Hydration Science: Understanding Dehydration

Lauren Motto

The human body is an incredible machine that needs to stay hydrated to keep moving. Because water is so important to all functions of the body, dehydration or the lack of water can quickly become very serious. Scientists have regularly studied the importance of hydration and in the mid-20th century, they started looking even closer at how people rehydrate. As part of this effort, researchers in several parts of the world contributed an advancement explored by notable, British medical journal, The Lancet, in 1978. Hailed as “potentially the most important medical advance” of the 20th century by the journal, this discovery was the formation of oral rehydration solutions, abbreviated ORS.

Early Hydration Science & Research

Earlier research conducted in the 1960s revealed that when glucose and water were combined with salt in a specific ratio, the liquid could be quickly absorbed through the intestinal walls. This happens through a specific protein known as the sodium-glucose cotransport mechanism, also known as the sodium glucose “pump.” This “pump” allows for water to be pulled into the body’s cells as opposed to only passing slowly through the digestive track and into the body.

sodium and dehydration
This cross section of the small intestine illustrates how the “pump” allows cells to open and absorb water (H2O) more quickly when the right balance of glucose (Glu) and sodium (Na) are introduced.


The Relationship Between Sodium and Dehydration

So how does this sodium glucose pump work? When drinking an oral rehydration solution, the amount of sodium in the liquid is different than the concentration in the body. Typically, we think of sodium as a dehydrating agent, but the amount of salt in an ORS is carefully controlled to avoid this. The right concentration of salt and glucose activates the sodium glucose pumps lining the intestine. When these pumps open, water can pass through with the sodium and glucose, leaving the intestines and entering the body’s cells to hydrate you.

hydration science
This 2D diagram represents how water will rush from the small intestine into the body when sodium and glucose activate the sodium glucose pump. The blue circles represent sodium, the green circles represent glucose, and the light blue arrows represent water.


hydrating with an ORS
When water enters the small intestine, a small amount may enter the body (left), compared to how much can flow through when an ORS presents the right sugar/salt ratio (right).


“If the body starts to experience mild to moderate dehydration, that’s when an ORS is really important.”

What is Tonicity and Why It Matters

Tonicity is also important when considering the effectiveness of an ORS. What exactly is it? Tonicity means the ability of a solution to move water into and out of a cell. In this case, we are talking about solutions and their effect on the human body.

The 3 types of tonicity:

  • Isotonic solutions maintain the current balance of water to salt concentration.
    • An example of an isotonic solution is a saline drip in a hospital.
  • Hypotonic solutions increase the movement of water based on too much salt present.
    • An example of a hypotonic solution is an ORS, such as Hydralyte.
  • Hypertonic solutions decrease the movement of water based on too little salt present.
    • An example of a hypertonic solution is sea water.
how tonicity works
How tonicity works.

How Hypotonic Drinks Work

Think of it this way: you have two glasses of water that are half filled. You put one drop of food dye into Glass A and two into Glass B. At this point, Glass B is much darker. To make the glasses the same shade, you can either add another drop of food dye to Glass A OR you can double the amount of water in Glass B. The process of doubling the water in Glass B could be achieved with a hypotonic solution.

In the example of the body, the food dye represents the amount of salt inside your body versus inside your small intestine. If you have a lot of salt in your body (your darker colored “glass B”) and not as much inside your intestines (your lighter colored “glass A”), you need to add more water to the body to make it the right “shade.” The only option here to dilute the body from too much salt (or, as in our example, to dilute the color of the darker Glass B) is to add water. So, your body automatically pulls water from the small intestine into the cells of the body to make the concentration (between the small intestine and the body, or between glasses A and B in our example) the same, “diluting” the body back to a normal state.

This example in the body is of a hypotonic solution because it increases how much water moves into the body. This is the way that hydration drinks and tablets rapidly hydrate you. An ORS, because it is hypotonic, prompts the body to require more water, which the small intestine provides with the ORS solution.

the right way to hydrate
The right ratio of sodium and glucose, in combination with a hypotonic solution, leads to rapid hydration.

The Science Behind Dehydration

Why is all of this so important? When water is able to pass through these open pumps, this allows for you to be hydrated faster than just drinking plain water. For most of us, water is the best hydration solution there is! Therefore, when a healthy, hydrated person drinks an oral rehydration solution they may not feel a difference, but if the body starts to experience mild to moderate dehydration, that’s when an ORS is really important and the difference in hydration can be felt immediately. Over 5.5 billion dollars in hospital charges are generated yearly due to dehydration, and using an ORS is a way to treat mild to moderate dehydration at home and avoid unnecessary ER visits.

Most people experiencing mild to moderate dehydration should consider choosing an ORS for rehydration to ensure clinical effectiveness when replacing lost fluids. While there will always be more to discover, the key to managing dehydration is here!