Lecture 8/9 Fibre Overview Flashcards
Define
Dietary Fibre
Plant derived non-starch polysaccharides indigestible by human enzymes
* Has edible polymers with 3 or more units but they cannot by hydrolyzed by enzymes in the SI of humans
What are commonly accepted factors for dietary fibre?
- naturally occuring in food
- obtained from food material and have a physiological benefit to health
- synthetic carbohydrate polymers that have a physiological benefit to health
What part of fibre cannot be hydrolyzed?
Humans lack enzymes that hydrolyze β-1,4 glycosidic bonds so they remain intact in the GI tract
* Ex. lacking cellulase for cellulose
What is the general distribution of fibre in plants?
- Non-digestible CHO (fibre) is often in the cell wall
- Digestible CHO is often found in the cell lumen
What does fibre contribute to the cell wall?
Usually provides protection and water proofing
What are some types of fibre in cell walls?
- cellulose
- hemicellulose
- lignin
- pectin
Distribution of fibre in a whole-grain wheat kernel
- 80% endosperm: ↑starch, ↑protein, ↓micronutrients
- 15% bran: ↑fibre, ↑micronutrients, ↑antioxidants, ↑phytochemicals
- 5% germ (plant embryo)
Relationship between whole grain consumption and decreased HD
Increased concentrations of whole grain consumption have show to reduce the risk of chronic diseases.
Cellulose
Description and food sources
insoluble fibre consisting of glucose molecules with β-glycosidic bonds; main structure component of plant cell walls
* whole wheat flour, bran, broccoli, cabbage, dried peas/ beans, apples, root vegetables
Hemicellulose
Description and food sources
Insoluble fibre consisting of a vareity of different monosaccharide molecules (e.g., glucose, arabinose, mannose, and xylose)
* Bran, cereals, whole grains
Pectin
Description and food sources
Soluble fibre found in the skin of ripe fruits that consists of a variety of different monosacchraide molecules, and is used commercially to make jams and jellies
* apples
* citrus fruits
* strawberries
* raspberries
β-Glucan
Description and food sources
A nonstarch polysaccharide that can be made synthetically and is composed of branched chains of glucose molecules
* mushrooms
* barley
* oats
Gums
Description and food sources
Highly soluble and viscous nonstarch polysaccharide used to thicken foods
* oatmeal
* dried beans
* other legumes
Lignin
Description and food sources
Insoluble nonpolysaccharide dietary fibre consisting of numerous alcohol units found within woodu portion of plants
* berries
* wheat
What is the dietary fibre content of various foods?
- Large variation
- usually a mix of different types
Dietary fibre vs. Functional fibre
- Dietary fiber: Fiber found naturally in foods.
- Functional fiber: Fiber that is extracted and isolated from whole foods, then added to processed foods
Total fibre = ?
total fibre = dietary fibre + functional fibre
Define
Resistant Starch
Resistant starch is a type of carbohydrate that doesn’t get digested in your small intestine. Instead, it ferments in your large intestine and feeds beneficial gut bacteria.
How much of starch ingested reaches the colon as resistant starch?
5-20%
Why is resistant starch not able to be digested?
not susceptible to enzymes - physically inaccessible
* food/ physical factors (fbire, proteins, high amount of starch)
* starch structure (dense, granular, retrograded, uncooked)
* fermentable by colonic microflora
How does resistant starch form?
May form during food processing, baking, cooling or storage
* amylose: broken down more efficiently that amylopection but more prone to irreversible retrogradation
* Amylopection: retrograde process can be revered with gentle heating
Properties of fibre
- ↑distention (stretch receptors), adding bulk with low energy density therfore ↑satiety & ↓daily kcals (of clinically sig.)
- properties vary between different fibre types and this partially determines how each fibre functions in the bodyand confers health benefits
How are fibres classified?
- composition: monosaccharides, β-bonds
- physical properties: water solubility, fermentability, viscositiy
How are fibres most often generalized?
