Eating a Mediterranean diet, which includes meals built around fruits and vegetables, boosts gut bacteria linked to 'healthy' ageing, while suppressing microbes associated with harmful inflammation, according to a study which may lead to better clinical food recommendations for old people.

The study, published in the journal Gut, noted that ageing is associated with deteriorating bodily functions and increasing inflammation, and the Mediterranean diet may act on gut bacteria in a way that helps curb the advance of physical frailty and cognitive decline in old age.

According to the researchers, including those from the University College Cork in Ireland, a poor diet, which is common among older people, particularly those in long term residential care, reduces the range and types of bacteria (microbiome) found in the gut, and speeds up the onset of frailty.

In the current study, they analysed the gut microbiome of 612 people aged 65 to 79, before and after 12 months of either eating their usual diet, or a Mediterranean diet rich in fruits, vegetables, nuts, legumes, olive oil and fish, and low in red meat and saturated fats.

According to the findings of the study, sticking to the Mediterranean diet for 12 months was associated with beneficial changes to the gut microbiome.

The diet was linked to an increase in the types of bacteria previously associated with several indicators of reduced frailty, such as walking speed, hand grip strength, and improved brain function, such as memory.

The study also noted that the diet was related to reduced production of potentially harmful inflammatory chemicals.

A detailed analysis revealed that the microbiome changes were linked to an increase in bacteria known to produce beneficial short chain fatty acids, and a decrease in bacteria involved in producing bile acids.

The researchers added that the overproduction of these bodily chemicals are linked to a heightened risk of bowel cancer, insulin resistance, fatty liver, and cell damage.

According to the scientists, the bacteria which proliferate in response to the Mediterranean diet may act as 'keystone' species, meaning they are critical for a stable 'gut ecosystem,' pushing out microbes associated with indicators of frailty.

They said the changes were largely driven by an increase in dietary fibre and associated vitamins and minerals -- especially, C, B6, B9, copper, potassium, iron, manganese, and magnesium.

The findings, the researchers said, were independent of the person's age or weight, both of which have been shown in earlier studies to influence the make-up of the microbiome.

However, they said, the study did not establish a causative role for the microbiome in health.

"The interplay of diet, microbiome and host health is a complex phenomenon influenced by several factors," the researchers reported.

"While the results of this study shed light on some of the rules of this three-way interplay, several factors such as age, body mass index, disease status, and initial dietary patterns may play a key role in determining the extent of success of these interactions," they added.