There's still some reason for caution when it comes to how unfiltered coffee might impact health. Research has shown that high consumption of unfiltered French press and boiled coffee may drive up LDL cholesterol, according to a review study published last year by van Dam in the journal Applied Physiology, Nutrition, and Metabolism . (Learn 10 ways to lower your LDL and increase your HDL cholesterol.) This is because of cafestol, a compound found in brewed coffee. Higher amounts of cafestol are found in boiled, Turkish, Greek, and French press coffee; intermediate amounts are found in espresso; and low amounts are found in instant and paper-filtered coffee.
Possible benefits. For people worried about developing type 2 diabetes, it might not be a bad idea to drink coffee, research suggests. In his review in Applied Physiology, Nutrition, and Metabolism , van Dam says, he found that "people who drank a lot of coffee had a substantially lower risk of type 2 diabetes than people who don't drink coffee." It's likely that another component of coffee—not caffeine—produces this effect since it was observed in all coffee drinkers, whether they drank caffeinated or decaffeinated coffee.
Coffee consumption also seems to decrease the risk of stroke and may help prevent skin cancer, research shows, and it may offer a protective effect against Alzheimer's disease, though more study is needed to confirm this result, according to a 2007 review published in Neurological Research. Regularly drinking coffee also appears to help prevent Parkinson's disease, but this benefit may not occur in postmenopausal women who are taking estrogen replacement therapy, reports a study published in 2004 in the American Journal of Epidemiology .
For those who already have diabetes, the news is less encouraging. "Giving people caffeinated coffee did exaggerate blood sugar response to meals, which suggested it may be more difficult for people with type 2 diabetes to keep glucose levels in check" if they drink coffee, van Dam says. A possible solution is for diabetics to switch to decaffeinated coffee.
Drinking coffee doesn't seem to increase the risk of bladder, gastric, renal, or breast cancer, and it may actually help provide some protection against liver cancer. Some research shows an association between drinking coffee and lower blood concentrations of biomarkers that indicate liver damage. This result was consistent whether or not people had a history of liver disease, and it's unclear whether this is due to caffeine or some other component of coffee. More research is needed to determine whether there is a true cause-and-effect relationship between drinking coffee and decreased risk of liver cancer.
The bottom line. For many people, there's nothing bad about enjoying a cup or two a day of coffee. Indeed, a moderate amount of caffeine—two to four cups of brewed coffee (200 to 300 milligrams) per day—is probably safe, according to the Mayo Clinic. But heavier amounts of coffee—say, four to seven cups (or 500 to 600 milligrams of caffeine) per day—may result in difficulty sleeping, nervousness, irritability, nausea, a fast or irregular heartbeat, headaches, anxiety, or muscle tremors.