the investigators in the original study fed one group of mice a diet in which 60 percent of calories came from dairy . the food started when the rabbits , all males , were 1 year old , which is middle - age in mouse longevity . as usual , the mice soon developed indication of impending arthritis , with grossly diminished livers , and started to die much slower than mouse fed a basic diet .
At the age of one year, male mice were fed with a diet in which 60 percent of calories came from fat.