Download 2015-05-25 1 Monosaccharides

2015-05-25
Unit 4. CARBOHYDRATES
Main Questions to Answer:
·
What are the different types of carbohydrates?
·
What are the best food sources for carbohydrates?
·
How are carbohydrates digested and absorbed by the body?
·
How do carbohydrates function in the body?
·
What are some of the problems and illnesses associated with
carbohydrate metabolism?
Types of carbohydrates
(mono-, di-, oligo-, poly-saccharides)
Monosaccharides
• Single units, simple sugars
• 6-carbon sugars (hexoses):
– Glucose (dextrose); Fructose (levulose); Galactose
• 5-carbon sugars (pentoses):
– ribose, xylose
• (Sugar alcohols, amines, acids)
Disaccharides
• sucrose, lactose, maltose
– Condensation reaction (releases water) of two
monosaccharides
– Simple sugars
Oligosaccharides
• fructo-oligosaccharides (FOS)
– Health benefits?
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Polysaccharides
• most abundant forms of carbohydrate; homo- & hetero-polymers
(plants: starches, fiber; animals: glycogen)
• Starches: Amylose-linear; Amylopectin-branched
• Dietary fibers (not all dietary fibres are carbohydrates):
– Soluble: pectins, gums,… readily dissolve in water
– Insoluble: cellulose, hemicelluloses…do not readily dissolve in
water
– Typically, not digested by human enzymes
– Can influence absorption (or absorpt. rate) of some nutrients
– Helps maintain healthy colon function
• Glycogen: Stored mainly in muscle & liver
Sources of carbohydrates and
recommended intakes
• sweetened & starchy foods, cereal products, fruits/vegetables….
• Recommendations are for complex carbohydrates (not simple
sugars; ‘empty calories’)
– 45-65% of total daily calories as carbohydrate (AMDR)
– A 2000 kcal diet can include about 300 g carbohydrates (DV)
– RDA, a minimum of 130 g/day for most adults
– Limit simple sugars to about 10-20% or less of carbohydrate intake
• Recommended dietary fibre intake
– 38 g/d for men and 25 g/day for women (AI, up to age 50)
– A food labeled as a ‘high source’ of fibre has at least 4 g/serving
– High fibre diets often require extra fluid
Sweeteners…from 0 to 4 kcal/g
• No calories (0 kcal/g)
– saccharin, acesulfame, sucralose, cyclamate…
– ADI (acceptable daily intake) values set by Health Canada
• Full calories (4 kcal/g)
– table sugar (sucrose), honey, corn syrup…
– honey and corn syrup are not recommended for infants
– aspartame typically contributes negligible energy to the diet
because it is very sweet
• A few calories (about 1-3 kcal/g)
– xylitol, mannitol, sorbitol…
– do not promote tooth decay
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Carbohydrate digestion
• Pre-digestive: cooking
• Mouth
– salivary amylase (starch→ maltose)
• Stomach
– Complex carbohydrates have slower rate of emptying from stomach
• Small intestine
– pancreatic amylase (→ mono-, disaccharides)
– disaccharidases (e.g., lactase) → monosaccharides
– absorbed monosaccharides go from enterocytes to liver
• Large intestine
– unabsorbed carbohydrates fermented ,and some fibre breakdown, by
bacteria
• Liver: monosaccharides → glucose…
Functions of carbohydrates in body
• Energy source
– Especially important for brain & erythrocytes
– Excess carbohydrate can be stored as fat…
• Antiketogenic
– ketones result from incomplete fatty acid oxidation
– Sufficient dietary carbohydrate prevents high fat use for energy
• May be problem with some low-carb diets
• Sparing of protein breakdown
– Dietary carb decreases gluconeogenesis from amino acids (protein
stores…extensive breakdown can weaken vital organs)
• Many structural and regulatory functions…
• Health of the GI tract: dietary fibre…
Possible functions of Dietary Fibre
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•
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Protection from diverticulosis or associated inflammation
Increased rate of transit through colon (insoluble fibres)
Prevention of constipation (stool softening…with fluid)
Prebiotic effect (also some oligosaccharides)…
• Decreased total plasma cholesterol (↓risk of some CVD?)
• Decreased glucose absorption rate..
• Increased satiety (…better weight management?)
• Decreased risk of some cancers??......
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Other potential problems and diseases
associated with carbohydrates
• Empty calories & Dental Caries
– Simple sugar-rich foods such as soft-drinks have few other nutrients
– Caries: influenced by types of carbohydrates and contact time with teeth
• Glycemic index and load
– Relative rating of the potential that foods have to increase blood glucose
• Diabetes (mellitus) & problems with glucose regulation
– Type 1: often juvenile onset; lack of insulin production
– Type 2: often mature adult onset; resistance to insulin action
• Other carbohydrate metabolic diseases …
• Lactose intolerance
•
Deficiency of lactase enzyme
• Very low carb diets…
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