Carbohydrates

Carbohydrates

• In this section we will discuss…

• Classification of carbohydrates

• Caloric value of carbohydrates

• Recommended daily allowances (RDA)

• Sources of carbohydrates

• Functions

• Digestion, absorption, storage and metabolism of carbohydrates

• Malnutrition of carbohydrates

Carbohydrates

• Carbohydrates are widely distributed in plants. Synthesized by photosynthesis

Carbohydrates

• Classification of carbohydrates

• 1. Monosaccharides

• 2. Oligosaccharides (Di, Tri, Tetra)

• 3. Polysaccharides

• Simplest carbohydrates group is monosaccharides group

• Also known as simple sugars

• ---ose is the suffix given in general

• Exist in Aldose and ketose form

Carbohydrates

• 1. Monosaccharides

• Classified depending on no. of “C” atoms

• Glucose is the most important player in metabolism

• Fructose is the sweetest of all

• Galactose is part of lactose present in milk

Carbohydrates

• 2. Disaccharides

• Formed by combination of two monosaccharides

SUCROSE

Glucose - Fructose

LACTOSE

Galactose - Glucose

MALTOSE

Glucose – Glucose

- Constituent of cane sugar

- Common table sugar

- Also called milk sugar

- Sole CHO source to

infants

-Product of starch

hydrolysis

-Found in germinating

seeds

Carbohydrates

• 3. Polysaccharides

• Complex carbohydrates composed of many single sugar units

• Generally tasteless and forms colloids in water

• Depending on composition 2 types

• Homopoly saccharides – same type of sugar unit

• Starch, glycogen, inulin, cellulose…

• Heteropoly saccharides – two or more type of sugar units

• Heparin, chondroitin sulfate…

Carbohydrates

Homopolysaccharides

Characters Starch Glycogen Cellulose

Monomer a-glucose a-glucose b-glucose

Type of bond

between

monomers

1,4 glycosidic bond

(amylose) + 1,4 and

1,6 glycosidic bond

(amylopectin)

1,4 and 1,6

glycosidic bonds1,4 glycosidic bond

Nature of chain

Amylose is coiled

unbranched

Amylopectin is long

branched chains,

some coiling

Short many

branched chains,

some coiling

Straight, long

unbranched chains form

H-bonds, with adjacent

chains

Occurrence In plantsIn animals and

fungiIn plants

FunctionCarbohydrate energy

store

Carbohydrate

energy storeStructural

General form Grains Small granules Fibres

Carbohydrates

heteropoly saccharides - play role in health

Carbohydrates

Summary of classification

Carbohydrates

• Calorie value

• 1gm of CHO releases 4 Kcal of energy

• RDA

• RDA refers to recommended daily quantities of nutrients for maintaining health and efficiency

Age Group % of calories from CHO

Infants 50-70

Pre-school children 40-60

Adolescent – school children 50-70

Adults 50-70

Pregnant / nursing women 40-60

Carbohydrates

• Sources

• Plant sources

• Cereals & grains

• Vegetables

• Fruits

• Sweets

• Animal sources

• Milk

• Glycogen from meat & poultry

Carbohydrates

• Functions

• Serve as major source of energy

• Precursors for many organic molecules

• In the form of glycoproteins and glycolipids participate in cellular functions like division, adhesion and fertilization…

• Serves as structural components (cell walls, exoskeleton) of organisms

• Serve as raw material for industries

• Serve as diagnostic aids in some diseases

• Example inulin – fructo-oligosaccharide

Carbohydrates• Digestion

• Main dietary carbohydrates are polysaccharides

• To some extent di-saccharides and very less amount of monosaccharides

• Polysaccharides get hydrated during the process of cooking which is essential for proper digestion

• Digestion of carbohydrates starts in mouth and mainly happens in intestine

• Digestion in mouth

• During mastication salivary α-amylase acts on starch

• Produces maltose, α-limit dextrins….

Carbohydrates• Digestion in small intestine

• Pancriatic α-amylase continues the digestion process

• The resulted products are maltose, isomaltose and oligosaccharides

• In upper jejunum di and oligosaccharides are converted in to monosaccharides by oligosaccharidases and disaccharadases

• Sucrase is of particularly important to digest table sugar

• Lactase is limited in humans in utilization of milk sugar

• Lactose intolerance

Carbohydrates• Absorption

• Final products of carbohydrate digestion are mainly glucose, galactose and fructose… have different absorption mechanisms

• Absorption mainly takes place in small intestine

• Galactose is most efficiently absorbed followed by glucose and fructose

• Glucose and galactose are absorbed by same mechanism

• Several glucose transporters with varying functions are in action

• Glucose / galactose transport is carrier mediated and energy requiring process

Carbohydrates• Absorption

• Glucose and Na+ share same transport mechanism (Symport)

• High concentration of Na+ out side the cells creates the driving force for its transport in to the cells

• Simultaneously glucose is also transported

• This is secondary active transport

• Maximum rate of glucose absorption is 120 g/hr

• Galactose is also absorbed in the same manner

Carbohydrates• Absorption

• Primary active transport

• Secondary active transport

Carbohydrates• Absorption

• Absorption of fructose is simple

• It doesn’t need energy and independent of Na+

• It is transported by facilitated diffusion

• Fibrous material which can not be digested by human enzymes or intestinal bacteria will be excreted

Glycemic index – concept

Alternative way to classify CHO foods

• Effect of CHO containing food on blood glucose levels is glycemic response. Depends on ease of digestion

• Rapidly increasing glucose level – High GI

• Slowly increasing glucose level – low GI

Carbohydrates• Absorption

Glycemic index – increase in blood glucose level above fasting level during 2 hr period following consumption of a 50g of CHO containing food compared with same amount of CHO in a reference food

the reference food can be glucose or white bread

Glycemic load - Glycemic index multiplied by amount of CHO in grams present in the food per portion

Carbohydrates

Carbohydrates• Metabolism & Storage

• Glucose is the central molecule in carbohydrate metabolism… all major pathways of metabolism are connected with it

• Conversion of glucose to glycogen - glycogenesis

• Conversion of glycogen to glucose - glycogenolysis

• Conversion of non-carbohydrates to glucose –gluconeogenesis

• Breakdown of glucose – glycolysis

• TCA cycle

• HMP shunt

• Uronic acid pathway – produce ascorbic acid

• Galactose to glucose, fructose metabolism

• Amino sugar and mucopolysaccharide metabolism

Carbohydrates• Malnutrition of carbohydrates

• Deficiency

• Carbohydrate deficiency is not common in adults

• Fats generally compensate for temporary deficiency – results in ketone bodies

• Deficiency may occur in children – “Marasmus”

• Generally appears in children of age 1year

• Marasmic children look like little old man with features of big head, huge eyes, wrinkled face and tiny body

Carbohydrates• Malnutrition of carbohydrates

• Over consumption

• Over consumption especially sucrose may increase dental caries

• Cause obesity

• Irritate GI-mucosa

• Depress apetite

• Increase in blood tri-glycerides leading to heart diseases….

Next class…..

• Lipids - nutrition