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Food Intake and Starvation Induce Metabolic Changes

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Food Intake and Starvation Induce Metabolic Changes
Figure 30.14. Electron Micrograph of Liver Cells. The liver plays an essential role in the integration of metabolism.
[Courtesy of Dr. Ann Hubbard.]
III. Synthesizing the Molecules of Life
30. The Integration of Metabolism
30.2. Each Organ Has a Unique Metabolic Profile
Figure 30.15. Insulin Secretion. The electron micrograph shows the release of insulin from a pancreatic β cell. One
secretory granule is on the verge of fusing with the plasma membrane and releasing insulin into the extracellular space,
and the other has already released the hormone. [Courtesy of Dr. Lelio Orci. L. Orci, J.-D. Vassalli, and A. Perrelet. Sci.
Am. 259 (September 1988):85 94.]
III. Synthesizing the Molecules of Life
30. The Integration of Metabolism
30.3. Food Intake and Starvation Induce Metabolic Changes
We shall now consider the biochemical responses to a series of physiological conditions. Our first example is the starvedfed cycle, which we all experience in the hours after an evening meal and through the night's fast. This nightly starvedfed cycle has three stages: the postabsorptive state after a meal, the early fasting during the night, and the refed state after
breakfast. A major goal of the many biochemical alterations in this period is to maintain glucose homeostasis that is, a
constant blood-glucose level.
1. The well-fed, or postabsorptive, state. After we consume and digest an evening meal, glucose and amino acids are
transported from the intestine to the blood. The dietary lipids are packaged into chylomicrons and transported to the
blood by the lymphatic system. This fed condition leads to the secretion of insulin, which is one of the two most
important regulators of fuel metabolism, the other regulator being glucagon. The secretion of the hormone insulin by the
β cells of the pancreas is stimulated by glucose and the parasympathetic nervous system (Figure 30.15). In essence,
insulin signals the fed state it stimulates the storage of fuels and the synthesis of proteins in a variety of ways. For
instance, insulin initiates protein kinase cascades it stimulates glycogen synthesis in both muscle and the liver and
suppresses gluconeogenesis by the liver. Insulin also accelerates glycolysis in the liver, which in turn increases the
synthesis of fatty acids.
The liver helps to limit the amount of glucose in the blood during times of plenty by storing it as glycogen so as to be
able to release glucose in times of scarcity. How is the excess blood glucose present after a meal removed? Insulin
accelerates the uptake of blood glucose into the liver by GLUT2. The level of glucose 6-phosphate in the liver rises
because only then do the catalytic sites of glucokinase become filled with glucose. Recall that glucokinase is active only
when blood-glucose levels are high. Consequently, the liver forms glucose 6-phosphate more rapidly as the bloodglucose level rises. The increase in glucose 6-phosphate coupled with insulin action leads to a buildup of glycogen
stores. The hormonal effects on glycogen synthesis and storage are reinforced by a direct action of glucose itself.
Phosphorylase a is a glucose sensor in addition to being the enzyme that cleaves glycogen. When the glucose level is
high, the binding of glucose to phosphorylase a renders the enzyme susceptible to the action of a phosphatase that
converts it into phosphorylase b, which does not readily degrade glycogen. Thus, glucose allosterically shifts the
glycogen system from a degradative to a synthetic mode.
The high insulin level in the fed state also promotes the entry of glucose into muscle and adipose tissue. Insulin
stimulates the synthesis of glycogen by muscle as well as by the liver. The entry of glucose into adipose tissue provides
glycerol 3-phosphate for the synthesis of triacylglycerols. The action of insulin also extends to amino acid and protein
metabolism. Insulin promotes the uptake of branched-chain amino acids (valine, leucine, and isoleucine) by muscle.
Indeed, insulin has a general stimulating effect on protein synthesis, which favors a building up of muscle protein. In
addition, it inhibits the intracellular degradation of proteins.
2. The early fasting state. The blood-glucose level begins to drop several hours after a meal, leading to a decrease in
insulin secretion and a rise in glucagon secretion; glucagon is secreted by the α cells of the pancreas in response to a low
blood-sugar level in the fasting state. Just as insulin signals the fed state, glucagon signals the starved state. It serves to
mobilize glycogen stores when there is no dietary intake of glucose. The main target organ of glucagon is the liver.
