However, the researchers uncovering the good news are all saying the same thing: while there appear to be some health perks from drinking coffee, there are also a few cautions, and the evidence is not solid enough to actively encourage people to go out and drink coffee.
Another reason to reserve some caution, is that although the evidence is shifting toward a more favorable view of coffee's effect on health, it is not based on cause and effect but on links for which there could be other explanations: it could be that regular coffee drinkers have something else in common, that studies have yet to discover, to account for the effect on health.
In this article we look at the shift in the research view on coffee consumption, touching on some of the key studies, and finish off with some facts and figures about coffee and caffeine.
Turning Point: Harvard Study 2008One of the turning points in media reporting on coffee and health came with the publication in 2008 of a Harvard-led study, that examined data on over 130,000 participants from the Nurses' Health Study and the Health Professionals Follow Up Study who were followed for about 20 years.
The results showed regularly consuming up to 6 cups of coffee per day (containing around 100 mg caffeine per 8 oz cup) was not linked with increased deaths in either men or women, from any cause, or death from cancer, or from cardiovascular disease.
This finding confirms the research picture that has been emerging in the last few years, says Rob van Dam, Assistant Professor in the Department of Nutrition, Harvard School of Public Health, one of the study researchers.
"For the general population, the evidence suggests that coffee drinking doesn't have any serious detrimental health effects," he adds.
Mayo Clinic preventive medicine specialist Donald Hensrud suggests one explanation for the apparent reversal in thinking about coffee, is that:
"Earlier studies didn't always take into account that known high-risk behaviors, such as smoking and physical inactivity, tended to be more common among heavy coffee drinkers at that time."
But perhaps what these more recent findings suggest, says van Dam, is that outside of certain groups, like pregnant women and those who have trouble controlling blood pressure, people should continue to enjoy their coffee in moderation and focus instead on other lifestyle factors, such as stopping smoking, getting more exercise, and eating more whole grains, as ways to reduce risk of poor health.
Heart Disease and StrokeSome of the evidence that has emerged in recent years suggests coffee consumption may lower the risk of heart disease and stroke.
A Kaiser Permanente study presented at an American Heart Association conference in March 2010, found coffee drinkers were less likely to be hospitalized for heart rhythm disturbances. The researchers examined data on of 130,000 health plan members and found people who reported drinking between one and three cups of coffee a day had a lower risk than non-drinkers, regardless of other risk factors.
Reported more recently, in 2012, a US study found that drinking coffee in moderation, may also protect slightly against heart failure.
For women, coffee drinking may mean a lower risk of stroke.
In March 2011, research led by the Karolinska Institute in Stockholm, Sweden, that followed over 30,000 women for 10 years, found those who drank more than one cup of coffee per day appeared to have a 22 to 25% lower risk of stroke, compared to non-drinkers. The researchers also found that "low or no coffee consumption was associated with an increased risk of stroke in women".
The findings were the same, regardless of other factors such as smoking, alcohol, body mass index, history of diabetes, and high blood pressure.
An earlier US study, published in 2009, involving 80,000 women from the Nurses' Health Study, had also found a 20% lower risk of stroke among coffee drinkers. None of the women had a history of stroke, coronary heart disease, diabetes, or cancer at the start of the study, and the researchers found the relative risk of stroke went down as coffee consumption went up.
However, a recent Harvard Health newsletter warns that while moderate coffee consumption (3 - 4 cups a day) may be linked to a lower risk for stroke, among infrequent coffee consumers the risk of a stroke just after drinking coffee could be higher.
One reason coffee consumption may lower longer term risk for heart disease and stroke, is because it appears to reduce the chance of developing type 2 diabetes, which is itself a risk factor for these diseases.
Type 2 DiabetesFrank Hu, nutrition and epidemiology professor at the Harvard School of Public Health, has been researching the effects of coffee on health since before the 2008 Harvard study, on which he was also a co-author.
