The entry of fuels into the body occurs during the digestive phase of the diet. Our diet consists of monomeric and polymeric forms of fuels, with the latter being converted into monomeric forms through digestion and absorption. The fuels present in our diet include monosaccharides (such as glucose, fructose, and galactose), long-chain free fatty acids (referred to as FFAs), amino acids (AAs), and possibly ethanol.

Glucose in the fed phase is utilized for ATP production and can also be stored as glycogen. During fasting, glycogen is broken down, and glucose is released into the bloodstream. In the fed phase, FFAs and triglycerides (TGs) are used to produce FFAs through de novo lipogenesis (DNL). In the fasting phase, FFAs and TGs are utilized for ATP production and to produce ketone bodies (KBs). In the fed phase, very low-density lipoprotein (VLDL) aids in the secretion of AAs, which are utilized in multiple anabolic pathways. In the fasting phase, AAs are utilized for gluconeogenesis.

During the digestive phase, the absorbed fuels are partitioned and used for different purposes. Insulin, which will be discussed in detail later, regulates various aspects of metabolism during this phase. Glucose serves as the primary fuel for energy production (ATP) during the digestive phase. Glucose is considered a universal fuel as most cells have the ability to trap and activate imported glucose. This is achieved by converting glucose into glucose-6-phosphate (G6P) through the activity of one or more hexokinases. G6P cannot pass through GLUT transporters, hence it gets trapped, and becomes a substrate for several enzymatic pathways.
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.