Thursday January 31 2013
It is important for vegetarians to eat a balanced, nutritious diet
A “vegetarian diet reduces heart disease risk by up to a third”, Channel 4 News reports. The news, which has been reliably covered by much of the media, is based on an impressive and wide-ranging UK study on nutrition.
Researchers recruited nearly 45,000 people in England and Scotland. They followed up with them for an average of 11 years, using hospital records and death certificates to determine how many of them developed coronary heart disease during that time (e.g. suffering angina, coronary heart disease or having a heart attack).
Compared to people who ate meat and fish at the start of the study, the vegetarians were less likely to be diagnosed with, or die from, coronary heart disease over the following years.
This association held even when the researchers adjusted for factors that are known to be linked to heart disease, including weight, sex, age and smoking status.
This study suggests that there are significant heart benefits to a vegetarian diet, which, the researchers argue, are probably due to the fact that eating a vegetarian diet involves eating less cholesterol than a typical meat eater. Also a vegetarian diet may lead to healthier blood pressure.
Although there may be other health and lifestyle factors not studied that could also be associated with both being vegetarian and having a lower risk of heart disease, overall this was a large, long-term study that suggests there are heart health benefits to a vegetarian diet.
But it is uncertain whether everyone would enjoy the same reduction in risk in the unlikely event that everyone in the UK turned ‘veggie’ overnight.
Where did the story come from?
The study was carried out by researchers from the University of Oxford and was funded by Cancer Research UK and the UK Medical Research Council.
The study was published in the peer-reviewed American Journal of Clinical Nutrition.
The media covered this research appropriately, with the Mirror rolling out the best pun of the day with its “Heart beet” headline.
What kind of research was this?
This was a prospective cohort study that examined the association of a vegetarian diet with the risk of developing coronary heart disease (e.g. angina or heart attack), which in this study the authors called by the alternative medical term of ischaemic heart disease (IHD). This is a general term to describe a number of conditions where the supply of blood the muscles of the heart becomes restricted.
The researchers say that while previous studies have looked at the link between vegetarianism and risk of dying from IHD, few prospective studies have looked at differences in both fatal and non-fatal IHD between vegetarians and meat eaters.
Ischaemic heart disease usually arises due to a thickening of the artery walls due to a build-up of fatty products, such as cholesterol, which restrict the flow of blood through the coronary arteries that supply the heart.
There are several factors that are known to increase one’s risk of IHD, some of which cannot be changed and these include age, sex and a family history of IHD. Other 'modifiable' risk factors for ischaemic heart disease are related to lifestyle, and thus are more easily altered, including smoking, high blood pressure and high cholesterol.
What did the research involve?
The study recruited individuals over the age of 20 years between 1993 and 1997. As the researchers were interested in the association between nutrition and health, they specifically recruited vegetarians and vegans, as well as the UK general population.
The participants completed a food-frequency questionnaire that asked them about what they ate over the previous year. Based on their responses, the researchers classified them as either non-vegetarian if they reported eating any meat or fish, or vegetarians if they reported eating no meat or fish (for the purpose of the study, no distinction was made between vegetarians and vegans).
This information was collected again five years into the study’s follow-up period.
At this time, data was also collected on the participants’ height and weight, smoking status, alcohol consumption, level of education, physical activity levels and socioeconomic status. Participants were also invited to have their blood pressure and cholesterol levels measured.
The researchers then examined hospital records, national audit records and death certificates to determine if the participants were treated for (non-fatal) or died of (fatal) IHD during the follow-up period. They used this information, along with the data on IHD risk factors collected at the beginning of the study, to compare the risk of developing or dying from IHD between vegetarians and non-vegetarians.
What were the basic results?
Overall, 44,561 participants were included in the study, 34% of whom were vegetarians at the beginning of the research, and 76% of whom were women. Over an average of 11.6 years follow-up, there were 1,235 cases of IHD (1,066 of these were hospital admissions, and 169 were deaths).
Overall, vegetarians tended to be younger than non-vegetarians, and were less likely to report receiving long-term medical treatment. Five years into the follow-up period approximately 85% of the vegetarian group reported that they were still vegetarians.
The researchers found that vegetarians had a 32% lower risk of developing IHD during the follow-up period compared to non-vegetarians (hazard ratio [HR] 0.68, 95% confidence interval [CI] 0.58 to 0.81).
In absolute terms, the probability of being hospitalised for, or dying of, IHD between the ages of 50 and 70 was 4.6% among vegetarians and 6.8% among non-vegetarians.
This reduced risk was seen among both continuous vegetarians and among those who were no longer vegetarians at five years’ follow-up.
When the researchers adjusted for BMI, the effect on IHD was lessened slightly to a 28% reduction among vegetarians compared to non-vegetarians (HR 0.72, 95% CI 0.61 to 0.85). The relationship also remained significant after adjusting for other risk factors associated with IHD such as smoking, alcohol, physical activity (or lack of it) and markers of socioeconomic status.
How did the researchers interpret the results?
The researchers concluded that vegetarians had a 32% lower risk of IHD than non-vegetarians, and that this is likely due to “reduced levels of well-established risk factors for IHD, such as non-HDL cholesterol concentrations and systolic blood pressure”.
This large and impressive prospective cohort study suggests that a vegetarian diet may benefit your heart, reducing the risk of IHD.
However, there are important limitations to the study that should be considered before assuming the results apply broadly to the whole of the UK.
First, this study specifically and actively recruited vegetarians and vegans. In addition to GP practice-based recruitment, the researchers “aimed to recruit health-conscious people living throughout the UK”. People who take the effort to get involved in research involving diet and health tend to be more health conscious than the population at large (this is what is known as selection bias). As such, it is a non-representative sample, and the absolute figures of IHD cases among the 50-70 year olds in this study (6.8% in non-vegetarians and 4.6% in vegetarians) may not reflect the absolute risk in the general population.
Additionally, while all participants were invited to have their blood cholesterol levels measured at the beginning of the study, less than half did so.
The researchers suggest that much of the difference in IHD risk between the groups is associated with non-HDL (‘bad’) cholesterol levels and blood pressure. But due to the lack of available blood cholesterol data for all participants, further research using a more complete data set would be needed to confirm this interpretation.
Despite these limitations, this was a well-conducted, large long-term study that suggests there are heart healthy benefits to a vegetarian diet.
Analysis by NHS Choices. Follow Behind the Headlines on twitter.