In 2005, he and his team published a paper in which they reviewed nine studies covering more than 193,000 people in the US and Europe that examined the link between coffee intake and type 2 diabetes. Their analysis found participants who reported drinking more than 6 or 7 cups of coffee a day were 35% less likely to have type 2 diabetes, compared with those who reported drinking under 2 cups a day. For those drinking 4 to 6 cups a day, the risk was reduced by 28%.
More recently, in 2009, an international study led by researchers in Australia, reviewed 18 studies covering nearly 458,000 people and found that for every extra daily cup of coffee consumed, there was a 7% reduction in risk for developing type 2 diabetes. There were similar reductions for tea and decaf coffee. However, the researchers warned that some of the studies they reviewed were small and less reliable, so the link between heavy coffee drinking and reduced risk of type 2 diabetes may be exaggerated. They called for randomized trials to investigate their finding more robustly.
In a comment to WebMD in 2011, Hu describes the evidence on coffee and type 2 diabetes, based on more than 15 published studies, as "pretty solid", and now it appears decaf may have the same benefit.
In February 2012, researchers from Mount Sinai School of Medicine who did a study on mice, wrote how they discovered decaffeinated coffee may improve brain energy metabolism associated with type 2 diabetes. This brain dysfunction is a known risk factor for dementia and other neurodegenerative disorders like Alzheimer's disease. The researchers said the new findings were evidence that some of the non-caffeine components in coffee provide health benefits in mice.
Hu also speculates that caffeine is unlikely to be the reason for the link between coffee consumption and reduced type 2 diabetes risk, and the more likely explanation is "the whole package" of nutrients. For instance coffee is rich in antioxidants, which are known to prevent tissue damage caused by oxygen-free radicals.
Coffee also contains minerals such as magnesium and chromium, both used by the body to regulate insulin which in turns controls blood sugar. People with type 2 diabetes have lost the ability to use insulin to regulate blood sugar properly.
Alzheimer's DiseaseIn 2009, researchers in Finland and Sweden reported a study that followed over 1,400 people over 20 years, and found that those who drank 3 to 5 cups of coffee a day in their midlife years had a 65% lower chance of developing dementia and Alzheimer's disease compared with those who reported drinking no coffee at all or only occasionally.
In June 2012, researchers from the University of South Florida (USF) and the University of Miami, published a paper describing how they monitored the memory and thinking processes of 124 people, aged 65 to 88, and found all those with higher blood levels of caffeine (mostly from drinking coffee) avoided the onset of Alzheimer's disease in the 2-4 year follow up. This was even true of those who had mild cognitive impairment (MCI), a precursor of Alzheimer's.
Lead author Chuanhai Cao, a neuroscientist at the USF College of Pharmacy and the USF Health Byrd Alzheimer's Institute, and colleagues, have been publishing papers on the links between caffeine, coffee and Alzheimer's disease since 2006.
For instance in 2009, Cao co-authored two significant papers, with colleagues from USF and other research centers in the US and the Saitama Medical University in Japan, that found giving aged mice with symptoms of Alzheimer's the caffeine equivalent of five cups of coffee a day, reversed two signs of the disease: memory impairment and the hallmark protein in the animals' blood and brains.
Earlier studies at USF had already shown that giving caffeine to elderly people who did not have dementia quickly affected their blood levels of beta-amyloid (a protein that forms stickly clumps of plaque in the brains of people with Alzheimer's), and found the same thing happened in the Alzheimer's mice.
"We are not saying that moderate coffee consumption will completely protect people from Alzheimer's disease. However, we firmly believe that moderate coffee consumption can appreciably reduce your risk of Alzheimer's or delay its onset," Cao told the press at the publication of the June 2012 paper.
While they believe caffeine to be the key, Cao and colleagues don't think it acts alone, but in conjunction with another, yet to be identified component in coffee, that boosts blood levels of a critical growth factor that seems to fight off the Alzheimer's disease process.
