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A disclaimer: I do not own stock in Starbucks nor, to my knowledge, in any other company that sells coffee or its accouterments. I last wrote about America's most popular beverage four years ago, and the latest and largest study to date supports that earlier assessment of coffee's health effects.
Although the new research, which involved more than 400,000 people in a 14-year observational study, still cannot prove cause and effect, the findings are consistent with other recent large studies.
The findings were widely reported, but here's the bottom line: When smoking and many other factors known to influence health and longevity were taken into account, coffee drinkers in the study were found to be living somewhat longer than abstainers. Further, the more coffee consumed each day -- up to a point, at least -- the greater the benefit to longevity.
The observed benefit of coffee drinking was not enormous -- a death rate among coffee drinkers that was 10 percent to 15 percent lower than among abstainers. But the findings are certainly reassuring, and given how many Americans drink coffee, the numbers of lives affected may be quite large.
In decades past, experts repeatedly warned that a coffee habit could harm health and shorten lives. And, indeed, the new study did find that when the data were adjusted only for age, the risk of death was greater among coffee drinkers.
But when the researchers took into account other health-related characteristics among the participants, like smoking, alcohol use, meat consumption, physical activity, and body mass index, those who regularly drank coffee lived longer.
"Coffee drinkers shouldn't be worried," said Neal Freedman, an epidemiologist at the National Cancer Institute who directed the study. "Their risk is quite similar to that of nondrinkers."
Coffee drinkers who were relatively healthy when the study began were less likely than nondrinkers to die of heart disease, respiratory disease, stroke, diabetes, infections, injuries, and accidents.
The study, published in The New England Journal of Medicine, examined data on 402,260 adults in the National Institutes of Health-AARP Diet and Health Study. They were ages 50 to 71 and free of heart disease, cancer, and stroke when the study began in 1995. By 2008, 52,515 had died. Freedman and his co-authors examined why they died in relation to how much coffee they said they drank when the study began.
The risk of death gradually dropped as the number of cups the participants drank increased to four or five. At six cups or more each day, there was a slight rise in death risk, compared with that at four or five cups. But the chances of death remained lower than among people who drank no coffee.
Reflecting practices of the mid-1990s, the researchers considered a cup of coffee to be 8 to 10 ounces. The gargantuan cups now often served would count as more than one cup, Mr. Freedman said. Several of these extra-large cups can cause restlessness, irritability, sleeplessness, and anxiety (and might enable me to fly without an airplane).
Contrary to previous belief, at usual levels of consumption, coffee is not any more of a diuretic than the equivalent amount of water. Up to six cups a day can be counted toward one's recommended liquid intake.
Effects on health
Coffee is a complex substance that contains more than 1,000 compounds that may affect health. Caffeine, a stimulant, is the most studied and sought after. The amounts in coffee can vary greatly, from about 70 milligrams in a shot of espresso to about 100 milligrams in 8 ounces of brewed coffee.
But there can be wide variability in caffeine levels, even in similar beverages. As Jane V. Higdon and Balz Frei of Oregon State University reported in Critical Reviews in Food Science and Nutrition, when the same type of coffee was purchased from the same store on six different days, the caffeine content varied from 130 milligrams to 282 milligrams in an 8-ounce cup.
Nor is caffeine is the only compound in coffee important to health. In the new study, little or no difference was found in death rates among those who drank predominantly caffeinated coffee or decaffeinated coffee. Other substances -- like antioxidants and polyphenols -- probably also play a health-related role, the researchers noted.
Their findings should reassure people concerned about possible harm from substances long used to remove caffeine from coffee. Fear of these chemicals prompted many manufacturers to switch to the Swiss water method for removing caffeine.
But how coffee is brewed can make a health difference. Two prominent chemicals in coffee beans, cafestol, and kahweol, are known to raise blood levels of cholesterol and especially artery-damaging LDL cholesterol. These substances are removed when coffee is prepared through a filter but remain in espresso, French press, and boiled coffee. Single-serving coffee pods, like those used in a Keurig, contain filters.
Even though coffee can cause a temporary rise in blood pressure, the new study, like those before it, found the risk of heart disease to be lower among otherwise healthy coffee drinkers. Other benefits suggested by recent studies include a reduced risk of Type 2 diabetes, liver disease, and Parkinson's disease. Some research has found a reduced risk of depression, dementia, and Alzheimer's disease among coffee drinkers.
People who engage in strenuous physical activities can also benefit, but only if their coffee contains caffeine, which helps muscles use fatty acids for energy and blunts the effect of adenosine, extending the time before muscles fatigue. Post-exercise soreness is also reduced and recovery time shortened.
Whether coffee poses a risk to pregnant women remains controversial. A causal relationship between coffee consumption and miscarriage has not been demonstrated at caffeine intakes of less than 300 milligrams a day, but some studies have found increased risk of low birth weight associated with consuming more than 150 milligrams a day.
Keep in mind, too, that caffeine is a drug. Some medications, including Tagamet, Diflucan, Luvox, Mexitil, estrogens and antibiotics like Cipro and Levaquin, interfere with the metabolism of caffeine and can increase its effects.
In other cases, caffeine can enhance the effect of drugs like aspirin and acetaminophen (a benefit for pain relief). Caffeine can be toxic if used with prescribed doses of the antipsychotic medication clozapine.