"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.
Where did the story come from?
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.
What kind of research was this?
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.
What did the research involve?
The research included:
- 73,710 women (aged 30 to 55 years) involved in the Nurses' Health Study (1984 to 2012)
- 92,329 women (aged 25 to 42 years) involved in the Nurses' Health Study 2 (1991 to 2013)
- 43,259 men (aged 40 to 75 years) taking part in the Health Professionals Follow-up Study (1986-2012)
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:
- An overall plant-based diet index (PDI) was created by assigning positive scores to plant foods and reverse scores to animal foods.
- A "healthful plant-based diet index" (hPDI) was created by giving positive scores to healthy plant foods such as whole grains, fruit, vegetables, nuts, oils and tea. Both animal foods and less healthy plant foods such as juices, refined grains, fries and sweets received a negative score.
- An "unhealthful plant-based diet" (uPDI) was created by giving positive scores to less-healthy plant foods, such as sweets, cakes, chips and crisps, and scores to animal and healthy plant-based foods.
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:
- physical activity
- multivitamin use
- family history of coronary heart disease
- margarine intake
- energy intake
- high blood pressure
- high cholesterol
- body mass index
- post-menopausal hormone use and oral contraceptive use in women
What were the basic results?
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:
- Highest adherence to the healthy plant-based diet reduced risk of heart disease by 25% compared with a low adherence to this diet (i.e. consuming an unhealthy plant-based diet, including meat) (HR 0.75, 95% CI 0.68 to 0.83).
- Highest adherence to an unhealthy plant-based diet increased risk of heart disease by 32% compared with lowest adherence to this diet (i.e. consuming a healthy plant-based diet, including meat) (HR 1.32, 95% CI 1.20 to 1.46).
How did the researchers interpret the results?
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:
- The cohort included only health professionals from the US so might not be representative of wider populations in the UK or elsewhere.
- The study can't provide information on the benefits or otherwise of this diet in people with established coronary heart disease, stroke or cancer as these people were excluded.
- The questionnaire was self-reported and asked for recall of food habits over the previous year so there might be some inaccuracies in reporting. Also, people might not want to admit to consuming less healthy foods – although if unhealthy foods were under-reported, this could have meant an even bigger difference in results.
- Heart disease outcomes were mainly self-reported and then verified, so some cases may have been missed.
- Although analyses adjusted for various health and lifestyle factors, there are likely to be many other confounding variables influencing likelihood of coronary heart disease, such as education, occupation or stress levels.
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.