CARBOHYDRATES
Carbohydrates
are a major source of energy for humans, providing approximately 45% to 80% of
the total caloric intake in different income groups. Since they are a
relatively inexpensive source of energy compared to fats and proteins, they
form the bulk of the diet of humans throughout the world.
They
are mainly present in food in the form of sugars, starches, and fibres. A study
of the various types of carbohydrates is necessary because the kind and
proportion of different forms of carbohydrate present in food have direct
bearing on our health.
Three
groups of carbohydrates are important in out diet from the nutritional point of
view, namely, sugars, starches and fibres. The sugar and starch that we consume
is ultimately broken down to glucose in the digestive tract and absorbed into
the blood circulation. In the human body, glucose is removed from blood by the
tissue cells and used as a source of energy. Some glucose is converted to
glycogen, also called animal starch, and stored in the muscle and liver as a
reserve store of energy.
Glucose Oxidised
in Energy + CO2 ↑ + H2O
Tissues waste
products of metabolism
Classification
of Carbohydrates
Carbohydrates
which are of importance in the diet are classified on the basis of the number
of sugar units present in them (Fig. 13.1, Table 13.1). they may also be
classified as:
Available
Carbohydrates : Carbohydrates which can be digested in the human body and yield
energy when they are oxidized in the body.
Unavailable
Carbohydrates : Carbohydrates which cannot be be digested because the human
body does not contain the enzymes necessary for their breakdown. Unavailable
carbohydrates do not provide any energy to the body but are necessary as they
perform some important functions in the body such as regular elimination of
faecal waste.
Digestion,
Absorption, and Metabolism
Complex
carbohydrates and sugars are too large to be absorbed through the intestinal
wall. They need to be broken down into their constituent monosaccharides can be
absorbed into the blood stream. The mechanical and chemical digestion of starch
begins in the mouth.
Fig.
13.1 Classification of Dietary carbohydrates
Table
13.1 Classification of Carbohydrates
Sr. No.
|
Category
|
Name of Carbohydates
|
Sources
|
1. Simple carbohydrates or
sugars
|
|||
(a).
|
Monosaccharides
|
(single
sugar unit)Glucose (dextrose)
Fructose
(levulose)
GalactoseFruits,
vegetables
Honey
On
hydrolysis of lactose(b).Disaccharides
(two
sugar units)Sucrose (glucose + fructose)
Maltose
(glucose + glucose)
Lactose
(glucose + galactose)Sugar cane, sugar beet,
Sprouted
and malted grains, acid hydrolysis of starch.
Milk
is the only source.2. Complex carbohydrates or polysaccharides(a).AvailableStarch
Glycogen
(animal starch)
DextrinCereals,
pulses, roots, tubers, veggies, and under-ripe fruits.
Liver
and muscle of freshly slaughtered animals.
Partial
breakdown of starch by dry heat or digestion.(b).Unavailable or dietary
fibres1.Water insolubleCellulose
Hemicelluloses
*LigninStructuralfibre
in whole grain cereals, nuts, wheat bran, figs,
vegetables,etc.2.WatersolublePectins
Gums
MucilagesNon-structural
fibres in apples, citrus fruits, guava, oats, barley, pulses, seaweeds,etc.
*Lignin
is not a carbohydrate.
Sources
Daily
diet should provide up to 50-70% kCal of energy from carbohydrate, which means
that the diet of an individual who needs 2,400 kCal should consume 60% of
2,400, i.e., 1,440 kCal or 360g of carbohydrates/day. Carbohydrates are not
only an economical source of energy but are also readily available and easy to
store as they have a long shelf life.
All
foods of plant origin contain carbohydrates in varying amounts. With the
exception of milk, animal foods do not contain carbohydrate. Although milk is
not consumed as a source of carbohydrate, some milk products, such as khoa and
milk powder, contain significant amount of carbohydrate lactose.
The
important sources of carbohydrates in the diets of Indians are cereals and
millets, roots, tubers, pulses, sugar and jiggery. (table 13.2)
All
sugars provide 4 kCal/g of energy. the carbohydrate and calorie content of a
food can be reduced by using sugars which are sweeter than sucrose so that the
quantity of sugar required will be less.
