In general, the cause of obesity in cats is not the food, but rather how they are fed. Studies have shown that even when cats are given diet food to eat, the effect on their weight loss is negligible. 

In the wild, cats go after a dozen mice every 24 hours, burning lots of calories. In the house, domesticated kitties jump from the couch to the food dish, back to the couch, onto the window sill, back to the food dish, etc. You should not free feed cats unless they are kittens or are suffering from illness that necessitates free feeding.
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