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This is your body on energy drinks

The anatomical science behind sipping turbo-powered fuel.

Whether to ward off a 3 p.m. energy drop or to fuel a lethargic workout, the should-I-or-shouldn't-I surrounding energy drinks can arise at one time or another.

But what's the impact of slugging one back? An 8-12 ounce energy drink has 72-150 mg of caffeine. Drank throughout the day, every so often, this isn’t necessarily problematic. (After all, studies have found that up to 4 cups of coffee a day—which could top off around 400 mg of caffeine—can have positivehealth benefits like lowering your risk of diabetes, upping your mood, and slashing cardiovascular disease risk.)

But a large energy drink bottle can flood up to 294 mg of caffeine into your body—in one sitting. Look at a label and you’ll also notice that your average Monster contains more than your morning cup o’ joe: namely sugar and other ingredients like taurine, ephedrine, guarana, and ginseng. All those "extras" act as stimulants, enhancing caffeine and sugar’s effects, and throwing your body into overdrive. Another problem: The effectiveness—and health risks—of those additives are largely unknown, says Maria Pagano, M.S. R.D., C.S.C.S. and Equinox Tier 4 coach.

So before you reach for an artificial pick-me-up, Pagano explains what’s going on under the hood while you’re sipping.

your brain

Caffeine blocks the effects of adenosine, a brain chemical involved in sleep (which is why too much of the pick-me-up can lead to insomnia). But the immediate effect of caffeine entering your body is this: neurons in your brain fire off to keep you alert, causing your pituitary gland to initiate the “fight or flight” response. That’s your body’s natural reaction to prepare for a threat.

your bloodstream

After a “fight or flight” response is activated, your pituitary gland releases adrenaline, which prepares your body for what’s to come. The release of adrenaline sends a signal to your liver: Pump more glucose into the bloodstream for more energy.

your heart

Adrenaline makes your heart beat faster and your eyes dilate—again a way to get ready for what’s to come. And the heart effects can last longer than you think: In fact, a recent German study found that healthy people who drank caffeine and taurine-packed energy drinks saw increased heart contraction rates up to an hour later.

back to your brain

The physical effects of your drink—more glucose in your bloodstream and a fight or flight response—affect levels of a chemical called dopamine in your brain. Basically, because of how your body is reacting, your brain is led to believe that you have more energy than you really do.

your skin

Sweat excessively while exercising and you’ll lose water and electrolytes—both of which sports drinks work to replace. Reach for an energy drink after that, though, and you won’t replenish what you lost, but rather, dehydrate yourself more.

your body

Too much caffeine can produce a diuretic effect, which can also translate to dehydration. If you overdo it, you could feel jittery, anxious, and irritable from too much of the stimulant and a lack of H20.