Coffee contains a complex mix of molecules, including caffeine, antioxidants, and carcinogenic substances.
Despite this, there are also more cautious studies. These tend to report on the risks of consuming caffeine, especially in children and young people, pregnant and lactating mothers, and individuals with underlying health conditions.
As you are sipping your first or your umpteenth cup of the day, find out what the latest research says about the safety of coffee and what coffee does to our bodies to cause the health miracles that everyone is talking about.
How much caffeine are we consuming?
Caffeine occurs naturally in tea leaves, coffee beans, and cocoa beans. Synthetic caffeine is also added to other drinks, some food products, and certain medications.
The levels of caffeine in coffee depend on the type of coffee used and how it was prepared. An average 8-ounce cup of brewed coffee contains between 95 and 165 milligrams of caffeine, while a single shot of espresso contains between 47 and 64 milligrams.
In the United States, 89 percent of adults consume caffeine - mostly in the form of coffee, tea, and, to a lesser extent, soft drinks. Men consume slightly more caffeine (on average, 240 milligrams per day) than women (who drink, on average, 183 milligrams per day).
Energy drinks make up only a small proportion of the caffeine intake of adults, but consumption did increase significantly between 2001 and 2010.
The majority of U.S. children (79 percent) also consume caffeine, with older children consuming more than younger ones.
Children below the age of 12 tended to consume caffeine in the form of tea, soda, and flavored dairy products, while older children mostly consumed coffee.
What happens inside our bodies?
Coffee contains a remarkably complex collection of molecules. Chief among them is, of course, caffeine, the central nervous system (CNS) stimulant that helps many of us to wake up in the morning and get through our day.
But there are also other antioxidant substances that help to mop up free radicals in our cells and activate DNA repair, as well as anti-mutagen molecules that stop cancer-causing DNA mutations from occurring. However, it's not all good news: coffee also contains carcinogens.
So what happens to the caffeine? Caffeine spreads throughout the body after it is taken up in the intestine. It takes quite a long time to be metabolized, which means that it is present in our bodies for some time after we consume it.
How we metabolize caffeine depends on our age. The half-life - that is, the amount of time taken to break down half the caffeine in our system - in adults is estimated to be between 3 and 7 hours.
But in newborns this is much higher, with numbers cited to be in the range of 65 to 130 hours.
Genetic variations make some people more susceptible to the effects of caffeine, by affecting both how quickly it is broken down and by how strong an effect it has on organs. Other things also affect caffeine metabolism.
The enzymes that break down caffeine are also responsible for breaking down steroids. In women, oral contraceptives are thought to double the half-life of caffeine in the body. In pregnant women, caffeine also stays in the body significantly longer.
What happens to the other molecules in coffee? The chief antioxidant and flavor molecules in coffee are chlorogenic acids. These are also found in apples, pears, and other fruits, as well as in vegetables and plants.
They are absorbed in the intestine and partly metabolized by our gut microbes. They are now able to exert their strong antioxidant effects on a range of cells.
The health benefits of coffee have been attributed to both the caffeine and the other antioxidant molecules in the drink. But what is the scientific evidence behind these claims?
The science behind the health benefits
A recent study compared the effect of coffee and caffeine from other sources on the length of telomeres, which are caps that protect the ends of chromosomes during cell division. Telomeres are used as a measure of aging; the structures get shorter when cells age.
Short telomeres are associated with a higher risk of death. As caffeine consumption increased in U.S. adults, telomeres were significantly shorter.
However, with increasing coffee consumption, telomeres were longer. Coffee consumption might therefore prolong life, while caffeine consumption could shorten it.
But caffeine itself has also been linked with potential health benefits. For example, researchers recently showed that caffeine can boost the levels of an enzyme that might prevent Alzheimer's disease.
Others have investigated the anti-inflammatory properties of caffeine and its metabolites. Older adults who had lower levels of inflammation were healthier than those with higher levels of inflammation, and the low inflammation group had significantly greater levels of caffeine metabolites in their blood.
If coffee is this good for us, shouldn't we all be drinking it?
Caffeine consumption is thought to be safe for most consumers - but not for all. The groups most at risk of potential negative effects from caffeine consumption include, as aforementioned, expectant mothers and lactating women, children and teenagers, and possibly patients with underlying conditions, such as cardiovascular disease.
A comprehensive review recently published in Frontiers in Psychiatry examined the adverse effects of caffeine in these groups.
In women trying to conceive, caffeine consumption greater than 400 milligrams per day was associated with an 11 percent increased risk of spontaneous abortion. Pregnant mothers who consumed caffeine were at greater risk of giving birth to a baby with low birth weight.
But there was some discrepancy between the levels reported by different studies, ranging from 50 milligrams per day to 300 milligrams per day, before this effect was seen. How this occurs has not been established and may either be due to caffeine or other factors.
When lactating mothers drink coffee, some of the caffeine is passed on to the baby through breast milk. As caffeine metabolism is much slower in babies than in adults, even small levels can have significant effects on the baby. However, studies are not conclusive.
Some studies reported increased irritability in babies, especially when mothers consumed very high levels of coffee, such as 10 cups or more per day. Research on other outcomes in infancy, such as IQ and childhood obesity, was largely inconclusive, as there are contradictory reports.
Children and other high-risk groups
Young children are also vulnerable to the effects of caffeine because they weigh less than adults. This means that when they do consume caffeine, its concentration in the body is higher per kilogram of body weight, and its effects will last longer. This is because children's bodies metabolize caffeine more slowly.
There is some evidence of sleep disturbance, violence, and anger in teens who consumed caffeine daily. Also, energy drink consumption in children and teenagers has been linked to changes in cardiovascular function, such as high blood pressure, irregular heart rate, and increased risk of severe cardiac events.
In fact, the American Academy of Pediatrics recommend that children be discouraged from consuming caffeine and should never drink energy drinks.
The jury is also still out on whether patients with cardiovascular disease should avoid coffee or not. Scientists disagree over the evidence, with some studies disputing the link between caffeine and irregular heartbeat and others confirming it.
But there is evidence to suggest that caffeine can interact with some medication taken for high blood pressure, making the drug less effective.
For healthy adults, however, the news is good. Moderate consumption of around 400 milligrams of caffeine per day does not seem to pose any health risks and may contribute to overall health and longevity.
So, if you are enjoying your first cup of the day, you can relax. If you are on your umpteenth cup, however, you may want to think about cutting back.