An average meal contains all three of the major nutrients—
carbohydrate, protein, and fat—with carbohydrate consti-
tuting most of the meal’s energy content (calories). Recall
from Chapter 15 that carbohydrate and protein are absorbed
primarily as monosaccharides and amino acids, respectively,
into the blood supplying the gastrointestinal tract. The blood
leaves the gastrointestinal tract to go directly to the liver by
way of the hepatic portal vein. This allows the liver to alter
the nutrient composition of the blood before it returns to the
heart to be pumped to the rest of the body. In contrast to
carbohydrate and amino acids, fat is absorbed into the
as triglycerides in chylomicrons. The lymph then drains into
the systemic venous system. Thus, the liver cannot ﬁ rst modify
absorbed fat before it reaches other tissues.
Some of the carbohydrate absorbed from the gastrointestinal
tract is galactose and fructose. Because these sugars are either
converted to glucose by the liver or enter essentially the same
metabolic pathways as glucose, we will simply refer to absorbed
carbohydrate as glucose.
Glucose is the body’s major energy source during the
absorptive state. Much of the absorbed glucose enters cells and
is catabolized to carbon dioxide and water, providing energy for
ATP formation. Skeletal muscle makes up the majority of body
mass, so it is the major consumer of glucose, even at rest. Skeletal
muscle not only catabolizes glucose during the absorptive phase,
but also converts some of the glucose to the polysaccharide gly-
cogen, which is then stored in the muscle for future use.
Adipose tissue cells (adipocytes) also catabolize glucose
for energy, but the most important fate of glucose in adipo-
cytes during the absorptive phase is its transformation to fat
(triglycerides). Glucose is the precursor of both
phosphate and fatty acids, and these molecules are then linked
together to form triglycerides.
Another large fraction of the absorbed glucose enters the
liver cells. This is a very important point: During the absorp-
tive period, there is net
of glucose by the liver. It is
either stored as glycogen, as in skeletal muscle, or transformed
-glycerol phosphate and fatty acids, which are then used
to synthesize triglycerides, as in adipose tissue. Some of the
fat synthesized from glucose in the liver is stored there, but
most is packaged, along with speciﬁ c proteins, into molecular
aggregates of lipids and proteins called
aggregates are secreted by the liver cells and enter the blood.
They are called
very-low-density lipoproteins (VLDL)
because they contain much more fat than protein, and fat is
less dense than protein. The synthesis of VLDLs by liver cells
occurs by processes similar to those for the synthesis of chylo-
microns by intestinal mucosal cells, as Chapter 15 described.
Because of their large size, VLDL complexes do not read-
ily penetrate capillary walls, once in the bloodstream. Instead,
their triglycerides are hydrolyzed mainly to monoglycerides
Almost all tissues
O + energy
Energy + CO
Major metabolic pathways of the postabsorptive state. The central focus is regulation of the blood glucose concentration. All arrows between
boxes denote transport of the substance via the blood.