Carbohydrates
Iva Klimešová
• Carbohydrates are defined as sugars and their
derivatives.
• Carbohydrates play a major role in supplying
energy for bodily function.
• Carbohydrates, which are stored in limited amounts
as blood glucose, liver glycogen, and muscle
glycogen, serve:
– as a major source of energy,
– to spare the breakdown of proteins,
– as a metabolic primer for fat metabolism,
– and as a fuel for the central nervous system
• Well-balanced diet contains about 60% of daily
calories as carbohydrates, predominantly in
unrefined, complex form.
• Excellent sources are:
– rice, bread, pita bread, whole wheat spaghetti,
cereals, potatoes, legumes...
Legumes are full of complex
carbohydrates, proteins, fiber and
vitamins and minerals.
There are a lot of polysaccharides
resistant to human digestive
enzymes in legumes (lens, beans..).
Consummation of legumes should
lead to a digestive problem
(flatulence or diarrhoea).
• This polysaccharides are soluble in
water and you can reduce amount of
them by macerating in water for 1 day
or by sprouting.
• Simple sugar consists of a chain of three
to seven carbon atoms with hydrogen
and oxygen attached in a ratio of 2 to 1.
• There are three kinds of carbohydrates:
– monosaccharides (sugars such as glucose,
fructose, and galactose);
– disaccharides (sucrose, lactose, and
maltose);
– and polysaccharides (starch, fiber, and
glycogen).
Monosaccharides
• Glucose – the most common simple sugar, also called
dextrose or blood sugar
Sources: honey, fruits
• Fructose – fruit sugar, is the sweetest of the sugars
Sources: honey, fruits
• Galactose – is produced in the mammary glands of
lacting animal
Sources: in milk as a part of lactose
• Glucose is an important form of usable food energy
and the body easily convert both fructose and
galactose to glucose.
Disaccharides
• The combination of two monosaccharides
makes a disaccharide.
• Lactose – is formed from glucose + galactose
(enzyme lactase breaks down lactose)
Sources: milk
• Sucrose – is formed from glucose + fructose
(enzyme sucrase)
Sources: sugar beets, sugar cane
Maltose – is composed of glucose + glucose
(enzyme maltase)
Sources: germinated grain, they are formed in
digestive breakdown of large carbohydrate
molecule
The monosaccharides and disaccharides
collectively make up what are commonly referred
to as the simple sugars.
Lactose intolerance
• In about 25% of people, intestinal level of the enzyme
lactase decrease when they get older. Occurrence of
this intolerance is common among African-American and
Hispanic people.
• They are unable to digest the lactose in dairy products.
• This undigested lactose passes through their digestive
tract until it is eventually fermented by the bacteria that
normally live in everyone’s large intestines.
• When this happens it often produces “gas,” and may
cause the person to have cramps and other unpleasant
symptoms. These people are called lactose intolerant
(this is different than an allergy).
• Some of these people may be able to eat yogurt,
cheese, or other dairy products in which bacteria have
already broken down the lactose.
• Different sugars don’t all taste
the same.
• Some taste more or less sweet
than each other.
• For example fructose is almost
twice as sweet as sucrose.
• The main health benefit in
fructose-sweetened products is
that they contained less sugar.
Sugar
fructose
sucrose
glucose
maltose
galactose
lactose
Sweetness
173%
100%
74%
33%
33%
16%
Some people have been critical of dairy products
containing extra powdered milk (“non-fat milk solids”)
because of the fact that lactose is so “un-sweet” that a
person could consume larger amounts without a lot of
sweet taste to warn of its presence.
Polysaccharides
• The term polysaccharides is used when
three or more sugar molecules combine.
• The most common polysaccharides are
starch, fiber, and glycogen.
Starch
• it‘s a plant polysaccharide
• Sources: grains, pastries, bread, corn,
beans, peas, potatoes...
Fiber
• it‘s a plant polysaccharide too
• The most common fiber are cellulose, hemicellulose,
lignins, pectins, and gums.
• Fiber is resistant to digestive enzymes and hence leaves
some residue in the digestive tract.
There are two categories of dietary fiber:
• water soluble
• water insoluble
Water soluble fiber (pectin, guar gum, inulin) presents in
oats, rye, beans and other legumes, pulp of fruits and
vegetables
• This kind of fiber hold water in digestive tract and this
moisture expansion gives feeling of saturation,
• depress absorption and synthesis of cholesterol in the
gut,
• depress absorption of carbohydrates,
• help in defecation.
Water insoluble fiber (cellulose, hemicellulose, lignin)
presents in brown rice, corn, wheat bran (outer layer of
the whole grain), peelings of fruits and vegetables
• Increasing stool weight and volume
• Mechanically clean the small and large intestines – to
prevent cancer
• Take energy for bacteria in the large intestine. This
bacteria product short chains of fatty acids who take
energy for function of intestines’ cells and is thought
to be protective in regard to colon carcinogenesis.
• Some plant have both types of fibers – e.g. apple pulp is
good source of soluble fiber (pectin) and apple peel is full
of insoluble fiber (cellulose).
• The recommended fiber intake is about 20 to 35 g
a day.
• Eating a variety of plant food will insure that all
types of fibers are consumed.
• The ratio of insoluble and soluble should be about
3 to 1 and should be derived from foods, not from
synthetic fiber supplement.
• An excess of dietary fibers carries the risk of
intestinal distress, and decreases absorption of
the minerals and vitamins.
Glycogen, or animal starch, is formed by
glucose molecules.
