"Being vegetarian isn't always healthy: Plant-based diet may raise the risk of heart disease," the Daily Mail reports. A US study found a vegetarian diet based on less healthy food options, such as refined grains, could increase the risk of heart disease.
The researchers behind the latest study made the point that many previous diet and health studies "lumped together" all types of vegetarian diets as plant-based, without considering the actual content of specific diets. And not all plant-based diets are healthy and nutritious.
The researchers looked at data involving 200,000 health workers from the US and tried to analyse any link between diet and coronary heart disease.
Overall a high plant-based diet wasn't linked with a clear benefit for heart disease risk compared with a low plant-based/high meat-based diet.
When the plant-based diets were broken down and analysed further, the researchers found interesting differences.
Those eating a "healthy" plant-based diet high in wholegrains, fruits, vegetables and healthy fats were less likely to get heart disease than people eating "unhealthy" plant-based diets including foods like potatoes, refined grains and sweets.
While the study can't rule out the possibility that other health and lifestyle factors such as stress, job type and education could have influenced the links, the association between unhealthy plant-based diets and heart disease is plausible.
The diet advice for vegetarians is the same for everyone else: eat a balanced diet with at least five portions of fruit and vegetables every day, eat less sugar, salt, and saturated fat, and choose wholegrain carbohydrates where possible.
The study was carried out by researchers from Harvard T.H. Chan School of Public Health, AbbVie (a pharmaceutical company), and Brigham and Women's Hospital, all in the US. It was funded by the US National Institutes of Health, US Department of Agriculture/Blueberry Highbush Council and the California Walnut Commission, and Metagenic. One author has served on the Scientific Advisory Committees of IKEA, Take C/O, and SPE, and another is also an employee of AbbVie.
The study was published in the peer-reviewed medical journal the Journal of the American College of Cardiology.
The Daily Mail's reporting was generally accurate, but the statement claiming "refined grains and potatoes lead to a higher risk of cardio-metabolic disease" is not entirely representative. These were just two of a wide variety of foods included in the "unhealthy plant-based diet." Neither does this statement account for the fact that there may be many other health and lifestyle factors other than diet contributing to coronary heart disease risk.
This was a study pooling data from three large cohort studies of health professionals. It aimed to see whether consuming a plant-based diet or a diet including meat was associated with risk of coronary heart disease.
Coronary heart disease is the general term used to describe when the arteries supplying the heart become clogged by a build-up of fatty substances. Complete blockage of the arteries causes heart attack, a major cause of death both in the UK and worldwide.
A prospective cohort study is a good way of looking at the link between an exposure (such as diet) and an outcome (like heart disease) as you can examine a large number of people over a long period of time.
However, you are unable to control the diets or all other lifestyle factors that could be having an influence, such as smoking and exercise. A randomised controlled trial would be needed for this, but it is not really possible to make sure people stick to a specific diet for a long period of time.
The research included:
This study only included participants who, at the start of the study, did not have coronary heart disease, stroke and cancer.
Information on diet was collected every two to four years using a food frequency questionnaire. Participants recorded how often on average they consumed a specified portion of any of 130 food items in the past year. This ranged from "never or less than once a month" to "six or more times a day".
Three versions of a plant-based diet were created from these questionnaires based on intake of 18 main food groups:
The researchers looked at participant reports of coronary heart disease during follow-up assessments, and validated this through checking medical records. Deaths were identified through next of kin and a search of the US National Death Index.
Results were adjusted for the following confounding factors:
During follow-up 8,631 people developed coronary heart disease.
High adherence to an overall plant-based diet (PDI) showed a trend for reduced risk compared to low adherence to a PDI and a mainly animal-based diet, but this fell just short of statistical significance (hazard ratio [HR] 0.92, 95% confidence interval [CI] 0.83 to 1.01).
However, when analysing "healthful" versus "unhealthful" plant-based diets separately:
The researchers concluded that a "higher intake of a plant-based diet index rich in healthier plant foods is associated with substantially lower CHD risk, whereas a plant-based diet index that emphasizes less-healthy plant foods is associated with higher CHD risk."
They further add that "dietary guidelines and lifestyle interventions could recommend increasing intake of healthy plant foods, while reducing intake of less healthy plant foods and certain animal foods for improved cardiometabolic health."
This large pooled cohort study seems to demonstrate an association between a healthy plant-based diet and reduced risk of coronary heart disease, and an increased risk of heart disease with an unhealthy plant-based diet.
This adds to the evidence base supporting the possible benefits of healthy plant-based diets in protecting against certain illnesses. However there are some limitations to the research:
Nevertheless the study supports general understanding about the benefits of wholegrains, fruits and vegetables and healthy sources of fat.
Eating a purely plant-based, but unhealthy, diet may be good for your conscience but not so good for the heart.
Read more about healthy vegetarian diets.