Tuesday June 4 2013
A vegetarian diet brings a range of health benefits
“Veg diet key to living longer,” reports today’s Daily Express front page headline.
Its story is prompted by a large, well-designed, long-term study into vegetarian dietary patterns and their effects on reported mortality (death). The main finding was that vegetarians had a 12% reduction in the risk of death from any cause compared with non-vegetarians.
However, the researchers’ definition of ‘vegetarian’ was quite broad and may horrify some vegetarians as it included people who ate meat and fish once a week or less. Dietary patterns were only measured at the start of the study, and these can change over time. The study also had a relatively short follow-up to determine whether dietary patterns might affect the risk of death.
It’s also worth noting that vegetarians tended to live healthy lifestyles, and this could have influenced the results.
While this study cannot show direct cause and effect between diet and death risk, it highlights an important point. Even if you do not want to stop eating meat there is plenty you can ‘borrow’ from the ‘vegetarian lifestyle’ to improve your health, such as eating plenty of fresh fruit and vegetables.
Where did the story come from?
The study was carried out by researchers from Loma Linda University, California and was funded by the US National Cancer Institute and National Institute of Food and Agriculture. It was published in the peer-reviewed journal, JAMA Internal Medicine.
The results of the research were generally well covered in the media. However, the Mail Online website presented speculation as fact in its headline: it states that people who avoid meat have better health due to low blood pressure. Although this is a possible and plausible explanatory factor, the current study did not investigate the blood pressure of vegetarians.
What kind of research was this?
This was a prospective cohort study set in the US that aimed to evaluate the association between vegetarian dietary patterns and mortality (death).
A cohort study is the ideal study design to address this question. However, cohort studies cannot show cause and effect, as it is possible that other factors (confounders) are responsible for the associations seen. Ideally, the effects of a particular diet on a clinical outcome would be assessed through a randomised controlled trial. However, this is unlikely to be feasible when investigating an outcome such as mortality, which would need a long duration of follow-up; and also it would be difficult to randomise people to eating meat, or not eating meat, which comes down to personal choice.
What did the research involve?
The diet of 73,308 men and women participating in the Adventist Health Study 2 was assessed. The Adventist Health Study 2 is an ongoing cohort study involving members of the Seventh Day Adventist church (a Christian denomination) where the promotion of a healthy diet and lifestyle is a tenet of Adventist teaching.
The people’s diets were assessed when they entered the study (between 2002 and 2007) using a food frequency questionnaire. Based on the results of the questionnaire, the participant’s diet was classified into one of five dietary patterns:
- non-vegetarian (ate fish and meat more than once per week)
- semi-vegetarian (ate fish and meat but not more than once per week)
- pesco-vegetarian (ate seafood at least once per month but all other meats less than once per month)
- lacto-ovo-vegetarian (consumed dairy and egg products but fish and all other meats less than once per month)
- vegan (consumed eggs, dairy, fish and all other meats less than once per month)
For some analysis, vegan, lacto-ovo-vegetarian, pesco-vegetarian and semi-vegetarian were combined as “vegetarian”.
Deaths that occurred up until the end of 2009 were identified from the US National Death Index.
The researchers analysed the relationship between vegetarian dietary patterns and all-cause and cause-specific mortality.
What were the basic results?
During a mean (average) follow-up of 5.79 years there were 2,570 deaths.
The results were adjusted for the following confounders:
- smoking status
- marital status
- alcohol intake
- geographical region
- amount of sleep per night
After adjustments, vegetarians (all vegetarians combined) had a 12% reduction in the risk of death from any cause compared with non-vegetarians (hazard ratio (HR) 0.88, 95% confidence interval (CI) 0.80 to 0.97).
Vegetarians also had a reduced risk of death from causes other than cardiovascular disease or cancer (HR 0.85, 95% CI 0.73 to 0.99). More specifically, vegetarians had a reduced risk of death from kidney problems and deaths from hormonal (endocrine) problems. Male vegetarians only also had a significantly reduced risk of death from ischaemic (coronary) heart disease and from cardiovascular disease overall.
The different classes of vegetarians were then considered separately. The researchers found that compared to non-vegetarians:
- Pesco-vegetarians had a significantly reduced risk, in both sexes combined, of death from any cause (HR 0.81, 95% CI 0.69 to 0.94), death from ischaemic heart disease (HR 0.65, 95% CI 0.43 to 0.97) and non-cardiovascular, non-cancer death (0.71, 95% CI 0.54 to 0.94).
- Lacto-ovo-vegetarians had a significantly reduced risk in both sexes combined for all-cause mortality (HR 0.91, 95% CI 0.82 to 1.00).
- Vegans had a significantly reduced risk, in both sexes combined, of deaths from non-cancer, non-cardiovascular causes (HR 0.74, 95% CI 0.56 to 0.99).
When men and women were examined separately, reductions in risk were larger and more often significant for men than for women.
How did the researchers interpret the results?
“Vegetarian diets are association with lower all-cause mortality and with some reductions in cause-specific mortality. Results appeared to be more robust in males. These favourable associations should be considered carefully by those offering dietary guidance.”
This large, well-designed cohort study has found an association between vegetarian dietary patterns and reduced risk of death.
This study has the advantage that it included a large number of participants consuming various diets.
However, as this study is a cohort study, it cannot show cause and effect, as it is possible that other factors are responsible for the associations seen. Although the researchers adjusted for many of these factors, it was found that the vegetarian groups tended to be older, more highly educated and more likely to be married, to drink less alcohol, to smoke less, to exercise more and to be thinner. The reduced risk of death may be due to other lifestyle factors of vegetarians rather than diet.
In addition, the study has some other limitations, which were pointed out by the researchers:
- The study had a relatively short follow-up. Although the average follow-up was almost six years, this is quite short to address how dietary patterns might affect the risk of death.
- Dietary patterns were only measured at baseline, and it is possible that dietary patterns change over time.
- The researchers included in the ‘vegetarian’ category people who eat fish and meats, but not more than once per week. This is not what a vegetarian diet is traditionally taken to mean.
This study was also conducted in a select population sample of Seventh Day Adventists, who have particular health and lifestyle characteristics. They tend to be much healthier than the population at large (for example, smoking and alcohol are discouraged among Adventists), and enjoy a greater average life expectancy.
This may mean that the differences observed between vegetarians and non-vegetarians in this study may differ from what would be observed in study of other population samples.
Analysis by Bazian. Edited by NHS Choices. Follow Behind the Headlines on Twitter.