Thursday December 16 2010
Eating five a day lowers your risk of serious health problems
“At least 33,000 lives could be saved each year if the nation ate more healthily,” reported the Daily Mail. “Eating five portions of fruit and vegetables a day would by itself prevent 15,000 early deaths, including 7,000 from coronary heart disease, almost 5,000 from cancer and more than 3,000 from stroke,” the newspaper continued.
The story is based on research that modelled what would happen if the average diet in the UK followed the recommendations for food consumption. In 2007, none of the UK countries met these recommendations. The study supports previous findings that good diet, together with other measures such as regular exercise and stopping smoking, cuts the risk of chronic disease and increases lifespan.
As the authors note, the modelling technique has some flaws and involves considerable estimation and assumptions, which may have led to an overestimate of the number of lives that would be saved if healthy eating guidelines were met. However, despite these flaws and even though the study cannot predict how diet influences risk for individuals, it does indicate that keeping to dietary recommendations reduces the risk of disease.
Where did the story come from?
The study was carried out by researchers from the University of Oxford. Funding was provided by the British Heart Foundation and the National Heart Forum. It was published in the peer-reviewed Journal of Epidemiology and Community Health.
The study was covered accurately in the media. The BBC and the Daily Mail both included comments from independent experts.
What kind of research was this?
This study used a computer model, called the Dietron model, to measure systematically the health impact of dietary changes at a population level. While research has already shown that diet is associated with the risk of cardiovascular disease and cancer, the researchers point out that such a model is needed to estimate accurately the effects of measures taken to improve diet in the population. This model linked the consumption of specific dietary components with disease risk factors (for example blood pressure, cholesterol and obesity) and subsequent mortality from coronary heart disease, stroke and cancer.
Modelling studies such as this always involve a considerable degree of estimation of the effect that an intervention (in this case dietary change) will have on the overall level of disease in a population. The links between diet and the risk of disease were taken from research reviews that had pooled the results of different studies in this area. The individual studies pooled in the reviews are likely to have been in a variety of populations, and used different methods, dietary interventions and follow-up assessments. In addition, the individual studies may not all have taken into consideration other potential confounding factors, particularly lifestyle factors such as smoking and exercise. Therefore, the risk associations that were put into the model and then used to link diet with actual diseases (coronary heart disease, stroke and cancer) may involve some degree of inaccuracy.
What did the research involve?
The researchers needed to obtain several sets of data to fill their model. Data for UK deaths from coronary heart disease, stroke and cancer were obtained from the Office for National Statistics, the General Register Office for Scotland and the Northern Ireland Statistics and Research Agency. Information on the population’s intake of foods and nutrients was obtained from two sources: the average intake of fatty acids, fibre, and fruit and vegetables for 2005–7 was derived from the Expenditure and Food Survey, while estimates of salt intake came from the National Diet and Nutrition Survey, 2006.
The modelling also incorporated several meta-analyses of individual studies looking at diet and disease risk factors. The researchers looked at reviews that had pooled data from randomised trials, cohort studies or case-control studies, giving priority to meta-analyses of randomised trials. These different studies were combined in the model to calculate the change in risk of disease for an individual who changes his or her diet. To estimate the change in health outcomes with a change in diet at a population level, the model used the difference between current average consumption levels and recommended levels of different foods in the UK.
What were the basic results?
In a general summary of the main findings, the researchers calculated that:
- About 33,000 deaths a year would be avoided if UK dietary recommendations were met.
- There would be a reduction in deaths from coronary heart disease of 20,800 (95% credible interval 17,845 to 24,069), a reduction of 5,876 for deaths from stroke (3,856 to 7,364) and a reduction of 6,481 for deaths from cancer (4,487 to 8,353).
- About 12,500 of these avoided deaths would be in people aged 75 or under.
- About 18,000 of the avoided deaths would be men and 15,000 would be women.
- More than 15,000 of the avoided deaths (nearly half the total figure) would be due to increased consumption of fruit and vegetables.
- Reducing average salt intake to 6g a day would avoid 7,500 deaths annually.
- The greatest number of deaths avoided would be in Northern Ireland and Scotland, whose populations are furthest from achieving dietary recommendations.
How did the researchers interpret the results?
The researchers say their study suggests that increasing average consumption of fruits and vegetables to five portions a day is the target likely to offer most benefit in terms of deaths avoided. They also say that reducing recommended salt levels to 3g daily and saturated fat to 3% of total energy would achieve a similar reduction in mortality.
They conclude that their calculations based on the Dietron model are robust, pointing out that their estimate of deaths avoided is lower than a previous government survey which calculated that 70,000 deaths a year could be avoided if government dietary recommendations were met. The estimates could be used in calculating the allocation of resources for interventions aimed at reducing chronic disease.
This well-conducted modelling study used various data sources to link consumption of different dietary components with disease risk factors (for example blood pressure, cholesterol and obesity) and subsequent mortality from coronary heart disease, stroke and cancer. The study supports previous research showing that diet plays a crucial role in health and that a diet with plenty of fruit and vegetables, fibre and low fat and salt levels can reduce the risk of chronic disease, in particular coronary heart disease. However, its predictions are made at the population level. A model such as this cannot predict individual risk, which will depend on many factors, including family history, smoking and other lifestyle habits.
It is important to note that the figures are based on the estimates and assumptions made when using a mathematical model, and not on reality. As the authors themselves note, the modelling technique they used may have led to “some degree of double counting” and that, therefore, their estimate of reduced mortalities if dietary recommendations were met is likely to be an overestimate. Also, the accuracy of the model depends to some extent on the quality of the meta-analyses that were included, and the quality of the individual studies that were pooled within these reviews in order to establish associations between diet and particular disease risk factors.
Overall, this study supports current dietary recommendations and even though it cannot predict how diet influences risk for individuals, it does indicate that keeping to dietary recommendations reduces the risk of disease.
Dietary recommendations include eating five portions of fruit and vegetables a day (about 440g) and 18g of fibre (provided by wholegrain foods and some fruit and vegetables). It is recommended that salt intake is limited to a maximum of 6g a day and that a third of total energy is provided by fats, with saturated fat comprising 10%. The researchers point out that in 2007, according to the estimated average intakes in the sources they used, none of the UK countries met these recommendations.