CHO Powerpoint (1)

Chapter 4

The Carbohydrates: Sugars, Starches, and Fibres

The Chemist’s View of Carbohydrates

• Carbohydrates are made of carbon, hydrogen and oxygen atoms.

• These atoms form chemical bonds that follow the laws of nature.

The Simple Carbohydrates• Monosaccharrides are single sugars (most are

hexoses).– Glucose serves as the essential energy source,

and is commonly known as blood sugar or dextrose.

– Fructose is the sweetest. It occurs naturally in honey and fruits, and is added to many foods in the form of high-fructose corn syrup.

– Galactose rarely occurs naturally as a single sugar.

The Simple Carbohydrates• Disaccharides are pairs of monosaccharides,

one of which is always glucose. – Condensation reactions link monosaccharides together.– Hydrolysis reactions split molecules and commonly

occur during digestion. – Maltose consists of two glucose units. It is produced

during the germination of seeds and fermentation.– Sucrose is fructose and glucose combined. It is refined

from sugarcane and sugar beets, tastes sweet, and is readily available.

– Lactose is galactose and glucose combined. It is found in milk and milk products.

The Complex Carbohydrates

• Few (oligosaccharides) or many (polysaccharides) glucose units bound/linked together in straight or branched chains.


• Glycogen– storage form of glucose in the body– provides a rapid release of energy when

needed.• Starches– storage form of glucose in plants– found in grains, tubers, and legumes.


• Dietary fibre provide structure in plants, are very diverse, and cannot be broken down by human enzymes.– Soluble fibres are viscous and can be digested

by intestinal bacteria (this property is also known as fermentability). These fibres are found in fruits and vegetables.

– Insoluble fibres are non-viscous and are not digested by intestinal bacteria. These fibres are found in grains and vegetables.


• Fibre Sources– Dietary fibre is found in plant foods.– Functional fibres are health-benefiting fibres

that are added to foods or supplements.– Total fibre considers both dietary and

functional fibres.• Resistant starches escape digestion and are

found in legumes, raw potatoes and unripe bananas.

• Phytic acid or phytate has a close association with fibre and binds some minerals.

Digestion and Absorption of Carbohydrates

• Carbohydrate Digestion– In the mouth, the salivary enzyme

amylase begins to hydrolyse starch into short polysaccharides and maltose.

– In the stomach, acid continues to hydrolyse starch while dietary fibre delays gastric emptying and provides a feeling of fullness (satiety).


• Carbohydrate Digestion– In the small intestine, pancreatic

amylase among other enzymes (maltase, sucrase, and lactase) hydrolyzes starches to disaccharides and monosaccharides.

– In the large intestine, dietary fibre remains and attract water, soften stools and ferment.


• Carbohydrate Absorption– primarily takes place in the small

intestine– glucose and galactose are absorbed by

active transport– fructose is absorbed by facilitated

diffusion.


• Lactose Intolerance– Symptoms include bloating, abdominal

discomfort, and diarrhoea.– Causes include lactase deficiency due to a

natural decrease that occurs with aging or damaged intestinal villi.

– Prevalence• lowest in Scandinavians and northern Europeans• highest in Southeast Asians and native North

Americans.


• Lactose Intolerance– Dietary changes

• increase consumption of milk products gradually• mix dairy with other foods• spread dairy intake throughout the day• use of acidophilus milk, yoghurt, and other fermented

products)• use of enzymes• individualisation of diets• must be careful that vitamin and mineral deficiencies

do not develop.

Glucose in the Body• A Preview of Carbohydrate Metabolism– The body stores glucose as glycogen in liver

and muscle cells.– The body uses glucose for energy if glycogen

stores are available.– If glycogen stores are depleted, the body

makes glucose from protein. • Gluconeogenesis is the conversion of protein to

glucose.• Protein-sparing action is having adequate

carbohydrate in the diet to prevent the breakdown of protein for energy.

Glucose in the Body• A Preview of Carbohydrate Metabolism– Ketone bodies are made from fat fragments.

• The accumulation of ketone bodies in the blood is called ketosis.

• Ketosis upsets the acid-base balance in the body.– The body can use glucose to make body fat

when carbohydrates are consumed excessively.

Glucose in the Body• The Constancy of Blood Glucose–Maintaining glucose homeostasis• Low blood glucose may cause dizziness and

weakness.• High blood glucose may cause fatigue.• Extreme fluctuations can be fatal.

Glucose in the Body• The Constancy of Blood Glucose– The regulating hormones

• insulin moves glucose into the cells and helps to lower blood sugar levels

• glucagon brings glucose out of storage and raises blood sugar levels

• epinephrine acts quickly to bring glucose out of storage during times of stress.

– Balance glucose within the normal range by eating balanced meals regularly with adequate complex carbohydrates.

