Some people can't get going in the morning until they've had a cup - or two or three - of coffee. Others swear they feel much better since they stopped drinking it. What's clear is that coffee has a strong effect on our body - or more precisely, caffeine does, its most active component.
"Caffeine has a pharmacological effect," says Dr Georg Wechsler, president of the Association of German Physicians for Nutritional Medicine (BDEM). In other words, it's similar to a drug.
"Nevertheless, people react differently to caffeine," he adds.
As nutritionist Harald Seitz, spokesman for Germany's Federal Centre for Nutrition (BZfE), puts it: "Every body is wired differently."
Here are answers to some common questions about coffee:
A chemical compound, caffeine stimulates the central nervous system, Seitz says. "It can increase concentration, enhance performance capability and help you to stay awake."
Wechsler points out that caffeine increases blood pressure and heart rate. "The theine in tea has the same effect as coffee, but it's more restrained," he says, using another name for caffeine, especially when present in tea. "Tea acts for hours, but less strongly. Coffee acts very briefly and quickly."
Coffee has a diuretic effect, as many people can attest. They drink a mug of coffee and before they know it have to go to the toilet.
"Caffeine stimulates blood circulation, causing a massive increase in blood flow to the kidneys," explains Wechsler.
Since the kidneys have the job of filtering waste products out of the bloodstream, they step up the process and consequently produce more urine through which the waste is excreted. This is why you have a strong urge to pee not long after drinking coffee.
Coffee isn't composed only of caffeine, of course. "There's hardly another food that contains as many basic chemical substances as coffee," Wechsler says.
Among them are antioxidants. "These are substances that have a protective effect in the body," says Seitz. "They scavenge free radicals [unstable molecules made during normal cell metabolism], which can damage the cells."
Coffee is also thought to reduce the risk of certain types of cancer. "These are long-term studies though, and it's not clear whether coffee was the sole cause," Seitz notes.
What's more, drinking coffee is often credited with enhancing the quality of life of people with low blood pressure or adynamia, a loss of vital power or strength, especially as the result of illness. "Being a natural stimulant, coffee helps them," says Wechsler, as it boosts blood circulation and metabolism.
Consumed in moderation, coffee lowers the risk of a heart attack as well, he says. By "moderation," he means up to three cups of filter coffee daily. "More than that is unfavourable," because too much caffeine can raise cholesterol levels and potentially increase the risk of cardiovascular diseases.
If you're healthy, he recommends two or three cups of coffee a day as a good guideline.
If you have a sensitive stomach, you may not tolerate coffee well because its roasting by-products stimulate production of gastric acid, which can cause heartburn. Wechsler advises drinking coffee with milk in this case, as it binds the roasting by-products.
"Coffee can trigger hypertension in people predisposed to it," he warns. "If you have high blood pressure, you should drink coffee in small amounts and see how you react." You may need to switch to decaffeinated coffee.
A chemical suspected of being carcinogenic, acrylamide can form in some foods during high-temperature cooking processes, including roasted coffee beans.
There are no legal limits on acrylamide in foods, only guidelines, Seitz says. "But roasting processes have been aligned European Union-wide, so as things now stand there's no need to worry if you drink normal amounts of coffee."
The acrylamide content in coffee depends on the brewing method. Since the chemical is highly water-soluble, its concentration increases the longer the coffee is in contact with water. Espresso contains considerably less acrylamide than French press, according to the BZfE, and instant contains the most.
"A certain habituation, and a certain addictive effect, can indeed occur with caffeine, since it's a drug in the broadest sense of the word," Wechsler says.
It's not truly addictive though, he adds, since abstaining from coffee doesn't result in severe withdrawal symptoms. "It's probably more of a psychological dependency."
"Actual physical addiction hasn't been proved," concurs Seitz, who also says the attachment is more in the coffee drinker's mind. However, he doesn't completely rule out a physical dependency "in cases of extremely high consumption over an extremely long period of time."
But this can't be tied to any particular amount. After all, as he says, every body is wired differently.