Is coffee healthy or unhealthy?
For healthy adults, moderate coffee consumption (3 to 5 cups per day) is more likely to be healthy than unhealthy; if you are pregnant or prone to anxiety, moderation or consultation with your doctor is advisable.
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Moderate coffee consumption -- around 3 to 5 cups per day -- is associated in large, long-running population studies with a lower risk of cardiovascular disease, type 2 diabetes, stroke, liver disease, and certain cancers. People who drink that much coffee also live longer on average. This is all observational research: the association is convincing and consistent, but a direct causal relationship has not been proven.
Both caffeinated and decaffeinated coffee appear to share these benefits. This suggests that caffeine alone is not responsible, and that other compounds in coffee -- such as chlorogenic acids (a type of antioxidant) -- also play a role. Adding sugar and cream may reduce the benefits, although the evidence for that is limited.
In the short term, caffeine improves alertness, concentration, and mood at intakes of up to around 400 mg per day, which corresponds to roughly 5 cups of coffee. Food safety authorities consider this safe for healthy adults. Earlier concerns about high blood pressure, heart arrhythmias, and cancer are not confirmed by current research. In fact, a recent small randomised trial found that patients with a specific heart arrhythmia who continued drinking coffee normally had a 39% lower chance of it recurring compared with those who stopped entirely.
There are exceptions and limits, however. Too much coffee can cause anxiety, sleep problems, palpitations, and possibly bone loss. If you already have a tendency toward anxiety, moderation is sensible. If you are pregnant, the advice is to consume no more than 200 mg of caffeine per day -- about 2 cups -- because of risks to the unborn child. Regarding a protective effect against depression, studies still contradict each other too much to draw a firm conclusion.
Long-term consumption is also associated with a lower risk of cognitive decline, Parkinson's disease, and Alzheimer's disease, but again this is observational: people who have been drinking coffee for decades differ from non-drinkers in more ways than one.
All claims are based on large prospective cohort studies, a number of systematic reviews, and one randomised clinical trial (DECAF, n=200). Most of the evidence is observational; causal relationships have only been partially established.