Summary Background The association of gut microbiota with obesity and its cardio‐metabolic complications in paediatric populations is still controversial. Objective We investigated the association of obesity and cardio‐metabolic traits with gut microbiota on 167 and 163 children with normal weight and obesity from Mexico City and Oaxaca, Mexico. Methods Anthropometric and biochemical traits were measured. The microbial communities were determined by high‐throughput sequencing of bacterial 16S rRNA gene v3‐v4 region. Results The gut microbial community structure was associated with obesity and fasting plasma insulin (FPI) in Mexico City (PObesity = 0.012, PFPI = 0.0003) and Oaxaca (PObesity = 0.034, PFPI = 0.016), and with triglycerides (TG) in Oaxaca (P = .0002). The Firmicutes/Bacteroidetes ratio was positively associated with TG in Oaxaca (P = .003). Firmicutes and Bacteroidetes phyla were positively and negatively associated with obesity (Mexico City: PFirmicutes = 0.013, PBacteroidetes = 0.009) and TG (Oaxaca: PFirmicutes = 0.002, PBacteroidetes = 0.004). In Oaxaca, Verrucomicrobia was negatively associated with obesity (P = .004). In Mexico City, the bacterial genus Fusicatenibacter, Romboutsia, Ruminococcaceae, Ruminiclostridium, Blautia, Clostridium, Anaerostipes and Intestinibacter were associated with obesity and FPI, while in Oaxaca, Bacteroides, Alistipes and Clostridium were associated with TG. Conclusion The gut microbial community structure in children is associated with obesity and FPI in Mexico City, and with obesity, FPI and TG in Oaxaca.

Context Rare partial/complete loss-of-function mutations in the melanocortin-4 receptor (MC4R) gene are the most common cause of Mendelian obesity in European populations, but their contribution to obesity in the Mexican population is unclear. Objective and Design We investigated whether deleterious mutations in MC4R contribute to obesity in Mexican children and adults. Results We provide evidence that the MC4R p.Ile269Asn (rs79783591) mutation may have arisen in modern human populations from a founder event in native Mexicans. The MC4R Isoleucine 269 is perfectly conserved across 184 species, which suggests a critical role for the amino acid in MC4R activity. Four in silico tools (SIFT, PolyPhen-2, CADD, MutPred2) predicted a deleterious impact of the p.Ile269Asn substitution on MC4R function. The MC4R p.Ile269Asn mutation was associated with childhood (Ncontrols = 952, Ncases = 661, odds ratio (OR) = 3.06, 95% confidence interval (95%CI) [1.94–4.85]) and adult obesity (Ncontrols = 1445, Ncases = 2,487, OR = 2.58, 95%CI [1.52–4.39]). The frequency of the MC4R p.Ile269Asn mutation ranged from 0.52 to 0.59% and 1.53 to 1.59% in children and adults with normal weight and obesity, respectively. The MC4R p.Ile269Asn mutation co-segregated perfectly with obesity in 5 multigenerational Mexican pedigrees. While adults with obesity carrying the p.Ile269Asn mutation had higher BMI values than noncarriers, this trend was not observed in children. The MC4R p.Ile269Asn mutation accounted for a population attributable risk of 1.28% and 0.68% for childhood and adult obesity, respectively, in the Mexican population. Conclusion The MC4R p.Ile269Asn mutation may have emerged as a founder mutation in native Mexicans and is associated with childhood and adult obesity in the modern Mexican population.