"Eating more than five a day has 'no extra health benefit'," reports The Independent. The paper reports on a review that combined the results of previous research looking at the effect of increasing the amount of fruit and vegetables people eat.
One of the things they specifically wanted to look at was whether there is a dose-dependent effect. The study did find there was a 5% reduction in risk of death on average from any cause for each additional serving of fruit or vegetables a day.
However, a threshold was observed at around five servings a day, after which the risk of death did not reduce further.
This result would appear to contradict a UK study covered by Behind the Headlines in April, which suggested we should be eating seven a day to achieve the maximum benefit.
This earlier study was not included in the new review, so it is unknown what effect its findings may have had on the results.
Many of us struggle to eat at least five a day, let alone seven a day, so the results of both studies reinforce the importance of including lots of fruit and vegetables in our diet.
Where did the story come from?
The study was carried out by researchers from Shandong University and Huazhong University of Science and Technology in China, and the National Institutes of Health and Harvard School of Public Health in the US.
It was funded by the National Natural Science Foundation of China and the US National Institutes of Health. No conflicts of interest were reported.
The results of this study were well reported in the media. BBC News' and The Guardian's reporting of the study was particularly useful, as both included interviews with the researchers involved in the April study on fruit and vegetable consumption.
What kind of research was this?
The review aimed to examine and quantify the potential dose-response relationship (the effect of increasing consumption) between fruit and vegetable consumption and:
- risk of death from any cause
- risk of death from cardiovascular disease, such as heart attack or stroke
- risk of death from cancer
Systematic reviews aim to identify all the evidence related to a specific research question and synthesise the findings from individual studies or reports in an unbiased way.
Meta-analysis is a mathematical technique for combining the results of individual studies.
A systematic review, when performed well, should give the best possible estimate of the true effect of an association between fruit and vegetable consumption and risk of death.
However, this systematic review only included cohort studies, as randomisation of dietary habits is generally not feasible.
Cohort studies may suffer from confounding. As the researchers note, the link between fruit and vegetable consumption and risk of death could be related to people who eat more fruit and vegetables having a generally healthier lifestyle.
What did the research involve?
The researchers searched databases of published literature to identify prospective cohort studies that had looked at the association between levels of fruit and vegetable consumption and death (from any cause, cardiovascular causes, or cancer). Some, but not all, studies adjusted or controlled for other risk factors.
Once they had identified relevant trials, the researchers assessed them to see if they were well performed and then extracted data.
The results of all the trials were combined to produce a bottom line on the association between levels of fruit and vegetable consumption and risk of death.
What were the basic results?
Sixteen prospective cohort studies including 833,234 people were included in the systematic review. During follow-up periods ranging from 4.6 years to 26 years, there were 56,423 deaths (11,512 from cardiovascular disease and 16,817 from cancer).
Higher consumption of fruit and vegetables was significantly associated with a lower risk of death from any cause. For each extra serving per day of fruit and vegetables, the risk of death from any cause was reduced by 5%.
There was a threshold around five servings of fruit and vegetables a day, after which the risk of death from any cause did not reduce further.
The risk of death from cardiovascular disease also decreased with increasing fruit and vegetable consumption. The risk of death from cardiovascular disease was reduced by 4% for each extra serving per day of fruit and vegetables.
Higher consumption of fruit and vegetables was not appreciably associated with a risk of death from cancer.
How did the researchers interpret the results?
The researchers concluded that: "This meta-analysis provides further evidence that a higher consumption of fruit and vegetables is associated with a lower risk of all-cause mortality, particularly cardiovascular mortality."
This systematic review of cohort studies has found higher consumption of fruit and vegetables is associated with a reduced risk of death from any cause, with an average reduction in risk of 5% for each additional serving per day.
There was a threshold observed at around five servings per day, after which the risk of death did not reduce further.
Greater fruit and vegetable consumption was associated with decreased risk of death from cardiovascular disease, but higher consumption was not appreciably associated with death from cancer.
As many of the news stories point out, this threshold at around five servings per day is slightly different from the findings of an English study Behind the Headlines covered back in April.
This study concluded benefits were seen in up to seven or more daily portions of fruit and vegetables. But it was not included in the earlier systematic review, as it was published after the search for studies was completed.
Analysis by Bazian
Edited by NHS Website
Links to the headlines
The Independent, 30 July 2014
BBC News, 30 July 2014
The Guardian, 29 July 2014
The Times, 30 July 2014
ITV News, 30 July 2014
Links to the science
The BMJ. Published online July 29 2014