Research Check: will a coffee a day really keep heart attacks at bay?

A recent headline in the Australian newspaper claimed “A short black a day can keep heart attack at bay”:

American scientists have unearthed fresh evidence that coffee exerts protective effects against heart failure and stroke.

According to the researchers, for every extra cup of coffee drunk per week, there was a 7% reduction in risk of heart failure and an 8% risk reduction for stroke.

So, is this more good news for coffee lovers, or a case of be careful what you read?

As the researchers explain in the media article:

We don’t know if it’s the coffee, compounds in the coffee or behaviour associated with drinking coffee.

The data comes from observational studies showing an association between coffee consumption, and heart failure and stroke. It does not prove causation. It shows that people who drank more coffee had lower rates of heart failure and stroke, not that drinking more coffee was responsible for reducing this risk.

There may be other reasons why people with heart failure and those who have had a stroke drink less coffee, for example, being on fluid restrictions for medical reasons, or not being able to move independently enough to make a cup of coffee.

Read more: Clearing up confusion between correlation and causation

That doesn’t mean you should avoid having another cup of coffee. A review of 20 observational studies from 2014 found those who drank the most coffee had longer life expectancies than those who drank the least or no coffee.

Again, these studies showed correlation not causation, but the evidence to suggest coffee is good for you is mounting.

How was the research conducted?

This story came from an abstract of a presentation to the American Heart Association’s 2017 Scientific Sessions on November 14. The researchers used data from more than 12,000 adults in the Framingham Heart Study to look for eating and drinking habits associated with heart disease.

The study used a powerful new statistical approach called random forest machine-learning methods. This uses all the individuals’ data to construct multiple decision trees and work out what the common patterns are when predicting their health outcomes. The researchers said this technique was a bit like the algorithms used in the marketing programs that predict our shopping behaviours.

The researchers confirmed that high blood pressure, high blood cholesterol and older age increased the risk of heart disease. They also identified that higher intakes of coffee predicted a lower risk of heart failure and stroke.

Lastly, the researchers created a statistical model that included the well-documented heart disease risk factors – age, sex, total and HDL (good) blood cholesterol levels, blood pressure, smoking and diabetes – that are used to calculate a person’s Framingham Risk Score. This is a person’s ten-year probability of developing cardiovascular disease, including stroke, heart failure and atherosclerosis (fatty deposits that clog arteries).

This analysis found that including coffee consumption in the equation improved the accuracy of the Framingham Risk Score in predicting heart failure and stroke by 4%.

The researchers reported finding similar trends – the 7% reduction in risk of heart failure and 8% risk reduction for stroke – in two separate studies.

What does this mean?

The study in the media headline was not about heart attack, it looked at heart failure and stroke, which are very different conditions:

• Heart attack is triggered by short-term lack of blood and oxygen to the heart muscle causing some muscle cells to die
• Heart failure means the heart can’t pump blood around the body adequately
• Stroke is when the blood supply to the brain is interrupted by either a blockage or a burst blood vessel.

This difference is important because while something might be good for the heart muscle itself, it’s not necessarily good for the blood vessels in the heart and brain.

Read more: Tom Petty died from a cardiac arrest – what makes this different to a heart attack and heart failure?

The data was from a conference abstract only. So it includes very limited details of the methods and results, and misses important information such as:

• which variables were adjusted for in the statistical analyses (external factors that might skew the results)
• how coffee intake was assessed
• whether decaffeinated coffee was included, and
• exactly how much was consumed each day or over the week.

While it’s great to hear about early research findings, the data has not gone through the full peer review publication process and so we will have to wait to eventually read the full paper.

Most importantly, this data comes from observational studies and shows an association between coffee consumption and heart health. It does not prove causation.

So is coffee good for your health?

If you are a smoker, it’s wise to avoid regular coffee. A review of the best evidence found a higher risk of lung cancer for smokers who drank regular coffee, although drinking decaffeinated coffee was suggestive of a lower risk.

Read more – Health Check: four reasons to have another cup of coffee

Among those with high blood pressure, caffeine in coffeedoes lead to an immediate increase in blood pressure that can last a few hours. However, there is no evidence of an overall higher risk of heart disease.

For a host of other reasons including a lower risk of type 2 diabetes, prostate cancer, liver cancer, and a longer life expectancy, drinking coffee is now on the list of things to consider to improve your overall health. – Clare Collins

Blind peer review

This is a fair and accurate assessment, and accords with the data from two studies published this year on death from any cause (and heart disease and stroke). –Ian Musgrave

Clare Collins is affiliated with the Priority Research Centre for Physical Activity and Nutrition, the University of Newcastle, NSW. She is an NHMRC Senior Research and Gladys M Brawn Research Fellow. She has received research grants from NHMRC, ARC, Hunter Medical Research Institute, Meat and Livestock Australia, Diabetes Australia, Heart Foundation. She has consulted to SHINE Australia, Novo Nordisk, Quality Bakers and the Sax Institute. She was a team member conducting the systematic reviews to inform the 2013 revision of the Australian Dietary Guidelines and the 2017 evidence review on dietary patterns and heart disease for the Heart Foundation.

Ian Musgrave receives funding from the National Health and Medical Research Council to study adverse reaction to herbal medicines and has previously been funded by the Australian Research Council to study potential natural product treatments for Alzheimer's disease. He has collaborated with SA water on studies of cyanobacterial toxins and their implication for drinking water quality.