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. My 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 “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 Sullivan

Brandon Sullivan, PhD

So, allow me to introduce Brandon Sullivan. Sully is a former teammate of mine on 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! 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 drinks

types of carbohydrates in energy drinks

The 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 visualization

Gastic emptying

Consider 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.