It isn’t present to any large extent in the foods we
eat. In addition, when we slaughter animal for
meat, their tissue enzymes break down most
glycogen within 24 hours.
Glycemic index (GI)
• The GI is a measure of the power of foods (or specifically the
carbohydrate in a food) to raise glucose levels after being eaten.
• Foods with a high GI score contain rapidly digested carbohydrate,
which produces a large rapid rise and fall in the level of blood
glucose.
• In contrast, foods with a low GI score contain slowly digested
carbohydrate, which produces a gradual, relatively low rise in the
level of blood glucose.
• In particular, foods containing high amounts of refined sugars have
a high GI.
• Foods with a high GI will lead rapidly to hyperglycemia, which will
cause a hyperinsulinemia, which will cause a reactive
hypoglycemia (fatigue, hungry, inceasing of food intake).
• On the other hand, foods high in fiber generally have a low GI.
Low GI = 55 or less
Medium GI = 56 - 69
High GI = 70 or more
Other factors that influence how quickly the carbohydrates in food raise
blood sugar include:
•
Fiber content. Fiber shields the starchy carbohydrates in food immediate
and rapid attack by digestive enzymes. This slows the release of sugar
molecules into the bloodstream.
•
Ripeness. Ripe fruits and vegetables tend to have more sugar than unripe
ones, and so tend to have a higher glycemic index.
•
Type of starch. Starch comes in many different configurations. Some are
easier to break into sugar molecules than others. The starch in potatoes, for
example, is digested and absorbed into the bloodstream relatively quickly.
•
Fat content and acid content. The more fat or acid a food contains, the
slower its carbohydrates are converted to sugar and absorbed into the
bloodstream.
Physical form. Finely ground grain is more rapidly digested, and so has a
higher GI, than more coarsely ground grain, mashed potatoes have higher
GI than potatoes boiled in their peals.
•
•
All these elements lead to sometimes counterintuitive results. Some foods
that contain complex carbohydrates, such as potatoes, quickly raise blood
sugar levels, while some foods that contain simple carbohydrates, such as
whole fruit, raise blood sugar levels more slowly.
Measuring the Glycemic Index of Foods
• To determine the glycemic index of a food, volunteers are typically given a
test food that provides 50 grams of carbohydrate and a control food
(white bread or pure glucose) that provides the same amount of
carbohydrate on different days.
• Blood samples for the determination of glucose are taken prior to eating
and at regular intervals after eating over the next several hours.
• The changes in blood glucose over time are plotted as a curve.
• The glycemic index is calculated as the area under the glucose curve after
the test food is eaten, divided by the corresponding area after the control
food is eaten.
• The value is multiplied by 100 to represent a percentage of the control
food.
SUGARS
GI
ROOT CROP
GI
Glucose
Fructose
100
20
Beet
Carrot
64
92
Maltose
Sucrose
105
59
Potatoes - boiled
Sweet potatoes
80
51
CEREAL GRAINS
GI
LEGUMES
GI
White bread
Rice - brown
69
66
Beens
Soyabeens
40
15
Rice - white
Spaghetti
72
50
Peas
Lens
51
29
Corn
49
Cornflakes
80
Apple
39
Oat flakes
49
Banana
62
Wheat flakes
DAIRY
67
GI
Orange
Grapefruits
40
26
Ice cream
36
Raisins
64
Milk - skimmed
32
Milk - whole
34
FRUITS
OTHER
Honey
GI
GI
87
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Low GI diets help people lose and control weight
Low GI diets increase the body's sensitivity to insulin
Low GI carbs improve diabetes control
Low GI carbs reduce the risk of heart disease
Low GI carbs reduce blood cholesterol levels
Low GI carbs can help you manage the symptoms of
premenstrual syndrom.
• Low GI carbs reduce hunger and keep you fuller for
longer
• Low GI carbs prolong physical endurance
• High GI carbs help re-fuel carbohydrate stores after
exercise – muscle glycogen.
a useful address: http://www.glycemicindex.com/
Glycemic load
• Glycemic Index alone does not provide enough information about the
glycemic affect of a food.
• For instance a carrot is high on GI, but you would have to eat pounds of
this vegetable to have any affect on blood glucose. This is due to that the
amount of carbs in a carrot is very small.
• To calculate glycemic load - multiply the GI by the amount of carbohydrate
and divide by 100.
• For example:
an 100g serve of carrot with a GI of 92 has 4.2 g per serve.
92 x 4.2 / 100 = 3.9
an 100g serve of water melon with a GI of 72 has 5,6 g per serve.
72 x 5,6/100 = GL = 4
Foods with a low GI usually have a low GL.
Low GL = 10 or less
Medium GI = 11-19
High GI = 20 or more
Digestion and absorption
• The mouth: the enzyme of salivary amylase attacks starch
and begins to reduce it to the simpler disaccharide form..
• The small intestine: pancreatic amylase, in conjunction with
other enzymes, completes the breakdown of starch into
simple monosaccharides.
• Monosaccharides are absorbed from the small intestine into
bloodstream and are transported to the hepatic-portal vein,
which feeds directly to the liver.
• The colon is the final stop for undigested carbohydrates,
including fibrous substances.
• The stool consists of undigested fragments of plant fiber and
connective tissue from animal foods, plus bacteria and water.
• Most ingested carbohydrates are initially converted
into blood glucose and used for energy or stored
as liver and muscle glycogen, but excess
carbohydrates may be converted into fat.
• The body can make glucose from certain byproducts of protein and fat.