It isn't necessarily a bad link of thinking though with this. Higher CO2 levels can reduce cognitive function [1]. Reduced cognitive function could mean reduce

It isn't necessarily a bad link of thinking though with this. Higher CO2 levels can reduce cognitive function [1]. Reduced cognitive function could mean reduced energy expenditure (a common figure is the brain is responsible for 20% of your energy demands), it may not though. I can see this as being a valid mental leap worth investigating. I suspect however in this case it's a combination of the things the article suggests as well as the thermic effect of food. A reduction in the amounts of nuts/seeds/spices/whole plant foods consumed, combined with highly processed foods that require less work to digest (a piece of fruit vs a bunch of table sugar), likely means diets with a lower thermic effect from the foods. Lower thermic effect means less caloric expenditure, less caloric expenditure should mean a lower body temperature. [1] https://ehp.niehs.nih.gov/doi/10.1289/ehp.1510037 https://ehp.niehs.nih.gov/doi/10.1289/ehp.1510037