Tangentially it's not the main subject of the book, but in Burn by Herman Pontzner he discusses how the human body burns a set number of calories as measured by double labelled water studies and he speculated that the immune system goes into overdrive in sedentary individuals since we eat a certain amount each day and the immune system will be more active since they have excess calories to burn. In a South American hunter gatherer society they found they had so many infections that they speculated that immune system activity was using a significant percentage of their daily energy. There was an African hunter gatherer studied that did not have the issue with infections but were more active daily.