Vegetarian diets are widely touted as a healthier option than eating meat.
Eating a vegetarian diet isn't always healthy and some may increase the risk of heart disease, scientists say.
But experts said that it can lead to a higher risk of heart disease – if vegetarians eat lots of refined grains, potatoes and sweets, and indulge in sweetened drinks.
Researchers from Harvard University designed separate diets which focused on plant food with a reduced animal food intake and a vegetarian diet that emphasised the intake of healthy plant foods such as whole grains, fruits and vegetables.
They also studied a third which was based on unhealthy diet of less healthy plant foods like refined grains.
Eating a vegetarian diet isn't always healthy and some may increase the risk of heart disease
Dr Ambika Satija, a postdoctoral fellow at the Harvard T.H. Chan School of Public Health said: 'When we examined the associations of the three food categories with heart disease risk, we found that healthy plant foods were associated with lower risk, whereas less healthy plant foods and animal foods were associated with higher risk.
'It's apparent that there is a wide variation in the nutritional quality of plant foods, making it crucial to take into consideration the quality of foods in a plant-based diet.'
How was the study carried out?
Researchers studies samples from more than 200,000 participants and followed up with a questionnaire every two years for 20 years on their lifestyle, health behaviours and medical history.
Lead author Dr Satija chose to exclude participants with coronary heart disease, stroke, cancer and coronary artery surgery.