Over 18,000 studies have looked at coffee use in the past few decades. Lately more and more are reporting real health benefits for coffee drinkers—but they must be balanced against the brew's possible bitter effects, especially in higher, caffeinated doses. An ideal "dose" of java is hard to determine, since people’s perceptions of "a cup of coffee" vary as widely as coffee-mug sizes do. But the good news is that many of the benefits are associated with around two to four (8-ounce) cups a day—"and that’s what most Americans drink anyway," notes Joe Vinson, Ph.D., a coffee expert at the University of Scranton, Pennsylvania. Some intriguing findings:
Moderate coffee drinking—between 1 and 5 cups daily—may help reduce risk of dementia and Alzheimer’s disease, as well as Parkinson’s disease, studies suggest. How? Coffee’s antioxidants may prevent some damage to brain cells and boost the effects of neurotransmitters involved in cognitive function, say experts. Preliminary studies have noted that as coffee (or tea) intake rises, incidence of glioma, a form of brain cancer, tends to drop. Some researchers speculate that compounds in the brews could activate a DNA-repairing protein in cells—possibly preventing the DNA damage that can lead to cells becoming cancerous.
Studies link frequent coffee consumption (4 cups per day or more) with a lowered risk of developing type 2 diabetes. Scientists suspect that antioxidant compounds in coffee—cholorogenic acid and quinides—may boost cells’ sensitivity to insulin, which helps regulate blood sugar. While most of the research didn’t assess whether the brews were caffeinated, decaf may be even better, since other studies have found that caffeine tends to blunt the insulin-sensitivity boost.
Some studies show that moderate coffee drinkers (1 to 3 cups/day) have lower rates of stroke than non-coffee-drinkers; coffee’s antioxidants may help quell inflammation’s damaging effects on arteries. Some researchers speculate that the compounds might boost activation of nitric oxide, a substance that widens blood vessels (lowering blood pressure). More java isn’t better: a 5-cup or more daily habit is associated with higher heart disease risks. Researchers believe excessive caffeine may sabotage the antioxidants’ effects.
Though the research is limited at best, it appears that the more coffee people drink, the lower their incidence of cirrhosis and other liver diseases. One analysis of nine studies found that every 2-cup increase in daily coffee intake was associated with a 43 percent lower risk of liver cancer. Possible explanation: caffeine and antioxidant chlorogenic and caffeic acids in coffee might prevent liver inflammation and inhibit cancer cells.
If you’re sensitive to caffeine, it can cause irritability or anxiety in high doses (and what’s "high" varies from person
to person). How? Chemically, caffeine looks a lot like adenosine, a "slow-down" brain chemical associated with sleep and
relaxation of blood vessels. Caffeine binds to adenosine receptors on nerve cells, leaving no room for adenosine to get
in—so nerve cell activity speeds up, blood vessels constrict—and you get a caffeine buzz (or irritable jitters).
Of course, if you caffeinate yourself daily, you’ll likely develop tolerance to its effects and the jitters will subside. But that also means that eventually you’ll need a regular caffeine fix just to reach your baseline level of alertness. And your body will adapt by producing more adenosine receptors, making you more sensitive to the effects of adenosine. So if you don’t have your daily cup, you’ll likely develop withdrawal symptoms like extreme fatigue and splitting headaches (caused by constricted blood vessels).
If you’re having trouble sleeping it might help to cut down on caffeinated coffee, or to drink it only early in the day. Generally it takes about 6 hours for the caffeine to clear your system, although it varies from person to person. The sleep-robbing effects may worsen as we age, too, a recent study suggests.
Boiled or unfiltered coffee (such as that made with a French press, or Turkish-style coffee) contains higher levels of cafestol, a compound that can increase blood levels of LDL ("bad") cholesterol. Choose filtered methods instead, such as a drip coffee maker.
The American College of Obstetrics and Gynecology says it’s safe for pregnant women to get a moderate amount of caffeine (no more than 200 mg, equivalent to 2 cups of coffee per day), but warns that it’s still not clear if higher intakes could increase risk of miscarriage. Since caffeine can pass into breast milk, nursing moms should cut down if their babies are restless or irritable.