The Glycemic Index not the most effective nutrition therapy intervention.

Based on the above reviews, the ADA position is that there is not strong supporting evidence for the use of a low-GI diet as a primary nutrition therapy strategy. The meta-analysis by Brand-Miller et al. provides support for this recommendation. They report that implementing a low-GI diet will have a small effect on overall glycemic control in diabetes; low-GI diets reduce overall HbA_1c by 0.43% units compared with high-GI diets (representing a 7.4% reduction in HbA_1c/fructosamine). In patients with type 1 diabetes, HbA_1c was reduced by ~0.4% units and in subjects with type 2 diabetes by ~0.2% units. This latter result is surprising, as one would expect a greater response in subjects with type 2 diabetes who, because of loss of first-phase insulin response, might be expected to benefit more from a low-GI diet than subjects with type 1 diabetes in whom mealtime insulin can be adjusted to control postprandial glycemic responses.

 http://care.diabetesjournals.org/content/26/8/2466.full