AbstractBackgroundEffect of intensive lifestyle intervention (ILI) on A1C in participants with diabetes is underestimated. A1C improvement is presumed to be dependent on the amount of weight loss. Here, we evaluate the magnitude of A1C change in relation to baseline A1C and the amount of weight loss in participants with diabetes who underwent ILI over 13 years in real‐world clinical practice.MethodsA total of 590 participants with diabetes were enrolled in the Weight Achievement and Intensive Treatment (Why WAIT) program, a 12‐week multidisciplinary ILI program designed for real‐world clinical practice between September 2005 and May 2018. We stratified participants based on baseline A1C into three groups: group A: A1C ≥ 9%, group B: A1C 8 to <9%, and group C: A1C ≥6.5% to <8%.ResultsAfter 12‐weeks of intervention, body weight decreased in all groups, and pairwise comparisons of A1C changes showed that: group A had 1.3% greater A1C reduction than group B (p= 0.0001) and 2% greater than group C (p= 0.0001), while group B had 0.7% greater A1C reduction than group C (p= 0.0001).ConclusionWe conclude that ILI may decrease A1C by up to 2.5% in participants with diabetes. At similar magnitude of weight loss, A1C reduction was more prominent in participants with higher baseline A1C. This may be valuable for clinicians to set a realistic expectation of A1C change in response to ILI.