Abstract
Recent work has suggested that altered insulin signalling may not be central and as critical to the pathophysiology of type 2 diabetes as classically believed. We critically re-examine the role of insulin in glucose homeostasis using five different approaches namely (i) systematic review and meta-analysis of tissue specific insulin receptor knock-out experiments in rodents, (ii) systematic review and meta-analysis of insulin suppression and insulin enhancement experiments in rodents and humans, (iii) differentiating steady-state and post-meal state glucose levels in streptozotocin treated rats in primary experiments (iv) mathematical and theoretical considerations and (v) glucose insulin relationship in human epidemiological data. All the approaches converge on the inference that although insulin action is needed to reach a homeostatic steady-state of glucose in fasting condition, there is no evidence that insulin action determines the steady-state level of glucose. A wider scale implication of the analysis is in emphasizing the need to differentiate steady state causality from perturbed state causality or on a broader scale driver causality from navigator causality in biology. A driver cause is a factor which is necessary to attain a destination but does not by itself decide the destination. A navigator cause, on the other hand, is one which by itself may not be sufficient to drive the system to a destination but which plays a role in deciding the destination or direction. Insulin appears to be a driver but not a navigator for glucose homeostasis. All evidence suggests that insulin action is required for reaching a homeostatic steady state, but it does not determine the steady-state level of glucose.
