In a study involving 52 sedentary, overweight, middle-aged men and women (aged 45-65), both exercise and calorie restriction had positive — and equal — effects on insulin sensitivity, but a combination of both had twice as much benefit for glucoregulation as either single approach. All three programs were designed to achieve about the same amount of weight loss (6-8%).
The study also indicated that both exercise and calorie restriction improve regulation of glucose levels through weight loss, but also through mechanisms that are independent of weight loss.
What all this suggests is that, even if you're maintaining a healthy weight, how much you eat, and whether you exercise, are factors that have health implications.
Weiss, E. P., Albert, S. G., Reeds, D. N., Kress, K. S., Ezekiel, U. R., McDaniel, J. L., … Villareal, D. T. (2015). Calorie Restriction and Matched Weight Loss From Exercise: Independent and Additive Effects on Glucoregulation and the Incretin System in Overweight Women and Men. Diabetes Care, 38(7), 1253–1262. http://doi.org/10.2337/dc14-2913