The events that contribute to the expansion of β-cell mass and enhanced β-cell function in insulin-resistant states have not been elucidated fully. Recently, we showed that β-cell adaptation failed dramatically in adult, insulin-resistant POKO mice, which contrasts with the appropriate expansion of β cells in their ob/ob littermates. Thus, we hypothesised that characterisation of the islets in these mouse models at an early age should provide a unique opportunity to: (1) identify mechanisms involved in sensing insulin resistance at the level of the β cells, (2) identify molecular effectors that contribute to increasing β-cell mass and function, and (3) distinguish primary events from secondary events that are more likely to be present at more advanced stages of diabetes. Our results define the POKO mouse as a model of early lipotoxicity. At 4 weeks of age, it manifests with inappropriate β-cell function and defects in proliferation markers. Other well-recognised pathogenic effectors that were observed previously in 16-week-old mice, such as increased reactive oxygen species (ROS), macrophage infiltration and endoplasmic reticulum (ER) stress, are also present in both young POKO and young ob/ob mice, indicating the lack of predictive power with regards to the severity of β-cell failure. Of interest, the relatively preserved lipidomic profile in islets from young POKO mice contrasted with the large changes in lipid composition and the differences in the chain length of triacylglycerols in the serum, liver, muscle and adipose tissue in adult POKO mice. Later lipotoxic insults in adult β cells contribute to the failure of the POKO β cell. Our results indicate that the rapid development of insulin resistance and β-cell failure in POKO mice makes this model a useful tool to study early molecular events leading to insulin resistance and β-cell failure. Furthermore, comparisons with ob/ob mice might reveal important adaptive mechanisms in β cells with either therapeutic or diagnostic potential.