Parkinson's DiseaseFor Parkinson's Disease, another neurodegenerative disorder, it appears there is also a link between higher coffee consumption and decreased risk. And like Alzheimer's, this also seems to be due to caffeine, but it is less clear how it works. However, one study of caffeine and risk of developing the two diseases that was published in Journal of Alzheimer's Disease in 2010, by Xuesong Chen and colleagues from the University of North Dakota in the US, suggest it might be something to do with a protective effect that caffeine has in preserving the blood-brain-barrier.
In another Journal of Alzheimer's Disease study also published in 2010, João Costa of the University of Lisbon, Portugal, and colleagues present an analysis of 26 studies that suggests an inverse association between coffee drinking and the chance of developing Parkinson's disease. For every increase of 300 mg per day in caffeine intake, they found a drop of 24% in the relative risk of developing Parkinson's. Among those who regularly drank two to three cups of coffee a day, there was a 25% lower chance of developing the disease compared to non-coffee drinkers. However, among women coffee drinkers only, this fell to 14%. The researchers said their findings could "hardly by explained by bias or uncontrolled confounding".
CancerStudies have also suggested coffee consumption is linked to a lower risk for some cancers, including endometrial, aggressive prostate, estrogen-negative breast cancer, liver cancer, and a common form of skin cancer, but not others (eg esophageal).
In 2011, researchers working with data from the Nurses' Health Study published findings that showed coffee drinkers who consumed more than four cups a day had a 25% lower risk of developing endometrial cancer.
Senior researcher Edward Giovannucci, professor of nutrition and epidemiology at the Harvard School of Public Health, said coffee is starting to emerge as a protective agent in cancers that are linked to obesity, estrogen and insulin.
He and his colleagues suggest antioxidant and anti-inflammatory substances in coffee may be responsible for anti-cancer activity. Giovannucci said lab tests show coffee has more antioxidants than most fruits and vegetables.
Giovannucci was also co-author of another 2011 study that found men who regularly drink coffee appear to have a lower risk of developing an aggressive, lethal form of prostate cancer. They also found the lower risk was the same for caffeinated as for decaffeinated coffee.
A link with coffee consumption and lower risk of estrogen-negative breast cancer was made in a Swedish study that also appeared in 2011.
When they first looked at their data, the researchers from Karolinska Institute found women who drank coffee had a lower incidence of breast cancer than women who rarely drank coffee, but when they took into account other risk factors, including lifestyle and age, they found the lower risk was only measurable for estrogen-negative breast cancer.
The case for linking reduced risk of liver cancer to coffee drinking has been building steadily for a while.
In 2007, a study led by the Mario Negri Institute for Pharmacological Research in Milan, Italy, that did a pooled analysis of ten studies that included over 2,200 people with hepatocellular carcinoma (HCC), reported that among coffee drinkers overall, there was a 41 per cent reduction in risk of HCC compared to those who never drank coffee. HCC is the most common liver cancer and accounts for about 90% of them.
But the researchers concluded that while they found this link, they could not say if it was coffee that was reducing the risk of liver cancer, or if it was that people with liver cancer tended to drink less coffee for other reasons.
Then in 2008, there followed the publication of a new large, prospective population-based study involving over 60,000 Finns followed for a median of 19 years, that confirmed higher coffee consumption was linked to lower risk of developing liver cancer.
Researchers led by Gang Hu at the University of Helsinki noted a significant inverse relationship between coffee drinking and the risk of primary liver cancer. The more coffee people drank, the lower their risk. But the authors said the biological mechanism behind this link was not known, and in an accompanying editorial, Carlo La Vecchia of Milan said that while the study solidly confirmed the link between coffee drinking and lower risk of liver cancer, it "remains difficult" to translate it into potential ways to prevent of liver cancer by increasing coffee consumption.
More recently, a large US study of over 110,000 people found that the more caffeine there was in their diets, the lower their risk of developing basal cell carcinoma, the most common type of skin cancer.
