A method for increasing gastric tract acid (GTA) production in a mammalian subject. The method comprises administering a therapeutically-effective amount of a composition comprising at least one liveor attenuated culture of a microbial species selected from the genus Blautia, species Faecalibacterium prausnitzii, genus Bacteroides, family Ruminococcaceae, family Lachnospiraceae, genus Coprococcus, genus Roseburia, genus Oscillospira, species Ruminococcus bromii, genus Ruminococcus, family Costridiaceae, species Dorea formicigenerans, species Bacteroides uniformis, genus Dorea, genus Streptococcus, order Clostridiales, genus Anaerostipes, genus Dialister, species Bifidobacterium adolescentis, family Coriobacteriaceae, genus Faecalibacterium, genus Sutterella, species Bacteroides ovatus, genus Parabacteroides, genus Ruminococcus, species Bacteroides faecis, species Eubacterium biforme, genus Phascolartobacterium, and family Enterobacteriaceae; or a prebiotic composition which increasesgrowth and / or viability of said microbial species in the gut. Administering the composition increases the synthesis of at least one GTA dicarboxylic fatty acid metabolite in said subject. Also described are method for determining gastrointestinal inflammation status and kits for detecting and treating a gastric tract acid (GTA) insufficiency.