Adipose tissue does more than store triglycerides. It has become increasingly apparent that fat tissue serves as an extensive endocrine organ. According to Iichiro Shimomura and his team at Osaka University in Japan, a newly discovered metabolic factor produced predominantly by visceral fat mimics the effects of insulin, lowering glucose levels in vivo and in vitro and activating the insulin receptor.1

The protein, which they renamed visfatin, was a known growth factor for early-stage B lymphocytes. "We confirmed that visfatin has a glucose-lowering effect that mimics that of insulin," Shimomura writes in an E-mail. Morris Birnbaum, of the University of Pennsylvania and a Howard Hughes Medical Institute investigator, says he found the data "very convincing" but was puzzled by some unexplained results, such as the positive relationship observed between visfatin and insulin sensitivity. "The paper shows that the more visceral fat they had, the more visfatin the fat produced,...

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