A general idea of carbohydrates in the eye, Glycemic Index and different classifications of carbohydrates based on chemical nature their digestion and absorption, carbohydrate metabolism in the eye and its transport

1. Unit: III
Lipids and eye
Carbohydrates and eye
Vitamins and eye
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2. Unit: III
Carbohydrates and eye
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3. Glycosaminoglycans
• The cornea and vitreous contain a macromolecular structure composed of collagen
proteins, which form fibres (with diameters which are thinner than half the
wavelength of light)
• interfibrillar spaces filled with polysaccharides known as glycosaminoglycans
(GAGs) -reduce the effects of diffraction.
• These polysaccharides consist of repeating disaccharide units forming long
polysaccharide chains.

4. Glycosaminoglycans
• The major GAG of the vitreous is hyaluronic acid – made of disaccharide unit of
glucuronate and N-acetyl glucosamine.
• The cornea contains four different types of GAG:
(1) keratan sulphate
(2) Chondroitin
(3) chondroitin sulphate
(4) dermatan sulphate

5. Proteoglycans
• Proteoglycans (mucoproteins) are proteins which contain one or more GAG
covalently linked to it
• these proteins are usually non-collagenous proteins, some GAGs have been found
to be linked to certain collagen types and are referred to as ‘part-time’
proteoglycans
• proteoglycans in ocular matrices interact with collagen in a functional manner and
(1) Regulates collagen fibril diameter
(2) Maintains the fibril distance in the collagen network

6. Cornea
The cornea consists of five layers:

7. Corneal Stroma
• Types of collagen in stroma.
Stromal Collagen
Type I (60%) Type V (10%) Type VI (30%)
Type III
(low conc.)

8. Corneal Stroma
• The collagen fibrils are 25–35 nm in diameter
• transparency of the corneal stroma is due to the organised lattice structure of the
collagen fibrils
• The structure is responsible for destructive interference, where light scattered by
neighbouring fibrils in predictable and opposing directions cancels each other out,
except in the primary visual axis
• fibril size and interfibrillar space play important roles in maintaining transparency
of this tissue.

9. Corneal Stroma
• Light-scattering occurs when the regional fluctuation in refractive index exceeds
the dimension equal to one-half the wavelength of light (200 nm) (>200 nm in case
of corneal swelling)
• proteoglycans are responsible for maintaining the relative positions of the fibrils
and for restricting fibril lateral growth
• proteoglycans control fibril diameter which both play an important role in ensuring
correct fibrillogenesis and corneal transparency.

10. The Vitreous
• The vitreous is highly hydrated ( >98% Water ) and transparent ( 90% of visible of
light)
• It is composed of hydrogel structure maintained by a network of thin unbranched
collagen fibrils with GAGs (hyaluronan) filling the spaces of the network.
• This is needed for maintaining corneal transparency.
• The peripheral or cortical vitreous contains hyalocyte cells(believed to originate
from blood Macrophages)
• The hyalocytes are secretory cells responsible for the synthesis of hyaluronan and
collagen which then move toward the central vitreous for matrix assembly.

11. The Vitreous

12. Hyaluronan
• The concentration of adult vitreous hyaluronan has been estimated between 65
and 400 mg/ml.
• subject to age dependent concentration changes, involving an initial increase in the
first 20 yrs - relatively constant concentration up to 70 yrs - then further increase
• Its high molecular mass and numerous mutually repelling anionic groups give rise
to a highly hydrated viscous matrix
• at high concentrations they form hydrodynamic interactions which give rise to
networks which can act as a molecular sieve, excluding cells and large molecules,
thereby contributing to transparency

13. Hyaluronan

14. Molecular structure of Vitreous

15. Age-Related Change in Vitreal Connective Tissue
• The human vitreous is subject to age-related changes, and morphologically two
distinct but simultaneous structural alterations occur:
(1) collapse of the collagen network (syneresis)
(2) formation of collagen free, liquid-filled spaces (synchysis)
• The vitreous undergoes liquefaction with age
• The liquefaction is due to a decrease in gel volume, believed to occur by syneresis
(contraction) of the collagen fibrillar network.
• The collagen fibrils aggregate into large bundles
• These structural changes occur by the time the eye has reached adult size- 14–15
yrs (20% liquefied) - 40–50 years - 80–90 years ~50% of the vitreous is liquefied

