Reply to comment on: Godsland IF, Jeffs JA, Johnston DG (2004) Loss of beta cell function as fasting glucose increases in the non-diabetic range. Diabetologia 47:1157–1166

Abstract

To the Editor: As we stated in our original paper [1], our intention was to define the level of glycaemia at which pancreatic insulin secretion begins to decline. We were careful throughout to emphasise that we were concerned not with causal relationships but with identifying the principal accompaniments of deteriorating fasting plasma glucose (FPG) levels in the sub-diabetic range. We agree with Abdul-Ghani and DeFronzo that the decline in IVGTT first-phase insulin secretion that accompanies the rise in FPG does not necessarily indicate a causal relationship [2]. Nevertheless, in our study, the decline in first-phase pancreatic insulin secretion was by far the most striking feature that accompanied increasing glucose levels. As Abdul-Ghani and DeFronzo affirm, the important aspect of this decline was that it occurred within ranges of glucose concentrations hitherto considered normal.In drawing attention to glucotoxicity and the sensitivity of pancreatic insulin secretion to rising glucose levels, Abdul-Ghani and DeFronzo raise an important mechanistic possibility. This does, however, pose a potential 'chicken and egg' problem, since glucose concentrations would have to rise for there to be deficient pancreatic insulin secretion, but there might have to be deficient insulin secretion for glucose concentrations to rise. Which comes first? One possibility, which they allude to, is that it is not decreasing insulin secretion that sets glucose levels rising but decreasing insulin sensitivity. Our finding that there was little change in insulin sensitivity with increasing FPG appears contrary to the weight of evidence in this respect.Abdul-Ghani and DeFronzo raise the possibility that our IVGTT protocol was not sensitive enough to detect impaired insulin sensitivity [2]. That this is not the case is illustrated by our previous publications, which attest to the ability of our protocol to detect diminished insulin sensitivity in a variety of circumstances and to detect significant associations with metabolic correlates of insulin sensitivity at a strength equivalent to that seen with other protocols and methodologies (see [3][4][5][6] for examples). Abdul-Ghani and DeFronzo also question the relevance of conclusions regarding pancreatic insulin secretion based on the non-physiological intravenous route of administration. However, they point out that, consistent with our results [1], in their own studies using the OGTT [7] there was a decline in the OGTT plasma insulin response with increasing FPG at levels >5.6 mmol/l. It is also noteworthy that in the study of Ozaki et al. of 8,923 men and women in Japan, in which OGTT insulin concentrations at 30 and 120 min provided indices of early-and late-phase insulin secretion, relationships were noted between plasma insulin concentrations and FPG levels in the range 4.4-8.3 mmol/l that were similar to those we distinguished between pancreatic insulin secretion and FPG [8].With regard to the apparent lack of association between insulin sensitivity and FPG in our study, the crit...