Now the scientists are turning their attention to the cafetiere and the espresso machine. The theory is the same, that the basic process of immersing ground coffee in boiling water can leach high concentrations of chemicals called terpinoids into the drink.
Two particular terpinoids – kahweol and cafestol – have been well known for many years to be capable of substantially increasing LDL cholesterol, even more than the best known culprit, saturated fat.
Cafestol, which is only found in coffee beans, is believed to affect the body's ability to metabolise and regulate LDL cholesterol, with some scientists dubbing it 'the most potent cholesterol-elevating compound identified in the human diet.' LDL cholesterol is known as the 'bad cholesterol' as high levels have been found to raise the risk of heart disease and stroke.
"It is known that some compounds in coffee, such as cafestol, can increase blood cholesterol," says Tracy Parker, senior dietitian at the British Heart Foundation. "Unfiltered coffees such as French press, Turkish or Scandinavian boiled coffee are linked to high levels of cafestol, while negligible levels are found in drip-filtered, instant and percolator coffee."
Online coffee aficionados have already suggested some hacks, such as getting rid of cafestol by running your cafetiere coffee through a paper filter, or using espresso pods, which contain lower quantities of the chemical.
However, while there is some credence to the Tromso Heart Study's findings, a lot of the risk posed by either cafetiere-prepared or espresso coffee depends on how much you are drinking. Research suggests that you can only really consume enough cafestol to have a serious impact on your cholesterol if you are drinking excessive amounts of coffee.
As an example, one study found that drinking coffee made in a cafetiere could raise your blood cholesterol levels by between five and eight per cent, but this was based on people who were drinking five cups of coffee every day, for four weeks. The Tromso Heart Study's findings were most significant for people who consumed six cups of espresso or cafetiere coffee per day.
"It does not really matter what type of coffee you drink if you only have one or two cups a day but it is important if you drink more," says Tom Sanders, emeritus professor of nutrition and dietetics at King's College London.
"For most people, a moderate amount of coffee, three to five cups a day, is fine," says Parker. "Consuming more coffee seems to be linked to higher cholesterol. In randomised control trials, six cups of unfiltered coffee per day compared with filtered coffee predicted an 11 per cent higher risk of major cardiovascular events."
Scientists also point out that a lot of this research is observational rather than causal. People who drink any form of coffee excessively may also have a more stressful lifestyle, eat unhealthily, or do not get sufficient sleep, which could all be having a greater impact on their cholesterol than the coffee itself. There's also the question of how coffee is consumed – for example, with milk or cream or sugar or while smoking or before exercise. These things can interfere with the findings.
"The observed associations may be attributable to confounding environmental or lifestyle factors," says Dipender Gill, NIHR clinical lecturer in clinical pharmacology and therapeutics, St George's, University of London. "Any preferred method of brewing coffee may also be associated with lifestyle traits that affect cholesterol levels."
As an example, people who brew coffee in a cafetiere may be more likely to live a more sedentary lifestyle at home.
Research has even suggested that moderate amounts of coffee can have positive effects on health – some of the claims which have been made include a reduced risk of type 2 diabetes and liver disease, although this remains contentious.
Dieticians say that the main categories of people who should be wary about drinking coffee are those who already have high blood pressure or an abnormal heartbeat, while pregnant or breastfeeding women should only drink coffee in moderation.
For people who are concerned about their coffee drinking habits, Duane Mellor, a dietician and researcher at Aston Medical School, suggests it is more worthwhile to consider cutting down on many of the ingredients we commonly add to coffee which tend to be high in saturated fat.
"Overall, moderate intake of coffee – up to about three cups per day, with a cup being 150-200ml – seems not to be linked to increased risks," says Mellor. "It is also worth remembering that it's often what else goes in our coffee, such as sugar, flavoured syrups and cream, or with it in the form of snacks and cakes, which can have a greater impact on our health."
