The Short Version
The absolute claim that insulin resistance "prevents" fat loss is not supported by the evidence. High-authority mechanistic studies show insulin resistance preserves antilipolytic signaling, making fat loss harder — but multiple clinical studies demonstrate that insulin-resistant individuals do lose fat through caloric restriction and exercise, sometimes at rates equal to or exceeding non-insulin-resistant groups. The accurate statement is that insulin resistance impedes or complicates fat loss, not that it categorically blocks it.