Breeding itself shortens a rats life, and the more often they breed, the more taxing it is on the rats longevity. The goal of high quality rat feed is to provide everything a rat needs to thrive and breed well. Just because they die sooner does not mean the food itself is the direct cause of the shortened life span, but because they breed.

Look at fancy rat keepers. They recognize the value of a quality lab block in a rats diet like Mazuri or Harlan, yet their animals can live for up to 4 or remarkably even 5 years, because their main goal is to breed for companions and may only breed a rat once it's whole life, not to produce as many as possible.

It's the stressful lifestyle we put on rats to raise our feeders that shortens life span, not the food itself.