José Manuel Fernández-Real
Head of the Nutrition, Eumetabolism and Health Group at IDIBGI and CIBEROBN, Professor of Medicine at the University of Girona and Head of the Endocrinology Section at the Dr. Josep Trueta Hospital
The press release does not fully reflect the study with precision. It is essentially a summary of a summary, which, in itself, does not contain all the information. For instance, in one of the cohorts, it is stated:
"...capturing sufficient data for 26 diseases (i.e., >50 cases and controls for each disease) with 11,807 cases and 17,118 controls. The analysis revealed that most diseases (14/26) were significantly associated with predicted microbial load (FDR < 0.05). Nine of the significantly associated diseases showed negative associations with predicted microbial load, while five showed positive associations with predicted microbial load."
This indicates substantial heterogeneity in the association between microbial load and diseases.
The study is of good quality. The number of samples analyzed is very high, and the methodology is solid.
What is not emphasized enough is that only 7-17% of diseases are misclassified if microbial load is not considered. It is an important but not critical factor.
The study highlights a known fact, although it is not always taken into account. An indirect way to control for microbial load is to restrict the analysis to a count threshold of the different species to be analyzed and to consider compositional data.
Microbiota studies should consider this confounding factor when attributing or not attributing its associative significance to a particular phenotype or disease.