CARBOHYDRATES
The term “carbohydrate” refers to a group of molecules
composed of carbon, hydrogen, and oxygen.
There are two groups:
Simple carbohydrates (also called “sugars”)
Complex carbohydrates
The term “carbohydrate” refers to a group of molecules
composed of carbon, hydrogen, and oxygen.
There are two groups:
Simple carbohydrates (also called “sugars”)
Complex carbohydrates
In fact, they are related:
1. The complex carbohydrates are formed by
linking together many simple carbohyrates
and
2. The simple carbohydrates can be formed by
breaking apart the complex carbohydrates
We get carbohydrates in our diets from eating
plants and, to smaller degree from milk and meat.
Simple Carbohydrates:
The simple carbohydrates are the monosaccharides
and the disaccharides
Simple Carbohydrates:
The simple carbohydrates are the monosaccharides
and the disaccharides
Although there are dozens of known monosaccharides
and disaccharides, only a small number are important
nutritionally because they are found in our food.
Monosaccharides: All have formula C6H12O6
Simple Carbohydrates: Monosaccharides
Glucose (also called “dextrose”) is
the most abundant and most important
monosaccharide.
Simple Carbohydrates: Monosaccharides
Glucose (also called “dextrose”) is
the most abundant and most important
monosaccharide.
All green plants produce glucose
from carbon dioxide, water, and sunlight (energy)
Gives food a sweet taste
Glucose is the primary “fuel molecule” our cells use to
produce energy
so
It is the sugar carried around the body in the blood
Simple Carbohydrates: Monosaccharides
Fructose (also called “levulose” or
“fruit sugar”) is also very sweet
Simple Carbohydrates: Monosaccharides
Fructose (also called “levulose” or
“fruit sugar”) is also very sweet
Occurs naturally in fruits, vegetables,
honey, and corn syrup.
Some cells can use fructose as a fuel molecule, but
most cells convert it to glucose instead
Simple Carbohydrates: Monosaccharides
Galactose - rarely found alone in nature
Usually combined with glucose
Simple Carbohydrates: Monosaccharides
Galactose - rarely found alone in nature
Usually combined with glucose
Found in milk
Cells convert galactose to glucose
for energy
May be part of cell membranes
Simple Carbohydrates: Disaccharides
Disaccharides are two monosaccharides linked together
Simple Carbohydrates: Disaccharides
Disaccharides are two monosaccharides linked together
Three disaccharides are dietarily important:
Sucrose
Lactose
Maltose
All have formula C12H22O11
Simple Carbohydrates: Disaccharides
Sucrose = glucose + fructose
Found in sugarcane, sugarbeets
“Table Sugar” – only sweetener that can be
called “sugar” on food labels in United States
Simple Carbohydrates: Disaccharides
Lactose = glucose + galactose
“Milk Sugar”
Simple Carbohydrates: Disaccharides
Maltose = glucose + glucose
Found in partially digested starch
and in germinating cereal grains
Complex Carbohydrates
Chains of two or more monosaccharides
Oligosaccharides = shorter chains
Polysaccharides = longer chains
Complex Carbohydrates: Oligosaccharides
Consist of 3-10 monosaccharides
Found in legumes, milk
Forms receptors on many cell membranes
Not digestible by humans = fiber
May be added to food: inulin
oligofructose
raffinose
maltotriose
Complex Carbohydrates: Polysaccharides
Starch:
How plants store energy
Found in grains, legumes, and tubers
Long chains of glucose units:
Amylose—straight chains
Amylopectin—branched chains
Complex Carbohydrates: Polysaccharides
Glycogen
How animals (including humans) store energy
Found in meat, liver
Long, branching chains
of glucose units:
Formed in liver and muscle when
blood glucose levels are high
Can be broken down to provide
glucose when blood glucose levels get low
Complex Carbohydrates: Polysaccharides
Cellulose
Indigestible chains of monosaccharides = “fiber”
- speeds up gastrointestinal movement
- increases fecal mass
- slows breakdown of starch
- slows absorption of glucose
Found in all plants,
“Functional” fiber: isolated and added to foods
Can be use as a supplement
Complex Carbohydrates: Polysaccharides
Other forms of fiber
= Indigestible chains of monosaccharides
Pectins
Gums
Lignans
Beta glucans
Chitins
Found in all plants,
“Functional” fiber: isolated and added to foods
Can be use as a supplement
Complex Carbohydrates
Complex Carbohydrates:
Dietary fiber:
Possible role in weight control
Better control of blood glucose
Reduced risk of heart disease
Healthier gastrointestinal functioning
Negative effects of excess fiber
Increased water consumption
Can bind small amounts of minerals
Carbohydrate Digestion:
Complex carbohydrates (amylose, amylopectin) broken
down to form disaccharides and monosaccharides:
Mouth: Salivary amylase begins digestion
Stomach: No carbohydrate digestion
Small intestine: Pancreatic amylase finishes digestion
Carbohydrate Digestion:
Disaccharides (sucrose, maltose, lactose) broken down
to form monosaccharides:
Small intestine: Pancreatic enzymes
Enzymes from intestinal cells
(sucrase, maltase, lactase)
Carbohydrate Digestion:
These enzymes are highly specific
For example:
- Amylase digests amylose but not amylopectin
- Lactase digests lactose but not maltose or sucrose
- Maltase digests maltose but not lactose or sucrose
- Sucrase digests sucrose but not maltose or lactose
Remember: Some carbohydrates
can not be digested;
fiber & “resistant” starch
Carbohydrate Absorption:
Monosaccharides are absorbed from the small intestine
Carbohydrate Absorption:
Monosaccharides are absorbed from the small intestine
and carried by the blood to the liver
Carbohydrate Absorption:
Monosaccharides are absorbed from the small intestine
and carried by the blood to the liver
Liver: Converts other
monsaccharides
to glucose
Stores excess
glucose as
glycogen
Carbohydrate Absorption:
Monosaccharides are absorbed from the small intestine
and carried by the blood to the liver
Liver: Converts other
monsaccharides
to glucose
Stores excess
glucose as
glycogen
Liver: Breaks down glycogen to
release glucose back into the
blood when needed.
