Thursday March 19 2015
The intervention group were given a Mediterranean-style diet
"Sensible diet cuts heart attack risk in months," The Times reports after a randomised controlled trial found evidence that following current UK diet guidelines can reduce cardiovascular disease risk factors such as blood pressure and cholesterol levels.
We know that being a healthy weight and not smoking can help lower the risk of cardiovascular diseases such as heart attacks and strokes, but the evidence that healthy people benefit from low-salt, low-fat diets is weaker.
One of this study's strengths is its randomised design, which is actually uncommon with diet studies. This helps reduce the possibility that other factors influenced the results. Limitations include the fact this was a small study (165 participants) carried out over a relatively short time period.
Blood pressure and cholesterol measures are good indicators of the chances of someone having a heart attack or stroke, but not as reliable as waiting to see whether people in the study actually did so. It would be difficult (and possibly unethical) to do a dietary study that lasted long enough to show these outcomes.
The study suggests that if healthy middle-aged people follow current UK dietary recommendations, there may well be benefits, but we can't be sure of the size of the protective effect.
Where did the story come from?
The study was carried out by researchers from King's College London and was funded by the UK Food Standards Agency, the Department of Health, and the National Institute for Health Research.
It was published in the peer-reviewed medical journal The American Journal of Clinical Nutrition. It has been published on an open access basis, so it is free to read online.
A number of the authors have worked, or are currently working, for food manufacturers and medical companies, which could represent a conflict of interest.
The UK media reported the study with enthusiasm, with the Daily Mirror describing fruit and veg as a "lifesaver".
The Guardian did a good job of reporting the different outcomes, but did not report that some of the key measures did not show any improvement.
But none of the papers questioned how the overall "one-third" reduction figure mentioned by the researchers was calculated.
What kind of research was this?
This was a randomised controlled trial that compared the effects of following two types of diet.
One was based on a nutritionally balanced standard UK diet. The other included current UK nutritional guidelines, which recommend reduced salt, saturated fat and sugar intake, and an increased consumption of oily fish, fruit, wholegrain and vegetables.
Randomised controlled trials are a good way of comparing the true effects of a treatment or diet. However, the 12-week study could only look at the effects the diets had on markers such as blood pressure and cholesterol levels, but not long-term outcomes such as heart disease and stroke.
What did the research involve?
The researchers recruited 165 healthy non-smoking UK volunteers aged 40 to 70. They all had health checks at the start of the study, and were then split randomly into two groups. One group was asked to follow a standard UK diet while the others followed a diet based on healthy eating guidelines.
After 12 weeks, the health checks were repeated and the researchers looked for differences in blood pressure, cholesterol and other measures of heart attack risk that could have been caused by the different diets.
People selected to be in the study had an average risk of having a heart attack or stroke in the next 10 years.
The researchers ensured health check measurements, such as blood pressure, were reliable by using 24-hour blood pressure monitors rather than just taking one-off measurements.
Volunteers also had urine tests throughout the study so the researchers could estimate how well they were sticking to their allotted diets by checking their nutrient levels.
The dietary guidelines group were given dietary advice to help them reach the salt, fat, sugar and other targets in the healthy guidelines, and were advised to choose low-fat dairy products and lean cuts of meat.
The standard group were advised to eat a balanced "British" diet with no salt or sugar restrictions, based on bread, pasta and rice, potatoes with meat, limited oily fish, and wholegrain cereals. They were asked to eat full-fat dairy products.
Both groups were asked to limit their intake of sweets, cakes, biscuits and crisps, and to drink alcohol within safe limits.
Before the study began, the researchers agreed to look for three main outcomes, which they said would indicate an important change in people's heart attack risk. These were:
- a reduction in daytime systolic blood pressure of 4mmHg (the higher blood pressure figure, which shows the pressure of the blood when it is pumped out of the heart)
- a 5% change in the ratio of total cholesterol to HDL (or "good") cholesterol
- a 1% reduction in blood vessel stiffness (flow mediated dilation)
While they reported on many other outcomes in the study, these are the key ones to look at. The researchers reported the effects of treatment as the comparison between the diet groups at the end of the study, adjusted to take account of differences between the participants before the study began.
However, it is not clear from the study how the researchers calculated the overall reduction in risk of having a heart attack or stroke from all the changes combined.
What were the basic results?
The main result was that people who followed healthy dietary recommendations reduced their daytime blood pressure measurement by an average of 4.2mmHg compared with the standard diet group, which was more than the researchers had hoped.
However, the average change in cholesterol ratio was less than expected – 4%, below the hoped-for 5%. Though there was a 10% reduction in LDL cholesterol ("bad cholesterol).
The people following the dietary recommendations lost weight compared with the standard diet group (average difference 1.9kg), even though that was not the intention of the study.
How did the researchers interpret the results?
The researchers said the change in blood pressure was "remarkable" and would "suggest" a reduction in the risk of having a fatal stroke of 54%, as well as a 39% drop in the risk of getting heart disease, for people following the healthy diet, depending on age.
They attribute about half of the drop in blood pressure to the effect of eating less salt. They say the change in cholesterol levels for the dietary guidelines group, although "modest compared with drugs such as statins", would still reduce the risk of heart disease by about 6%.
They concluded that, "selecting a diet consistent with current dietary guidelines compared with a traditional United Kingdom dietary pattern" would be likely to cut the chances of having a heart attack or stroke for people in the general population by 30% based on previous research.
This study showed that following dietary recommendations closely for 12 weeks can reduce blood pressure by a significant amount, which is likely to cut the chances of having a heart attack or stroke for an average healthy middle-aged person. The diet also affects cholesterol levels, but the overall effect of this may be modest.
The study appears to have been carefully conducted to avoid biasing the results. The researchers gave butter or margarine spread and cooking oil to people in both groups, for example, and asked everyone to fill out food diaries, as well as taking urine samples for nutrient analysis.
This may have improved the chances of people sticking to the diet they were allocated to. The methods used to analyse blood pressure and other health checks were rigorous and likely to produce reliable results.
However, it is disappointing that the study report is not clear about how the researchers reached the headline figure of a one-third drop in the risk of a heart attack or stroke.
The report includes much detail about changes to individual risk factors, such as different ways to measure cholesterol, but does not explain how the researchers calculated the overall risk reduction.
That said, this is a well-conducted study that offers good-quality evidence of the effects of following the current UK dietary recommendations.
Analysis by Bazian. Edited by NHS Choices. Follow Behind the Headlines on Twitter. Join the Healthy Evidence forum.