Osmolarity and glucose

How can you explain osmolarity and the glucose transport mechanism for adding a type and amount of carbohydrates to your sports drink?

The osmolal concentration of a solution becomes osmolality (which is not osmolarity) when the concentration is expressed in osmoles per kg of water.

In contrast, this osmolal concentration becomes osmolarity when expressed in osmoles per liter of solution.

• In the context of bodily fluids, the two terms can be used, as the differences are tiny at the biological level.
• Scientific calculations are based on osmolarity, and that is how we know it.

Osmolarity is measured in osmoles per liter (osm/L or OsM) or milliosmoles per liter (mosm/L or mOsM), which means the number of particles in a solution, although it does not represent their composition.

What is osmolarity?

In biology, osmosis is defined as the movement of a solvent, such as water, through a semipermeable membrane.

• This membrane may be the intestinal wall, from a less concentrated solution to a more concentrated one.

If, for example, there were water on both sides of the membrane, there would be no net flow from one side to the other, and the two solutions would be considered isotonic.

During digestion, large amounts of water are secreted into the small intestine in response to osmotic gradients.

When large food particles (or supplemental ingredients), such as proteins and starch, are digested into thousands of smaller particles, such as maltose, glucose, and amino acids, the concentration of particles increases dramatically in our gastric fluids.

• In other words, the osmolality of the gastric solution increases dramatically and becomes hypertonic.
• This causes water to be drawn from the blood into the intestine.

Then, as glucose and fructose molecules are absorbed through the intestinal wall and into the bloodstream, the concentration of particles in the intestinal solution decreases until it becomes hypotonic (less concentrated than the blood).

This low osmolality of gastric fluids causes water to flow into the bloodstream.

The osmolality of fuel for training

The purpose of endurance fuel during a workout is to address the two main causes of fatigue: depletion of the body's carbohydrate stores and dehydration.

However, fuel is only effective if it promotes voluntary fluid intake, gastric emptying, and rapid absorption.

The fuel used to provide muscular/aerobic endurance during a workout depends on its composition to do so.

Here is the problem;

• The higher the concentration of carbohydrates in a beverage, the more hypertonic it becomes, making it harder for the water in the beverage to be absorbed.
• It reduces the time it takes to empty from the stomach, and
• It reduces the time it takes to be absorbed in the intestine.

For this reason, decades of research have been conducted to try to discover ways to maximize carbohydrate delivery without compromising water absorption.

Another glucose transport mechanism

The good news is that there is another way for fluid to move through the intestinal wall and into the bloodstream.

There are several proteins in the intestinal wall that actively transport glucose and fructose molecules from inside the intestines into the bloodstream; when this happens, sodium and water follow that pathway.

Research demonstrated that it is important to consider not only the osmolality of sports drinks, but also the types of carbohydrates included in the formula.

In other words, it is not only the concentration of carbohydrates that matters, but also the variety of carbohydrates used.

Fuel that includes simple sugars using at least two different transport proteins, such as glucose and fructose, allows greater fluid absorption than fuel using only one type of carbohydrate.

There is a great deal of misinformation about beverages needing to have a carbohydrate concentration of 4–6%.

In fact, researchers demonstrated that osmolality is not the most important characteristic governing water absorption in the small intestine when multiple simple sugars are present.

In fact, it can reach a concentration of up to 8% before affecting fluid absorption.

This has led to a general recommendation across the food and supplement industry of 6–8%.

Sources

• Journal of Applied Physiology – 1998: Effect of beverage osmolality on intestinal fluid absorption during exercise
• Medicine and Science in Sports and Exercise – 1995: Effect of Sodium Concentration in a Carbohydrate-electrolyte Solution on Intestinal Absorption.
• Indianapolis, IN – 1990: Gastric emptying of fluids during exercise.