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Study: Mice Gain Weight In Cold Temperatures Due To Gut Changes (economist.com)
Beeftopia writes with the results of a study described in The Economist: Mice were separated into two groups, one temperature maintained at 6C, the other at 22C. Researchers expected the cold mice to lose weight as they burned stored fat to stay warm. And for the first few days they did. But after five to ten days, in spite of their rations not increasing, the cold mice begain to put on weight. When scientists examined the gut microbiome of the previously identical mice, they found they were radically different. Additionally, the intestine had grown villi 50% larger than those of the warm temperature mice. Finally, after transplanting the gut microflora into a new batch of aseptic mice kept at warm temperatures, those mice showed the increased insulin sensitivity, cold tolerance, and villi length of the cold mice.
This explains why I gain weight when pouring beer after cold beer into my gut.
But it is a simple matter of calories in/out. The study even says so. They looked at the calorific value left in the faeces.