# Endurance training in the heat: why osmolality and taste decide how much energy you absorb

> In the heat it is not the muscle that limits you, but the stomach. What research shows about gastric emptying, osmolality and voluntary fluid intake - and how WOO® Fast Endurance Lemon answers it.

At 28 degrees in the shade, a well-planned session quickly turns into a fight against your own circulation. It is rarely the legs that limit you first - it is your ability to absorb fluid and energy at all. Those levers sit in the stomach, not in the muscle.

## Why heat changes energy absorption

Under heat stress the body shifts a substantial share of its blood volume towards the skin to dissipate heat. Splanchnic blood flow - the supply to stomach and intestine - drops accordingly, core temperature rises, **gastric emptying** slows. Exactly when the demand for fluid and carbohydrate is highest, absorption capacity is at its lowest.

## Osmolality: concentration is what counts

Vist and Maughan showed back in the 1990s that gastric emptying depends strongly on carbohydrate concentration and **osmolality**. Hyperosmolar drinks stay in the stomach longer and additionally draw water into the intestinal lumen - short term, the body loses fluid instead of gaining it.

High-molecular-weight corn starch has a physical advantage here: because a very large number of glucose units hang on one single long molecule, the number of osmotically active particles is far lower for the same amount of energy. Piehl Aulin and colleagues found faster glycogen resynthesis for high-molecular-weight solutions than for low-molecular-weight ones.

## Taste governs how much you actually drink

How much someone drinks voluntarily depends measurably on flavour. Passe and colleagues compared sports drinks, diluted orange juice and water during exercise - **voluntary intake** differed clearly, in favour of the beverages rated as more pleasant. In the heat, where sweetness quickly becomes cloying, an acid-forward citrus profile is functional: less flavour fatigue, a steadier drinking frequency across the whole session.

## Sodium and magnesium at high sweat rates

Sweat contains primarily sodium and chloride. At sweat rates above one litre per hour - nothing unusual in summer - plain water becomes a problem: plasma sodium is diluted, thirst fades even though the deficit keeps growing. Sodium maintains the osmotic gradient and supports glucose uptake via the SGLT1 transporter. Magnesium **contributes to normal muscle function**.

## How WOO® Fast Endurance Lemon fits the summer setup

**[WOO® Fast Endurance Lemon](/en/shop/woo-fast-endurance-lemon-can)** is built for this scenario. A 40 g serving delivers 56 g of carbohydrate from high-molecular-weight corn starch combined with maltodextrin, glucose and sucrose - immediately available and longer-lasting fractions in a solution with low osmolality. Plus 300 mg of sodium and 25 mg of magnesium per serving.

The lemon profile is what sets this variant apart from the forest-fruit and orange versions: refreshingly tart, little sweetness on the finish, and therefore still drinkable in the second hour. Vegan, gluten- and lactose-free, without artificial additives - ideal before and during training, with dosing you can adapt individually.

## Frequently asked questions

**How concentrated should I mix it in summer?**
In the heat, fluid demand is higher relative to energy demand. A slightly thinner mix - the same serving in more water - increases the gastric emptying rate and is usually better tolerated.

**Why lemon specifically for long sessions?**
Because flavour fatigue is real. Acid-forward profiles stay pleasant for hours longer than sweet ones, and voluntary intake is the strongest lever for a stable fluid balance.

**Isn't water plus a salt tablet enough?**
For sessions under an hour, often yes. As soon as carbohydrate enters the picture, composition matters: sodium and glucose are transported together in the gut.

## Sources

Vist GE & Maughan RJ (1995), Journal of Physiology - Osmolality, carbohydrate content and the rate of gastric emptying of liquids in man.
Piehl Aulin K et al. (2000), European Journal of Applied Physiology - Muscle glycogen resynthesis with carbohydrates of low and high molecular mass.
Passe D et al. (2004), Int. Journal of Sport Nutrition and Exercise Metabolism - Palatability and voluntary intake of sports beverages during exercise.
Sawka MN et al. (2007), Medicine & Science in Sports & Exercise - Exercise and Fluid Replacement (ACSM Position Stand).

*WOO® Fast Endurance Lemon in the NEMAPO shop.*