Carbohydrate Metabolism
The primary role of carbohydrate metabolism is the maintenance
of blood glucose at a range between 70 to 120 mg/dl. The sources of blood
glucose are from both carbohydrate and noncarbohydrate sources and blood glucose
is the most common sugar found in the blood. Dietary starches and simple
carbohydrates provide blood glucose after digestion and absorption; glycogen
stored by the liver and muscle tissue is converted back to glucose in a process
called glycogenolysis. Intermediate carbohydrate metabolites are also a source
of blood glucose. These metabolites include lactic acid and pyruvic acid, which
are created when muscle glycogen is used for energy.
Noncarbohydrates can also produce blood glucose. Gluconeogenesis is the process
of producing glucose from fat and protein. It is not as efficient as using
carbohydrates when it comes to producing glucose though. As fat is metabolized
into fatty acids and glycerol, the smaller glycerol portion can be converted by
the liver into glycogen, which is then available to be converted to glucose
through glycogenolysis. Protein, which is composed of different combinations of
amino acids, can also be used as a source of glucose. Some amino acids are
glucogenic, which means if they aren’t used to form protein structures, they can
be metabolized to form glucose.
All the cells of the body get their energy from blood glucose. Glucose can be
used for immediate energy needs or can be converted to glycogen or fat stores;
both conversions provide energy for future use. Although glycogen can be
converted back to glucose, the conversion of glucose to fat cannot be reversed.
Glucose cannot be formed again but is stored as fat and if it is needed at a
future date is metabolized as fat.
Brain function and cell formation are dependent on glucose, particularly during
pregnancy and growth. Because the body can form glucose through Gluconeogenesis
from protein and fat, glucose is not technically an essential nutrient.
Gluconeogenesis can provide some glucose but not enough to meet essential needs
if dietary carbohydrates are insufficient. To compensate for this ketone bodies
can also be used for energy. Ketone bodies are created when fatty acids are
broken down for energy when there are not enough carbohydrates available for
energy needs. The problem is that this process of fat metabolism is not
complete. As dietary carbohydrates continue to remain insufficient a buildup of
ketones will result which can cause ketosis.
Metabolism of glucose and regulation of blood glucose levels are controlled by a
sophisticated hormonal system. Insulin is a hormone produced by the pancreas
that works to lower blood glucose levels. It is able to do this by enhancing the
conversion of excess glucose to glycogen through glycogenesis or to fat storage.
It also helps the cells of the body absorb glucose to be used as energy.
While insulin helps to lower blood glucose levels there are other hormones that
raise glucose levels. Two of these include glucagon and somatostatin. Glucagon,
produced by the pancreas, stimulates conversion of liver glycogen to glucose,
assisting the regulation of glucose levels during the night. Somatostatin from
the hypothalamus and pancreas helps to inhibit the function of insulin and
glucagon. The adrenal glands also have a role in raising blood glucose levels.
Epinephrine enhances the fast conversion of liver glycogen to glucose. Steroid
hormones function against insulin and promote glucose formation from proteins.
Growth hormone and adrenocorticotropic hormone (ACTH), which are produced by the
pituitary gland, also function as insulin inhibitors. Thyroxine, a thyroid
hormone, affects blood glucose levels by enhancing intestinal absorption of
glucose and releasing epinephrine.
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