PregnancyPregnant women are advised not to drink too much coffee. In 2010, the American College of Obstetricians and Gynecologists (ACOG) put out a statement that said drinking less than 200 mg of coffee a day (which they equate to 12 oz of coffee), doesn't seem to increase risk of miscarriage, or premature delivery, but above this level it is not clear what the risks might be.
William H Barth, Jr, chair of ACOG's Committee on Obstetric Practice, told the press:
"After a review of the scientific evidence to date, daily moderate caffeine consumption doesn't appear to have any major impact in causing miscarriage or preterm birth."
"Given the evidence, we should reassure our pregnant patients and let them know that it's OK to have a cup of coffee," he added.
The ACOG statement pointed out that caffeinated tea and soft drinks also contain caffeine, although less than coffee, and so do chocolate candy bars.
Global Consumption of CoffeeCoffee is one of the most widely consumed beverages in the world, and the most commonly consumed psychoactive drug. Some its behavioral effects, such as arousal, are not dissimilar to those of other stimulants like cocaine and amphetamines.
From 1950 to 1990, world production of coffee doubled, and despite the economic downturn, consumption is still rising. Overall, the world consumes about 7.4 million metric tons a year, or 1.3 kg per person per year, ranging from nearly zero in countries like China and India, to upwards of 7 kg per person per year in Switzerland, Iceland and Scandinavia, with the Finns being the biggest coffee consumers (12 kg per person per year).
The United States consumes 1.3 metric tons per year, at a rate of 4.2 kg per person. The latest figures for 2012 suggest 65% of American adults drink coffee, placing the beverage "neck and neck with soft drinks", says the National Coffee Association.
How Much Caffeine in a Cup of Coffee?It can be confusing when you read about coffee consumption in cups because of the difference in cup sizes from country to country. For instance, in the US, coffee is typically served in an 8 oz (240 ml) cup, which is twice the amount in a typical European serving.
It can be even more confusing if you want to know how much caffeine is in a cup, because that varies depending on the beans, how they are roasted, and how the coffee is prepared.
For example, a restaurant-style serving of Espresso in a 1 oz (30ml) cup can contain from 40 to 75 mg of caffeine. Even a decaffeinated Espresso can contain up to 15 mg of caffeine.
On the other hand, an 8 oz (240 ml) cup of generic instant coffee can contain any amount from 27 to 173 mg of caffeine, while a Starbucks Pike Place 16 oz (480 ml) cup of brewed coffee contains 330 mg of caffeine.
Tea has about half as much caffeine as coffee.
How Much Is a Moderate Intake of Caffeine?A moderate intake of caffeine is probably around 300 mg per day. This is roughly 3 to 4 cups of ground roasted coffee or 5 cups of instant.
For pregnant women, this level would be considered excessive, and they are advised to keep their consumption below 200 mg a day.
Coffee is not the only source of caffeine in the diet. 300 mg is also the amount of caffeine in 5 or 6 servings of tea and some colas, and the average chocolate candy bar has about 35 mg.
Some painkillers also contain caffeine, because it can make them 40% more effective in treating headaches. The range here also varies; for instance, from 16 mg per capsule (Dristran) to 200 mg (Vivarin), among popular over-the-counter painkillers in the US.
In the American diet, coffee accounts for about 75% of the adult intake of caffeine.
Caffeine is probably the most investigated substance in coffee, but there are many others, which is probably why coffee seems to have good sides and bad sides, and the overall effect may depend on how much they cancel each other out.
After being absorbed in the stomach and small intestine, caffeine travels to the rest of the body and the brain. The amount in the bloodstream peaks about 30 to 45 minutes after ingestion, petering out some 10 hours later, as it is metabolized in the liver.
As yet no study has said that coffee does more good than harm and therefore drinking it should be recommended. But perhaps that is just a matter of time, and the meantime, those who thought they should give it up for the good of their health, unless their doctor advises them differently, can continue to enjoy it, and focus on other ways to improve health.
Written by Catharine Paddock PhD