It is often stated that the DCCT proved that tight control or lowered
HbA1c reduces complications. This is not the case. The controlled
variable in the DCCT was intensive vs conventional therapy, and
intensive therapy was defined by several factors including a team of
skilled health care professionals acting in partnership with the
patient. The results show that intensive therapy results in both
lowered HbA1c and fewer complications, but do not show that one causes
the other. The lead authors provide a good summary of this point in a
followup (NEJM 330:642, March 3, 1994):

     We want to stress that the most valid interpretation of the trial
     is that intensive therapy, with the **goal** of achieving blood
     glucose concentrations as close to the nondiabetic range as
     possible, delays the onset and slows the progression of long-term
     diabetic complications. The secondary analyses support the notion
     that lower glycosylated hemoglobin values are associated with a
     lower risk of progression of retinopathy, but they do not prove
     that hyperglycemia in itself causes retinopathy. [emphasis added]

Many of us believe, and believed before the DCCT, that actually
achieving good control aids our health. The DCCT adds weight to this
case but does not prove the point.