Want to get hydrated? Drink milk, not water


No, this is not clickbait, no, I am not high, and yes, I do study biology.

Let’s start from the top. What is hydration exactly?

Your body needs water. Every one of the 50 trillion cells in your body requires water to go about their business. They use it to remove waste products, maintain your body’s temperature, and a host of other functions. So if being hydrated means having water in your body to function efficiently, why would drinking it not be the most beneficial way to stay hydrated? Science, my friend. Science.

Let me take you back to the good old days of GCSE Biology, where we learnt one of the most crucial aspects of human physiology: homeostasis. Besides mitochondria being the “powerhouses” of the cell, or respiration being unequivocally different to breathing, you might also remember that our kidneys are the “filters” for human blood, and they heavily rely on the process of diffusion.

For the A-students, hopefully you remember the principles of diffusion, and therefore don’t need to read the next sentence. For those high-school students who were more inclined to remembering images and facts rather than definitions, diffusion is the process whereby the molecules of a substance move from a region of high concentration to a region of low concentration, until the substance’s concentration becomes uniform or equal throughout.

Let’s take a closer look at what happens to your blood when it enters the kidneys. It gets filtered into the functioning unit of the kidneys, known as the nephron. Most of the contents of the blood (ions, sugars etc.) get squeezed into the nephrons, leaving behind the substances that were too large to pass through, such as proteins and cells. This creates a concentration gradient — a gradual change in concentration between two regions — between the nephron tubule and the surrounding blood vessels.


So we can think of the kidney as having two tubes next to each other. One which contains the contents of the blood, and the other, which is the nephron. At the end of these tubes (once they leave the kidney), the blood will recirculate around the body, and the nephron fluid is what passes into the urine. Salts, water, and other substances will pass between the two, and the mechanisms of diffusion and active transport (pumping substances against their concentration gradient), will determine the exact concentrations of the salts in both the blood and urine.

Now you’re probably wondering why I’ve spent so much time rambling about the kidney, and not talking about milk in this entire article. It’s important that you understand the key mechanisms behind hydration, and how your body stays hydrated. That way, when you start mentioning ludicrous ideas about how water isn’t the best substance for hydration, you can back it up. Now that you know, it should seem pretty obvious right?

No? All right, I’ll keep going then. Let’s imagine you consume a large amount of water, say 2 litres. Your blood becomes extremely diluted, making the relative concentrations of salts in the nephrons higher than in the blood. To resolve this, you end up with more water in the nephron (which leads to your bladder), and less in the blood. All you’ve actually done though, is increase your desire to run to the toilet and relieve yourself more frequently (you pee more).

Let’s say you drink some milk instead. This has high concentrations of all sorts of salts and ions, most famously calcium. Your blood, relative to the kidney nephrons, now has a much higher concentration of salts than water. Thanks to our pal homeostasis, your body will try retain more water in the blood to balance out these concentrations. This means more water circulates the body, which means less peeing time, which ultimately means you are more hydrated. Congrats!

So, next time you’re out and about working up a sweat, and someone offers you a bottle of water, slap it out of their hand and say, “no thanks, I’ll have some milk.”

Disclaimer: this whole process has been simplified. It’s a bit more complicated in reality.

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Hasan MZI

Hasan is a medical student at University College London. He's an aspiring surgeon who loves Arsenal, and he skips leg day at the gym. No abuse please, his mum is protective.

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