16. Age-Related Change in Vitreal Connective Tissue
• If the hyaluronic acid (HA) molecules are no longer attached or associated with the
collagen, it will clump, and stick to itself squeezing water molecules out.
• With age, there is a depolymerisation of hyaluronic acid, causing these molecules
to release their water and form lacunae i.e., pockets of liquefied vitreous.
• The collagen 'filaments' aggregate to form larger 'fibrils', causing further collapse
of the vitreous gel structure.
• This process is known as vitreous degeneration and 'syneresis’.
• The collagen fibrils may 'float' within the liquid vitreous pockets, giving the patient a
sensation of floaters.

17. Age-Related Change in Vitreal Connective Tissue

18. Age-Related Change in Vitreal Connective Tissue
• The posterior border of the vitreous base separates from the retina as the gel-like
structure of the vitreous collapses, leading to posterior vitreous detachment (PVD)
• PVD is common with ageing, and usually makes a clean break away from the retina.
• the vitreous adheres tightly to the retina in certain places and a small, horseshoe-
shaped tear in the retina can result from persistent tugging by the vitreous, leading
to retinal detachment

19. Glucose Metabolism in Cornea
• much of the glucose is metabolised by the hexose monophosphate pathway (the
pentose shunt) with no production of ATP.
• The products are ribose-5-phosphate and NADPH (2)
• The corneal epithelium is permeable to oxygen - give rise to reactive oxygen species
- oxidise the sulfhydryl (-SH) groups of proteins

20. Glucose Metabolism in Lens
• Three main pathways :
(1) glycolysis (80–85%)
(2) TCA (Krebs) cycle (~5%)
(3) hexose monophosphate (pentose shunt) (10– 15%)
• Glucose yields – 2 ATP under aerobic condition and 36 ATP under anaerobic
condition
• The maintenance of the lens structural integrity requires:
(1) osmotic balance provided by Na+,K+-ATPase
(2) redox balance provided by glutathione (and GSH reductase)
(3) protein synthesis necessary for growth and maintenance of the tissue.

21. Glucose Metabolism in Lens
• GSH maintains unaggregated state of lens proteins by reducing or reversing
oxidative damage by UV
• HMP in the lens provides the NADPH required by GSH reductase, which is
important in maintaining GSH and redox balance

22. NADPH NADP+
Glucose Metabolism in Lens
• Glucose can be converted to sorbitol by auto-oxidation or enzymatically by aldose
Reductase which utilises NADPH supplied by HMP
• Sorbitol can be converted to fructose by sorbitol dehydrogenase;
• the ratio of the two enzymes in the human lens favours sorbitol production
• Less than 5% of glucose is converted to sorbitol
• it slowly diffuses out to the aqueous humour and doesn't accumulate in the lens

23. Glucose Metabolism in Lens
• high external glucose concentrations lead to increased lens glucose, which saturates
the normal metabolic pathways, leading to increased sorbitol synthesis
• Sorbitol accumulation leads to increased osmolarity of the lens - affects the
structural organisation of crystallins and promotes denaturation and aggregation,
leading to increased scattering of light and cataract
Sorbitol accumulation
↑ Glucose accumulation ↑ Osmolarity
Changes in
crystallin
organization
Cataract
↓ Glucose accumulation
(infantile hypoglycemia)
↓ NADPH ↑ ROS

24. Glucose Metabolism in Lens
• one of the most metabolically active tissues
• The blood–retinal barrier is overcome by carrier-mediated facilitated diffusion
through specific plasma membrane glycoproteins, the glucose transporters GLUT 1
and GLUT 3.
GLUT1
GLUT3
Glucose
BloodRetinalBarrier
Pigmented epithelium

25. Glucose Metabolism in Lens
GLUT1
GLUT3
Glucose
BloodRetinalBarrier
Pigmented
Epithelium
Glycolysis
Glucose
Lactate
Pyruvate
TCA
Cycle
Cone Photoreceptor
Muller Glia
Lactate

