A novel obesity index reveals obesity-associated microbiota changes after weight loss intervention in a Spanish population

Gut microbiota specific taxa influencing a higher or lower risk for developing obesity and related metabolic parameters remain unclear. Here, we aimed to identify gut microbiome fingerprints associated with obesity using an integrated obesity index (OBIndex), and to evaluate how these profiles change after a hypocaloric nutritional intervention with two different diets. 295 Spanish adult volunteers participating in the Obekit study were classified into LOW or HIGH degree of obesity according to a novel obesity index (OBIndex), based on the combination of three parameters: body mass index (BMI), fat mass (%), and waist circumference (WC). Differential abundant taxa between the two groups were investigated, at baseline together with the changes in their abundance after a weight-loss intervention with two different hypocaloric diets (Moderately high protein (MHP) and low fat (LF)). The classification of participants using the OBIndex effectively differentiated obesity status, overcoming the inherent limitations of BMI, WC, and fat mass when considered individually. MHP diet was associated with an improvement in obesity status in women, while LF diet showed better outcomes in men. In relation to these improvements, changes in gut microbiota were observed. We found that Mediterraneibacter levels decreased only in men who experienced a significant improvement in their obesity status. Additionally, Agathobacter levels in participants who improved their physiological condition were reduced to match those of individuals who already exhibited a healthier status at baseline. Alongside these findings, we identified two poorly studied genera, Pseudobutyricicoccus and Intestinimonas, which were associated with a healthier profile at baseline and increased in abundance following the nutritional intervention. Our OBIndex helped to achieve a more accurate separation of our volunteers regarding their obesity status and, thus, identifying four obesity strong related bacteria. Particularly, Mediterraneibacter and Agathobacter were related to a worse obesity status whereas Pseudobutyricicoccus and Intestinimonas showed a negative association with obesity status suggesting a potential protective or anti-obesity role. The study protocol was approved by the ethical committee at the University of Navarra (Registration No. IR.MUI.RESEARCH.REC.1401.370) and registered online at clinicaltrials.gov (identifier NCT02737267).