- Soluble fibre
- Insoluble fibre
Properties of soluble fibre
attract and dissolve in water to form a gel making them highly viscous
* generally delays gastric emptying & slows down digestion by ↑ transit time which ↑satiety & lowers postpranial blood glucose levels
* Tend to be more fermentable by bacteria
What fibres are included as soluble?
some hemicelluloses, pectins, b-glucans, gums, polysaccharides
* oatmeal, oat cereal, lentils, apples, oranges, pears, oat bran, strawberries, nuts, flaxseeds, beans, dried peas, blueberries, psyllium, cucumbers, celery, carrots
Properties of insoluble fibre
Do not dissolve in water and pass through GI tract relatively intact
* ↓transit time, speeding up passage of food and waste
* laxative effect, adding bulk to food bolus which helps prevent constipation
* poorly fermentable but considered gut health to get things moving through
Sources of insoluble fibre
mainly found in whole grains and vegetables and includes lignin, cellulose, some hemicelluloses
* whole wheat, whole grains, wheat bran, corn bran, seeds, nuts, barley, couscous, brown rice, bulgur, zucchini, celery, broccoli, cabbage, onions, tomatoes, carrots, cucumbers, green beans, dark leafy vegetables, raisins, grapes, fruit, root vegetable skins
What is the result of the water binding capacity of soluble fibre?
A viscous, slow moving solution is formed that may trap nutrients to slow digestion/absorption such that the different aspects of the food matrix being presented to the SI will interact with enzymes differently when combined with fibre
* Delayed (slowed) gastric emptying - ↑satiety
* ↓Mixing of GI contents with digestive enzymes - ↓enzyme function (physical barrier + ↓contact with brush border due to high viscosity)
* ↓Nutrient diffusion rates into bloodstream (delayed absorption) so ↓glycemic response
insoluble fibre has some water binding capacity but not as much
How do insoluble fibres reduce enzyme function?
Insoluble fibres can also reduce enzyme function (physical barrier) & delay absorption of nutrients
* ↓transit time so not enough time for efficient digestion/absorption to occur
How macromolecule can fibres bind to?
Many fibre types (e.g. lignin, gums, pectins, some hemicelluloses) bind to fatty acids, cholesterol & bile acids
* interferes with micelle formation, ↓absorption of FA
* ↑Fecal bile acid excretion
* ↓serum cholesterol concentrations
* Altered mineral & carotenoid absorption (may interfere with mineral bioavailability)
What fibres are mostly fermentable vs. slowly fermentable?
- mostly fermentable: fructans, pectin, gums, psyllium, polydextorse, resistant starch
- more slowly fermentable: cellulose, hemicellulose
What bacteria ferment fibre?
different fibres are metabolized by different bacteria encouraging their growth, and the bacteria produce particular metabolites which have specific effects.
What are some common metabolites produced from the bacteria feeding on fibre?
- lactate → ↓pH
- propionate (SCFA) → energy source, metabolized in liver, ↓HMGCoA reductase
- Acetate (SCFA) → energy source, metabolized in the liver
- Butyrate (SCFA) → important energy source for coloncytes
What is the net effect of ↑SCFAs production?
- ↑water & sodium absorption in colon
- ↑mucosal cell proliferation
- provision of energy
- Acidification of luminal environment
How do SCFA provide energy?
- 10% to 15% of ingested CHO may be fermented in the colon
- 1.5-2.5 kcal/g
What is the benefit of of SCFA acidifying the luminal environment?
- ↑excretion of bile: ↓solubility of bree bile acids, ↑Ca binding
- ↓rate of conversion of primary bile acids to secondary bile acids
How does fibre in fecal bulk increase?
with ↓fermentability
What makes up fecal bulk?
unfermented fibres, salts, water, bacterial mass
What is the best source of fibre to add to fecal bulk?
wheat bran (↑cellulose) have greated effect
* psyllium, inulin, oligosaccharides also effective
What is the overall effect of ↑ fecal bulk from fibre?
- ↑frequency of defecation & ↓intestinal transit time & ↓intraluminal pressure
- Detoxification by diluting effect of toxins and decreasing their contact time with the luminal wall which also may decrease carcinogen formation from bacteria which would scavenge toxins
What are prebiotics?
Prebiotics are special plant fibers that help healthy bacteria grow in your gut.
bacteria food
How are prebiotics classified?
- resistance to digestion/ absorption in the SI
- partial or complete fermentation by microbiota in the colon
- ability to stimualte growth of select bacteria
What bacteria type dominates human gut flora?
bacteroides & firmicutes
* fermentable fibre shifts gut microbial populations by providing substrates for bacterial fermentation
* some soluble/ fermentable fibres seem to ↑ fecal populations of bifidobacteria and lactobacillus… which is good
Possible mechanisms by which fibers lower serum cholesterol
- Delayed gastric emptying
- interference with digestive enzymes
- interference with micell formation
- interference with mixing of intestinal contents
- inhibition of cholesterol biosynthesis
See notes for the effect of these mechanisms
Summary of fibre functionality
Summary
Fibre DRIs