Table
13.2 Some rich sources of carbohydrates
Cereals
|
Pulses
|
Fruits and vegetables
|
Nuts and oilseeds
|
Miscellaneous
|
Wheat
|
Rice
Jowar
Bajra
Ragi
Oats
Barley
CornAll
whole grain and dehusked pulses and their by-products,
e.g.,
rajma, Bengal gram, whole green gram, lentils, and besan.Mango
Chikoo
Jackfruit
Custard
apple
Banana
Green
peas
Beans
Potato
Yam
ColocasiaCashew
nuts
Coconut
Ground
nuts
Garden
cress
Seeds
GingellyseedsSugar
Jaggery
Honey
Sago
Tapioca
Dates
Raisins
Skimmed
milk powder
Functions
Carbohydrates
have many functions in the body.
1. The
chief function of carbohydrate is to provide energy to the body so that it can
carry out day-to-day work and maintain body temperature. All carbohydrates
except fibre provide 4kCal/g of energy. it is the cheapest source of energy
available.
2. Glucose
is the only form of energy used by the central nervous system. When blood
glucose levels fall, the brain does not receive energy and convulsions may
occur.
3. Carbohydrates
spare proteins from being broken down for energy and are used for bodybuilding
and repair. In carbohydrate deficient diets, proteins meant for bodybuilding
and repair are oxidised to meet the most important and first need of the body,
i.e., energy.
4. They
are required for complete oxidation of fat. In a deficiency, fats broken down
rapidly for energy and intermediate products such as ketones are formed in
large amounts resulting in a condition called ketosis.
5. Carbohydrates
can be converted into non-essential amino acids, provided a source of nitrogen
is available.
6. The
sugar lactose helps in the absorption of the minerals calcium and phosphorous.
7. Lactose
helps certain bacteria to grow in the intestines. This bacterial flora is
capable of synthesizing B-complex vitamins in the gut.
8. Dietary
fibre plays an important role of increasing faecal mass by absorbing and
holding water, stimulating peristalsis, and eliminating faecal waste.
9. Fibre
also helps in lowering blood cholesterol levels by binding bile acids and
cholesterol.
Deficiency
The
daily diet should not contain less than 100g of carbohydrate. Carbohydrate
deficiency is uncommon in our country as diets are cereal based. A deficiency
of carbohydrate in the diet results in utilization of fat for energy. In severe
deficiency, incomplete oxidation of fats causes ketone bodies to accumulate in
the blood.
Excess
Carbohydrates
1. Excessive
consumption of refined sugars could be one of the causes of dental caries or
tooth decay.
2. Excessive
sugar depresses the appetite, provides hollow calories, and could result in
malnutrition.
3. High
intake of sugar and refined carbohydrates increase the blood triglyceride
levels leading to heart diseases.
4. When
excessive carbohydrates are consumed they are converted into fat and deposited
in the adipose tissue, whic could lead to obesity, i.e., body weight of 20% or
more than desirable weight.
5. Excessive
fibre could irritate the intestinal lining causing cramps or bloating due to
gas formation.
6. Excessive
fibre interferes with the absorption and availability of mineral elements such
as iron and calcium.
Role
of Dietary Fibre in Prevention and Treatment of Disease
Dietary
fibre refers to the total amount of naturally occurring material in plant
foods, which is not digested. The terms roughage, bulk, and unavailable
polysaccharides are synonymous with fibre. Fibres cannot be digested by human
enzymes.
Dietary
fibre or roughage does not provide humans with energy but performs many
important functions in the body (Table 13.3). Fibre can absorb and hold water
thereby increasing faecal bulk. This acts as a laxative and reduces
intraluminal pressure in the colon preventing diverticulosis. Insoluble fibre
prevents constipation by stimulating peristalsis in the large intestine. The
contraction of muscular walls of the digestive tract is stimulated by fibre.
Fibre increases water absorption, forming a larger, softer stool that rapidly
passes through the colon. Soluble fibre binds bile acids and cholesterol and is
beneficial to people suffering from coronary heart disease. Fibre reduces the
triglyceride and cholesterol levels in blood.
Table
13.3 Functions and sources of dietary fibre
Sr.no.
|
Type
|
Functions
|
Food source
|
||
Insoluble fibres
|
|||||
1.
|
Cellulose
|
1. Insoluble fibre
|
|||
2. Holds
water
3. Increases
stool bulk
4. Reduces
intraluminal colonic pressure
5. Prevents
constipation
6. Binds
minerals such as Ca and Fe.