26. Glucose Metabolism in Lens
• In the absence of glucose (hypoglycemia, aglycemia) the retina can metabolise
exogenous lactate or pyruvate
• The hexose monophosphate pathway (HMP) is also active in the retina: produce
NADPH for Glutathione and Ribose phosphate for DNA and RNA
Sorbitol accumulation
↑ Glucose accumulation ↑ Osmolarity
Fluid
retention
and hypoxia
Retinopathy

27. Glycemic Index (GI)
• A relative ranking of carbohydrate in foods
according to how they affect blood glucose
levels.
• Carbohydrates with a low GI value (55 or less)
are more slowly digested, absorbed and
metabolised and cause a lower and slower rise
in blood glucose and, therefore insulin levels
• Foods with a high GI are those which are rapidly
digested, absorbed and metabolised and result
in marked fluctuations in blood sugar (glucose)
levels.

28. Glycemic Index (GI)
• There are three classifications for GI:
• Individual food portion:
• Low: 55 or less
• Moderate: 56 – 69
• High: 70+
• Low GI carbohydrates – is one of the secrets to long-term health, reducing your risk
of type 2 diabetes and heart disease. It is also one of the keys to maintaining
weight loss

29. GI to Age-Related Macular Degeneration
• In the first study published in 2006, women with high dietary GI compared had an
approximately 2.7-fold increased risk for early AMD, mainly pigment abnormality
• Consuming lower GI diets appears to provide ophthalmic benefit in addition to that
gained from currently known dietary factors.
• Under hyperglycemia, aldose reductase (AR) reduces glucose to sorbitol which is
later oxidized to fructose.
• In this process, the AR consumes cofactor NADPH.
• Therefore, the hyperglycemic polyol pathway consumes NADPH and hence results
in the depletion of reduced glutathione (GSH).
• This increases intracellular oxidative stress.

30. GI to Age-Related Macular Degeneration

31. GI to Age-Related Macular Degeneration
Hyperglycemia
• Oxidative stress
• Angiogenesis
• Capillary occlusion
• Blood flow abnormalities
• Inhibition of cell division of retinal associated cells
• Induces inflammation and apoptosis

32. Diabetic eye disease
• Diabetic retinopathy affects blood vessels in the retina that lines the back of the eye. It
is the most common cause of vision loss among people with diabetes and the leading
cause of vision impairment and blindness among working-age adults.
• Diabetic macular edema (DME). A consequence of diabetic retinopathy, DME is
swelling in an area of the retina called the macula.
• Cataract is a clouding of the eye’s lens. Adults with diabetes are 2-5 times more likely
than those without diabetes to develop cataract. Cataract also tends to develop at an
earlier age in people with diabetes.
• Glaucoma is a group of diseases that damage the eye’s optic nerve—the bundle of
nerve fibers that connects the eye to the brain. Some types of glaucoma are associated
with elevated pressure inside the eye. In adults, diabetes nearly doubles the risk of
glaucoma..

33. Carbohydrates
Carbohydrates are polyhydroxy aldehydes, ketones, alcohols, acids, their simple
derivatives and their polymers having linkages of the acetal type. They may be
classified according to their degree of polymerization and may be divided initially into
three principal groups, namely sugars, oligosaccharides and polysaccharides
Carbohydrates
Monosaccharides Disaccharides Polysaccharides
Complex
polysaccharides

34. Classification on the basic of degree of polymerization
• Monosaccharides: consisting of single unit of sugar and also known as simple
sugars (DP: 1)
• Disaccharides: consisting of (2) monosaccharide's (DP: 1-2)
• Oligosaccharides: each moleculeconsisting of (3-9) monosaccharide units (DP: 3-9)
• Polysaccharides: each molecule containing more than 9 but usually several
monosaccharaides units (DP: >9)

35. Monosaccharides
Biose (C2H4O2)
• Glycolic aldehyde is the only member of this group
• Crystalline and is easily soluble in water
• Sweet in taste
Trioses (C3H6O3)
• Two compounds of this category are: Glyceraldehyde and dihyroxyacetone
• They occur in plant and animal tissue in small amounts and are derived from
breakdown of glucose

