Adipose tissue secretes a variety of molecules into the circulation. These adipocyte-derived proteins are involved in the maintenance of energy homeostasis, insulin action, glucose and lipid metabolism and therefore sytemic levels were analyzed before and 2 h after glucose uptake. Because most studies focus on overweight probands or patients that may not reflect the normal situation, plasma levels of several adipose tissue secreted proteins were analyzed in the fasted state and 2 h after the ingestion of 75 g glucose in young, slim and highly insulin-sensitive adults.
In contrast to some studies that describe an induction or no alterations of leptin [2–4], systemic leptin was found reduced 2 h after glucose ingestion to 85 ± 8% in our study group and this is in accordance with the findings of Masuo et al. , where insulin-sensitive probands also showed lower leptin levels. Leptin did not change in insulin-resistant subjects in their study  indicating an altered response of systemic leptin in insulin-resistant probands. Leptin is suggested to increase insulin sensitivity or to cause insulin resistance  and obesity is associated with leptin resistance and a decreased transport of leptin across the blood-brain barrier . Leptin appears to exert its function mainly in the hypothalamus. Leptin increases serotonin turnover by inhibition of nitric oxide (NO) synthesis in the brain and endogenous hypothalamic serotonin (5-hydroxytryptamine) is well described to control appetite and caloric intake . Injection of leptin into the ventromedial hypothalamus increases glucose uptake and decreases hepatic glycogen without any alterations of plasma insulin or glucose levels suggesting that peripheral effects of leptin on glucose metabolism are mediated by the central nervous system . Bradykinin and NO were identified to be involved in leptin-mediated glucose uptake in skeletal muscle .
Leptin is not suggested to regulate short-term food intake and central administration of leptin in monkeys reduced food intake the following day . Therefore reduced circulating leptin after carbohydrate ingestion may be an important factor in controlling body weight in slim adults.
In contrast to leptin, adiponectin and resistin were not altered 2 h after glucose uptake and this is in agreement with recent studies [10, 11] indicating that systemic adiponectin and resistin are not influenced upon acute hyperglycemia and/or hyperinsulinemia. Adiponectin enhances insulin signaling by reducing p70 S6 kinase-mediated serine phosphorylation of IRS-1  but systemic adiponectin levels are not influenced by carbohydrate uptake. The mRNA expression of the adiponectin receptors AdipoR1 and AdipoR2 is reduced by refeeding in mice and by insulin in-vitro indicating that adiponectin effects are regulated at the receptor level in the postprandial phase . Resistin is regarded as a proinflammatory protein and was recently shown to block insulin signaling in human hepatocytes  but al least systemic resistin is not influenced by glucose uptake in insulin-sensitive probands.
The adiponectin paralog CORS-26 was shown to be highly abundant in human plasma for the first time whereas it was not detected in mouse serum . Furthermore, similar to adiponectin, CORS-26 circulates as a HMW complex that is converted to a lower molecular weight form by denaturating conditions. Systemic CORS-26 was similar before and after glucose uptake although immunoblot analysis may only be suitable to detect changes of at least 20% and a suitable ELISA is needed to further investigate systemic CORS-26. So far the physiological role of CORS-26 is unclear. E. coli produced CORS-26 stimulates the proliferation of chondrogenic precursors and chondrocytes thereby regulating chondrogenesis and cartilage development . In contrast to adiponectin that is only synthesized by adipocytes, CORS-26 is also expressed in kidney cells and monocytes . Nevertheless CORS-26 may exert similar functions as adiponectin, and antiinflammatory effects  and the formation of HMW complexes, by both proteins further supports this hypothesis. Therefore the effect of recombinant CORS-26 on insulin signaling has to be investigated to further evaluate the physiological role of this protein.
Omentin, initially identified as intelectin-1 expressed in intestinal paneth cells [28, 29], is highly abundant in the stroma vascular fraction of omental adipose tissue . Omentin was detected in the sera of three donors and the concentration was determined to be 100 ng to 1 μg/ml . This protein recognizes galactofuranose and plays a role in the recognition of bacteria-specific components in the host . In the current study omentin was easily detected in plasma by immunoblot but levels were not markedly influenced upon glucose uptake nor related to the gender of the donor. Omentin enhances insulin-stimulated glucose uptake in subcutaneous and visceral human adipocytes in-vitro  and therefore systemic omentin may influence postprandial glucose levels. However, systemic omentin levels did not correlate to postprandial blood glucose indicating that at least circulating omentin is not directly associated with postprandial glucose levels in vivo.