7. Binds
bile acids
8. Reduces
transit timeBran, whole grain cereals, specially wheat, rye, apples, pears,
tomatoes, cabbage, beans.2.HemicellulosesBran, whole grain cereals, specially
millets – jowar, bajra, ragi.3.Lignin (non-carbohydrate source)Whole grain
cereals, pears, peaches, plums, mature vegetables.Soluble
fibres1.Pectins1. Soluble fibre
2. Binds
cholesterol and bile acids
3. Holds
water
4. Fermented
in the colon to volatile fatty acids and gas by the normal bacteria flora of
the colon.Guava, apple, citrus fruits, wood apple, berries, carrots and green
beans.2.GumsOatmeal, pulses, and beans, dinkacheladoo, processed
foods.3.Mucilages, seaweeds and algae.Thickener in food products, stabilizer,
gelling agent in puddings.
Fibre
is beneficial to people on weight reduction regime. It provides satiety value
to the meal because of more chewing required and at the same time does nto add
to the calorific value of the meal.
It
helps in lowering blood sugar levels in diabetic individuals by slowing down
carbohydrate absorption and lowers the insulin requirement. Regular intake of
fibre may prevent cancers of the colon and rectum.
Although
fibre is not a nutrient, because it cannot be digested by humans, it is
nutrionally important. Foods such as whole grain cereals, fruits, and
vegetables, specially when the peel and seeds are edible, are rich sources
of fibre.
The
fibre content of the daily diet should be approximately 30-40 g/day (Table
13.4)
Recommended
dietary intake for adults
Fibre
– 40g/day is desirable
Carbohydrates
Minimum
– 100g
Maximum
– less than 70% of total calories from carbohydrates.
Artificial
sweeteners
These
are also known as non-nutritive sweeteners. Artificial sweeteners are 100-350
times as sweet as sucrose and provide no or very negligible calories. A wide
variety of sweetening agents are available in the market and are used for
low-calorie products such as diet coke. These processed products are specially
manufactured for obese individuals, weight watchers, and diabetic patients.
They are used quite successfully in bakery items such as cakes, biscuits
cookies, Indian sweetmeats, confectionery products, beverages,
puddings and chewing gum. Saccharin, aspartame, sodium cyclamate and sugar
are some of the commonly used substitutes for sugar. Their use is not
recommended in soft drinks and other food consumed by children as these foods
may be a substitute for essential.
Alcohol
Ethyl
alcohol is produced by yeast fermentation of carbohydrates under anaerobic
conditions. Different carbohydrates are used to manufacture alcoholic
beverages.
Alcoholic
beverages do not supply necessary nutrients but contribute significant amount
of energy (Table 13.5). alcohol contributes 7kCal/g or 5.6kCal/ml and in people
who consume alcoholic beverages, up to 10% of total energy needs may be derived
from alcohol. Some chronic alcoholics may consume insufficient food and suffer
from malnutrition while the reverse may be observed in the case of social
drinkers who consume large amount of high-calorie foods such as starters, nuts,
and wafers along with their drinks. These high-calorie snacks are rich in
carbohydrates, fats, and sodium. Alcohol is absorbed rapidly, directly into the
blood stream. Drinking on an empty stomach increases the alcohol level in blood
twice as fast as on a full stomach. It is a good rule to have some light snacks
along with alcoholic beverages.
Yeast
C6H12O6 2C2H5OH + 2CO2
anaerobically (ethyl
alcohol)
Percentage
of alcohol is proof divided by 2
86
proof whiskey = 43% alcohol
Calorie
content of one peg or 30ml of whiskey = 43 x 30 x 5.6
100
= 78
kCal
If
alcohol is taken along with antidepressants or tranquilizres, it prolongs the
sedative effect of these medicines.
Excessive
consumption of alcohol accompanied by decreased intake of other nutrients can
lead to malnutrition and serious liver disorders such as cirrhosis of the
liver.
Table
13.5 Calories supplied by alcohol
Beverage
|
Amount
|
Calories
|
Alcohol (%)
|
Lager beer
|
240ml
|
110
|
4 – 15
|
Ale
|
240ml
|
150
|
4 – 12.5
|
Gin 80 proof
|
30ml
|
65
|
40
|
Rum
|
30ml
|
70
|
42 . 8
|
Whiskey 86 proof
|
30ml
|
78
|
43
|
Wine red
|
90ml
|
145
|
12 – 15
|
Wine dry
|
90ml
|
90
|
12 – 15
|
Vermouth
|
30ml
|
50
|
18 – 22
|
Martini
|
90ml
|
140
|
12 – 13
|
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