No proof that the Mediterranean diet reduces frailty...

Mail Online reports that eating a Mediterranean diet "for just one year reduces frailty and keeps mind sharp in old age", while the Express carried a similar message.

Despite these positive reports, the study in question provides no proof that eating a Mediterranean diet reduces frailty or improves memory.

Researchers were doing a secondary analysis of a trial carried out a few years ago. The original trial was called the NU-AGE project. It included 1,294 older adults (average age 71) from 5 European countries and assigned them to follow a Mediterranean diet for 1 year. This original trial mainly aimed to see if the Mediterranean diet had any effect on inflammation and frailty (bone mineral density). Researchers found it had no effect.

The current study looked at 612 people (half the original trial sample) who provided poo samples at the start and end of the 1-year study. The researchers tested these samples to look at the type of bacteria they had in their gut (microbiota).

The researchers found that the Mediterranean diet was associated with higher numbers of "positive" bacteria. These bacteria have been linked with health benefits in other studies. The researchers also found that people who were less frail tended to have higher levels of these "positive" gut bacteria.

While this study had some interesting results, it does not show that the Mediterranean diet directly boosted "positive" gut bacteria or directly reduced frailty. The study is also based on results from a previous study that was looking at a different outcome, so its results are less reliable.

But the benefits of a diet high in fruit and vegetables and low in saturated fat, like the Mediterranean diet, are well established.

Where did the story come from?

The study was carried out by researchers from University of East Anglia; University College Cork, Republic of Ireland; Queen's University Belfast, Ireland; University of Bologna, Italy; CHU Clermont-Ferrand and Université Clermont Auvergne, France; University of Life Sciences, Poland; and Wageningen University, Netherlands. It was funded by Science Foundation Ireland. The study was published in the peer-reviewed medical journal Gut, part of the BMJ group, and is free to access online.

The media has taken these findings at face value and used misleading headlines.

What kind of research was this?

The original NU-AGE project was a randomised controlled trial where older adults were randomly assigned to follow a Mediterranean diet or control diet for 1 year.

This latest study was a secondary analysis looking at the change in gut bacteria before and after the diet.

A randomised controlled trial is the best way to determine whether a treatment (in this case, the Mediterranean diet) is effective. The randomisation process should cancel out any differences between people in the study. In this trial, researchers were looking for changes in the level of the inflammatory marker called C-reactive protein (CRP). The additional analyses looking at changes in gut bacteria and the effect on frailty are less reliable.

What did the research involve?

Details of the original NU-AGE trial are not given in the current study. The 2018 publication reports that the trial recruited 1,294 adults aged 65-79 years (average 71), who did not have any long-term conditions. They were from 5 countries: the UK, Italy, France, Netherlands and Poland.

The Mediterranean diet they were following was based on dietary guidelines. It involved tailored advice, which was given 9 times during the course of the 1-year study, by phone or in person.

Participants were also given foods to help them meet the dietary guidance, including:

wholegrain pasta
olive oil
margarine that was high in monounsaturated and polyunsaturated fats
low-fat, low-salt cheese

They were also given vitamin D supplements. Compliance was assessed through 7-day food diaries administered at the start and end of the 1-year study. The control group continued their normal diet.

The current study looked at 47% of the original NU-AGE trial sample (612 adults: 323 in the intervention group and 289 controls) who had given poo samples at the start and end of the study.

The researchers looked at the types of bacteria found in the poo samples. They then looked for links between this and:

markers of inflammation and frailty
type of diet (Mediterranean and control)
adherence to the diet

What were the basic results?

At the start of the study, participants from the UK and France carried similar gut bacteria, as did those from Poland and The Netherlands, while people from Italy were more distinct.

After the 1-year study, the balance of gut bacteria had changed and there was a clear link with the diet consumed. In the Mediterranean diet group, changes were driven by an increased consumption of fibre, vitamins and minerals. Changes in the control group were influenced mainly by an increased consumption of fat.

The more closely the participants followed the Mediterranean diet (greater adherence), the greater the diversity of their gut bacteria. This included many types of bacteria previously reported to be linked with positive health effects. For example, gut bacteria with anti-inflammatory or anticancer properties. Meanwhile, greater adherence was linked with lower levels of some bacteria that have negative health links, such as associations with diabetes or cancer.

The researchers found that older adults with higher bone density (less frailty) tended to have higher levels of the "positive" bacteria in their gut compared with the frail individuals.

They also found that having more "positive" bacteria was linked with lower levels of certain inflammatory markers and improvement in certain cognitive tests (for example, memory scores).

How did the researchers interpret the results?

The researchers conclude: "Our findings support the feasibility of improving the habitual diet to modulate the gut microbiota which in turn has the potential to promote healthier ageing."

Conclusion

This study found that sticking to the Mediterranean diet was linked with having more "positive" gut bacteria which are linked to health benefits. They also found that greater numbers of these "positive" gut bacteria showed links with slightly lower frailty.

However, there's need for caution before drawing firm conclusions from these findings.

This is a secondary analysis of a previously published randomised controlled trial. The original trial did not set out to look at changes in the gut bacteria of participants, or how changes were linked to frailty. So, while these later analyses may be interesting for further study, they are less reliable.

The original NU-AGE trial found that the Mediterranean diet had no significant effect on either inflammation or on frailty. Following the diet closely was also not associated with these outcomes. Therefore, the overall conclusion from the NU_AGE trial was that a Mediterranean diet had no effect on bone density.

The authors acknowledge that findings from this study that link the levels of "positive" bacteria in individuals to their level of frailty, inflammation or memory scores, are weak.

This study is also based on only half of the original trial sample. The sample size for a trial is calculated based on how many people you need to include to be able to reliably detect a difference in the main outcome of the trial (CRP level).

Also, the study does not show whether there were any differences between the half of the population studied, and the other half who were excluded (who didn't provide poo samples).

Furthermore, as with all dietary trials, it can be difficult to know how closely people followed the diet, even when they completed food diaries.

Overall, a diet high in fruit and vegetables and low in saturated fats, like the traditional Mediterranean diet, is known to be associated with health benefits. However, there is no proof from this study that a Mediterranean diet will reduce frailty in old age.

Read more about how to get a healthy balanced diet using the Eatwell Guide.

Page last reviewed: Tue Feb 2020 Next review due: Tue Feb 2020