Eating a Mediterranean diet for a year boosts the types of gut bacteria linked to 'healthy' ageing, while reducing those associated with harmful inflammation in older people.

The five-country study, published online in the journal Gut, looked at whether a Mediterranean diet might maintain the microbiome in older people's guts, and promote the retention or even proliferation of bacteria associated with 'healthy' ageing.

The researchers suggested that as ageing is associated with deteriorating bodily functions and increasing inflammation, both of which herald the onset of frailty, this diet might act on gut bacteria in such a way as to help curb the advance of physical frailty and cognitive decline in older age.

Previous research suggests that a poor/restrictive 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 helps to speed up the onset of frailty. 

They analysed the gut microbiome of 612 people aged 65 to 79, before and after 12 months of either eating their usual diet (n = 289) or a Mediterranean diet (n = 323), rich in fruits, vegetables, nuts, legumes, olive oil and fish and low in red meat and saturated fats, and specially tailored to older people (NU-AGE diet).

The participants, who were either frail (n=28), on the verge of frailty (n=151), or not frail (n=433) at the beginning of the study, lived in five different countries: France, Italy, Netherlands, Poland, and the UK.

Sticking to the Mediterranean diet for 12 months was associated with beneficial changes to the gut microbiome.

Findings were independent of the person's age or weight

It was associated with stemming the loss of bacterial diversity; an increase in the types of bacteria previously associated with several indicators of reduced frailty, such as walking speed and hand grip strength, and improved brain function, such as memory; and with reduced production of potentially harmful inflammatory chemicals.

More detailed analysis revealed that the microbiome changes were associated with an increase in bacteria known to produce beneficial short chain fatty acids and a decrease in bacteria involved in producing particular bile acids, overproduction of which are linked to a heightened risk of bowel cancer, insulin resistance, fatty liver and cell damage.

What's more, the bacteria that proliferated in response to the Mediterranean diet acted as 'keystone' species, meaning they were critical for a stable 'gut ecosystem,' pushing out those microbes associated with indicators of frailty.

The findings were independent of the person's age or weight (body mass index), both of which influence the make-up of the microbiome. And while there were some differences in the make-up of a person's gut microbiome, depending on country of origin to start with, the response to the Mediterranean diet after 12 months was similar and consistent, irrespective of nationality.