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.

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.

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 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.

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.

The natural daily rise and fall of the stress hormone in the mice’s poop becomes flatter and stays higher all day when they eat a fatty diet.