Glucagon stimulates glycogen breakdown and inhibits glycogen synthesis by triggering the cyclic AMP cascade leading
to the phosphorylation and activation of phosphorylase and the inhibition of glycogen synthase (Section 21.5). Glucagon
also inhibits fatty acid synthesis by diminishing the production of pyruvate and by lowering the activity of acetyl CoA
carboxylase by maintaining it in an unphosphorylated state. In addition, glucagon stimulates gluconeogenesis in the liver
and blocks glycolysis by lowering the level of F-2,6-BP.
All known actions of glucagon are mediated by protein kinases that are activated by cyclic AMP. The activation of the
cyclic AMP cascade results in a higher level of phosphorylase a activity and a lower level of glycogen synthase a
activity. Glucagon's effect on this cascade is reinforced by the diminished binding of glucose to phosphorylase a, which
makes the enzyme less susceptible to the hydrolytic action of the phosphatase. Instead, the phosphatase remains bound to
phosphorylase a, and so the synthase stays in the in-active phosphorylated form. Consequently, there is a rapid
mobilization of glycogen.
The large amount of glucose formed by the hydrolysis of glucose 6-phosphate derived from glycogen is then released
from the liver into the blood. The entry of glucose into muscle and adipose tissue decreases in response to a low insulin
level. The diminished utilization of glucose by muscle and adipose tissue also contributes to the maintenance of the
bloodglucose level. The net result of these actions of glucagon is to markedly increase the release of glucose by the liver.
Both muscle and liver use fatty acids as fuel when the blood-glucose level drops. Thus, the blood-glucose level is kept at
or above 80 mg/dl by three major factors: (1) the mobilization of glycogen and the release of glucose by the liver, (2) the
release of fatty acids by adipose tissue, and (3) the shift in the fuel used from glucose to fatty acids by muscle and the
liver.
What is the result of depletion of the liver's glycogen stores? Gluconeogenesis from lactate and alanine continues, but
this process merely replaces glucose that had already been converted into lactate and alanine by the peripheral tissues.
Moreover, the brain oxidizes glucose completely to CO2 and H2O. Thus, for the net synthesis of glucose to occur,
another source of carbons is required. Glycerol released from adipose tissue on lipolysis provides some of the carbons,
with the remaining carbons coming from the hydrolysis of muscle proteins.
3. The refed state. What are the biochemical responses to a hearty breakfast? Fat is processed exactly as it is processed in
the normal fed state. However, this is not the case for glucose. The liver does not initially absorb glucose from the blood,
but rather leaves it for the peripheral tissues. Moreover, the liver remains in a gluconeogenic mode. Now, however, the
newly synthesized glucose is used to replenish the liver's glycogen stores. As the blood-glucose levels continue to rise,
the liver completes the replenishment of its glycogen stores and begins to process the remaining excess glucose for fatty
acid synthesis.
30.3.1. Metabolic Adaptations in Prolonged Starvation Minimize Protein Degradation
What are the adaptations if fasting is prolonged to the point of starvation? A typical well-nourished 70-kg man has
fuel reserves totaling about 161,000 kcal (670,000 kJ; see Table 30.1). The energy need for a 24-hour period
ranges from about 1600 kcal (6700 kJ) to 6000 kcal (25,000 kJ), depending on the extent of activity. Thus, stored fuels
suffice to meet caloric needs in starvation for 1 to 3 months. However, the carbohydrate reserves are exhausted in only a
day.
Even under starvation conditions, the blood-glucose level must be maintained above 2.2 mM (40 mg/dl). The first
priority of metabolism in starvation is to provide sufficient glucose to the brain and other tissues (such as red blood
cells) that are absolutely dependent on this fuel. However, precursors of glucose are not abundant. Most energy is stored
in the fatty acyl moieties of triacylglycerols. Recall that fatty acids cannot be converted into glucose, because acetyl CoA
cannot be transformed into pyruvate (Section 22.3.7). The glycerol moiety of triacylglycerol can be converted into
glucose, but only a limited amount is available. The only other potential source of glucose is amino acids derived from
the breakdown of proteins. However, proteins are not stored, and so any breakdown will necessitate a loss of function.
Thus, the second priority of metabolism in starvation is to preserve protein, which is accomplished by shifting the fuel
being used from glucose to fatty acids and ketone bodies (Figure 30.16).
