Dahl (bred or genetically modified) salt-sensitive rats. Breeding is a form of genetic manipulation. Its not clear wether the rats were genetically modified to be sensitive to salt or just bred.
Is there any evidence that the genetic variants themselves caused the sensitivity not only to salt but also fat?
If the fats fed to the rats are not natural to the rat. You will expect negative results. If you feed the rat beef fat, its system would not know what to do with it. As most rats live on grains a high fat diet would give adverse effects one would think.
Humans are omnivorous. But most people would be more carnivorous. Thus, we have been consuming fat since we learned to hunt. Yet we are all still here. I don't think fat is natural to the diet of rats. Perhaps this experiment should be repeated on a carnivore.
Type of fat matters. Fish fat is healthy and protective for humans. We also know the trans fats undo all the good work fish fats do.