Integrated Overview of Energy Metabolism
Fuels enter the body from the diet during the digestive phase. Our diet includes both monomeric and polymeric forms (the latter are converted into monomeric forms during digestion and absorption) of the following fuels: (1) monosaccharides, including glucose, fructose, and galactose; (2) long-chain free fatty acids (referred to in this chapter as simply FFAs); and (3) amino acids (AAs). The diet may also include other fuels such as ethanol.
Glucose,fed•UtilizationforATPphase•Storageasglycogen
Glucose,fasting•Breakdownofglycogenandphasereleaseofglucoseintoblood
FFA/TG,fedphase•MakeFFAsfromglucosebyDNL
FFA/TG,fasting•UtilizationforATPphase•UtilizationtoproduceKBs
SecretionofVLDLAAs,fedphase•Utilizationformultipleanabolic pathwaysAAs,fastingphase•Utilizationforgluconeogenesis
UtilizationforketogenesisKBs,fedphase•ShouldbeabsentKBs,fastingphase•SynthesisfromFFAsandsome
AA,aminoacid;ATP,adenosinetriphosphate;DNL,denovolipogenesis;FFA,freefattyacid;G3P,glycerol-3-phosphate;KB,ketonebody;TG,triglyceride;VLDL,verylow-densitylipoprotein.
During the digestive phase, absorbed fuels are partitioned and used for different purposes. Insulin (discussed in detail later) regulates essentially every aspect of metabolism during the digestive phase. Glucose is the primary fuel used for energy (i.e., ATP production) during the digestive phase (
Fig. 39.3A ). Glucose is considered a universal fuel in that most cells can perform the following: 1.
2.
“trap” and “activate” imported glucose by converting glucose into glucose-6-phosphate (G6P) through the activity of one or more hexokinases. G6P cannot pass through GLUT transporters (“trapping”) and is now a substrate for several enzymatic pathways (“activation”).
3.