Carbohydrates Flashcards
Artificial sweeteners
Nutritive - polyhydric alcohols or polyols (sorbitol, mannitol, xylitol) - properties Absorbed more slowly than sugars Converted to fructose Associated with decreased cariogenicity
Non- nutritive
- saccharin
- aspartame (nutrasweet)- aspartic acid + phenylalanine
- acesulfame-K (sunette/ sweet one)
- cyclamate
- sucralose (Splenda)
Sugars
Monosaccharides- glucose, fructose (sweetest), galactose- occur in raw fruit, berries and veggies
Disaccharides-
sucrose- found widely in fruit, berries,vegetables, sugar cane, beets. (Glucose and fructose)
Lactose- main sugar in milk/dairy products. (Glucose and galactose)
Maltose- sprouted wheat, barley. (Glucose and glucose)
Polysaccharides
Non- available CHO resistant starch - dietary fibre
Polyols/ sugar alcohols
Alcohols of glucose and other sugars eg sorbitol
Found naturally in some fruits
Made commercially using aldose, reductase to convert the aldehyde group of the glucose molecule to alcohol
Used as a sucrose replacement to make “sugar free” products
Neither sugars not alcohols
They are carbohydrates with a chemical structure that partially resembles sugar and partially resembles alcohol, but they don’t contain ethanol as alcohol beverages do
They are incompletely absorbed and metabolised by the body, and consequently contribute fewer calories
Laxative effect of polyols
Have osmotic effect- agtract and bind water. When a large quantity of osmotically bound water is present, the result may be diarrhoea.
Undigested polyols may be metabolised by the micro organisms that inhabit the large intestine. When this happens, one of the products may be gas, so flatulence is a common result. Microbial metabolism can also produce short chain fatty acids; these can also have osmotic effects.
Olgiosaccharides
Short chain carbohydrates
Degree of polymerisation 2-10
Maltodexrins
Others: raffinose, verbacose eg in lentils/ beans
Non- digestible:
Not susceptible to brush border enzyme break down
Fructans- insulin, fructo-olgiodsccharides eg in artichokes
Become known as prebiotics
Polysaccharides
Starch polysaccharides (alpha glucans) Granules comprised of two polymers- amylose and amylopectin Digestion depends on type A/B/C Starch not digested in the small intestine= resistant starch
Non- starch polysaccharides- mostly from plant cell walls Cellulose 10-30% of these. Hemicellulose Pectins Gums and mucilages Algal polysaccharides Fermented in the large intestine
Carbohydrate availability
Major sources: Cereals Root crops Sugar cane and beet Pulses Vegetables Fruit Milk products
Digestion / absorption
Begins in mouth
Most absorbed in the upper part of small intestine(pancreatic enzymes), hydrolysed to monosaccharides prior to absorption (brush border enzymes),
Some CHO eg most oligos, some starch and all NSP, resist digestion and pass into the large bowel where they are fermented (bacterial enzymes)
Carbohydrate metabolism
Concentration of blood glucose in humans is carefully controlled- insulin key player
But these can rise and fall depending on eating/ fasting
Careful control is maintained through process of:
Glycolysis- breakdown of glucose
Gluconeogenesis- conversion of non-carbs to glucose
Glycogenolysis- conversion of stored glycogen to glucose
Glycogenesis- synthesis of glycogen from glucose
Blood glucose transported to liver for glycogen synthesis and gluconeogenesis, to muscles to be converted into glycogen, converted into triglycerides to form free fatty acid, and to other tissue.
CHO dependent
Red blood cells
Brain
Nervous system
Glucose stored as glycogen
Blood glucose-
Continually replenished by liver glycogen- 85g
Used for various functions (nervous system, blood) anywhere in the body
Muscle glycogen- used only in the muscular site- approx 6000kJ of stored energy
Recommended CHO intake
No RDI/EAR or AI for carbohydrate
- FAO recommends a min of 180g/d to prevent ketosis
- 45-65% of energy from CHO to total energy in the diet, from low energy density, low GI foods
Dietary fibre
Resistant to hydrolysis by enzymes of stomach and small intestine
Includes resistant starch
Includes non-starch polysaccharides
- soluble (non-cellulose): all vegetable tissue gums, mucilage (eg oats, legumes, pectin)- storage components
- insoluble (cellulose): lignin,
Woody vegetables 70-75% of fibre in diet. Structural components.
Indigestible by human enzymes, but can be digested by bacteria in the gut
Fibre in the colon
Bacterial growth Gas production (flatus)- CO2, hydrogen, methane Production of short chain fatty acids - acetic, proprionic, butyric
What causes flatus
Swallowing air
- oxygen and nitrogen swallowed when we eat- some exerted burping, reabsorbed from stomach or passes to bowel flatulence
Foods
- undigested starch and foods high in fibre, particularly beans and cruciferous vegetables (cauliflower, cabbage)
- lactose intolerance or disaccharide malabsorption
- CHO modifies artificial sweetener
Properties of dietary fibre
Soluble
- absorbs ware into GI tract- increases faecal bulk and softens stool
- binds with Bile salts- promotes elimination of cholesterol
- affects the rate of digestion- slow, decreased transit time in gut
- produces low molecular weight organic acids
Insoluble
- affects the rate of digestion - increases transit time in gut
- increases the bulk of the stool
Potential clinical uses of fibre
Weight control- satiety, reduced energy density, slows gastric emptying
Diabetes mellitus
- high fibre low GI diet improves BGL
? Prevention of cancer
Bowel diseases- prevents constipation which can cause diverticulae and infection
Contains cholesterol lowering properties, thus assisting prevention of CVD
Too much fibre
Inhibits mineral absorption- binds calcium and iron forming insoluble compounds
High in bulk- reduced energy density- adverse for elderly and young children
side effects- flatulence
Increases requirements for fluid
50g/d no serious health effects
Carbohydrates and disease
Blood glucose and diabetes
Dental caries
Lactose intolerance/malabsorption
FODMAPs
High sugar intake and associated disease
Low CHO diet- ketosis
High CHO diets- improvement in weight control
Glycemic response
Plasma glucose levels- rise 5-45 mins after any meal that contains sugar or digestible starch
Return to fasting 2-3 hours later
Influenced by level of processing, components of food
Glycemic load: amount available CHO x GI/100
Glycemic index
Incremental area under the blood glucose response curve following a 50g carb portion of a test food, expressed as a % of the response to the same amount of CHO from a standard food eg. Glucose/ white bread
Ranking of CHO on scaled of 0-100 according to the extent to which they raise BGL after eating
