In today’s edition of Dear Mark , I cover a topic near and dear to many of your hearts: caffeine . But I don’t just cover caffeine; I explore whether caffeine truly does act as a diuretic, especially during exercise, and whether or not caffeine can actually be helpful to athletic performance. Should we all be downing mugs of joe or cups of tea before we hit the gym or head outdoors?
Let’s find out.
I’ve been told that drinking coffee prior to, or during workouts is a big no-no, because it’s a diuretic and will lead to dehydration, which is no good for performance (or health). But I love an iced coffee right before my workouts. I feel like it helps. It could just be placebo, but if it’s not hurting, I’m okay, right?
I wonder if you could give me the lowdown on what the literature says. Thanks!
First, let’s tackle the dehydration question. It has undoubtedly become “common knowledge” that coffee is a potent, perhaps the most potent, diuretic, that drinking it is like drinking negative water, and that if you’re stuck on a desert island you’d be better off drinking your own saliva than that steaming cup of joe from the Starbucks that inexplicably decided to set up shop on a desert island. Yeah, there are a lot of scary stories about coffee, but does it hold up to scrutiny?
No. A quick search on PubMed turns up a couple German-only studies with vociferously and unambiguously worded titles but no abstracts (“Coffee does not cause dehydration! ” and “Coffee does not dehydrate. New studies of Germany’s favorite addiction: coffee .”), as well as some English ones with abstracts:
- One from the University of Connecticut  measured fluid, electrolyte, and renal indices of hydration over eleven days of caffeine consumption in human subjects. Doses of up to 6 mg caffeine per kilogram of body weight had no effect on body mass, urine osmolality (urine concentration), urine specific gravity (concentration of excreted materials in urine), urine color, urine volume, sodium excretion, potassium secretion, creatinine content, blood urea nitrogen (forms when protein breaks down), and serum levels of sodium and potassium, causing the researchers to conclude that caffeine does not cause dehydration.
- Another  compared hydration markers in patients who consumed either caffeinated beverages (coffee and cola), non-caffeinated beverages (coffee and other sodas), and/or water. The effect on hydration status was essentially uniform across all beverage categories, regardless of caffeine content.
- And finally, a review  from the American College of Sports Medicine found that not only does caffeine not reduce hydration nor induce electrolyte imbalances, it has no effect on heat tolerance during exercise.
I think that settles that. Caffeine does not dehydrate you or cause you to overheat. It’s “safe.” It’s not bad for the active athlete.
But is it actually good? Does it do anything except fail to dehydrate you?
Oh, yeah. Let’s dig into the literature to find out what it can do for your athletic performance.
Most of exercise/caffeine literature centers on endurance training and performance. I remember back when I was running, the most oft-cited benefit to caffeine before a race or training was that it would increase the oxidation of fat, thus sparing muscle glycogen . That sounds nice and tidy, and it would be awesome if it were true, but the most recent evidence  suggests that caffeine has little, if any, effect on fat or glycogen metabolism  during endurance  exercise. So what are we to make of the older evidence  that does show a difference in fat oxidation after caffeine ingestion? Or the 1992 study  that found caffeine reduced the tendency of muscle to burn glycogen early on during extended bouts of exercise, thus “sparing” it for later on?
It may be that caffeine simply makes exercise  more tolerable, makes muscles work harder and better, and allows those exercising to do so harder. One study  found that while pre-workout caffeine did not spare glycogen, it did boost the endorphin  response to exercise. If endorphins are high, exercise is more tolerable, even enjoyable. If caffeine can increase the runner’s high, it’s also going to make exercise more effective and more self-perpetuating.
Whatever the case may be, the literature is pretty clear that caffeine improves endurance performance , perhaps by enhancing fuel partitioning or making exercise more tolerable and enjoyable.
The extent of research into the effects of caffeine on anaerobic performance – think sprints , weight lifting, and interval training – is limited, but useful literature exists. One review , from 2009, noted that while caffeine appears beneficial to speed endurance training (in the realm of 60 to 180 seconds) and high intensity interval training  (HIIT), it has limited use in power sports. It may help lower body muscle endurance, but it appears to have a minimal effect on the upper body. The authors propose a number of mechanisms for caffeine’s action, including enhanced calcium transport and the old fat utilization/glycogen sparing thing, but the most promising idea is that caffeine simply stimulates the central nervous system  enough to blunt adenosine receptors, increase pain tolerance, and dampen perceived exertion.
What about resistance training? In one study, caffeine ingestion boosted trained women’s 1RM in the bench press (PDF ). In another , caffeine seemed to have no effect. A review  from 2010 determined that short-term, acute ingestion of caffeine is beneficial in team-based and power sports, but mostly in individuals who did not routinely ingest caffeine. Six of eleven resistance training studies reviewed in the study showed benefits to caffeine ingestion, so the evidence remains fairly equivocal.
Thus, when you drink coffee before lifting heavy things or sprinting, your performance will not suffer – and it may even improve.
It may even be a simpler, less exciting explanation than anything overtly physiological: that the “benefits” of caffeine to physical performance may actually be a cessation of the negative effects of caffeine withdrawal. As Sweat Science points out  in a recent post, before double-blind trials on the effect of caffeine on performance, participants must abstain from caffeine for a day or two. If they’re habitual caffeine fiends (as many people are), by the time they begin the study they’re already suffering withdrawals. Studies on cognitive performance and caffeine have found that when you account for the withdrawal effect, caffeine has little to no benefit to performance. Researchers have yet to examine the withdrawal effect in studies on athletic performance, but it appears a likely candidate for at least some of the reported benefit to caffeine consumption.
It’s also important not to lose sight of the fact that most of us are drinking coffee, not popping pure caffeine pills. Coffee contains tons of polyphenols, bioactive compounds that could have beneficial (or negative) effects on exercise performance. Most of the studies are looking at caffeine, so they have to isolate it. But if you’re drinking coffee, shouldn’t you look for studies that examine coffee? There’s a recent one that found ingesting coffee polyphenols increased fat oxidation (PDF ). Of course, the caffeine, polyphenol , and other bioactive compound contents of coffee are not stable. Coffee is a food made up of hundreds of factors. It’s not just a source of caffeine. Based on soil conditions, climate, elevation, roast, and variety of bean, two cups of coffee can display remarkably different characteristics, and it’s likely that the effects of each on exercise performance will also differ.
Bottom line, though: if coffee makes you perform better, keep drinking it before, during, or after you workout. At least we can say for sure that it’s not dehydrating you.
How does coffee affect your workouts? Better? Worse? No effect? Let me know in the comments!