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Coffee is a drug I enjoy consuming, and I want to know what science has to say on its effect of my health.
For example, The Atlantic recently published "The Case for Drinking as Much Coffee as You Like", an article that presents coffee as a panacea. According to them, coffee diminishes the likelihood of type 2 diabetes, pain, coronary heart disease, stroke, Alzheimer's disease, cancer and alcoholism to name only a few.
However, a little bit of research reveals that the main source from this article is sponsored by the following:
"""A grant from a consortium of coffee-producing countries (Brazil, Colombia, Mexico, and Guatemala and a coalition of Central American nations) under the auspices of the Association of Coffee Producing Countries (ACPC), the National Coffee Association of the USA, and the All Japan Coffee Association. Subsequently, the work of the ICS has also received support from the International Coffee Organization and the USA corporate sector (NCA, Kraft Foods, Nestle, Sara Lee, Starbucks)."""
I do not doubt that the source of the article is well-meaning and does good work, but let's put our skeptic glasses and inquire on our own what the scientific literature has to say on the topic.
The methodology consists of searching the scientific literature for recent articles that are highly cited and from credible sources.
Coffee and health: a review of recent human research (2006, 304 citations) is a comprehensive review of the effect of coffee on health. The results is summarized in "For adults consuming moderate amounts of coffee there is little evidence of health risks and some evidence of health benefits.". Good news, your daily coffee is probably not killing you. However, "some evidence of health benefits" is far from the claim that the article from The Atlantic suggests.
Here are more details from the study:
Coffee may help prevent several chronic diseases, including type 2 diabetes mellitus, Parkinson's disease and liver disease. At present, there is little evidence that coffee consumption increases the risk of cancer and coffee consumption is not significantly increased cardiovascular disease risk.
Coffee consumption is associated with increases in several cardiovascular disease risk factors, including blood pressure and plasma homocysteine. Some group of people are more vulnerable to the ill effects of caffeine (people with hypertension, elderly, adolescents, ...). Do not drink more than 300mg of caffeine if you are pregnant, it can impair fetal growth.
If you enjoy drinking coffee, keep on -- it most probably does not impair your health unless you suffer from other medical conditions (in which case, ask your doctor). If you want to be healthy, eat well, exercise, maintain strong social ties, sleep well and avoid stress; coffee probably only has a marginal positive impact on health.
This one is a bit more complicated since mental performance can be interpreted in many ways. We split the investigation in the long and short term effects of coffee consumption on mental performance.
"Does drinking coffee for years keep your brain in better shape?". Coffee Consumption and Cognitive Function among Older Adults (2002, 87 citations) attempts to answer that question by administering standardized tests to old people and comes to a rather surprising conclusion: "No relation was found between coffee intake and cognitive function among men or between decaffeinated coffee intake and cognitive function in either sex. Lifetime and current exposure to caffeine may be associated with better cognitive performance among women, especially among those aged 80 or more years."
Intuitively, most coffee drinkers know that coffee helps when one is very tired. Effects of caffeine, sleep loss, and stress on cognitive performance and mood during U.S. Navy SEAL training (2002, 256 citations) confirms this intuition: caffeine significantly improved visual vigilance, choice reaction time, repeated acquisition, self-reported fatigue and sleepiness with the greatest effects on tests of vigilance, reaction time, and alertness **for sleep-deprived individuals in a stressful environment. These effects lasts up to 8 hours after consumption.
But what about when someone is in a "normal situation": not necessarily sleep deprived or stressed. Researchers investigated if some of caffeine's positive effect could be cause by the absence of withdrawal rather than really boosting mental performance: Effects of caffeine on performance and mood depend on the level of caffeine abstinence (2002, 86 citations). They come to a striking conclusion: "Caffeine reliably improved performance on a sustained attention task, and increased rated mental alertness, in moderate caffeine consumers who were tested when caffeine-deprived. However, caffeine had no such effects when consumers were no longer caffeine deprived."
This is an outstanding result; it means that there is no short-term cognitive benefit to drinking coffee (under "normal conditions"), there is only an absence of the negative effects of withdrawal on people addicted to coffee!
Over time, you will likely not perform better at your job if you drink coffee. If you need a sudden boost of attention in a situation where you are likely to get tired such as in a long drive, coffee is helpful. Also, if you usually drink coffee, do not skip it on the morning of an exam!
There seems to be some long-term cognitive and health benefits to coffee drinking for certain group of people, but the effect is small. There are some marginal positive health effects and it can help you momentarily escape the ill effects of fatigue, but mostly drink coffee if you enjoy it -- do not expect strong cognitive or health benefits from it.
Mayo clinic has an excellent table with lots of detail on that topic. But it is long and precise, here is a quick reference that abstracts the details:
Drink | Caffeine Content in mg ------------------------------ 5-Hour Energy: 200 Brewed Coffee: 150 Red Bull: 78 Expresso: 60 Black tea: 40 Green tea: 32 Coca-Cola: 32 Brewed, decaf: 7