Immunity-Based Evolutionary Interpretation of Diet-Induced Thermogenesis.

Diet-induced thermogenesis (DIT) has often been argued to be a physiological defense against obesity, but no empirical proof of its effectiveness in limiting human body weight gain is available. We here propose an immune explanation of DIT-i.e., that it results from the coevolution of host and gut microbiota (especially Firmicutes) that ferment ingested food and proliferate, causing periodic, vagally mediated increases in thermogenesis aimed at curtailing their expansion. Because of this evolutionary adaptive significance related to the immune system, DIT is not effective as an "adaptation" to maintain a certain body mass. Were DIT an effective adaptation to prevent obesity, the current obesity epidemic might not have occurred.