Is HEED an isotonic or hypotonic drink? The answer lies in how it's prepared.

Recently, several people have asked us whether Hammer HEED is actually an isotonic or hypotonic sports drink. The question is perfectly valid, as the "ISO" designation is often found on sports drinks, while HEED's laboratory osmolality results clearly fall into the hypotonic range.

To answer this, we first need to clarify an important detail: the sports drink powder in the box is neither hypotonic nor isotonic by itself. This property is determined by the osmolality of the prepared drink, which is created by the ratio of the powder to water and the molecular properties of the ingredients.

Different concentrations of drinks can be made from the same sports drink powder. With more water, a weaker solution with lower osmolality is obtained. With less water or a larger amount of powder, the osmolality increases, and the drink may move towards the isotonic range.

What does osmolality mean?

Osmolality shows how many osmotically active particles are present in one kilogram of solvent. Its unit of measurement is mOsm/kg.

The osmolality of body fluids is approximately 280–300 mOsm/kg. Compared to this, three main groups are distinguished.

A hypotonic drink has a lower particle concentration than body fluids. An isotonic drink's concentration is close to that of body fluids, while a hypertonic solution has a higher osmolality.

Therefore, the precise classification is not solely determined by how many grams of carbohydrates are in the drink. The type of carbohydrate from which this amount is derived also matters.

What do HEED's laboratory results show?

HEED's osmolality was measured under laboratory conditions according to the USP <785> test method. One serving of powder was dissolved in 20 US fluid ounces, or approximately 591 ml of water.

These values are well below the 280–300 mOsm/kg range of body fluids. Therefore, the HEED tested, prepared with 591 ml of water, is clearly a hypotonic sports drink.

The small differences observed between the various flavors can be attributed to the flavoring and ingredients used for each variant. All tested variants fall into the same hypotonic category.

How can its osmolality be so low despite so many carbohydrates?

Maltodextrin plays a key role in this.

The main energy source in HEED 2.0 is tapioca-derived maltodextrin. One serving provides 28 grams of carbohydrates and 110 kcal of energy, yet an extremely low osmolality drink can be made from it.

Osmolality is primarily shaped by the number of particles in the solution. Simple sugars, such as glucose and fructose, are small molecules. For a given mass of carbohydrates, many separate sugar molecules enter the drink, each contributing to the osmotic load.

Maltodextrin, in contrast, is a larger carbohydrate molecule made up of linked glucose units. The same carbohydrate mass is associated with fewer distinct particles. Therefore, 28 grams of carbohydrates from maltodextrin can cause a lower osmotic load than the same amount of glucose or fructose.

Simply put: the drink can contain a significant amount of carbohydrates without the number of dissolved particles being proportionally high.

In the intestines, maltodextrin breaks down into glucose units that can be used for energy production. However, in the prepared state of the drink, the longer carbohydrate chains help keep osmolality low.

The relationship between the osmolality, energy content, and gastric emptying of sports drinks is complex. The process is influenced by the drink's energy content, volume, temperature, pH, carbohydrate composition, and exercise intensity. This relationship has already been demonstrated in previous human studies. Research on the relationship between the composition of sports drinks and fluid absorption examined osmolality and carbohydrate concentration as separate factors.

Why is it not enough to classify a sports drink based on its carbohydrate percentage?

We often encounter the simplification that a drink with less than 4% carbohydrate content is hypotonic, 4–8% is isotonic, and over 8% is hypertonic. This can be a useful guideline for drinks based on simple sugars, but it does not replace the measurement of osmolality.

HEED is a good example of this. One serving contains 28 grams of carbohydrates. If dissolved in 591 ml of water, the carbohydrate concentration is approximately 4.7%. Based on the percentage rule of thumb, this might seem like the lower end of the isotonic range.

However, the laboratory test showed values between only 60–88 mOsm/kg. The difference is largely explained by the larger molecular size of maltodextrin and the resulting lower particle count.

Therefore, it is more accurate to start from the measured osmolality of the prepared drink. The percentage concentration of carbohydrates alone does not tell you how the solution behaves osmotically.

Can HEED be both hypotonic and isotonic?

Yes, depending on the concentration of preparation, the same powder can produce drinks with different osmolalities.

The available laboratory results confirm that one serving of HEED prepared with 591 ml of water is strongly hypotonic. If the same serving is dissolved in less water, the concentration of dissolved particles increases. Adding more powder results in an even more concentrated drink.

In such cases, the osmolality can gradually approach the isotonic range. At a sufficiently high concentration, it can even reach it, and then with further thickening, it can become hypertonic. A separate laboratory measurement is required for the precise classification of each mixing ratio, because the change in osmolality cannot be reliably calculated by simple proportionality for all ingredients.

Based on the current results, therefore, we can state this precisely:

One serving of HEED dissolved in 591 ml of water is a laboratory-proven hypotonic sports drink. With a denser mix, its osmolality increases, and the drink may move towards the isotonic range.

The term "ISO" is often used broadly in the sports drink market as a categorical designation for drinks containing electrolytes and carbohydrates. Scientific classification is more precise: it always takes into account the actual osmolality of the prepared drink.

What are the benefits of hypotonic HEED?

A hypotonic drink has a lower particle concentration than body fluids. This can be beneficial in situations where fluid replacement is of paramount importance, such as in hot weather, during intense sweating, or during continuous endurance exertion.

According to a systematic review and meta-analysis examining the hydration effects of hypotonic carbohydrate-electrolyte drinks, these drinks may support hydration maintenance more favorably during continuous sports exertion than water, or isotonic or hypertonic drinks. This is a group-level research finding; individual reactions are still influenced by sweating, intensity, weather, and the amount consumed. The full research can be read here.

HEED's specialty is that it also provides 28 grams of carbohydrates per serving in addition to low osmolality. Thus, fluid replacement is accompanied by substantial energy replenishment. The full electrolyte profile includes sodium, potassium, calcium, magnesium, and chloride.

Low osmolality does not mean that the same mixing ratio will be ideal for all athletes. Fluid, energy, and electrolyte needs vary individually. In cooler weather or during shorter workouts, less fluid may be sufficient, while in hot weather, greater dilution and continuous sipping may be preferred.

When might a more concentrated mix be advantageous?

It may happen that an athlete wants to consume more energy from the same bottle. In cooler weather, fluid requirements are often lower, while the energy demand of the exertion remains. In such cases, a more concentrated HEED mix can be used.

Maltodextrin also offers an advantage in this situation. A larger amount of carbohydrates can be consumed with less increase in osmolality than with the same simple sugar load. This is one of HEED's most important formulation advantages: the athlete can adjust the drink to their own fluid and energy needs within certain limits by varying the ratio of water to powder.

Always try more concentrated mixing during training. The energy content of the drink, the amount consumed at one time, and the intensity of the workout can all affect gastric emptying and comfort. In addition to increasing the concentration, separate plain water may also be needed.

The correct designation: hypotonic electrolyte sports drink

Based on the available laboratory data, the most accurate designation for HEED, at the recommended 591 ml dilution, is:

low osmolality, hypotonic electrolyte sports drink with a maltodextrin-based energy source.

This says more from a professional standpoint than the "ISO sports drink" designation. It shows that HEED provides fluid, carbohydrates, and electrolytes, while its measured osmolality is particularly low.

Denser preparation allows the drink's concentration to shift towards the isotonic range. HEED's true uniqueness comes from maltodextrin: it can provide a significant amount of carbohydrates with a low osmotic load.