Carbohydrate Metabolism:
Glucose is our primary fuel source
so
It is important to maintain normal blood glucose levels
and
It is important for glucose to get from the blood into cells
Carbohydrate Metabolism:
Glucose is our primary fuel source
so
It is important to maintain normal blood glucose levels
and
It is important for glucose to get from the blood into cells
These processes are primarily regulated by two hormones
Insulin
Glucagon
Both are secreted by the pancreatic islets
Carbohydrate Metabolism:
Carbohydrate Metabolism:
Insulin promotes:
(i) Energy storage
glucose glycogen (glycogenesis)
(ii) Protein synthesis
glucose amino acids
(iii) Fat synthesis
glucose fatty acids
protein
triglycerides(fat)
Carbohydrate Metabolism: Disorders
Normal Blood Glucose: 70-110 mg/100 ml of blood
Carbohydrate Metabolism: Disorders
Normal Blood Glucose: 70-110 mg/100 ml of blood
Persistent high blood glucose levels = Hyperglycemia
Insufficient insulin
>125 mg/100 ml
Too much glucagon
Most common cause = diabetes mellitus
Carbohydrate Metabolism: Disorders
Normal Blood Glucose: 70-110 mg/100 ml of blood
Persistent high blood glucose levels = Hyperglycemia
Insufficient insulin
>125 mg/100 ml
Too much glucagon
Most common cause = diabetes mellitus
Persistent low blood glucose levels = Hypoglycemia
Too much insulin
<50 mg/100 ml
Insufficient glucagon
Most common causes = insulin overdose in diabetes
starvation
strenuous exercise
Carbohydrate Metabolism: Disorders
Diabetes Mellitus:
Glucose unable to enter cells
so
Blood glucose levels rise (hyperglycemia)
Complications: Hypertension
Heart damage
Kidney damage
Nerve damage
Blood vessel damage
Eye damage
Carbohydrate Metabolism: Disorders
Diabetes mellitus:
Type 1 or “insulin dependent” diabetes:
Lack of insulin production
Type 2 or “insulin independent” diabetes:
Cells are resistant to insulin
Carbohydrate Metabolism: Disorders
Diabetes mellitus:
Type 1 or “insulin dependent” diabetes:
Lack of insulin production
Type 2 or “insulin independent” diabetes:
Cells are resistant to insulin
Usually present as “prediabetes” first
Gestational diabetes: occurs during pregnancy
Carbohydrate Metabolism: Disorders
Diabetes mellitus:
Best prevention:
Healthful diet
Regular exercise
Healthy weight
Don’t smoke
Monitor blood sugar if you
have genetic predisposition
Carbohydrates:
Recommended carbohydrate intake:
Recommended Daily Allowance = 130 grams per day
Acceptable Macronutrient
Distribution Range
= 45–65% of calories
Carbohydrates:
Recommended carbohydrate intake:
Recommended Daily Allowance = 130 grams per day
Acceptable Macronutrient
Distribution Range
= 45–65% of calories
Choose carbohydrates wisely: fiber-rich fruits
vegetables
whole grains
high-fiber cereal
Because:
Not all carbohydrate sources have the same effect on
blood glucose levels
Carbohydrates:
Each type of food has a glycemic index which measures
its effects on blood sugar levels.
Carbohydrates:
Each type of food has a glycemic index which measures
its effects on blood sugar levels.
Carbohydrates that break down quickly
during digestion and release glucose
rapidly for absorption into the bloodstream
have a high glycemic index
Carbohydrates:
Each type of food has a glycemic index which measures
its effects on blood sugar levels.
Carbohydrates that break down quickly
during digestion and release glucose
rapidly for absorption into the bloodstream
have a high glycemic index
Carbohydrates that break down more
slowly, releasing glucose more gradually
for absorption into the bloodstream, have
a low glycemic index.
Carbohydrates:
Low glycemic index:
(55 or less)
Most vegetables, fruits, legumes,
whole grains, nuts, milk, citrus
juices, foods low in carbohydrates
Moderate glycemic index: Whole wheat products, honey,
(56 to 69)
brown or basmati rice, bran,
baked potato, sweet potato
High glycemic index:
(70 and above)
White bread, white rice, breakfast
cereals, waffles, chips, candy,
table sugar
(Pure glucose: glycemic index = 100)
Carbohydrates:
Moderating sugar intake:
Choose food with lower glycemic index
Use less added sugar
Limit soft drinks, sugary cereals, and candy
Choose fresh fruits or those canned in water or juice
Carbohydrates: Natural and Artificial Sweeteners
Nutritive sweeteners
Natural sugars
Sugar alcohols
Non-nutritive sweeteners
Saccharin
Aspartame
Acesulfame K
Sucralose
Let’s move on to the lipids next