I have used C57BL6J mice in multiple cohorts of high-fat diet induced obesity and metabolic syndrome using pelleted food from Research Diets (and have also collabed on a study using Wistar). They eat basic chow during the acclimation period, but for the duration of the study the control group eats a 10% fat diet while tx groups eat either 45% fat (MetS diet) or 60% fat diet. These diets are also very high in saturated fat. The big factor to consider is that the 10% fat diet is higher in sucrose than the 45/60% fat diets in order to be isocaloric. Also, rodents tend to eat more of the high-fat diet food compared to low-fat or chow. After 4-5 weeks we see insulin resistance in both diets during GTT/ITT. Hope that helps. :)