Metabolic profiling of the human response to a glucose challenge reveals distinct axes of insulin sensitivity

Abstract

Glucose ingestion after an overnight fast triggers an insulin‐dependent, homeostatic program that is altered in diabetes. The full spectrum of biochemical changes associated with this transition is currently unknown. We have developed a mass spectrometry‐based strategy to simultaneously measure 191 metabolites following glucose ingestion. In two groups of healthy individuals ( n =22 and 25), 18 plasma metabolites changed reproducibly, including bile acids, urea cycle intermediates, and purine degradation products, none of which were previously linked to glucose homeostasis. The metabolite dynamics also revealed insulin's known actions along four key axes—proteolysis, lipolysis, ketogenesis, and glycolysis—reflecting a switch from catabolism to anabolism. In pre‐diabetics ( n =25), we observed a blunted response in all four axes that correlated with insulin resistance. Multivariate analysis revealed that declines in glycerol and leucine/isoleucine (markers of lipolysis and proteolysis, respectively) jointly provide the strongest predictor of insulin sensitivity. This observation indicates that some humans are selectively resistant to insulin's suppression of proteolysis, whereas others, to insulin's suppression of lipolysis. Our findings lay the groundwork for using metabolic profiling to define an individual's ‘insulin response profile’, which could have value in predicting diabetes, its complications, and in guiding therapy. ### Synopsis The human body strives to control the concentration of blood glucose within a narrow range, by using counterbalancing hormones. When we ingest sugar, the surge in blood glucose triggers the release of the hormone insulin, which acts to dispose of the extra glucose. An inability to lower the glucose levels leads to diabetes, a disease whose prevalence is quickly rising. Insulin released in response to glucose ingestion also switches the body from a fasting state to a fed state, reducing the consumption of energy sources and triggering fuel storage. The processes involved in this transition are important to understand, as they determine the body's ability to adapt its metabolism to varying conditions. Focused studies over the past few decades revealed much on the fasting–feeding transition, but a global analysis of the metabolic processes involved in this transition has not been possible. Today, new technologies permit the simultaneous monitoring of large collections of metabolites (small molecules participating in metabolism). In the current study, we applied such a profiling technology to measure metabolite changes in blood of healthy individuals in response to an oral glucose challenge. We identified 21 metabolites whose levels change significantly ([Figure 1C][1]), and with the exception of glucose, all those alterations were present 2 h after the ingestion. 18 of these 2‐h metabolite changes also replicated reliably in an independent group of healthy individuals. Metabolites responding to glucose ingestion span multiple chemical classes and biological pathways, and interestingly, point to pathways that have not been previously linked to the maintenance of glucose levels, such as bile acid secretion, urea synthesis and nucleotide degradation. For insulin to drive the transition from a fasting state to a fed state, the processes involved in the transition must be sensitive to insulin's action. Disposal of blood glucose, for example, requires that the process of glucose uptake be sensitive to stimulation by insulin. Loss of sensitivity impairs the body's ability to adapt, and is an early sign of type II diabetes. While the sensitivity of glucose uptake to insulin has been studied extensively, less is known about the sensitivity of other processes. Through metabolic profiling of the response to glucose ingestion in healthy individuals, we have identified metabolite changes that reflect insulin's action in four distinct axes: suppression of fat breakdown, suppression of protein breakdown, suppression of ketone body synthesis and stimulation of glucose metabolism. We next asked how these metabolic responses are altered in pre‐diabetic individuals. In all four axes, we found that individuals with lower sensitivity (as determined by elevated fasting insulin levels) exhibited a blunted metabolic response ([Figure 5A][2]). Interestingly, there were individuals with similar elevations of fasting insulin who exhibited different profiles: some were less sensitive to insulin's suppression of fat breakdown, whereas others were less sensitive to insulin's suppression of protein breakdown ([Figure 5B][2]). These findings suggest the existence of multiple, uncorrelated dimensions of insulin sensitivity within pre‐diabetic populations. Our findings suggest that there might be value in measuring insulin sensitivity along several physiological dimensions—not only with respect to glucose metabolism. By coupling the measurement of only four metabolites to the common oral glucose tolerance test, ‘an insulin sensitivity profile’ could be readily defined for an individual. In future longitudinal studies, it would be interesting to explore the association between loss of sensitivity in a specific axis and the clinical outcome. Findings from such studies might guide the choice of therapy for pre‐diabetic individuals and help predict future complications. Mol Syst Biol. 4: 214 [1]: #F1 [2]: #F5