Two to three cups of coffee appears to be safe for most of health outcomes, according to a review of studies (albeit mostly observational ones).
On the plus side, coffee consumption may reduce the risks of type 2 diabetes and hypertension, as well as other conditions associated with cardiovascular risk such as obesity and depression. Drinking coffee is neutral when it comes to coronary heart disease, congestive heart failure, arrhythmias, and stroke. It may be related to anxiety, insomnia, tremulousness and palpitations, bone loss and possibly an increased risk of fractures.
Results appeared in the Journal of the American College of Cardiology.
In the evidence review, a meta-analysis of more than 400,000 patients in 21 independent prospective cohort studies from January 1966 to January 2008 suggested that moderate coffee consumption may decrease the long-term risk of coronary heart disease. Compared to light-to-absent coffee consumption (less than one cup a day in the U.S. less than two cups a day in Europe), moderate coffee consumption (more than one or two cups daily, respectively) was associated with significantly lower rates of coronary heart disease (relative risk [RR], 0.87; P=0.001).
Several studies suggested that it is safe for patients with established coronary heart disease to continue drinking coffee. A randomized, controlled trial of 103 patients with acute ST-segment elevation myocardial infarction found that drinking coffee was associated with an increase in parasympathetic tone, and coffee did not increase cardiac arrhythmia.
Coffee may reduce the risk of ischemic stroke. A recent meta-analysis of seven prospective studies with more than 440,000 participants, nearly 7,000 stroke events and follow-up of two to 24 years found that one to three cups of coffee were associated with a decreased risk of stroke (RR, 0.82; 95% CI, 0.74 to 0.90, P less than 0.001). There was a significant reduction of stroke risk in people drinking three to six cups of coffee per day (RR, 0.79; 95% CI, 0.68 to 0.92; P=0.003). Drinking more than six cups of coffee per day was not associated with any effect on stroke risk (RR, 1.00; 95% CI, 0.76 to 1.32; P=0.97).
Habitual coffee drinkers showed minimal impact on transient blood pressure in a meta-analysis of 10 randomized controlled trials and 5 cohort studies. Non-significant mean changes in systolic blood pressure of -0.55 mm Hg (95% CI, -2.46 to 1.36) and diastolic blood pressure of -0.45 mm Hg (95% CI, -1.52 to 0.61) were found in coffee drinkers when compared to the control group, with no clinically important effects. The Nurses' Health Study, with 1.4 million person-years of follow-up, demonstrated that daily intake of up to 6 cups of coffee was not associated with an increased risk of hypertension.
For diabetes, a systematic review of nine cohort studies compared coffee consumption of less than two cups a day to that of more than 6 six cups a day for risk of developing diabetes. The risk of developing diabetes was lowest in subjects who drank more than six cups daily (RR, 0.65; 95% CI, 0.54 to 0.78) and also significantly reduced for subjects who consumed four to six cups daily (RR, 0.72; 95% CI, 0.62 to 0.83). A prospective study of more than 88,000 women ages 26 to 46 years established a linear relationship between drinking coffee and lower rates of diabetes, whereby even small amounts of coffee on a daily basis conferred benefit, regardless of whether it was caffeinated or not.
Authors noted that coffee's possible advantages have to be weighed against potential risks (which are mostly related to its high caffeine content) including anxiety, insomnia, tremulousness and palpitations, as well as bone loss and possibly increased risk of fractures.
But, they added, "[S]ubstance abuse experts generally do not consider caffeine dependence a serious addiction. Indeed the tendency for coffee to promote habitual daily consumption may ultimately turn out to be advantageous if its myriad potential health benefits are confirmed. Caffeine, in moderate daily doses of about 300 milligrams, or about 3 cups of coffee, appears to be safe and harmless for healthy adults. ... Moderation, tending towards 2 or 3 with up to as much as 4 cups a day if tolerated, seems a reasonable suggestion."