Wednesday April 30 2014
Fibre can help lower cholesterol levels
"If you have had a heart attack, eat plenty of fibre," is the advice on the BBC News website. A US study has found that in a group of people who'd had heart attacks, those who ate a high-fibre diet were less likely to die than those who didn't.
This was a large long-term study using data from health professionals who'd had a heart attack. The researchers looked at whether people's reported fibre intake was associated with a reduced risk of death subsequently.
They found that the fifth of people who ate the most dietary fibre after a heart attack had a 25% lower risk of death than in the fifth of people who ate the least.
The study cannot prove that the dietary fibre reduced the risk of death as other factors could have been responsible, but many of these were taken into account.
However, a limitation of the study is potential inaccuracies in people's dietary recall, and it's unclear if the results would apply to the general population.
Overall, this study adds to the body of evidence proving the benefits of fibre as part of a healthy lifestyle.
Where did the story come from?
The study was carried out by researchers from Harvard School of Public Health, Brigham and Women's Hospital, and the Beth Israel Deaconess Medical Centre in the US, and was funded by the US National Institutes of Health.
It was published in the peer-reviewed British Medical Journal on an open access basis, so it is free to read online.
The story was reported accurately by BBC News.
What kind of research was this?
This was a prospective cohort study using data from two large US cohort studies – the Nurses' Health Study and the Health Professional Follow-up Study. It aimed to see if increased dietary fibre was associated with improved survival rates in people after they had a heart attack.
As it is a cohort study, it cannot prove causality – that the increased dietary fibre prolonged survival. But it can show an association with increased survival rates in people who eat more fibre.
There may be other factors (confounders) that are responsible for the association seen. A randomised controlled trial would therefore be required to show causality, but would be unfeasible over a long study period.
What did the research involve?
Researchers analysed data from more than 4,000 US health professionals who were participating in two cohort studies that had collected information over at least 22 years. They assessed dietary fibre intake before and after these people had suffered from a heart attack and calculated their survival rates, taking other factors into account.
The researchers used data from the Nurses' Health Study (a prospective cohort of 121,700 US nurses, which began in 1976) and the Health Professional Follow-up Study (a prospective cohort of 51,529 US male health professionals, which started in 1986). Participants filled in lifestyle and medical history questionnaires twice a year.
Food frequency questionnaires estimating average food consumption over the previous year were filled in every four years.
From these studies, researchers used the data of 2,258 women and 1,840 men who:
- had no cardiovascular disease, stroke or cancer when the studies began
- survived a heart attack during the study period
- had not had a stroke before the heart attack
- had not died before returning the next food frequency questionnaire after the heart attack
- had fully completed questionnaires
The researchers used the food frequency questionnaires to calculate the amount of dietary fibre that was usually consumed before and after the heart attack. They confirmed self-reported heart attack using medical records.
The researchers looked at how dietary fibre was associated with risk of death from any cause and with risk of death because of cardiovascular disease. Deaths were identified from vital records, the national death index, next of kin or the postal system. Cause of death was identified from medical records and autopsy reports.
They performed numerous statistical analyses to take into account other factors, such as medication use, medical history and lifestyle, including smoking history, alcohol consumption, physical exercise and calorie intake.
The following factors did not appear to have any effect on the results, so they were not accounted for in the final analysis:
- family history of heart attack
- protein intake
- polyunsaturated fat intake
- multivitamin use
- blood sugar levels
- features of the heart attack
What were the basic results?
The 2,258 women and 1,840 men were followed up for an average of nine years after their heart attack. During this time, 682 women and 451 men died.
After adjusting for possible confounders, 10g higher dietary fibre intake per day after a heart attack was associated with a 15% lower risk of death from any cause (hazard ratio [HR] 0.85, 95% confidence interval [CI] 0.74 to 0.97).
When men and women were analysed separately, the reduction in risk was of borderline statistical significance for women and not statistically significant for men:
- 28% lower for women, HR 0.82 (95% CI 0.68 to 1.00)
- not significant for men, HR 0.88 (95% CI 0.72 to 1.07)
The researchers then compared people in the highest fifth for consumption of dietary fibre after heart attack with people in the lowest fifth. People in the highest fifth had a 25% lower risk of death from any cause (HR 0.75 [95% CI 0.58 to 0.97]) compared with the lowest fifth.
Again, when men and women were analysed separately, the reduction in risk was only significant for women:
- 29% lower for women, HR 0.71 (95% CI 0.51 to 0.98)
- not significant for men, HR 0.82 (95% CI 0.54 to 1.25)
Cereal fibre was the only type of fibre that was significantly associated with lower all-cause mortality: a 27% lower risk when combining male and female results, HR 0.73 (95% CI 0.58 to 0.91).
The researchers also looked at associations with changes in fibre intake after a heart attack. People who increased their fibre intake by 10g after their heart attack had a 33% reduced risk of death from any cause, HR 0.77 (95% CI 0.68 to 0.90).
How did the researchers interpret the results?
The researchers concluded that, "Greater intake of dietary fibre after MI [myocardial infarction, or heart attack], especially cereal fibre, was inversely associated with all-cause and cardiovascular mortality. In addition, participants who increased their dietary fibre intake after MI had lower long-term rates of all-cause and cardiovascular mortality."
This was a well-designed study. Although it was a cohort study and so cannot prove causation, attempts were made to analyse the results while taking multiple factors into account.
Its strengths include that it used data from a large number of people and measured dietary habits over the previous year, which may be a more accurate assessment than snap-shot 24-hour food questionnaires. However, there will still be room for bias in people's recall and estimates of portion size.
There were a few limitations to the study:
- it was not able to take into account people who suffered from a heart attack and died before the next scheduled food frequency questionnaire, which may have been up to four years after the heart attack
- the participants were all health professionals who fully completed the dietary questionnaires, as those who did not fully complete them were excluded – this may indicate that the participants were more likely to take an interest in their health and so the results may not be applicable to the general population
- the researchers point out that it may be other factors related to fibre intake that are having beneficial effects, such as vitamins, minerals, antioxidants and phytochemicals
- they also acknowledge that people may have changed other aspects of their lifestyle after having a heart attack, which helped to lower their risk of death
With these limitations in mind, this study adds to the body of evidence that shows the benefits of dietary fibre as part of a healthy lifestyle.
Read more advice about recovering from a heart attack.
Analysis by Bazian. Edited by NHS Choices. Follow Behind the Headlines on Twitter. Join the Healthy Evidence forum.