Mediterranean diet associated with maintaining bacterial diversity and markers of reduced frailty in older HIV negative study
An international study of over 600 people aged 65 to 79 reported beneficial changes in gut microbiome in those who were randomised to a Mediterranean diet for more than 12 months, compared to those who had no dietary changes. 
The interventional diet had been specially tailored to older people (NU-AGE diet) and was rich in fruits, vegetables, nuts, legumes, olive oil and fish and low in red meat and saturated fats.
At baseline, the countries had significantly different microbiome profiles – linked to the closeness to the Mediterranean diets reported by the participants from Poland, Netherlands, UK, France and Italy. Although the study didn’t report within-country benefits based on the randomised arms, it did report that closer adherence to the Mediterranean diet was linked to loss of microbiome diversity (which has been linked in other studies to poorer clinical outcomes.
In a sub analysis looking at markers of frailty by baseline frailty status (using the Fried score to categorise participants as non-frail, pre-frail and frail) adherence to the Mediterranean diet was associated with reduced frailty, improved cognitive function and reduced markers of inflammation – high-sensitivity C reactive protein (hsCRP) and interleukin 17 (IL-17).
Beneficial changes in people who kept to the Mediterranean diet for 12 months included maintaining bacterial diversity including those beneficial short chain fatty acids and a decrease in bacteria linked to a heightened risk of bowel cancer, insulin resistance, fatty liver and cell damage.
The changes were largely driven by an increase in dietary fibre and associated vitamins and minerals (vitamins C, B6, B9 and copper, potassium, iron, manganese, and magnesium) and were independent of the age and weight (body mass index) which both influence the microbiome.
Although this is a complex paper and some of the hoped for differences by diet were not always significant, the associations with reduced frailty and inflammation across different countries is interesting.
While associations do not prove a causative link, the overlapping concerns for older HIV positive people might make the results relevant.
Ghosh TH et al. Mediterranean diet intervention alters the gut microbiome in older people reducing frailty and improving health status: the NU-AGE 1-year dietary intervention across five European countries. Gut, 17 February 2020. doi: 10.1136/gutjnl-2019-319654. Open access.