Carbohydrates, dietary fibre and their role in nutrition Flashcards
How is carbohydrates classified?
Digestible and non-digestible
Molecular size – based on the number of carbohydrate “units” (DP-degree of polymerisation)
Glucose, fructose = DP1
Sucrose, lactose = DP2
Starch, cellulose = DP-lot
Classification by molecular size
- Mono- and di-saccharides (free sugars)
- Oligosaccharides (DP 3-9)
- Polysaccharides (DP>9) includes starch (amylose, amylopectin, pectin, gums) and cellulose, hemicellulose
What are the main carbohydrates in food?
Glucose – fruit and veg Fructose - fruit and veg Sucrose - fruit and veg Lactose – dairy products Oligosaccharides – legumes, onions, cereals Starch – cereals, vegetables.
Describe the metabolism of carbohydrates.
Digestion and absorption in the small intestine.
Fermentation in the large intestine.
The metabolism in the body depends on their chemical properties, monosaccharide composition and several other factors.
What are the major steps in the digestion and absorption of dietary carbohydrates?
In the small intestine
Glucose, fructose, sucrose, lactose and starch & dextrins (gets degraded by pancreatic amylase) is affected by the brush border enzymes and then degraded to monosaccharides that can be used for energy.
In the large intestine
Lactose, fructose, raffinose, insulin, resistant starch, dietary fibre etc is fermented by gut microflora that creates volatile fatty acids that can be used for energy.
How is monosaccharides transported to the blood stream?
Disaccharides are split into monosaccharides at the brush border.
sugar transporters on enterocytes
Glucose and galactose transported via SGLT1(sodium-glucose transport protein) (by Na+/K+- ATPase)
Fructose is absorbed via a facilitated transport GLUT5 (limited diffusion in some humans)
All three monosaccharides transported to the blood by GLUT2
What are some effects of malabsorption of carbohydrates?
Malabsorption of carbohydrates I the small intestine may lead to stomach pain, flatulence, bloating, altered bowel habits.
If dietary sugars (fructose, lactose, sucrose, sorbitol) is malabsorbed in the small intestine it lead to osmotic effect of malabsorbed sugar and fermentation of the malabsorbed sugar, this leads to bloating etc.
Carbohydrates malabsorption:
- Primary lactose intolerance >75% of human adults? (5% in northern Europe, >90% in Afric & part of asia?)
- Secondary lactose intolerance
Diseases of the GIT -> impairment of brush-border and enterocyte function. - Glucose-galactose malabsorption syndrome
Mutation in the SGLT1 - Fructose malabsorption
Up to 60% of adults have limited capacity for facilitated diffusion of fructose.
What is the carbohydrate requirements?
The brain need about 120 g glucose/d
Pregnant and lactating women require 175-210 g glucose/d, including for their brains.
The body can produce glucose from protein and fat (e.g. gluconeogenesis) (but glucose from the diet is the preferred energy source, excessive use of fat and protein for gluconeogenesis can lead to loss of body stores).
Describe the carbohydrate storage.
The body stores very little carbohydrate
Glycogen in the liver = 100-120 g after a meal – used for maintaining blood glucose during fasting
Glycogen in skeletal muscle = approx. 2% - approx. 420 g in a 70 kg person, used for physical activity
Once these stores are used up, glucose must come from the diet or made via gluconeogenesis
Describe the regulation of blood glucose?
Regulation of blood glucose concentration
Pancreas
Secrete insulin – stimulates uptake of glucose in tissues and cells
Secrets glucagon – stimulates breakdown of glycogen, fat, protein and glucose synthesis
High blood glucose = higher insulin
Low blood glucose = higher glucagon
What is diabetes mellitus?
Insulin dependent diabetes mellitus (or type 1 diabetes mellitus) – autoimmune destruction og pancreatic beta-cells, not enough insulin is secreted
Non-insulin-dependent diabetes mellitus – insulin resistance (cellular insulin receptor are not so sensitive and/or fewer, more insulin secretion is required to have the same effect, loss og glucose homeostasis)
What are some causes of elevated insulin levels?
Hyperinsulinemia – impaired sensitivity of cells to insulin -> increases production and elevated circulating insulin levels
Basal insulin concentration increase with age
Strongly associated with obesity and low-level pf physical activity – physical activity can induce glucose uptake independent of insulin.
What is the glycaemic index?
Meal blood glucose response – the glycaemic index
Standardise comparison of blood glucose responses after intake of meals with different foods (with the same amount of digestible carbohydrates)
GI measurements
Subjects are fed 25-50 g of available carbohydrates (must be the same amount and repeated with the same subject)
Blood glucose samples taken from time 0 up to 120 or 180 min.
Area under the curve measured and compared to a reference (either glucose or white bread)
What are some limitations with GI?
Limitations with GI
The amount of available carbohydrates very different in different food products e.g. to have 50 g available carbohydrates from pasta, carrots or watermelon the following amounts are needed: 182 g boiled pasta, 575 g carrots, 666 g watermelon
Inter-individual variation in glycaemic responses to food (large number of subjects needs to be studies at standardise conditions)
Several factors in food influence the postprandial glucose response in the gastrointestinal tract.
Factors influencing postprandial glucose response
Type of starch: amylose (low GI)/ amylopectin (high GI), granular size and cellular structures, crystallinity.
Cooking/food processing: gelatinization, retrogradation.
Interactions with other nutrient/food factors.
What are some factors influencing digestibility?
Granular size
Crystalline structures: A-type (cereals), B-type (bananas, tubers, high-amylose starch), C-type (legumes)
Interactions with other nutrients
Granules can be enclosed inside a protein matrix, e.g. in pasta.
Resistant starch- escapes digestion in the small intestine.
RS 1: physically inaccessible starch mostly present in whole grains e.g. seeds, legumes, partially milled grains.
RS 2: raw starch granules e.g. banana starch.
RS 3: retrograded starch (after processing) e.g. breakfast cereals, cooked & cooled potato.
(RS 4: modified starch)
describe starch gelatinization and retrogradation.
Starch undergoes swelling or gelatinization when heated in the presence of water. Retrogradation takes place in gelatinized starch when the amylose and amylopectin chains realign. Crystalline structures are formed after cooling.
