Cat owners want their pets to be happy, but providing an abundance of food and snacks can have unintended consequences. A new study from the University of Illinois looked at what happens in the digestive system and gut microbiota when cats eat too much and found that once the cats were able to overeat, they immediately increased their food intake substantially and started to gain weight. As cats ate more and gained weight, gastrointestinal transit time was reduced, and so was digestive efficiency. When the body gets less food, it will be more efficient in extracting nutrients. But when the amount of food increases, it passes through the digestive system faster and fewer nutrients are extracted in the process. The researchers also found significant changes in gut microbial composition and the relative abundance of Bifidobacterium, which has antimicrobial activity, inhibits pathogens, and stimulates the immune system, increased, while Collinsella, which degrades fibre and has been linked to pro-inflammatory diseases, decreased. These results are opposite to what has been measured in overweight humans and suggest that their association to weight gain is complex.