The metabolic changes on the first day of starvation are like those after an overnight fast. The low blood-sugar level
leads to decreased secretion of insulin and increased secretion of glucagon. The dominant metabolic processes are the
mobilization of triacylglycerols in adipose tissue and gluconeogenesis by the liver. The liver obtains energy for its own
needs by oxidizing fatty acids released from adipose tissue. The concentrations of acetyl CoA and citrate consequently
increase, which switches off glycolysis. The uptake of glucose by muscle is markedly diminished because of the low
insulin level, whereas fatty acids enter freely. Consequently, muscle shifts almost entirely from glucose to fatty acids for
fuel. The β-oxidation of fatty acids by muscle halts the conversion of pyruvate into acetyl CoA, because acetyl CoA
stimulates the phosphorylation of the pyruvate dehydrogenase complex, which renders it inactive (Section 17.2.1).
Hence, pyruvate, lactate, and alanine are exported to the liver for conversion into glucose. Glycerol derived from the
cleavage of triacylglycerols is another raw material for the synthesis of glucose by the liver.
Proteolysis also provides carbon skeletons for gluconeogenesis. During starvation, degraded proteins are not replenished
and serve as carbon sources for glucose synthesis. Initial sources of protein are those that turn over rapidly, such as
proteins of the intestinal epithelium and the secretions of the pancreas. Proteolysis of muscle protein provides some of
three-carbon precursors of glucose. However, survival for most animals depends on being able to move rapidly, which
requires a large muscle mass, and so muscle loss must be minimized.
How is the loss of muscle curtailed? After about 3 days of starvation, the liver forms large amounts of acetoacetate and d-
3-hydroxybutyrate (ketone bodies; Figure 30.17). Their synthesis from acetyl CoA increases markedly because the citric
acid cycle is unable to oxidize all the acetyl units generated by the degradation of fatty acids. Gluconeogenesis depletes
the supply of oxaloacetate, which is essential for the entry of acetyl CoA into the citric acid cycle. Consequently, the
liver produces large quantities of ketone bodies, which are released into the blood. At this time, the brain begins to
consume appreciable amounts of acetoacetate in place of glucose. After 3 days of starvation, about a third of the energy
needs of the brain are met by ketone bodies (Table 30.2). The heart also uses ketone bodies as fuel.
After several weeks of starvation, ketone bodies become the major fuel of the brain. Acetoacetate is activated by the
transfer of CoA from succinyl CoA to give acetoacetyl CoA (Figure 30.18). Cleavage by thiolase then yields two
molecules of acetyl CoA, which enter the citric acid cycle. In essence, ketone bodies are equivalents of fatty acids that
can pass through the blood-brain barrier. Only 40 g of glucose is then needed per day for the brain, compared with
about 120 g in the first day of starvation. The effective conversion of fatty acids into ketone bodies by the liver and their
use by the brain markedly diminishes the need for glucose. Hence, less muscle is degraded than in the first days of
starvation. The breakdown of 20 g of muscle daily compared with 75 g early in starvation is most important for survival.
A person's survival time is mainly determined by the size of the triacylglycerol depot.
What happens after depletion of the triacylglycerol stores? The only source of fuel that remains is proteins. Protein
degradation accelerates, and death inevitably results from a loss of heart, liver, or kidney function.
30.3.2. Metabolic Derangements in Diabetes Result from Relative Insulin Insufficiency
and Glucagon Excess
We now consider diabetes mellitus, a complex disease characterized by grossly abnormal fuel usage: glucose is
overproduced by the liver and underutilized by other organs. The incidence of diabetes mellitus (usually referred
to simply as diabetes) is about 5% of the population. Indeed, diabetes is the most common serious metabolic disease in
the world; it affects hundreds of millions. Type I diabetes, or insulin-dependent diabetes mellitus (IDDM), is caused by
autoimmune destruction of the insulinsecreting β cells in the pancreas and usually begins before age 20. The term insulindependent means that the individual requires insulin to live. Most diabetics, in contrast, have a normal or even higher
level of insulin in their blood, but they are quite unresponsive to the hormone. This form of the disease known as type
II, or non-insulin-dependent, diabetes mellitus (NIDDM) typically arises later in life than does the insulin-dependent
form.
DiabetesNamed for the excessive urination in the disease. Aretaeus, a
Cappadocian physician of the second century a.d., wrote: "The
epithet diabetes has been assigned to the disorder, being something
like passing of water by a siphon." He perceptively characterized
diabetes as "being a melting-down of the flesh and limbs into urine."
From Latin, meaning "sweetened with honey." Refers to the
presence of sugar in the urine of patients having the disease.
Mellitus distinguishes this disease from diabetes insipidus, which is
caused by impaired renal reabsorption of water.
