Regulation of Organic Metabolism and Energy Balance
567
Events of the Absorptive and
Postabsorptive States
The regular availability of food is a very recent event in
the history of humankind, and indeed is still not univer-
sal. Thus, it should not be surprising that mechanisms have
evolved for survival during alternating periods of plenty
and fasting. There are two functional states or periods the
body undergoes in providing energy for cellular activities:
the
absorptive state,
during which ingested nutrients enter
the blood from the gastrointestinal tract, and the
post-
absorptive state,
during which the gastrointestinal tract is
empty of nutrients and the body’s own stores must supply
energy. Because an average meal requires approximately four
hours for complete absorption, our usual three-meal-a-day
pattern places us in the postabsorptive state during the late
morning, again in the late afternoon, and during most of
the night. We will refer to going more than 24 h without
eating as fasting.
During the absorptive period, some of the ingested nutri-
ents supply the energy needs of the body, and the remainder are
added to the body’s energy stores, to be called upon during the
next postabsorptive period. Total-body energy stores are ade-
quate for the average person to withstand a fast of many weeks,
provided water is available.
Figures 16–1
and
16–2
summarize the major pathways
to be described in this chapter. Although they may appear
formidable at fi rst glance, they should present little diffi culty
after we have described the component parts. You should refer
to these fi gures frequently during the following discussion.
SECTION A
Control and Integration of Carbohydrate,
Protein, and Fat Metabolism
Begin
Muscle
Glycogen
Glucose
Protein
Amino
acids
Triglycerides
Adipose tissue
Fatty acids
Glucose
Fatty acids Monoglycerides
α
-glycerol
phosphate
Almost all tissues
CO
2
+ H
2
O + energy
Glucose
All tissues
Amino acids
α−
ketoacids
NH
3
Triglycerides
(VLDL)
Liver
α
-glycerol
phosphate
CO
2
+ H
2
O + energy
Urea
Glucose
Glycogen
Fatty acids
GI tract
Glucose (galactose, fructose)
Triglycerides
Amino acids
(Chylomicrons)
Figure 16–1
Major metabolic pathways of the absorptive state. The arrow from amino acids to protein is dashed to denote the fact that excess amino acids
are not stored as protein (see text). All arrows between boxes denote transport of the substance via the blood. VLDL
=
very-low-density
lipoproteins. Energy
=
ATP.
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