Thousands adopt a vegetarian diet in the hope it will improve their health.

But ditching meat and fish may prove detrimental in later life, particularly in terms of dementia risk, new US research shows.

Scientists at Loma Linda University Health in California found most vegetarian diets are associated with a lower risk of diseases in middle-aged people. However the opposite was the case for those aged over 65.

Researchers observed slightly higher risks among very elderly vegetarians for conditions such as stroke, dementia and Parkinson's disease. 

However, when fish was added to the diet that risk was significantly reduced — and associated with the lowest risk of death.

The study, which was published in August in the American Journal of Clinical Nutrition, used dietary data from 88,000 people aged 30-85 from Canada and the US, including 12,500 deaths. Participants were recruited between 2002 and 2007 and followed-up in 2015.  

Participants were separated into five categories according to their diets: non-vegetarian, semi-vegetarian, pesco-vegetarian (those that ate fish), lacto-ovo-vegetarian (those that ate dairy and eggs), and vegan. 

Overall those that ate a vegetarian diet had about a 12 per cent less risk of diet in comparison to meat eaters. 

But people who ate a vegetarian diet but also ate fish (pesco-vegetarian) had an 18 per cent less risk of death. 

Researchers also looked at the benefit of adding dairy and eggs in a vegetarian diet and found the diet this dropped the risk of death a further 15 per cent.

Vegans, who don't eat any animal products, only had a three per cent decreased risk of death.

However, the study found that, although the vegetarian diet offered protection from risk of death for middle aged people, those in their 80s didn't see as much of a benefit. 

Principle investigator of the study Professor Gary Fraser explained that there is an increased risk of neurological conditions among vegetarians in their 80s. 

This could be because a vegetarian diet that does not include fish could be missing vital nutrients, like fatty acids, that can help the brain.   

The Mediterranean diet ,which also includes eating a variety of fruit and vegetables as well as oily fish, such as salmon and mackerel, is thought to be beneficial because it's high in antioxidants. 

The Alzheimer's society suggests that this may help protect against some of the damage to brain cells associated with Alzheimer's disease. 

Adding oily fish, which is rich in omega-3, could also help brain health. 

That's because omega-3, which is also a kind of fat found in cell membranes, is important for your brain from development in the womb to adulthood. 

It's thought omega-3 can help reduce inflammation and oxidative stress, both of which contribute to the development of Alzheimer's disease and dementia. 

Omega-3 can also be found in eggs, nuts seeds and walnuts, but at much lower levels than oily fish, according to the British Dietetic Association.