Complex sentence: This was done by studying animals which have their metabolism changed to reduce insulin uptake: it seemed that these animals can have a normal dietary intake, but retain the leanness of animals in the earlier studies, and have a similarly increased lifespan.
Split into simpler sentences: This was done by studying animals which have their metabolism changed to reduce insulin uptake, consequently retaining the leanness of animals in the earlier studies. It was observed that these animals can have a normal dietary intake, but gain a similarly increased lifespan.