Mice on a fatty diet not only weigh more, but also have a lot more body fat and a higher BMI, meaning they’re much fatter than mice on a normal diet.

At first, the mice on the fatty diet ate the same amount of food by weight as the others, but still got more calories because fat is energy-dense.

When female mice eat a diet full of fat for a month, they get much heavier than mice eating a normal diet, and this starts happening after just two weeks.

Even if they eat the same amount of food by weight, mice on a fatty diet take in way more calories because fat has more energy than other nutrients.

Mice that eat a high-fat diet start snacking more during the day—when they normally sleep—and eat less at night—when they normally are active.

Even though the mice on the fatty diet got heavier, the increase in stress hormone in their fur wasn’t linked to how fat they became—it seems the diet itself caused the change.