Because we have similar DNA and chemistry, low-level processes are very similar between mice and men. But because we’ve evolved for a long time to be different species occupying different ecological niches, macro-level things - like our diets - have the potential to be quite different. Even micro-level things have the potential to be dramatically different. It’s these tiny differences that make them mice, and us humans.
At a recent panel discussion I attended, John Glendinning spent some time reminding us that “mice are not small rats.” That is, physiological and biological differences exist that make rats and mice very different species with different responses to things like carbohydrates and non-calorie sweeteners, with the consequences that mice are a much better model for human study than rats (when it comes to this specific research area.) For example, to rats, maltodextrin is the sweetest stuff EVAR and they go nuts over it (people don’t find maltodextrin to be very sweet). Meanwhile, they don’t find asparthame sweet, perhaps not at all. Mice, on the other hand, respond much more like we do.
That having been said, the evolutionary trees of mice and rats only separated fifteen or twenty million years ago.
The ancient ancestors of mice/rats separated form the ancient ancestors of primates/humans more like eighty million years ago. So we’re more differenter from mice/rats than mice and rats are from each other.
Good science demands good research models, and good scientists work hard to ensure that we understand what species make for good research models for different aspects of the human condition.
As another example… you can learn a lot about the internal combustion engine by taking apart a gas-powered weed whacker. It’s safer and more convenient than taking apart a car. Many of the general principles are the same. But you shouldn’t assume the car will run well on the same oil/fuel mix as the weed wacker.