The Atlantic's Lindsay Abrams describes.
"What I tell patients is, if you like coffee, go ahead and drink as much as you want and can," says Dr. Peter Martin, director of the Institute for Coffee Studies at Vanderbilt University. He's even developed a metric for monitoring your dosage: If you are having trouble sleeping, cut back on your last cup of the day. From there, he says, "If you drink that much, it's not going to do you any harm, and it might actually help you. A lot."
Officially, the American Medical Association recommends conservatively that "moderate tea or coffee drinking likely has no negative effect on health, as long as you live an otherwise healthy lifestyle." That is a lackluster endorsement in light of so much recent glowing research. Not only have most of coffee's purported ill effects been disproven -- the most recent review fails to link it the development of hypertension -- but we have so, so much information about its benefits. We believe they extend from preventing Alzheimer's disease to protecting the liver. What we know goes beyond small-scale studies or limited observations. The past couple of years have seen findings, that, taken together, suggest that we should embrace coffee for reasons beyond the benefits of caffeine, and that we might go so far as to consider it a nutrient.
[. . .]
The most recent findings that support coffee as a panacea will make their premiere this December in the American Journal of Clinical Nutrition. Coffee, researchers found, appears to reduce the risk of type 2 diabetes.
"There have been many metabolic studies that have shown that caffeine, in the short term, increases your blood glucose levels and increases insulin resistance," Shilpa Bhupathiraju, a research fellow at the Harvard School of Public Health's Department of Nutrition and the study's lead author, told me. But "those findings really didn't translate into an increased risk for diabetes long-term." During the over 20 years of follow-up, and controlling for all major lifestyle and dietary risk factors, coffee consumption, regardless of caffeine content, was associated with an 8 percent decrease in the risk of type 2 diabetes in women. In men, the reduction was 4 percent for regular coffee and 7 percent for decaf.
The findings were arrived at rigorously, relying on data from the Nurses' Health Study and the Health Professionals Follow-Up Study, two prospective studies that followed almost 80,000 women and over 40,000 men from the 1980s through 2008. Although self-reported, the data is believed to be extremely reliable because it comes from individuals who know more about health and disease than the average American (the downside, of course, is that results won't always apply to the general population -- but in this case, Bhupathuraju explained that there's no reason to believe that the biological effects seen in health professionals wouldn't be seen in everyone else).
That there were no major differences in risk reduction between regular and decaf coffee suggests there's something in it, aside from its caffeine content, that could be contributing to these observed benefits. It also demonstrates that caffeine was in no way mitigating coffee's therapeutic effects. Of course, what we choose to add to coffee can just as easily negate the benefits -- various sugar-sweetened beverages were all significantly associated with an increased risk of diabetes. A learned taste for cream and sugar (made all the more enticing when they're designed to smell like seasonal celebrations) is likely one of the reasons why we associate coffee more with decadence than prudence.