Only half of diabetics with poor glucose control develop renal failulre, while 40% of those with good control develop diabetic neuropathy. (S. Rich, "Genetics of diabetes and its complications," Journal of the American Society of Nephrology, vol. 17, p. 353 (2006); M. Centofani, "Diabetes complications: more than sugar?" Science News, vol. 149, no. 26/27, p. 427 (1995))
Recently there has been something of a revolution in views about strict blood sugar control for type 2 diabetics, since studies have shown that there is no benefit of intensive control for them in cardiovascular disease and microangiopathy. Although children with type 2 diabetes have much better glucose control than those with type 1 diabetes, they have much worse macrovascular disease than type 1 patients. (V. Montori and M. Fernandez-Balsells, "Glycemic control in type 2 diabetes: time for an evidence-based about-face?" Annals of Internal Medicine, vol. 150, no. 11, p. 803 (2009; K. Shiga and N, Kikuchi, "Children with type 2 diabetes mellitus have more risks for macrovascular complications," Pediatrics International, May 27, 2009))
Rapid improvement of blood sugar control has long been known to worsen rather than improve diabetic complications. (M. Leow and J. Wyckoff, "Under-recognized paradox of neuropathy from rapid glycemic control," Postgraduate Medical Journal, vol. 81, no. 952, p. 103 (2005): G. Tauber, "Diabetes: paradoxical effects of tightly controlled blood sugar," Science, vol. 323 (5917) 1009 (2009)) It is interesting to compare this with the result noted in pregnant diabetic women, which is that they seem to have better outcomes when their blood sugar levels are kept stable, rather than when they are rendered more unstable by the effort to keep them closer to normal. Perhaps what the body needs most is just glucose homeostasis rather than normoglycemia. Hyperglycemia, because it creates more space for instability, may then be causing complications by worsening the irregularity of the body’s glucose supply rather than by simply elevating the blood sugar level.
Some have held that the worsening of complications from rapid improvement in blood sugar control is followed by an eventual net improvement in complications over time. But one study which followed cases of diabetic retinopathy for 41 months after rapid glucose tightening failed to detect the expected improvement. (O. Brinckmann-Hansen, et al, "The Response of diabetic retinopathy to 41 months of multiple insulin injections, insulin pumps, and coventional insulin therapy," Archives of Ophthalmology, vol. 106, no. 9, p. 1242 (1988)) Perhaps complications are in part due to the repeated, short-lived attempts by patients to bring their blood sugar levels under control, each of which causes damage, and since most people cannot maintain their blood sugar improvement (the DCCT follow-up showed that strict control group participants gave it up after the study), all they wind up doing is making things worse. Homeostasis is vitally important for many physiological functions, and blood pressure, oxygen, nitrogen, carbon dioxide, pH, and potassium all have to be kept constantly in a narrow range or damage occurs, so perhaps this is the case with blood sugar as well.