In the current study, we have applied metabolic profiling to investigate the kinetics of human plasma biochemicals in response to an oral glucose challenge. To our knowledge, this is the first study to apply a profiling technology to characterize this physiologic program. The systematic approach we have taken has enabled us to recapitulate virtually all known polar metabolite changes associated with the OGTT, while spotlighting some pathways never linked to this program. Importantly, simultaneous measurement of multiple metabolites made it possible to explore connections among metabolic pathways, providing novel insights into normal physiology and disease.
One of the most surprising findings of this investigation is the dramatic increase in bile acid levels following glucose ingestion. In response to eating, the gallbladder contracts and releases bile into the intestines. Bile acids are then absorbed and travel through the portal vein back to the liver, where the enterohepatic cycle completes. Although the uptake of bile acids to the liver is fairly efficient, a constant fraction (10–30%) reaches the systemic circulation (Angelin et al, 1982
). In a previous investigation, human subjects ingesting a standard liquid meal containing fat and protein exhibited an increase of 320–330% in individual serum bile acids (De Barros et al, 1982
). In the current study, we found that ingesting glucose alone elicits a bile acid response of similar magnitude (), which is sustained for 2 h. Our finding is supported by previous work showing that glucose ingestion can increase the plasma concentration of cholecystokinin, a hormone signaling the gallbladder to contract (Liddle et al, 1985
). Interestingly, a recent investigation has shown that bile acids are a signal for fat and muscle cells to increase their energy expenditure through activation of thyroid hormone (Watanabe et al, 2006
). Given these findings, and our observation of bile acid release following glucose ingestion, we tested the hypothesis that bile acids influence peripheral glucose uptake. We could not detect an effect of bile acids on basal or insulin-stimulated glucose uptake, however, in cultured adipocytes (data not shown). At present, the physiologic role of sustained bile acid release following glucose ingestion is unknown.
Although insulin resistance is traditionally defined as the reduced ability of insulin to promote glucose uptake or as the impaired suppression of gluconeogenesis, resistance can emerge in other insulin-dependent processes. In prior studies, for example, inadequate suppression of lipolysis was observed in women with a history of gestational diabetes (Chan et al, 1992
), and an elevated proteolysis rate was seen in individuals with HIV-associated insulin resistance (Reeds et al, 2006
). In obesity, manifestations of insulin resistance include elevated rates of lipolysis (Robertson et al, 1991
) and proteolysis (Jensen and Haymond, 1991
; Luzi et al, 1996
). We have used metabolic profiling to monitor insulin action across multiple axes (), and with markers of each axis in hand, were able to detect kinetic differences between them. In healthy individuals, insulin's suppression of lipolysis and ketogenesis is rapid compared to its suppression of proteolysis (), consistent with the different concentrations of insulin required to inhibit each of these processes (Fukagawa et al, 1985
; Nurjhan et al, 1986
). Such differences in the activation thresholds of metabolic pathways could be contributing to the diversity in the clinical presentation of insulin resistance. Mouse models of insulin-resistant T2DM provide another example for inter-pathway differences in insulin sensitivity: in the livers of these mice, gluconeogenesis is resistant to suppression by insulin, whereas lipogenesis is responsive, resulting in hyperglycemia and hypertriglyceridemia (Brown and Goldstein, 2008
). Collectively, these observations demonstrate that environmental or genetic perturbations could lead to selectivity in insulin sensitivity and contribute to pathogenesis.
Our study demonstrates that an individual's ‘insulin response profile', namely, the vector of sensitivities to insulin action along multiple physiologic axes, can be revealed by metabolite excursions in response to a glucose challenge. Specifically, we have shown that excursions in Leu/Ile and glycerol, reflecting the sensitivity of proteolysis and lipolysis to the action of insulin, are jointly predictive of fasting insulin, with each of the two excursions offering complementary and significant explanatory power (). These findings demonstrate that in two individuals, different axes could be responsible for the same elevation of fasting insulin, and simultaneous measurement of metabolites can distinguish between the two conditions. Monitoring the response to glucose ingestion across multiple axes in larger, prospective clinical studies of pre-diabetics could establish links between insulin sensitivity profiles and disease progression, thus helping to predict future diabetes and its complications as well as to guide therapeutic interventions.