A total of 48,000 people were studied over a period of 18 years for the research, which was published in the British Medical Journal.
The study is part of an EPIC-Oxford research project that analyses the long-term impact of people’s diets.
Half of participants, recruited between 1993 and 2001, were meat-eaters. Just over 16,000 vegetarian or vegan, and 7,500 described themselves as pescatarian (fish-eating).
Smoking, medical history and physical activity were taken into account when the participants were asked about their diets on joining the study, and again in 2010.
In total there were 2,820 cases of coronary heart disease (CHD) and 1,072 cases of stroke.
Three hundred of the strokes were haemorrhagic strokes, which happen when a weakened blood vessel bursts and bleeds into the brain.
Vegans and vegetarians were found to have a 22 per cent lower chance of CHD, while pescatarians were 13 per cent less likely to suffer from heart disease.
But the results of the study showed that those on plant-based diets were 20 per cent more likely to suffer from a stroke.
Is it unhealthy to be vegan or vegetarian?
In spite of this, the experts say that this does not indicate that vegan and vegetarian diets are unhealthy.
Dr Frankie Phillips, from the British Dietetic Association, explained that this was because the study was observational, meaning the results were based on association rather than direct cause-and-effect.
Other lifestyle choices not measured in the study could have had an impact on the number of people who suffered from strokes of CHD.
Dieticians recommend a varied diet, and say that diets with a lack of vegetables are unhealthy.
The nature of vegan and vegetarian diets have changed over the course of the study, too. Vegan and vegetarian diets are now becoming more mainstream, and processed and convenience plant-based foods are much more readily available.
It is known that processed foods come with health risks.
What is a healthy diet?
The NHS recommends the following to people looking to make sure they have a healthy diet:
- Eat at least five portions of fruit and vegetables a day
- Base meals around higher-fibre starchy foods such as potatoes, bread, rice or pasta
- Don't forget protein - from lean meat, fish, seafood, pulses, tofu or unsalted nuts
- Include dairy or dairy alternatives
- Foods high in fat, sugars or salt should be eaten less often and in small amounts
Vegans and vegetarians need to be careful that they get enough of the nutrients typically gained from eating meat and fish, however this is not difficult.
For example, meat-eaters often have plenty of vitamin B12, found in meat, which is essential for healthy blood and nervous systems. To avoid deficiency, vegans can find B12 in fortified cereals and yeast extracts.
Iron can also be difficult to get enough of if you eat mainly plants, however good vegan and vegetarian options include wholemeal bread and pulses.