Regulation of Organic Metabolism and Energy Balance
Endocrine and Neural Control
of the Absorptive and
We now turn to the endocrine and neural factors that control and
integrate these metabolic pathways. We will focus primarily on
the following questions, summarized in
: (1) What
controls net anabolism of protein, glycogen, and triglyceride in
the absorptive phase, and net catabolism in the postabsorptive
phase? (2) What induces the cells to utilize primarily glucose
for energy during the absorptive phase, but fat during the post-
absorptive phase? (3) What stimulates net glucose uptake by
the liver during the absorptive phase, but gluconeogenesis and
glucose release during the postabsorptive phase?
The most important controls of these transitions
from feasting to fasting, and vice versa, are two pancreatic
hormones—insulin and glucagon. Also playing a role are
the hormones epinephrine and cortisol from the adrenal
glands and the sympathetic nerves to liver and adipose tissue.
Insulin and glucagon are peptide hormones secreted
islets of Langerhans
(or, simply, pancreatic islets),
clusters of endocrine cells in the pancreas. There are several
distinct types of islet cells, each of which secretes a different
Summary of Nutrient Metabolism
During the Postabsorptive Period
Glycogen, fat, and protein syntheses are curtailed, and net
Glucose is formed in the liver both from the glycogen
stored there and by gluconeogenesis from blood-borne
lactate, pyruvate, glycerol, and amino acids. The kidneys
also perform gluconeogenesis during a prolonged fast.
The glucose produced in the liver (and kidneys) is released
into the blood, but its utilization for energy is greatly
reduced in muscle and other nonneural tissues.
Lipolysis releases adipose tissue fatty acids into the blood,
and the oxidation of these fatty acids by most cells and of
ketones produced from them by the liver provides most of
the body’s energy supply.
The brain continues to use glucose but also starts using
ketones as they build up in the blood.
Summary of critical points in transition from the absorptive state to the postabsorptive state. The term
could be replaced with
actions of insulin,
and the term
results of decreased insulin.
The numbers at the left margin refer to discussion questions
in the text.