Download Jennifer Atkinson October 14, 2013 HUN 3230 Section 81944

Jennifer Atkinson
October 14, 2013
HUN 3230 Section 81944
Carbohydrates and Type 2 Diabetes
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Carbohydrates are one of the main, essential sources of nutrients in the body and
provide the most amount of energy to one’s body by means of being consumed in the
production of ATP. Carbohydrates are composed of carbon, hydrogen, and oxygen
molecules. There are various types of carbohydrates that occur in the body and food sources.
There are simple carbohydrates that contain only one or two sugar molecules and complex
carbohydrates that contain three to hundreds or thousands of monosaccharides.
The simple carbohydrates, or simple sugars, are referred to as monosaccharides and
disaccharides. Monosaccharides have only one sugar molecule and are the sugars known as
glucose, fructose, and galactose. When these sugars combine with one another, they form the
two sugar molecules known as disaccharides, which include lactose, maltose, and sucrose.
Simple carbohydrates come from food sources such as soft drinks, candy, and fruit. The
complex
carbohydrates
incorporate
oligosaccharides
that
contain
three
to
ten
monosaccharides and polysaccharides that can consist of hundreds to thousands of glucose
molecules. Starch, glycogen, and most fibers are considered to be polysaccharides. Food
sources that are composed of complex carbohydrates include breads, potatoes, rice, nuts, and
flour. The RDA for carbohydrates is about “125-175 grams, which equals about 55-60% of
one’s total chloric intake”3. 1,2,3
Carbohydrate digestion begins right as it enters the body, in the mouth. Once food
has arrived in the oral cavity, the salivary glands release an enzyme known as salivary
amylase or alpha-amylase. A-amylase works as a catalyst to break down the starch and
transform it into sugar. “Amylase digests starch by catalyzing hydrolysis, which is splitting
by the addition of a water molecule. Therefore starch plus water becomes maltose (which is
equivalent to two joined glucose molecules).”4 When the food arrives in the stomach,
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carbohydrate digestion ceases for that time because the “gastric acid penetrates the food
bolus and lowers the pH sufficiently to inactivate the enzyme [a-amylase].”1 The digestive
process picks back up when the chyme enters the duodenum. The pancreas releases
pancreatic a-amylase to hydrolyze the polysaccharides into disaccharides. The small intestine
then produces the enzymes, lactase, sucrase, and maltase that break down the disaccharides
into monosaccharides.1,4
Now that the starch is completely broken down into monosaccharides, the small
intestine is ready to begin absorbing them. The glucose, fructose, and galactose that are
produced get absorbed by the enterocytes that are in the upper half of the villi in the small
intestine. Only D-glucose and D-galactose are actively absorbed in the small intestine. Dfructose is not actively absorbed but has a greater rate of absorption than would be expected
by passive diffusion5. Glucose and galactose are actively transported into the cytoplasm of
the enterocyte by the sodium dependent glucose transporter, SGLT1, and “exit across the
basolateral membrane by the glucose transporter GLUT2.”6 Once glucose, galactose, and
fructose cross the intestinal wall, they enter portal circulation where they go directly to the
liver. Once in the liver, the galactose and fructose are metabolized and converted into
glucose.5,6
Carbohydrate metabolism is composed of three pathways: Glycolysis, Krebs Cycle,
and the Electron Transport Chain (ETC). Glycolysis is the first step of carbohydrate
metabolism and it occurs in the cytosol. In this process, one six-carbon glucose molecule is
split into two three-carbon pyruvate molecules. The outcome of this process is two net ATP,
because four are created but two are used, and two NAD are hydrogenated (Table 1). Then
the pyruvate from glycolysis is converted into Acetyl CoA in the cytosol and then it is moved
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into the mitochondria. The mitochondria are the site for the Krebs cycle. The Krebs cycle
begins with the two-carbon Acetyl CoA and the four-carbon oxaloacetic acid to make a sixcarbon citric acid molecule. The citric acid then goes through a series of reactions where
NAD+ is hydrogenated to make NADH, ADP is used to make ATP, and two carbon dioxide
molecules are released. Since the two carbon dioxide molecules get released, the six-carbon
citric acid molecule is then made into a oxaloacetic acid molecule, which cycles back through
to meet up with another Acetyl CoA molecule and the Krebs cycle continues on (Table 2).
