From owner-metabolic-reg@net.bio.net Tue Aug 06 23:00:00 1996 Path: biosci!agate!spool.mu.edu!munnari.OZ.AU!news.ecn.uoknor.edu!news.cis.okstate.edu!vms.ocom.okstate.edu!banschbach From: banschbach@vms.ocom.okstate.edu (Martin Banschbach, Ph.D.) Newsgroups: bionet.metabolic-reg,sci.med,sci.med.nutrition Subject: Re: metabolic adaptations during prolonged fasting / starvation Date: 7 Aug 96 10:46:58 CST Organization: College of Osteopathic Medicine, O.S.U. Lines: 43 Message-ID: <1996Aug7.104658@vms.ocom.okstate.edu> References: <96123183wnr@concern1.demon.co.uk> NNTP-Posting-Host: vms.ocom.okstate.edu Xref: biosci bionet.metabolic-reg:809 sci.med:133825 sci.med.nutrition:48398 In article <96123183wnr@concern1.demon.co.uk>, steve collins writes: > Is there any evidence for metabolic adaptations to spare protein during > starvation / prolonged fasting? Much of what I read seems to be saying > that after the initial loss of labile protein the > utilistaion of protein / kg body weight (as reflected by urinary N > losses) is constant (i.e. no metabolic adaptations to spare protein). I > find this conclusion slightly at odds with the observations that obese > people survive very much longer during prolonged fasting than > leaner people. If anybody could shed any light on > where my thinking is going wrong I'd be most greatful -- steve collins Gluconeogenesis (the process of converting noncarbohydrate material to glucose) starts to increase about four hours after the last meal and it peaks at about 10 grams of glucose produced per hour about 2-3 days after the last meal is consumed. Gluconeogenesis then decreases to about 6 grams of glucose produced per hour until about 12 days after the last meal at which time it increases very slowly upto 40 days (the longest time period that I have) where it's producing about 8 grams of glucose per hour. The source of the carbon for the glucose production is amino acids (from the breakdown of skeletal muscle protein and also from internal organs if the fast lasts long enough), lactate, glycerol and odd chain fatty acids (only the last 3-carbon fragment). There is adaptation to decrease the breakdown of muscle protein during starvation (fasting) but the process never stops. It is possible to loose enough skeletal muscle mass to become too weak to move and therefore find food (or more importantly water) and die before all of the fat energy reserve is gone. In addition, a prolonged fast also decreases the weight of all internal organs. The main sparing element for protein is the production of ketone bodies from stored fat. As the stored fat reserve gets depleted, ketone body production decreases and more protein has to be used. That data I quoted is for non-obese individuals. For obese individuals, the fat reserve is large enough to go more than 12 days without seeing a reduction in ketone body formation (meaning that the production of glucose via gluconeogenesis would not start to increase at about 12 days and the protein would be "spared" for a longer time period). -- Marty B. "You are what you eat" .