36. Monosaccharides
Tetroses (C4H8O4)
• Two compounds belonging to this category are known: erythrose and threose
Pentoses (C5H10O5)
• Pentoses occurring commonly in nature are: 1) Arabinose, 2) Xylose, 4) Ribose
• Deoxyribose which occur free deoxyribonucleic acid contains one oxygen atom less
than ribose
• Arabinose: It does not occur free in nature. The polysaccharide Xylan present in
cherry gum or gum arabic is formed is formed by combination of a large number of
arabinose molecules

37. Monosaccharides
Pentoses (C5H10O5)
• Xylose: Xylose doesn’t occur free in nature. Polysaccharide xylan present in wood
gum and straws is formed by combination of a large number of arabinose
molecules.
• Ribose: It is present in adenylic acid and ribonucleic acid (RNA) occurring in plant
and animal tissues
• Deoxyribose: It is constituent of deoxyribonucleic acid (DNA) which is present in
the nucleus of plant and animal cells

38. Monosaccharides
Hexoses (C6H12O6)
• Divided into two groups:
• 1) Aldoses or sugar containing aldehyde group, e.g. glucose, galactose and
mannose
• 2) Ketose or sugars containing a ketone group, e.g. fructose and sorose
• Glucose – main source of energy. Normal human blood contains 80-100mg of
glucose
• Fructose: occurs in free state in many fruits. Readily utilized by body
• Galactose: present in milk

39. Disaccharides
Formed by combination of two monosaccharides
Sucrose
• Cane sugar, Formed by condensation of one molecule of glucose and one molecule
of fructose. Readily hydrolyzed by sucrase present in intestinal juice
Sucrose Glucose + Fructose
Sucrase

40. Disaccharides
Maltose
• Malt Sugar is present in malt made from barley, ragi, jowar etc.
• Formed from two molecules of glucose
Maltose 2 Glucose
Maltase

41. Disaccharides
Lactose
• Milk Sugar
• Formed by condensation one molecule of glucose and one molecule of galactose
• Digested by lactase in intestinal juice
Lactose Glucose + Galactose
Lactase

42. Polysaccharides
Starch
• Important source of starch are: cereals and millets, roots and tubers
• It consists of mixture of two polymers amylose and amylopectin
• Starch is hydrolyzed by dilute acids to glucose
• Amylase present in saliva or pancreatic juice convert starch to maltose
Starch Glucose
Acids
Starch Maltose
Amylase

43. Polysaccharides
Dextrins
• Polysaccharides formed y the partial hydrolysis of starch by acids or amylase
• They composed of a large number of glucose molecules
• They are soluble in water
Glycogen
• Is the reserve carbohydrate found in liver (3-7 %) and muscle (0.5-1%) of animal
and man
• It is made of large number of glucose molecules

44. Functions of Carbohydrates in the diet
1. They supply energy for body functions and for doing work
2. They are essential for the oxidation of fats, production of ribose sugar, ATP and
NAD(P)H
3. They exert a sparing action on proteins
4. They provide carbon skeleton for the synthesis of some non essential amino
acids
5. Carbohydrates are present in all tissues and cells
6. Its is needed for production and functioning of glycoproteins
7. They add flavor to the diet
8. Starch which forms the main source of carbohydrates in the average diets has a
land taste and is non-irritant and hence can e consumed in large amounts to
provide major part of the energy requirements of the body

45. Important source of carbohydrate in diet
Food item Carbohydrate in g/100g
Cereals and Millets 63-79
Pulses 56-60
Nuts an Oil seeds 10-25
Roots and Tubers 22-39
Arrow root flour 85-87
Cane sugar 99
Sago 87-89
Honey 79-80
Jaggery 94-95
Milk 4-5
Dried Fruits 67-77
Fresh Fruit 10-25

46. Carbohydrate Requirements
• The percentage calories derived from carbs in diets consumed by a vast majority
of people in developing countries is as high as 60-70%
• The carbohydrate calories should be at least 40% in well balanced diets
• The level of carbohydrate calories in the diet will also depend on the availability
of fat and the economic conditions of the people as the fat is about twice as
costly as cereals and millets in developing countries
Age group Optimal level of carbohydrate calories
as per cent of total calories
Adults 50-70
Expectant and Nursing Mothers 40-60
Infants (1-12 months) 40-50
Preschool children (1-5 yrs. ) 40-60
Older children and adolescents 50-70