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Milk - It Does the Body Good! The Surprises of the Beverage Hydration Index

As part of my nutrition coach certification through the National Academy of Sports Medicine, I was fascinated by one particular research study that I read about in the hydration section. I’m an avid runner and have experimented with various “hydration drinks” over the course of the past five years so I’m always interested in the topic of hydration. This post is going to be a bit geeky because I’m a science nerd. I promise it’s not complicated though.

Some basic information on how our body works before we jump into the hydration study: Water constitutes 50-70% of our total body mass. Roughly 2/3 of this fluid is located inside of individual cells and is referred to as intracellular fluid (ICF), and the remaining fluid can be found in the vascular system (plasma portion of blood) or in the space between cells (interstitial fluid). All of the fluid found outside of cells is called extracellular fluid (ECF). While there are many solutes, such as glucose and red blood cells, located within the body’s water compartments, the primary factor that determines what space body water will occupy is dependent predominantly on the concentration of two electrolytes: sodium (the main ECF solute) and potassium (the main ICF solute). So what this basically means is that the fluid inside your cells has lots of potassium, and the fluid outside your cells has lots of sodium. A neat sodium-potassium pump allows water to move into your cells based on how much potassium and sodium are in your ECF. Whew, enough of that and now on to that fascinating study!

In 2015, researchers (Maughan and seven others) set out to create a Beverage Hydration Index (BHI) where they compared the hydration effect of 13 different beverages. Over the course of a few months, they had their subjects drink a controlled quantity of various beverages over a specific amount of time and urinate and give blood samples at time intervals after drinking (up to a maximum of four hours after consumption). Low urine output was desired because that meant the water in that beverage stayed in the body and not lost through urine output. After the scientists gathered and analyzed the data, the results were (drumroll please…….)

MILK! MILK! MILK! It does the body good. Milk, Oral Rehydration Fluid (think hospital strength), and orange juice clearly beat out water, and some other drinks (cola and beer) were just a tad better than water. What! How is this so?

We’ve all heard that the key to hydration is electrolytes, specifically sodium and potassium for their importance in ICF and ECF contribution. Gatorade, Powerade, and Pedialyte are all touted for their hydration ability. Take a look at the chart below (from the same study) and look at the sodium and potassium content in various beverages.

The problem with water as a hydrator is that it has absolutely no electrolytes (sodium and potassium) in it, and that in order for it to be effective as a hydrator (not thirst quencher), you have to eat food that has sodium and potassium with it (bananas and salt tablets for you runners out there). Without the added potassium and sodium consumed through food, you just pee out that water! However, milk (and other drinks) have sodium and potassium in them that help retain the water in that beverage and are needed as part of intercellular and extracellular fluid in your body. Remember that potassium is the major electrolyte inside our cells so the beverages with more potassium were better hydrators. Milk even had one of the lowest water content percentages, but the electrolytes helped retain it in your body.

I’m not suggesting that you drink milk during your sweat depleting exercise because that would just be gross, but it’s a fantastic recovery drink (chocolate milk is also awesome because of the added carbs). When you are feeling especially dehydrated, you might want to reach for a glass of milk (or OJ) instead of that glass of water.

Happy Science Sunday!

A link to the article from The American Journal of Clinical Nutrition is here if you want to read the entire study.

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