Eating a Mediterranean diet could have a positive effect within just one year for older adults, increasing “good” bacteria in their guts and decreasing the “bad” ones, new research suggests.

A study of 612 people aged 65 to 79 in five European countries showed that by adhering to a Mediterranean diet for a year showed a wide range of positive effects on gut bacteria that indicate “healthy aging.”

Those who stuck to the diet slowed the loss of gut bacteria diversity. They also saw an increase in bacteria associated with better brain function, indicators of reduced frailty – like grip strength and walking speed – and reduced production of harmful inflammation. 

Analyzing the changes occurring in people’s gut bacteria, the researchers found those that stuck with the diet increased bacteria tied to the production of short chain fatty acids, which can play an important role in maintaining health and staving off disease, and decreased bacteria linked to some bile acids associated with cancer and other adverse effects.

The “good” bacteria they were producing also played the role of “keystone” species in their guts, increasing the stability of the gut microbiome and reducing the “bad” bacteria.

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The research was published Monday in the peer-reviewed British Medical Journal Gut.

The Mediterranean diet is full of vegetables, legumes, fruits, nuts, olive oil and fish and low in red meat, dairy products and saturated fats. It is consistently rated as one of the healthiest diets.

While previous studies have shown the diet’s correlation with decreased risk of disease, this research is one of the largest and longest studies that looks at the diet’s effect on gut bacteria, said Elisabetta Politi, the nutrition director at the Duke Diet and Fitness Center. Politi was not involved in the research.

This shows not just that the Mediterranean diet is associated with good health but how that is achieved within a person’s body, she explained.

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“It’s really fascinating to see that those who eat Mediterranean-like achieve these health benefits because they have a more diverse microbiota,” Politi told USA TODAY.

Measuring indicators of frailty is also important as longevity increases and changes in diet help increase lifespans, Politi said. As people enter their 80s and 90s, there are increased worries around strength and bone health. 

“We really need to emphasize the best diet that can strengthen our skeleton and muscles,” she said. 

The participants in the study were categorized by the level of their frailty and just over half were assigned a Mediterranean diet tailored for older people. The participants were from France, Italy, Netherlands, Poland and the United Kingdom.

A person’s age and weight did not affect the results, and while the nationality of a person affected their baseline gut microbiome, the changes in bacteria appeared to be consistent as diet changed.

While the observational study cannot show a causal relationship, its size, length and range of countries adds to the strength of results, Politi said.

More research is needed to show the effects of the diet beyond a year, she added, and clinicians need to work with patients to ensure changes in diet are long-lasting.

“It’s just hard to change the way we are raised and what we are accustomed to eating,” Politi said. “We can do it more easily for six months and a year, but for me, the really interesting question is are all these changes sustainable.”

Follow USA TODAY’s Ryan Miller on Twitter @RyanW_Miller