The Global Nature of the Fat Mouse Problem
Calorie restriction is very potent in mice and other laboratory species: even a seemingly modest decrease in calorie intake can have beneficial effects on health and longevity that are comparable to the effects of whatever medicine, compound, or biological mechanism is actually being tested. If your prospective treatment happens to reduce mouse appetite, easy to achieve accidentally, then it's likely that the resulting study data will appear beneficial - but not for the right reasons. This is why those of us interested in longevity science must carefully examine promising studies to see whether the authors controlled for the effects of varying calorie intake. Care on this front has been slow to arrive, sad to say, and a lot of past data has to be thrown out or taken with a grain of salt because of the ubiquitous effects of calorie restriction.
One wide-ranging and positive result of the modern interest in calorie restriction research is a greater rigor in the diet of laboratory animals. The more of that the better, as results will improve across the board.
This scale tilts the other way as well, however. Regular readers will know that a high and sustained intake of calories eventually overwhelms natural storage mechanisms - deposition of fat tissue - and leads to metabolic dysfunction and more serious conditions such as diabetes and dementia. It is possible to argue that "high and sustained" well describes the calorie intake of laboratory mice in many studies, meaning that there is as much pervasive distortion in this fashion as for calorie restriction elsewhere:
All-they-can-eat diet for lab mice and rats may foster inaccurate test results
The widespread practice of allowing laboratory rats and mice to eat as much as they want may be affecting the outcome of experiments in which scientists use these "test-tubes-on-four-feet" to test new drugs and other substances for toxicity and other effects. ... the millions of lab rodents used in laboratory studies each year have a nutritional status that is different from other test animals. While other test animals are fed meals, rodents have round-the-clock access to food. And eat they do, gaining more weight and more body fat than meal-fed rodents. The authors cite other research indicating that lab rodents with free access to food tend to develop abnormally high blood fat levels, high cholesterol, nerve and heart damage, cancer and other disorders.
Meal-Feeding Rodents and Toxicology Research
Most laboratory rodents used for toxicology studies are fed ad libitum, with unlimited access to food. As a result, ad libitum-fed rodents tend to overeat. Research demonstrates that ad libitum-fed rodents are physiologically and metabolically different from rodents fed controlled amounts of food at scheduled times (meal-fed). Ad libitum-fed rodents can develop hypertriglyceridemia, hypercholesterolemia, diet-induced obesity, nephropathy, cardiomyopathy, and pituitary, pancreatic, adrenal, and thyroid tumors, conditions likely to affect the results of toxicology research studies.
It should go without saying that overfed humans also tend to develop the conditions on that list, amongst others, to a far greater degree than their calorie-controlled peers - which is something to think on.