Dr Allen Lim and “drinking your calories”

Has he got enough bottles?

Looking through some of my regular favourite sites on the web I stumbled across this video from Allen Lim of Skratch Labs. He is an american exercise physiologist who has done alot of work with professional cycling teams (Garmin, Radioshack, Bontrager Trek). The video in the link below is a good little intro into a blog I have had sitting is draft for a wee while focussed on hydration and substance concentrations in nutrition products.

Follow the link here for the video:

It all sounds very simple right? Calories in fluids = dehydration. In reality this is not exactly true. The scientific principle demonstrated in the video is osmosis, movement of fluids across a partially permiable membrane. Osmosis is related to a concept called tonicity, basically the comparison of the concentration of osmotically active particles between two solutions. Differences in this osmotic concentration, or osmolarity, is one of the key drivers of fluid movement and absorption processes in our bodies.

Osmolarity is a concept very important to life itself. When thinking of fluids it is important to remember that our bodies are essentially made up of millions of small bags of fluid – cells. These cells contain proteins and and electrolytes that all interact with each other and the fluid that supports them. Furthermore the individual cells, or bags, exist in a bath of fluid. The concentrations of substances in cells and in the fluid that surrounds them are critical to life.

David Millar – dehydrated

The fluid inside cells is intracellular fluid. The fluid “bathing” or surrounding cells is extracellular fluid. Extracellular fluid can be divided into intravascular (within blood vessels) and interstitial fluid (outside of cells but not in blood vessels). These compartments each have different physiologic properties and different concentrations of substances within them.

The concentrations don’t just occur by accident, out body is filled with electrolyte pumps and mechanisms for transporting substances between compartments. The aim of these processes is to preserve an optimal equilibrium state. As well as these active processes, passive processes such as osmosis and diffusion also take place. Both active and passive processes are affected by the concentration of the substances suspended in the fluid (in other words the concentration of solutes in a solvent). The overall concentration of particles that affect osmosis, diffusion and the active transport processes can be called osmolarity.

Is my Vitasport isotonic or hypotonic? Maybe its hypertonic?

At this point it is important to note that moving a particle within a fluid from one compartment to another changes its concentration. The body strives to maintain equilibrium so water (the fluid bathing all cells) generally follows any movement of electrolytes. That is, if electrolytes are in a high concentration in the intravascular compartment for example, water will tend to move in from the extravascular compartment in order to level out the concentration.

Tonicity is a concept closely related to osmolarity. It is used to describe the relationship of fluid. A fluid that has a greater osmolarity than another is said to be hypertonic, two fluids with the same osmolarity are isotonic and one with a lower concentration is hypotonic when compared to the other fluid.

Still with me?

Basically – concentrated solutions make cells shrink (dehydrate as Lim puts it), dilute solutions make them swell (hydrate).

A simple diagram of tonicity/osmosis. Concentrated shrinks cells, dilute expands.

In the video Lim talks about calories, or sugars, as being the primary driver of this fluid movement. He only comments on the calorie concentration. This is too simplified. In reality electrolytes (salts) and proteins also come into play. Look at the contents of any sports drink and you will notice sodium, magnesium, potassium, etc. These affect osmosis in a big way.

The second limitation of the model is that the eggs are inactive metabolically, that is, over the 90 minutes of the test they did not consume any calories. Think about a race, if you are racing you are depleting your body stores of calories, and the fluid and food in your gut will be absorbed to try and replace these losses. The whole process is dynamic and depends on not only the concentration of the products you consume, but what is happening internally.

Salts can also be lost into the gut, so by drinking something very low in sugars and salts there can be a flux of salts across the gut, depleting stores of sodium and other salts. This can be a huge problem. But needs another article to explain it.

Lim raises some good points, and he is right in pointing out that concentrated solutions will dehydrate your gut. There is however a good middle zone, somewhere between hypotonic and isotonic that provides good hydration while also making available sugars/calories for absorption and required salts. In a future article I will explain the calculations behind this and suggest how to mix your sports drinks to get close to the ‘sweet spot’ for optimal hydration.

What’s Boonen got in his bottle?