Nocturnal free fatty acids are uniquely elevated in the longitudinal development of diet-induced insulin resistance and hyperinsulinemia

Abstract

RN. Nocturnal free fatty acids are uniquely elevated in the longitudinal development of diet-induced insulin resistance and hyperinsulinemia. Am J Physiol Endocrinol Metab 292: E1590 -E1598, 2007. First published January 30, 2007; doi:10.1152/ajpendo.00669.2006.-Obesity is strongly associated with hyperinsulinemia and insulin resistance, both primary risk factors for type 2 diabetes. It has been thought that increased fasting free fatty acids (FFA) may be responsible for the development of insulin resistance during obesity, causing an increase in plasma glucose levels, which would then signal for compensatory hyperinsulinemia. But when obesity is induced by fat feeding in the dog model, there is development of insulin resistance and a marked increase in fasting insulin despite constant fasting FFA and glucose. We examined the 24-h plasma profiles of FFA, glucose, and other hormones to observe any potential longitudinal postprandial or nocturnal alterations that could lead to both insulin resistance and compensatory hyperinsulinemia induced by a high-fat diet in eight normal dogs. We found that after 6 wk of a high-fat, hypercaloric diet, there was development of significant insulin resistance and hyperinsulinemia as well as accumulation of both subcutaneous and visceral fat without a change in either fasting glucose or postprandial glucose. Moreover, although there was no change in fasting FFA, there was a highly significant increase in the nocturnal levels of FFA that occurred as a result of fat feeding. Thus enhanced nocturnal FFA, but not glucose, may be responsible for development of insulin resistance and fasting hyperinsulinemia in the fat-fed dog model. obesity; diurnal IT HAS TRADITIONALLY BEEN BELIEVED that the development of insulin resistance associated with obesity is due to an increase in the level of circulating free fatty acids (FFA) resulting from an impairment of insulin's ability to suppress lipolysis in adipose tissue (4,7,21). Increased FFA levels have been shown to decrease insulin's ability both to suppress hepatic glucose output and to promote peripheral glucose uptake, which can then result in an increase in fasting glucose (14,15). It has traditionally been thought that this increase in fasting glucose resulting from insulin resistance is responsible for compensatory hyperinsulinemia. Thus increasing FFA by lipid infusion results in development of insulin resistance and a compensatory increase in insulin levels (9) in addition to causing mild fasting hyperglycemia due to stimulation of both glycogenolysis and gluconeogenesis (40). However, studies in several different animal models as well as in humans have not consistently demonstrated increases in fasting FFA or glucose during the development of insulin resistance and hyperinsulinemia during obesity (16,20,22,38,41). Studies conducted in our own laboratory (25, 31) using the fat-fed dog model have found development of insulin resistance with concomitant increases of 90 -150% in basal insulin with no significant changes in either fastin...

Orchestration of Glucose Homeostasis

“…Not only were fasting glucose levels either constant or slightly reduced by the fat intake regimen, but there was absolutely no change in 24-h glucose values (online appendix Fig. A6) (58). Neither did we observe increases in other putative signals, such as cortisol or active GLP-1.…”

“…We considered a group of blood-borne signals that could be put forth as mediating hyperinsulinemia in the face of fat-diet-induced insulin resistance (58). Among these candidates were GLP-1, known to cause pancreatic islet-cell proliferation in rodents, and growth hormone and cortisol, each of which can result in increased insulin.…”

Impaired beta cell glucose sensitivity rather than inadequate compensation for insulin resistance is the dominant defect in glucose intolerance

“…It remains to be firmly established whether chronic small elevations in plasma glucose concentration or other metabolites and hormones (e.g. NEFA or glucagon-like peptide-1 [7,18]) are responsible for revving up the secretory tone.…”

Exenatide Sensitizes Insulin-Mediated Whole-Body Glucose Disposal and Promotes Uptake of Exogenous Glucose by the Liver

“…Blood sampling. Samples for the determination of glucose, free fatty acids (FFAs), insulin, C-peptide, glucagon, and tracer were collected as previously described (16,23). All samples were immediately centrifuged, and plasma was separated and stored at Ϫ80°C until analysis.…”