Wednesday September 1 2010
The DASH diet is low in fat and high in fruit and vegetables
A diet combining fruit and vegetables with foods such as fish, poultry and nuts “can protect you against heart attack”, reported The Independent.
The news is based on a well-conducted trial which tested the DASH diet, a diet high in fruit and vegetables but low in saturated fat that is recommended by the US government. The study enrolled 459 healthy people with slightly high blood pressure and randomly assigned them to follow the DASH diet, a high-fat “American” diet or an American diet supplemented with more fruit and vegetables. After eight weeks, the DASH diet lowered blood pressure and cholesterol, and decreased the participants’ risk of developing heart disease in the next 10 years more than the other diets.
This study has numerous strengths, but it only estimated future heart disease risk rather than monitoring participants over 10 years. Additionally, the risk of heart disease at the start of this study was very low at only 1%, and the DASH diet reduced this risk by only a minimal amount. Despite these small limitations, this study demonstrates the importance of blood pressure as a risk factor for coronary heart disease and the role a balanced diet might play in reducing this risk.
Where did the story come from?
This study was carried out by researchers from Johns Hopkins University, Baltimore. This particular trial was funded by the US National Center for Research Resources, and individual researchers received various other grants and research awards. The trial used data from a previous study, the DASH trial, which was sponsored by the US National Heart, Lung and Blood Institute. The study was published in the peer-reviewed medical journal Circulation.
In general, The Independent accurately reflected the findings of this well-conducted study, but did not mention some important limitations.
What kind of research was this?
This randomised controlled trial investigated the effects of dietary pattern on the 10-year risk of coronary heart disease (CHD). A randomised controlled trail is the best way of investigating a treatment’s ‘efficacy’, i.e. its effectiveness under ideal test conditions.
Dietary studies frequently have an inherent limitation in that it is difficult to accurately control how well a person adheres to the experimental diet being tested. However, this trial had the benefit of providing all the participants’ meals.
What did the research involve?
The researchers analysed findings of the Dietary Approaches to Stop Hypertension (DASH) trial, a previous study that assessed how various short-term dietary interventions affected high blood pressure. The trial had enrolled 459 healthy people with an average age of 45 and blood pressure that was on the high side of normal (average 131/85 mmHg) but not yet considered to be high. The researchers excluded participants with any significant illness, high cholesterol, any cardiovascular event in the previous six months or a BMI higher than 35kg/m2 (a BMI over 25kg/m2 is above ideal weight).
Participants were randomly assigned to follow one of three dietary patterns for eight weeks:
- a control diet: a “typical American diet”, high in saturated fat and cholesterol, low in minerals such as calcium and magnesium
- the F/V diet: rich in fruit and vegetables but otherwise similar to the control diet
- the DASH diet: rich in fruit, vegetables and low-fat dairy, and with a higher ratio of polyunsaturated fat to saturated fat than the other diets
Diets were reportedly prepared in research kitchens, with lunch and dinner prepared on site and breakfast provided for participants in a cooler to be eaten at home. Participants were also asked to record any additional items they consumed, including drinks and added salt. Blood pressure was measured on five occasions during the last two weeks of the study and the average measure was calculated. Cholesterol was also checked.
This subsequent study took the data from the DASH trial and applied the Framingham heart risk tool, a recognised method for predicting an individual’s risk of developing coronary heart disease (CHD). The researchers estimated the 10-year CHD risk of each participant at the beginning of the study and after eight weeks of their assigned diet. This heart risk calculation method takes into account several factors that can contribute to CHD risk, including gender, age, blood pressure, smoking status and diabetes.
What were the basic results?
At the start of the study, all participants had a low risk of developing CHD within the next 10 years (0.98% on average). The researchers found that, compared with the control diet, following the DASH diet for eight weeks:
- lowered blood pressure
- lowered total cholesterol
- lowered LDL (“bad” cholesterol)
- lowered HDL (“good” cholesterol)
At the end of the eight-week study period, there was no significant difference in 10-year CHD risk between the control and F/V diet groups. However, the DASH group had a significantly greater decrease in their 10-year CHD risk compared to the control group and the F/V group.
Over the course of the trial, participants in the DASH group had:
- an 18% decrease in their CHD risk compared to those in the control group (relative risk [RR] 0.82, 95% confidence interval [CI] 0.75 to 0.90)
- an 11% decreased risk compared to those in the F/V group (RR 0.89, 95% CI 0.81 to 0.97)
How did the researchers interpret the results?
The researchers concluded that the DASH diet, which was low in saturated fat and high in fruit and vegetables, decreased the 10-year CHD risk more than a diet high in fruit and vegetables alone or a typically American control diet that was high in saturated fat.
This well-conducted trial benefits from its relatively large size, accurate provision of the three randomised diets and high study completion rates (95%). It also featured a reliable study outcome by using the average of a series of blood pressure measures, which is preferable to relying on a single blood pressure reading.
The study found that eight weeks of the DASH diet, which was rich in fruit and vegetables and low in saturated fat, lowered blood pressure and cholesterol. This contributed to a decrease in predicted 10-year CHD risk. The DASH diet reduced this risk by 18% compared to a high-saturated fat “American” diet and by 11% compared to a diet similar to the American diet but with higher intake of fruit and vegetables.
Some points to note when interpreting this study include:
- All participants in this study had a low risk of developing coronary heart disease in the next 10 years (only about 1%). The DASH diet lowered this 1% risk by approximately one-tenth compared with the diet high in fruit and vegetables and by approximately one-fifth compared to the high-fat diet. Therefore, although the DASH diet lowered risk further, the overall risk remained low in all groups and the differences in risk between the groups were small.
- Although the tool used to calculate 10-year CHD is fairly reliable and commonly used, it is still only an estimate. The people were not followed up over 10 years to see whether they developed heart disease.
- This was only a brief eight-week intervention period. The effects of continuing these diets in the long term are unclear.
- The content of these diets is unclear. Although the newspapers reported that a more balanced diet involving nuts, chicken and fish was the most beneficial, the researchers did not describe the particular foods the participants ate, the calorific content or how much fat and cholesterol the diets contained.
- There were some differences between the three dietary groups at the start of the study. Those in the DASH group had a lower starting cholesterol level than participants in the F/V and control groups. This is an important difference as it could affect blood pressure.
- 60% of the participants in the trial were of African-American ethnicity, 35% were white and the remainder were of other ethnicity. Subgroup analysis also demonstrated that there was a greater decrease in CHD-risk in African-American participants. Therefore, it seems that these results are most applicable to this population group, which should be taken into account when generalising results to all ethnic populations.
- As the researchers acknowledge, this study was too small to reliably predict how the diets tested may affect other population subgroups, such as postmenopausal women, people with higher CHD risk or people with existing CHD.
Overall, this well-conducted study demonstrates the importance of blood pressure as a risk factor for coronary heart disease. It also supports the benefits of fruit and vegetables and low saturated fat as part of a healthy lifestyle to further modify the risk of heart disease.