Nutrition and lifestyle interventions have been a critical component in three of the four largest clinical trials that focused on diabetes in the past two decades. Evidence of the effectiveness of nutrition and lifestyle interventions with resepect to achieving diabetes-related outcomes for diabetes prevention and a reduction in diabetes complications for people with diabetes is clearly mounting. The Diabetes Control and Complications Trial (1983–1993), the Diabetes Prevention Program (1996–2001) and the Look AHEAD (Action for Health in Diabetes) Trial (2001–2012) have been providing key evidence for the dietitian’s role in delivering effective nutrition and lifestyle interventions for people with prediabetes, type 1 and type 2 diabetes. The dietitians in these multicentre efficacy trials contributed to this evidence base by expanding their roles beyond implementing the protocol defined goal-based nutrition and lifestyle interventions to conducting ancillary research and using problem-solving strategies that tailor counselling approaches toward participants’ barriers to goal achievement. As lifestyle coaches and case managers, dietitians had the opportunity to work with the same group of study participants over extended periods of time and use their clinical and research expertise to uncover important insights and strategies that helped achieve clinical goals related to glycaemia control, weight loss and activity. The present review will ‘chart the course’ of how the evidence base for nutrition and lifestyle interventions emanated from these trials and discuss the implications for clinical dietetic practice. Dietitians can use insights gleaned from these experiences with the research process to expand their roles and guide cutting edge evidence-based clinical dietetic practice in diabetes.