a feed of gatorade every couple of hours.

    My response:

    Jen: maltodextrin and simple sugars are very similar in their effects on blood sugar. The advantage of maltodextrin has more to do with how they are processed in the gut (gastric emptying). The follow-up article will be posted tomorrow morning.

    Jen’s response:

    I’ll am looking forward to reading your article. I guess I question the value of more rapid gastric emptying (and therefore more rapid absorption). In diabetes land, we have the “rule of 15” — that it takes 15 grams of glucose 15 minutes to absorb. The idea is that if your blood sugar is low, you should take 15 grams and retest in 15 minutes.

    Every feed, I aim for 30 grams of carbs, which should absorb over 30 minutes. I don’t seem to experience peaks and valley — just nice steady absorbtion, which seems to be similar to the idea of superstarch.

    I can see the benefit of super-rapid absorption if you are a cyclist in a 3-hour race and you’re facing a steep hill, but I guess I question the need if you are a marathon swimmer maintaining your “forever” pace.

    Naturally, I’m no scientist, so these are just my experiences. But when I shared them with another marathon swimmer with diabetes, they worked extremely well for him as well.

    1. And here is Sully’s response (via email) to the above conversation:

      I think this falls into, “there is not enough science to know the answers for sure.”

      Look at the chart I included, there is very little difference in the gastric emptying of glucose and maltodextrin at low concentrations. So smaller feeds with consistency there is likely little to no difference between glucose and maltodextrin. Other than taste which is a personal preference. Also, to my knowledge there is little (or no) data that compares gastric emptying as a factor of activity level. It may actually be true that gastric emptying behaves differently under physical stress causing different rates of maltodextrin and glucose at lower concentrations.

      I don’t doubt that you can successfully complete a marathon swim with simple sugars. My hunch is that small differences (like gastric empyting rates) are amplified during stress, but I can’t prove that without NIH’s help. There’s also a lot of variables in play especially for the examples. Volume of feed, past timings, energy intake, etc.

      The 30 every 30 strategy = 240 Cal/hr. How does that compare to what you do? In general, I think we are talking about small differences, but intensity matters greatly. Someone swimming 25K for completion and someone racing 25K have very different physiology, so I would expect complex carbs to shine more under extreme conditions.

  3. More from Jen A. (via Facebook):

    As someone whose life literally hinges on the absorption and timing of carbohydrates, I was very interested in the article you mentioned, so I looked it up to read more. From the article, I understand that you graphed the amount of time it takes to half-absorb 600 ml of 4% solutions vs 19% solutions of glucose and maltodextrin. I thought, though, that 6-8% solutions were found to have best absorption?

    Also, from the article, I get the impression that glucose and maltodextrin — at the one hour mark, for 4% solution — have absorbed the exact same (within 10ml, or 2 teaspoons). And that’s intaking 600ml (~20oz) of fluid, which I think most swimmers would be hard-pressed to attempt to drink every feed.

    I’m guessing that the absorption of a 6-8% carb solution would be similar. So, if the absorption is pretty much the same, I don’t see the advantages to maltodextrin.

    (The original article is here, for anyone that wants to look at it:

    I know I am not a scientist, and the finer points of the research are lost on me. But I literally could die if I mess up on carb absorption, and so I am very grateful to you for furthering the discussion by writing the articles.

    1. Jen, I think you understood the fine parts of the article quite well! In my early reply to Evan, I more of less agreed with everything you said. The differences between the gastric emptying rates of glucose v. maltodextrin are very similar under the experimental conditions for dilute feeds. For a large percentage of marathon swimmers, glucose is sufficient – in fact, how many channel swims before 5 years ago were done on Gatorade and the like? Probably many if not the majority.

      Here are my subjective arguments for maltodextrin over glucose. First is volume versus concentration. The reason the glucose feeds and maltodextrin feeds emptied almost identically is that the volume was high (neither were very concentrated). As you point out – most marathon swimmers can not intake large amounts of fluid without bloating, etc. Maltodextrin allows one to swallow more energy in less volume – an osmolality effect. Second, I think taste can not be underestimated in distance swimming. I once swam a 7 hour practice session on a whim where I showed up with Powerade, water and Clif Bars. I brought 4 different flavors of Powerade, but after 4 hours I could not stomach it – I had to have a friend go buy me Coke for the taste/texture difference. I think it is invaluable to be able to use maltodextrin and flavor it how you would prefer. Third, I think the corroborating data suggests maltodextrin is better – even if we don’t entirely understand the physiology. More athletes are having success with designer drinks than simple sugars and it’s hard to imagine it’s all placebo – but maybe it is. If only we had a psychologist-marathon swimmer to weigh in!

      I’ll end by saying this. I would love to become a nutritional consultant for marathon swimmers. I could charge money for what boils down to a one sentence plan: Try a handful of strategies and stick with what works. There are too many variable and not enough science to assign concrete answers to these questions.

      1. “Maltodextrin allows one to swallow more energy in less volume” – Nail, meet hammer.

        Re: taste. Gatorade & Powerade are scientifically designed…. to sell as many bottles as possible to people with sweet tooths. NOT to maximize performance.

        Possibly the best comment ever written here, with the possible exception of the “ethic of self-navigation” guy. But you get the prize for concision 🙂

        Thanks, Sully.

  4. These 2 nutrition posts & their comments are keepers! They alone would be enough for a Readers’ Choice nomination.

    This discussion has been fascinating to me. I haven’t yet started swimming for long blocks of time but I hope to. I’m especially interested in Jen A’s comments. I’m not diabetic, my body responds to simple sugars (and their taste) in an exaggerated way — they tend to spike my blood sugar. So I don’t eat them.

    A friend whose body responds the same way as mine is a triathlete. On her long runs she fuels with a mixture of cooked oats, a little protein powder, and banana. She says it works well for her and keeps her steady and energized. It’ll be interesting to see what works for me as a swimmer.

    1. Bubbles, thanks for stopping by again, and I appreciate the compliment!

      I guess the bottom line is: If it (a) keeps you energized, and (b) doesn’t cause gastric distress, and (c) continues to do both A and B over a LONG stretch of CONSTANT effort – then it works.

      I do think there are a few reasons to be cautious about applying nutrition strategies for land-based sports to swimming. Swimming is different, especially open-water swimming, and especially ocean swimming. Your body is horizontal; you may be ingesting some quantity of saltwater (i.e., sodium); waves and chop may be jerking you around in unpredictable directions, leading to motion sickness. All issues that can make the digestive system surprisingly sensitive.

      That said: cooked oats, protein powder, and banana sound pretty delicious to me!

      1. Thanks, Evan, excellent points.

        Yep, I do understand about food used in land-based sports not necessarily working for swimmers. I’ve swum in my bay a few times soon after eating and have felt that sloshy tum…it definitely wouldn’t work for any serious amount of time or ocean conditions. I was thinking that some version of that concoction might work. The info you posted (along with the comments) is really helpful.

        It’ll be fun to experiment. I’ve never felt motion sickness (ever), so I don’t know how sensitive I will be in rough water.

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