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Flashcards in Carbohydrates Deck (36):

What is a carbohydrate?

A monomer or polymer of simple sugar units or saccharides


How are carbohydrates typically described?

In terms of the number of saccharide units they contain - mono, di, oligo (a few), or polysaccharides


What are the 3 major subdivisions of dietary carbohydrates?

1. Sugars
2. Starches
Both of which are readily digested and absorbed in the small intestine
3. Non-starch polysaccharides - these are resistant to digestion by gut enzymes and are therefore considered to be the unavailable carbohydrate. However they can yield up tp 2 kcal/g if they are fermented by bacteria in the large intestine.


How much energy do the available carbohydrates yield?

3.75kcal per gram


Which carbohydrates are monosaccharides?

fructose - fruit sugar
galactose - milk sugar
glucose from starch and disacchairdes


Which sugars are di and oligosaccharides?

sucrose - cane or beet sugar - (glu-fru)
lactose - milk sugar (glu-gal)
Products of starch digestion


Of the polysaccharides which are starches?

Polymers of glucose digested by alpha-amylase


Which carbohydrates are non-starch polysaccharides (NSP)?

Soluble and insoluble plant carbohydrates resistant to alpha-amylase but fermented by gut bacteria


What is the relative sweetness of the major dietary sugars?

The relative sweetness of sugars and artificial sweeteners is graded in relation to sucrose (relative sweetness 100)
Lactose 30
Glucose 50
Sucrose 100
Fructose 170


Describe lactose

Disaccharide found exclusively in milk
In most non-europeans populations lactase activity declines after about 4 years of age in most human populations - reducing the capacity to digest and absorb milk - primary lactase non-persistence
Lactose intolerance can also develop after intestinal infection or inflammation. - secondary lactase deficiency


Describe sucrose

Digested by enzyme sucrase - located in small intestine
Found in fruit and veg and in large quantities in sugar beet and sugar cane
Readily available in purified forms e.g. sugar, treacle, syrup
Term sucrose is used synonymously with sugar


Describe saccharin

Artificial sweetener
Absorbed and excreted unchanged
large intakes increase incidence of bladder tumours in rats
Bitter after taste
Destroyed by heating so cannot be used in cooking


Describe aspartame

Artificial sweetener
Dipeptide made up of the amino acids aspartic acid and phenylalanine
Not heat stable
In patients with phenylketonuria they lack the ability to metabolise dietary phenylalanine and so it can accumulate in the brain causing severe brain damage and learning disabilities
Some concern that consuming large quantities may cause brain damage in those without PKU.


Describe sucralose

Made from sugar - three of the hydroxyl groups or sucrose are replaced with chlorine atoms.
Tastes very similar to sugar
The compound is about 600 times sweeter than sucrose and most passes through the gut unchanged
Heat resistant


Do artificial sweeteners result in reduced energy consumption?

In theory they save sugar calories e.g. a diet soft drink would save 75 calories vs the sugar version.
However obesity rates have continued to rise since they became widely used
They don't provide any energy and have no satiating effect and therefore people tend to replace the lost sugar calories with more of their usual food mix, unless they are consciously restricting caloric intake.


What are sugar replacers?

They have a similar sweetness to sucrose
Typically sugar alcohols - e.g. xylitol, sorbitol
They yield fewer calories than sugar due to being incompletely absorbed or metabolised
They are used in similar amounts to sugar so provide the mouthfeel and textural properties
Don't promote dental caries
Large amounts in the bowel have an osmotic effect and increase bacterial fermentation resulting in diarrhoea and flatulence


What sugar exposure is the most cariogenic?

Between meals
Frequent consumptions of small amounts is more harmful than the same amount eaten in one go
sticky sugar is worse e.g. toffees


How do dental caries form?

The bacteria Streptococcus mutans in plaque produce acids by fermenting carbohydrates, especially sugars, in food. When the mouth is acidic enough (pH < 5.2) demineralisation of teeth occurs resulting in holes in the hard outer layer or enamel, thereby exposing the soft under layers of dentine, which allows decay to proceed rapidly throughout the tooth.
It's not a one way process and remineralisation occurs, with the balance between the two determining susceptibility to decay.


