Are vegetables as good for your heart as we thought?

Vegetables alone may not protect against developing heart and circulatory disease, according to new research.

A healthy diet is important to help to reduce the risk of developing many diseases, including heart and circulatory conditions. UK advice is that we should all eat eat at least five portions of fruit and vegetables every day as part of a balanced diet to maintain good health.

Evidence from previous research into the effect of eating vegetables on developing heart and circulatory conditions has been mixed. Researchers from the University of Oxford and elsewhere wanted to find out whether, after accounting for all the other factors that can influence health and wellbeing, eating vegetables does really stop you developing heart and circulatory disease.

They looked at whether there was any association between how many vegetables people ate (at the start of the study) and their risk of developing or dying from a heart or circulatory condition over the following 12 years.

The researchers found that, compared to those with the lowest intake, people with the highest total vegetable intake were 10 per cent less likely to develop a heart or circulatory condition. Eating raw vegetables seemed to have a stronger protective effect than cooked vegetables. They also found that people who consumed two or more heaped tablespoons of vegetables per day were less likely to die from a heart or circulatory condition.

However, when the researchers controlled their analysis for other factors that can affect your risk of developing a heart or circulatory condition, such as socioeconomic status, physical activity levels, smoking and alcohol consumption, they found that most of the protective effect of eating vegetables disappeared.

The researchers say that they “cannot rule out the possibility of a true causal protective effect” – in other words, we can’t draw firm conclusions from this study. They also say that eating vegetables is still likely to be protective against some other diseases.

How good was the research?

The research used data from 399,586 people who didn’t already have a heart or circulatory condition. These large numbers and relatively long follow-up are a major strength of the study. The data is from the UK Biobank study, which is linked to electronic health records, so the researchers can be fairly confident that they will have captured a true picture of heart and circulatory conditions in the group.

Despite this, there are a few important limitations to this research. Participants were only asked about their daily vegetable intake when they first signed up to the UK Biobank study. This was assumed to be their average intake over the whole 12 year follow-up. However, we can’t know for sure whether they estimated their vegetable intake accurately at the start, or whether the amount of vegetables they ate changed during that time.

In their paper the researchers say that the remaining protective effects of increased vegetable intake which the research found are likely to be explained by ‘residual confounding’. This means that although they adjusted for factors that could have influenced the results (like how much exercise they did, and how comfortably-off they were) they weren't able to account for every factor. However, some of the experts who commented on the research said it was also possible that the number and type of factors that were adjusted for could have meant that vegetables appeared to be less protective than they are.

The researchers themselves say that future research should be aware of the importance of assessing for these residual confounders or should use approaches that are less likely to be affected by confounding factors.

The average age of participants in this study was 56 at the outset (people were aged between 40 and 69 at enrolment). This means which means we can’t be certain that these findings will apply to people of all ages.

It’s also important to remember that this type of study can only show an association between a variable (the amount of vegetables eaten every day) and an outcome (developing a heart and circulatory condition) – it can’t show cause and effect.

How good was the media coverage?

The study was covered by the BBC, Mail Online, The Times and The Telegraph.

Overall the coverage of this study was good and offered a balanced summary of the research and its findings. Headlines from the BBC and The Telegraph emphasised that this study shows that a diet high in vegetables alone can’t guarantee long-term heart health.

All of the coverage included the recommendation that everyone should eat at least five portions of fruit and vegetables each day along with a quote from an independent expert who stressed that the findings from this research should not change current dietary advice.

The BBC, The Times and The Telegraph also included comments from experts not involved in the research which highlighted the difficulties with these types of studies, and pointed out that we can’t be sure whether these findings are totally reliable.

The BHF verdict

Victoria Taylor, Senior Dietitian at the British Heart Foundation (BHF), said: “Although this study found that eating more vegetables wasn’t associated with a lower rate of heart and circulatory diseases once other lifestyle and other factors were taken into account, that doesn’t mean we should stop eating vegetables.

“Fruit and vegetables remain an important source of vitamins, minerals and fibre, which make them an essential part of a healthy, balanced diet.

“There are multiple factors in our diet and lifestyle that can influence our chance of developing cardiovascular disease, and we need to consider all of these when it comes to lowering our overall risk. Adopting a whole diet approach to the way we eat – such as the traditional Mediterranean-style diet – as well as not smoking, being physically active and managing our weight, are important to reducing the risk of heart and circulatory conditions.”

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