Jules, I did way in the beginning, when I first started keeping rats. My vet asked me about their diets cause my crew was very unhealthy. I was following a Suebee's diet which is the UK's equivalent and dog food. My vet (exotic) was not impressed. She did some research into it, got an animal nutritionist and other international top exotic colleagues she came back with a no go on that diet. She told me it is very lacking in many nutrients. The dog food is simply not energy dense enough for our rats. Since then, I've done a lot of research on it. Found out that rats actually do far better on very low protein and very low fat diet. Being overweight/obese causes so many ailments in rats. Too much protein and fat causes kidney damage as well as fatty liver. Too much fat contributes to tumours, even pituitary tumours. Sugar causes/feeds cancer. So, my rule of thumb when feeding rats: low protein, low fat, no sugar and no salt. I stick with a high quality rat block and daily veggies and some fruits. I don't like to give to much fruits because of the fructose.
My rats went from obese, lazy, dull coats, porphyrin, leaky eyes, dying young to slim, active, shiny, bright happy rats. My rats used to die at around 20 months, many at 14 months, but since the big change, I've had rats live over 38 months, one at 41 months and Loki at 45 months. The other live beyond 30 months, closer to 36 months. I'm sure there are other variables that factor in. But when you think that my rats don't all come from one source, from all over, several cities, then it rules out strong genetics. My rats are rescue rats, not the best of beginnings so it's really got me sold on my choice of nutrition for rats.