In type I diabetes, insulin is absent and consequently glucagon is present at higher-than-normal levels. In essence, the
diabetic person is in biochemical starvation mode despite a high concentration of blood glucose. Because insulin is
deficient, the entry of glucose into cells is impaired. The liver becomes stuck in a gluconeogenic and ketogenic state. The
excessive level of glucagon relative to insulin leads to a decrease in the amount of F-2,6-BP in the liver. Hence,
glycolysis is inhibited and gluconeogenesis is stimulated because of the opposite effects of F-2,6-BP on
phosphofructokinase and fructose-1,6-bisphosphatase (Section 16.4; see also Figures 30.4 and 30.6). The high glucagon/
insulin ratio in diabetes also promotes glycogen breakdown. Hence, an excessive amount of glucose is produced by the
liver and released into the blood. Glucose is excreted in the urine (hence the name mellitus) when its concentration in the
blood exceeds the reabsorptive capacity of the renal tubules. Water accompanies the excreted glucose, and so an
untreated diabetic in the acute phase of the disease is hungry and thirsty.
Because carbohydrate utilization is impaired, a lack of insulin leads to the uncontrolled breakdown of lipids and proteins.
Large amounts of acetyl CoA are then produced by β-oxidation. However, much of the acetyl CoA cannot enter the citric
acid cycle, because there is insufficient oxaloacetate for the condensation step. Recall that mammals can synthesize
oxaloacetate from pyruvate, a product of glycolysis, but not from acetyl CoA; instead, they generate ketone bodies. A
striking feature of diabetes is the shift in fuel usage from carbohydrates to fats; glucose, more abundant than ever, is
spurned. In high concentrations, ketone bodies overwhelm the kidney's capacity to maintain acid-base balance. The
untreated diabetic can go into a coma because of a lowered blood pH level and dehydration.
Type II, or non-insulin-dependent, diabetes accounts for more than 90% of the cases and usually develops in middleaged, obese people. The exact cause of type II diabetes remains to be elucidated, although a genetic basis seems likely.
30.3.3. Caloric Homeostasis: A Means of Regulating Body Weight
In the United States, obesity has become an epidemic, with nearly 20% of adults classified as obese. Obesity is
identified as a risk factor in a host of pathological conditions including diabetes mellitus, hypertension, and
cardiovascular disease. The cause of obesity is quite simple in the vast majority of cases more food is consumed than
is needed, and the excess calories are stored as fat.
Although the proximal cause of obesity is simple, the biochemical means by which caloric homeostasis and apetite
control are usually maintained is enormously complex, but two important signal molecules are insulin and leptin. A
protein consisting of 146 amino acids, leptin is a hormone secreted by adipocytes in direct proportion to fat mass. Leptin
acts through a membrane receptor (related in structure and mechanism of action to the growth-hormone receptor; Section
15.4) in the hypothalamus to generate satiation signals. During periods when more energy is expended than ingested (the
starved state), adipose tissue loses mass. Under these conditions, the secretion of both leptin and insulin declines, fuel
utilization is increased, and energy stores are used. The converse is true when calories are consumed in excess.
The importance of leptin to obesity is dramatically illustrated in mice. Mice lacking leptin are obese and will lose weight
if given leptin. Mice that lack the leptin receptor are insensitive to leptin administration. Preliminary evidence indicates
that leptin and its receptor play a role in human obesity, but the results are not as clear-cut as in the mouse. The interplay
of genes and their products to control caloric homeostasis will be an exciting area of research for some time to come.
III. Synthesizing the Molecules of Life
30. The Integration of Metabolism
30.3. Food Intake and Starvation Induce Metabolic Changes
Figure 30.16. Fuel Choice During Starvation. The plasma levels of fatty acids and ketone bodies increase in
starvation, whereas that of glucose decreases.
III. Synthesizing the Molecules of Life
30. The Integration of Metabolism
30.3. Food Intake and Starvation Induce Metabolic Changes
Figure 30.17. Synthesis of Ketone Bodies by the Liver.
III. Synthesizing the Molecules of Life
30. The Integration of Metabolism
30.3. Food Intake and Starvation Induce Metabolic Changes
Table 30.2. Fuel metabolism in starvation
Amount formed or consumed in 24 hours (grams)
Fuel exchanges and consumption
Fuel use by the brain
Glucose
Ketone bodies
All other use of glucose
Fuel mobilization
Adipose-tissue lipolysis
Muscle-protein degradation
Fuel output of the liver
3d day
40th day
100
50
50
40
100
40
180
75
180
20
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