“Two cups of coffee a day can reduce the risk of heart failure (but five can be bad for you),” the Daily Mail reported today. This seemingly contradictory headline is actually a fair but slightly overenthusiastic reflection of a review of research that looked at the relationship between regular coffee drinking and risk of heart failure.

Heart failure is a condition  in which the heart cannot pump blood to meet the body’s demands, this can have many causes, the most common being a heart attack. This review of five large studies questioned people about how much coffee they drank and then examined whether they went on to develop heart failure. The combined results suggested that, compared to no coffee consumption, four to five servings of coffee a day was associated with an 11% lower risk. Drinking excessive amounts of coffee had no benefit – and is likely to give you the jitters.

Because some of the studies included people who had already had a heart attack, it is not clear whether some may have already had some stage of heart failure when their coffee drinking was assessed.

Although a systematic review can provide good evidence, the results of this one should be viewed with some caution. All of the included studies relied on self-reported coffee drinking and this can affect the reliability. Coffee consumption was only measured once and did not take into account any changes in consumption over the years. It is also unclear how the studies defined the outcome of heart failure. Because some of the studies included people who had already had a heart attack, it is not clear whether they were currently in some stage of heart failure when their coffee drinking was assessed. The studies were inconsistent in how they adjusted for other factors that can be related to heart failure risk, such as high blood pressure and obesity.

The type, strength and volume of coffee consumed by people in this study will also have varied widely, making it hard to get any clear message about the benefits or risks of coffee consumption.

Overall, this review does not prove that coffee is good for the heart - only a possible association between coffee drinking and the risk of heart failure. The relationship between coffee consumption and heart health remains confusing, with previous studies showing inconsistent results. As in most things, moderation is key.