9 Surprising Ways Caffeine Affects Your Health

Caffeine is a known performance booster. Not only does it ease the pain and fatigue of a workout, but it’s believed to help muscles burn fat as an energy source too.

In a small Brazilian study of endurance cyclists, caffeine supplements gave athletes an edge: They were able to pedal longer and faster than when they took a placebo instead. What’s more, they benefited from the caffeine boost–the supplement contained about 400 milligrams of the stuff–no matter how much caffeine they got from their daily coffee habit.

A caffeine buzz can really snap you out of a sleepy lull. In doses up to 300 milligrams, studies suggest it enhances attention, reaction time, and “vigilance,” meaning you’re able to stick with “lengthy, boring, or tedious tasks,” according to one review of caffeine’s effects.

Whether caffeine is a boon to higher-level mental tasks is less clear, researchers note. Its impact on problem solving and decision making, for example, “are often debated.”

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Caffeine is routinely administered to preterm infants in the neonatal intensive care unit to coax their brains and lungs to keep breathing.

Studies show this natural stimulant works by reducing “apnea,” or pauses in preemies’ breathing, and staving off intermittent drops in their blood oxygen levels.

Caffeine is not recommended for heart patients because it can induce an abnormal, rapid heart rate, especially at high doses. But what if you’re otherwise healthy?

The evidence on caffeine’s cardiovascular effects is varied and incomplete, and there are many unanswered questions, according to a 2013 workshop convened by the Institute of Medicine. It could be, for example, that some people are genetically susceptible to caffeine's heart health effects while others are not.

How caffeine affects your heart may also depend on whether you’re a habitual or infrequent user of caffeine, whether you have other preexisting health conditions, and what medicines you may be taking.

Young people may be more vulnerable to these effects, especially when consuming large amounts of caffeine. Media reports of adolescents whose hearts stopped suddenly after downing popular highly caffeinated energy drinks have raised serious safety questions.

The American Medical Association supports a ban on the marketing of high-caffeine beverages to anyone under age 18, and the American Academy of Pediatrics says energy drinks “should never be consumed by children or adolescents.”

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Caffeine slightly reduces calcium absorption, studies suggest. Other studies link the caffeine and phosphorus in colas (but not other soft drinks) to bone loss.

As long as you’re getting enough calcium in your diet and you’re otherwise healthy, there’s no solid evidence that consuming up to 400 milligrams of caffeine will harm your bones. Studies to date have shown no significant fall or fracture risk, nor heightened risk of bone loss among healthy adults with adequate calcium intake.

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Caffeine doesn’t cause ulcers, but experts suggest avoiding caffeine if you have one. That’s because the jolt sparks stomach acid production, which might in turn aggravate ulcers, open sores that sometimes develop on the lining of the stomach, esophagus, or small intestine.

Likewise, caffeine alone doesn’t cause gastroesophageal reflux disease (GERD), also known as acid reflux. According to the American College of Gastroenterology, there have been no studies to show that quitting caffeine improves GERD symptoms. Yet many doctors advise heartburn sufferers against consuming caffeine because it may worsen symptoms.

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Coffee drinkers seem to have a lower risk of developing type 2 diabetes. But if you already have the disease, you may want to be careful with caffeine. Studies show caffeine drives up blood sugar levels and impairs insulin sensitivity, frustrating efforts to keep diabetes under control.

Researchers at Duke University Medical Center, who studied the effects of giving caffeine capsules to people with type 2 diabetes, suspect that caffeine may interfere with sugar metabolism or trigger the release of adrenaline (the fight-or-flight hormone), which is known to boost blood sugar.

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Avoiding caffeine as much as possible is the safest bet if you’re pregnant, says the American Pregnancy Association. Caffeine can raise your heart rate and blood pressure. It can boost urination, which can dehydrate you. And it can cross the placenta, affecting your baby’s movement and sleep patterns.

Some studies link caffeine to miscarriage, while others do not, according to the March of Dimes.

However, moderate caffeine consumption–less than 200 milligrams a day–“does not appear to be a major contributing factor in miscarriage or preterm birth,” adds the American College of Obstetricians and Gynecologists.