Phasting versus dessert

When fasting (meaning, in this case, not eating any carbohydrates), the pancreas releases a polypeptide hormone, glucagon, that stimulates gluconeogenesis in the liver (1). Glucagon functions via a regulating bifunctional enzyme (1). The enzyme dephosphorylated acts as phophofructokinase-2 (PFK-2) and glucagon induces phosphorylation to produce fructose bisphosphatase-2 (FBPase-2) (1). The suppression of PFK-2 and increased activity of FBPase-2 reduces concentration of fructose 2,6-biphosphate (1).

Fructose 2,6-biphosphate’s presence regulates glycolysis and gluconeogenesis (1). It is the key positive modulator for glycolytic enzyme phophofructokinase (PFK) (not same as PFK-2), and PFK’s increased activity stimulates glycolysis by suppressing activity of fructose biphophatase (not same as FBase-2) (1). The reduced presence of fructose 2,6-biphosphate and, thus, increased activity of gluconeogenesis restores blood glucose levels successfully (1).

Phew! Now on to dessert.

In response to restored blood glucose levels from gluconeogenesis or when breaking a fast, the picture is reversed. The presence of glucose stimulates the pancreas to release insulin (1). Insulin reduces fructose 2,6-biphosphate via dephosphorylation of phosphofructokinase-2, and positively affects activity of glycogen synthase (also through dephosphorylation) stimulating glycogenesis (1).

So just remember, dessert equals dephosphorylation and phasting equals phosphorylation!

Reference List

1. Gropper SS, Smith JL, Groff JL. Advanced Nutrition and Human Metabolism. Belmont, CA: Thomson Wadsworth, 2009.

Published by David Despain, MS, CFS

David is a science and health writer living on Long Island, New York. He's written for a variety of publications including Scientific American, Outside Online, the American Society for Nutrition's (ASN) Nutrition Notes Daily, and Institute of Food Technologists' (IFT) Food Technology magazine and Live! blog. He's also covered new findings reported at scientific meetings including Experimental Biology, AAAS, AOCS, CASW, Sigma Xi, IFT, and others on his personal blog "Evolving Health." David is also an active member of organizations including the National Association of Science Writers (NASW), the American Association for the Advancement of Science (AAAS), the American Society for Nutrition, the Institute of Food Technologists, and the National Audubon Society. David has a master's degree in human nutrition from the University of Bridgeport, and a bachelor's degree in English from University of Illinois at Springfield. He also earned his Certified Food Scientist credential from the Institute of Food Technologists.

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