A Primer on Osmolality

A Primer on Osmolality

What is Osmolality? Why should you care?

One of the obvious realities of being a runner is that the harder we work, the more energy our bodies burn; thus the more energy we need to replace in order to feed the engine. The ironic twist to that equation is that the harder we work, the more reluctant our body is to absorb the calories it requires. The science is pretty clear; at the pointy end of the output spectrum, our stomach goes offline (to spare blood for use in our legs), and both the quantity and form of calories that we can process becomes limited.

Let's look at an exaggeration: while out for an easy hike, you can probably handle bacon and eggs in your gut, but while going full gas in a 3 hour race, you're pretty limited to carbohydrates diluted in an appropriate amount of water. Those are obviously the extremes but for a longer ultramarathon, you will likely fall somewhere in the middle.

At a certain output (different levels, for different people) the stomach almost entirely goes offline, and digestion is pretty much an impossibility. There's a nifty work-around that the body allows for though; you can bypass digestion and go straight to absorption… but the conditions need to be right!

This is where osmolality (or concentration) comes in. An isotonic drink is one that has a similar concentration of carbohydrates and electrolytes to your blood — which allows for easier absorption. One significant characteristic of concentration is that it is based on the number of molecules, and not the size of the molecules.

An isotonic drink with bigger molecules (like maltodextrin or starch) will have more carbohydrate (by weight) in it than an isotonic drink with smaller molecules (like glucose or fructose). All of those carbohydrate sources having the same energy content of approximately 4 calories per gram.

Here are the rough numbers: maltodextrin is isotonic at 300g/1000ml. Simple sugars like Glucose, Fructose, and Sucrose are isotonic at 52g/1000ml. This means that you need to dilute the simple sugars with six times as much water as maltodextrin to hit that isotonic ‘sweet spot’. If the concentration is too high, absorption is impeded, which can start off an unpleasant chain of events.

So what does this mean?

From my experience, I’m almost always trying to get the most amount of calories into my system with the least amount of water. A drink that contains 150 cal/L is almost useless as a fuel source. The truth is that I would be hard pressed (except under extreme heat) to take in 1L/ hour… which would mean less than 150 cal/hour in this example. More likely, I'm aiming for the 300 cal/hour zone, which requires some specific fine tuning of the diluted carbohydrates. Which brings us back to molecule size and osmolality. Maltodextrin and starches are bigger molecules, and thus they allow for more caloric density in an isotonic drink (compared to the simple sugars). So when caloric density is the big consideration, those carbohydrates (maltodextrin and starch) are your primary choices.

Two of the brands we carry at Capra are Carbo-Pro (maltodextrin) and Vitargo S2 (starch). It's tougher to go wrong when using science as the starting point for your fuelling equation.

Not everyone is trying to solve the same nutritional hurdles, and fuelling plans will be widely varied for individuals, but hopefully this primer gives you a little more insight into the science behind absorption vs digestion and why a nutrition strategy is so important for longer races/runs.

We do our best to offer a wide variety of nutritional products at the shop; products that cover a range of macronutrient sources, electrolyte concentrations, and taste. Come in and talk to us if you need some help choosing a fuel source.