A real-life lesson in why hydrating with caffeine is a dumb idea

On a run last week I was exploring with my sister whether donating plasma is a creepy or reasonable thing to try, given that I’d come across a coupon purporting to pay $300 over five visits.

Even allowing two hours for the first session, where you undergo a physical, that works out to around $50 an hour.

As a journalist I do a lot of research, but I was interested in Traci’s gut feeling as a health care professional. “Think that’s a bad idea?” I asked.

“No,” she said, “it’s actually a good thing to do,” noting that plasma is a much-needed resource.

We weren’t planning to run the next day so I decided to try it then, figuring that would give me plenty of time to rehydrate beforehand and recover before our next run.

Consumed as I was with whether this was a good move for me, it never crossed my mind that I might not be good enough for them. Not finding easy enough access to the prerequisite number of veins, the nice guy in scrubs told me I could try again in a month – but to make sure that next time I drank more water beforehand.

“But I had like 96 ounces of fluids the day before!” I whined to my sister.

Knowing me all too well, she asked: “So how much of that was coffee?”

Well … I probably had at least as much coffee as water. And then there was that 48 ounces of Diet Mountain Dew I guzzled after Tuesday’s run. Stupidly, I thought I might drink more if I was drinking something with, you know, flavor.


“Well, there ya go,” she said, rolling her eyes like I was dumber than a box of rocks, which is a look she’s been giving me ever since she was 3 and I was 11. “Caffeine dehydrates you. However much caffeine you take in, you need to drink twice that much water.”


Well, that would explain the weird sensation of feeling like my stomach was so full of liquid I thought I might bust, yet feeling like I was still kinda thirsty.  Suddenly I did feel like a world-class idiot. I guess I did know that caffeine had at least a mild diuretic effect, but that factoid was so deeply buried in the clutter of my mind that I hadn’t factored it into the equation of a busy day. Or maybe it’s just part of an ingrained bias delusion; for somebody who considers herself a health writer, I drink a ridiculous amount of coffee. (Some days that’s about all I drink, though I’ve been trying to work on that.)

Could I really have been a bit dehydrated after drinking so much fluid? It’s not like Traci conspired with the folks at Biolife to teach me a lesson. But some of the studies I looked up suggested maybe it wasn’t quite that simple.

“Several studies have challenged the assertion that caffeine could contribute to a severe fluid deficit,” wrote the researchers in a 2015 meta-analysis of 78 studies reported in The Journal of Science and Medicine in Sport

Interestingly, however, the researchers discovered that females were nearly six times more susceptible to the diuretic effects of caffeine – a finding which they attributed to differences in how caffeine is metabolized.

So yeah, I’m definitely going to be drinking more water and less coffee from here on out. Regardless of how appealing that makes my veins look to a lab tech, I’m hoping that will pay off during our runs – where it just so happens that I’m always the one who’s thirsty first.

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