Describe NSPs

Structural components of plant cell walls - cellulose and hemicellulose, and viscous soluble substances found in cell sap e.g. actin and gums
The term is used interchangeably with the term dietary fibre (but this also includes lignin and resistant starch


What is the dietary reference value for NSPs?



What are the 2 major fractions of NSPs?

Soluble - form gums when mixed with water
Insoluble - cellulose and heme-cellulose


Discuss the fermentation of NSPs

Soluble NSPs are more readily fermentable
Cellulose remains largely unfermented
The by-products of fermentation are gases and short-chain fatty acids (acetate, butyrate and propionate)
The short chain fatty acids can be absorbed and used as an energy substrate


What physiological effects does increasing dietary NSP have?

Slows rate of glucose absorption - thought to be due to the mechanical effects of the soluble NSPs
Reduced transit time and increased stool volume - bacterial mass and water - which also softens
May lower cholesterol - may prevent absorption or reabsorption of dietary cholesterol and bile acids
High starch diets tend to have lower energy density


Which foods are predominantly soluble and insoluble NSPs?

Insoluble - wheat, rice and maize bran
Oats and most fruits and veg - contain higher proportions of soluble NSP


Do NSPs prevent the absorption of micronutrients?

Yes NSP (and some substances associated with is) tend to bind or chelate minerals thus hindering their absorption.If the NSP is fermented in the colon then this releases bound minerals.


What is resistant starch?

Starch that resists digestion by alpha-amylase and enters the large bowel undigested
Its fermented in the large bowel and behaves like a fermentable component of NSP


What are the 3 reasons why a starch is resistant to digestion?

-it's inaccessible to digestive enzymes because it's enclosed within unbroken cell walls or in partly milled grains or seeds
-some forms of raw crystalline starch e.g. in raw potatoes or green bananas are resistant to amylase digestion. During cooking it gelatinises and becomes digestible but during cooling some of it recrystallises (retrogradation) and becomes indigestible again
-retrograded starch in food that has been cooked and cooled is indigestible.


What is a glycemic carbohydrate?

One that is digested to sugars and absorbed in the small intestine


What is the generic formula for carbohydrates?

Carbs are stored energy, synthesised by plants from CO2 and water using the suns energy


Which carbohydrates do humans make?

Limited capacity to make carbohydrates
Lactose and oligosaccharides can be made for milk
The storage carbohydrate glycogen is also made from glucose and is found in the liver and muscles


What is maltose?

A disaccharide derived from starch and found in sprouted wheat and barley


What is an oligosaccharide?

A carbohydrate with a degree of polymerisation between 2 and 10. Although carbs with a DP of 2 are considered disaccharides and the continuation with polysaccharides is somewhat blurry.


What are the 2 food groups of oligosaccharides?

1. Maltodextrins which are derived from starch and contains alpha-glucans. They are widely used by the food industry as sweetness, fat substitutes and to alter texture of foods.
2. Oligosaccharides that aren't alpha-glucans. These include raffinose and stachyose. They are sucrose joined to varying numbers of galactose molecules and are found in a variety of plant seeds including peas, beans and lentils.
This group also includes inulin and fructo-oligosaccharides. They are fructans and are the storage carbodydrate in artichokes and also to smaller degrees in wheat, rye, asparagus and onions,leeeks and garlic.


Describe fructans

Storage carbohydrate in artichokes, asparagus, leeks, onions, garlic, wheat and rye.
Their chemical bonds are not susceptible to pancreatic or brush border enzymes and they are therefore non digestible.
They have unique properties in the gut and are known as prebiotics.


What is an intrinsic sugar?

One that is incorporated into a plant cell wall, that is occurs naturally and is accompanied by other nutrients.
Extrinsic - sugars added to food
Lactose doesn't really fit into either and the term non milk extrinsic sugar has been used for extrinsic sugars


What are 'free sugars'?

A term recommended by the WHO to describe mono and disaccharides added by manufacturers and consumers, plus the sugar in honey, syrups and fruit juices