It could, however, hurt their hearts. Studies show an average blood pressure increase of 4/2 millimeters of mercury (mmHg) after taking caffeine, according to a 2004 review of research published in the journal Psychosomatic Medicine. That small spike won't matter much for most people, but it could exacerbate the condition of someone on the edge of heart problems, says Jim Lane, a psychophysiologist at Duke University Medical Center in Durham, N.C., who has extensively studied the relationship between caffeine, stress and health.
People report feeling more stressed out while caffeinated, Lane says, with good reason. In a study published in the journal Psychosomatic Medicine in 2002, he found that, after taking a caffeine pill equivalent to the amount in four or five cups of coffee, people showed an average 32% rise in the stress hormone adrenaline and a 3-millimeter rise in blood pressure. These effects lasted throughout the day after people took caffeine. A 5-millimeter rise in blood pressure leads to a 34% increase in the risk of stroke and a 21% increased risk of coronary heart disease, according to a review of nine large studies, and each millimeter rise adds an equivalent amount of risk, Lane says.
His studies also show that taking on difficult, mind-bending math problems while caffeinated produces more stress than either caffeine -- or math -- alone. Chronically high levels of stress can damage the blood vessels, kidneys and heart. "Caffeine exaggerates the effects of stress on the body as if the stress were worse," he says. "It doesn't just add stress, it multiplies."
Studies show a slight drop in blood pressure when people stop drinking coffee for a few days, Lane says, but scientists have yet to prove a link between caffeine and cardiovascular problems. "It is consistently reliable that caffeine raises blood pressure, and we know that higher blood pressure is associated with a greater risk of heart disease," he says. "To really test that scientifically, you would have to take a large number of people, assign them to drink coffee every day or never drink coffee, and follow them for 5 to 10 years. No one is going to do that study."
A number of long-term studies have, however, compared the health of people who choose to guzzle coffee and tea with those who avoid the stuff. And those studies are turning up perplexing results that contradict the isolated effects of caffeine.
Some data, for example, suggest that drinking coffee might help prevent heart disease. In a study published in February in the American Journal of Clinical Nutrition, for example, scientists from the City University of New York and State University of New York Health Science Center at Brooklyn followed more than 6,500 healthy adults for nearly nine years. In people older than 65, results showed, those who drank four or more cups of coffee a day were about half as likely to die from cardiovascular disease as their coffee-free peers. Similar studies have found similar trends. Coffee contains thousands of compounds that might be responsible for such benefits, but polyphenols, similar to the healthful compounds found in chocolate and red wine, as well as antioxidants are the most likely candidates.
Several large studies have linked large daily doses of coffee with lower rates of Parkinson's disease, as well. One theory is that regular caffeine consumption makes the brain more sensitive to adenosine, which has been shown to help protect the brain from mini-strokes caused by oxygen and sugar deprivation, Rogers says. Another possibility is that people with less addictive personalities have a genetic profile that makes them more likely to develop the degenerative brain disease. It's also possible that people who drink coffee tend to have healthier lifestyles in some other way. Only men seem to get the brain protection from caffeine, and estrogen may work against caffeine's brain-buffering effects in women.
Many painkillers also contain caffeine, which can ease the pain of migraine headaches by dilating blood vessels in the brain. And coffee drinking has also been linked to lower rates of liver cancer, Type 2 diabetes and gout, among other ills. However, scientists warn, correlations such as these don't prove that the drink itself (or any ingredient in it) is responsible for the protective effect. In some cases, the benefits probably come from other ingredients.