Friday April 15 2011
Fruit and vegetables play an important role in all diets
“Veggie diet cuts heart attack risk by a third,” according to the Daily Express, which today reported that vegetarians are a third less likely to suffer heart problems, diabetes or stroke than meat eaters.
The results come from a small study that looked at how different dietary patterns related to the prevalence of metabolic syndrome. Metabolic syndrome is a cluster of disorders, including raised blood pressure, cholesterol and blood sugar, which increase the risk of cardiovascular disease and diabetes. The research was conducted in 773 members of the Seventh-day Adventist faith, a Christian denomination that places emphasis on staying healthy and limiting intake of meat. The researchers found that 35% of participants who considered themselves vegetarian were less likely to have metabolic syndrome or its associated risk factors than non-vegetarians.
This relatively small study is of limited value due to both its size and the fact that it assessed a very specific group of people who may not be representative of the population as a whole. Also, it only looked at people at one point in time, meaning that we cannot tell if their past behaviours influenced the prevalence of metabolic syndrome.
It has long been recognised that there may be health benefits from following a diet low in saturated fats and high in vegetables, fruit and unsaturated fats such as nut and seed oils. These health benefits include a reduction in the risk of obesity, high blood pressure and diabetes. This study does not change current healthy eating advice.
Where did the story come from?
The study was carried out by researchers from the Karolinska Institute in Sweden, Loma Linda University and the School of Public Health, Loma Linda, California. Funding was provided by the US National Institutes of Health. The study was published in the peer-reviewed medical journal Diabetes Care.
The news stories have, in general, not considered the numerous limitations of this cross-sectional study, including the fact that the study examined a very select population that may not reflect the behaviours or health of the general British population. Additionally, it is not clear where the 36% reduction in the risk of metabolic syndrome in vegetarians quoted in the newspapers came from. The study quoted an odds ratio of 0.44 for metabolic syndrome in vegetarians relative to non-vegetarians, which equates to vegetarian participants having a 56% lower chance of metabolic syndrome than their non-vegetarian counterparts.
What kind of research was this?
This was a cross-sectional survey of participants taking part in The Adventist Health Study 2, an ongoing research project studying followers of the Seventh-day Adventist religious denomination. People who follow this Christian belief system have been studied in dietary research because many adhere to special dietary habits, for example not consuming meat. Their religion also places emphasis on looking after health, particularly through avoiding habits such as smoking and drinking. Their tendency to avoid certain unhealthy lifestyle choices means that researchers can potentially discount the influence of these behaviours when performing analyses.
In this study researchers surveyed the dietary patterns of 773 participants (average age 60 years) and assessed how their diets related to their risk of metabolic syndrome or their risk of having its individual composite risk factors (for example, cholesterol, blood pressure and high BMI). Metabolic syndrome is a cluster of disorders associated with an increased risk of diabetes and cardiovascular disease.
Studies with a cross-sectional design (which look at factors at only a single point in time) can give us proportions only, but cannot demonstrate changes or cause and effect relationships because the participants were not followed over time. Also, this particular cross-sectional study took a sub-sample of people taking part in another study, the Adventist Health Study 2, in which all of the participants were Seventh-day Adventists who are known to have different lifestyle and dietary habits from the general population. The selection and inclusion criteria used when enrolling people to The Adventist Health Study 2 may mean they are not representative of the general population.
What did the research involve?
The Adventist Health Study 2 included 96,000 people from the US and Canada, all of whom are Seventh-day Adventists, with the aim of assessing links between their lifestyle, diet and disease. At enrolment all were examined in a clinic where height, weight and blood pressure were measured and blood samples were taken to test for glucose and cholesterol levels.
Metabolic syndrome was defined according to established cut-off levels for glucose (fasting glucose above 100mg/dL), and they considered people to have high blood pressure or diabetes if they were taking medications appropriate to these conditions.
A food frequency questionnaire was administered and people were classed as either:
- vegetarian, if meat, poultry or fish was eaten less than once per month
- semi-vegetarian, if any amount of fish was eaten, but meat less than once per month
- non-vegetarian, if meat or poultry was eaten more than once per month, and in total any type of meat was eaten more than once a week
A telephone assessment was also made to record details of alcohol consumption, smoking and exercise. The current study considered 773 of these people who had appropriate clinical and dietary information available.
What were the basic results?
The average age of participants was 60 years. Some 35% were vegetarians, 16% semi-vegetarian and 49% non-vegetarian. Body mass index (BMI) was lower among the vegetarians (25.7kg/m2) than in the semi- (27.6kg/m2) and non-vegetarians (29.9kg/m2). A BMI of 18.5 to 25 is considered to be ideal weight, and a BMI of over 25 is considered to be overweight.
Risk factors for metabolic syndrome included high levels of cholesterol or glucose, high blood pressure, a large waist circumference or a high BMI. Vegetarians were less likely to have metabolic risk factors (12% of the group had three or more risk factors), compared with semi- and non-vegetarians (in both of these groups 19% had three or more risk factors). After adjusting for other lifestyle risk factors, age and sex, the researchers found that levels of blood cholesterol, blood glucose, blood pressure, waist circumference and BMI were all significantly lower among vegetarians compared with non-vegetarians. There was also a significantly higher prevalence of metabolic syndrome among non-vegetarians than among vegetarians (39.7% vs. 25.2%). Relative to non-vegetarians, vegetarians had a 56% reduced odds of having metabolic syndrome (odds ratio OR 0.44, 95% confidence interval 0.30 to 0.64, p<0.001).
How did the researchers interpret the results?
The researchers conclude that “a vegetarian dietary pattern is associated with a more favorable profile of metabolic risk factors and a lower risk of metabolic syndrome”.
This relatively small, cross-sectional study has found a lower prevalence of metabolic syndrome or its composite risk factors among vegetarians compared with non-vegetarians. The study report itself is brief and there are several important limitations to bear in mind:
- As this is a cross-sectional survey, cause and effect cannot be implied. Too little is known about these people, their past diets, their medical history and family history to know what may have contributed to their current state of health.
- The dietary categories were quite broad and the definitions used for vegetarian, semi- vegetarian and non-vegetarian may not be consistent with other ideas of what constitutes such a dietary pattern.
- Non-vegetarians were studied as a single group containing anybody who ate meat more than once per month. Therefore, the people in this group may have had a wide range of meat-eating behaviours, with the study making no differentiation between people who ate meat twice a month and those who might, for example, eat meat every day.
- Disease outcomes, for example, heart disease, stroke and diabetes, are not reported here. Therefore, the one-third reduction in metabolic syndrome among vegetarians does not necessarily equate to one-third lower risk of having a heart attack.
- Importantly, this was a cross-sectional assessment of a sub-sample of a very select population group taking part in a wider study examining the diet and lifestyle behaviour of Seventh-day Adventists, and how this affects their health and disease risk. The findings in this group may, therefore, not be applied more generally to the wider population.
It has long been considered that a diet low in saturated fats and high in vegetables, fruit and unsaturated fats, such as nut and seed oils, has health benefits, such as reducing risk of obesity, high blood pressure and diabetes. This study does not affect current healthy eating advice.