From the Krebs cycle, six NADH, two ATP, and two FADH2 are produced. The final step in
carbohydrate metabolism is the Electron Transport Chain. The ETC occurs in the inner
mitochondrial membrane of the cell. In this process, NADH and FADH2 are oxidized and
their electrons are donated to O2, which gets reduced to H2O. The energy released from
reducing oxygen to water is used to phosphorylate mitochondrial ADP to ATP (Table 3). In
total, the metabolism of one glucose molecule creates 38 ATP from these three cycles.1, 2,3
Carbohydrates are essential to one’s body. The primary role that carbohydrates play
is their use for short-term energy storage. Shown in tables 1-3, the metabolism of
carbohydrates produces a vast amount of energy for one’s body that can be used right away.
A secondary function is intermediate-term energy storage. When a lot of carbohydrate is
consumed at once and one’s blood sugar level is adequate, the excess of glucose can be
stored in the liver and muscles as glycogen and used if one’s blood-sugar level begins to
decline. Other carbohydrates are involved in the structural components in cells.1,2
Because the amount of carbohydrates one intakes greatly affects one’s blood-glucose
levels, Type 2 Diabetes and the intake of carbohydrates are very much linked together. Type
2 Diabetes is the most common form of diabetes and it occurs when one’s blood-glucose
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levels rises higher than normal, also called hyperglycemia, or lower than normal, which is
hypoglycemia. In type 2 diabetes, either one’s body does not produce enough insulin or the
cells deny the insulin. Insulin is essential for one’s body to be able to transform glucose into
energy. Once polysaccharides are chemically broken all the way down into glucose, the
glucose must get into the cells to be able to make ATP. Glucose from the blood is assisted
into the cells by insulin. If glucose is unable to get into the cells, the body will lack in energy
and could potentially shut down completely and seize or fall into a coma.7
If one’s blood-glucose level rises higher than normal, they may have frequent
urination and an increase in thirst. When hypoglycemia occurs, one may become shaky,
lightheaded, or weak; all indicating that one needs to check his/her blood-glucose level.
Those with Type 2 Diabetes need to have a specific diet planned out for them to know how
many carbohydrates they should eat to maintain healthy blood-glucose levels. To prevent
hyperglycemia, one should have a regular exercising plan, one should limit the amount of
food they intake, and be sure to drink a lot of water to maintain homeostasis within the blood
and cells. For both high and low blood-glucose levels, one needs to be very in tune with
one’s body and be able to detect any abnormal sensations within one’s body.7
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Table 1
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Table 2
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Table 3
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Reference List
1. Gropper SS, Smith JL. Advanced Nutrition and Human Metabolism. Wadsworth,
Cengage Learning. Belmont, CA; 2013; 63-96
2. Thompson JL, Manore MM, Vaughan LA. The Science of Nutrition. Pearson; 2014; 118151
3. Georgia Highlands College Web site. Nutrition and Metabolism.
http://www.highlands.edu/academics/divisions/scipe/biology/faculty/harnden/2122/notes/
metab.htm. Accessed October 9, 2013.
4. Vance, Nena. Effects of Salivary Amylase. http://biology.clc.uc.edu/students/114Fall99/Amylase.htm. Accessed October 7, 2013.
5. FAO Corporate Document Repository. Digestion, absorption and energy value of
carbohydrates. http://www.fao.org/docrep/w8079e/w8079e0k.htm Accessed October 12,
2013.
6. Levin RJ. PubMed Web site. http://www.ncbi.nlm.nih.gov/pubmed/8116552. Accessed
October 12, 2013.
7. American Diabetes Association Web site. http://www.diabetes.org/?loc=logo. Accessed
October 12, 2013.