– Blood glucose can fall outside the normal range with hypoglycemia or diabetes.

Glucose in the Body• The Constancy of Blood Glucose– Diabetes

• type 1 diabetes is the less common type with no insulin produced by the body

• type 2 diabetes is the more common type where fat cells resist insulin

• prediabetes is blood glucose that is higher than normal but below the diagnosis of diabetes.

– Hypoglycaemia is low blood glucose and can often be controlled by dietary changes.

Glucose in the Body• The Constancy of Blood Glucose– Glycaemic response is how quickly the blood

glucose rises and elicits an insulin response.• Glycaemic index classifies foods according to their

potential for raising blood glucose.• Glycaemic load refers to a food’s glycaemic index

and the amount of carbohydrate the food contains.• The benefit of the glycaemic index is controversial.

Health Effects and Recommended Intakes of Sugars

• Sugar poses no major health problem except dental caries.

• Excessive intakes may displace nutrients and contribute to obesity.

• Consuming foods with added sugars should be limited.

• Naturally occurring sugars from fruits, vegetables and milk are acceptable sources.


• Health Effects of Sugars – Foods with added sugars have sugars listed as

a first ingredient. – Nutrient deficiencies may develop from the

intake of empty energy. • Just because a substance is natural does not mean it

is nutritious (e.g. honey).– Dental caries may be caused by bacteria

residing in dental plaque and the length of time sugars have contact with the teeth.


• Controversies Surrounding Sugars – Excessive sugar intake can contribute to the

development of body fat.– Sugar may be able to alter blood lipid levels

and contribute to heart disease in some.– There is no scientific evidence that sugar

causes misbehaviour in children and criminal behaviour in adults.

– There is a theory that sugar increases serotonin levels, which can lead to cravings and addictions.


• Recommended Intakes of Sugars– Because added sugars deliver energy

but few or no nutrients, the Dietary Guidelines for Australian Adults urges consumers to consume only a moderate amounts of sugars and food containing added sugars.

Health Effects and Recommended Intakes of Starch and Fibres

• Health Effects of Starch and Dietary Fibre– may be some protection from heart disease and

stroke• soluble dietary fibre binds with bile and thereby lower

blood cholesterol levels• dietary fibre may also displace fat in the diet

– reduces the risk of type 2 diabetes by decreasing glucose absorption

– enhance the health of the GI tract which can then block the absorption of unwanted particles

– may protect against colon cancer by removing potential cancer-causing agents from the body.


• Health Effects of Starch and Dietary Fibre– They promote weight control because

complex carbohydrates provide less fat and added sugar.

– Harmful effects of excessive dietary fibre intake• displaces energy and nutrient-dense foods• abdominal discomfort and distention• may interfere with nutrient absorption.


• Recommended Intakes of Starch and Dietary Fibre– The RDI for carbohydrate is set at 130 grams

per day, based on the average minimum amount of glucose used by the brain.

– The Dietitians Association of Australia recommends eating 30 grams of dietary fibre daily.


• Recommended Intakes of Fibre– FDA recommends 25 grams for a 2000-kcalorie

diet.– DRI at 14 grams per 1000-kcalorie intake (28

grams for a 2000-kcalorie diet)– American Dietetic Association recommends 20

to 35 grams per day.– World Health Organization suggests no more

than 40 grams per day.


• From Guidelines to Groceries– bread, cereals, rice, pasta and noodles – when

selecting from this group of foods, be sure to make at least half wholegrain products

– vegetables – the amount of carbohydrate depends primarily on its starch content.

– fruit – vary in water, dietary fibre and sugar content.– milks, yoghurt and cheese – contain carbohydrate;

cheese is low– meat, fish, poultry, eggs, nuts and legumes –

meats are low but nuts and legumes have some carbohydrate

– food labels list grams of total carbohydrate, dietary fibre and sugar.

Alternatives to Sugar

Artificial Sweeteners• Artificial sweeteners are also called sugar

replacers and non-nutritive sweeteners.• Saccharin– used primarily in soft drinks and as a tabletop

sweetener– rapidly excreted in the urine– does not accumulate in the body– has been removed from list of cancer-causing

substances.

Artificial Sweeteners• Acesulfame-K (acesulfame potassium)– research confirms safety

• Sucralose–made from sugar– passes through digestive tract

• Alitame and cyclamate– very few food items use these

sweeteners.

Artificial Sweeteners• Acceptable Daily Intake (ADI) is the level of

consumption, maintained every day and still safe by a wide margin. – Moderation and variety are still recommended.

• Artificial sweeteners and weight control– Much research is still being done.– Using artificial sweeteners will not

automatically reduce energy intake.

Sugar Replacers• also called nutritive sweeteners,

sugar alcohols, and polyols• maltitol, mannitol, sorbitol, xylitol,

isomalt, and lactitol• absorbed more slowly and

metabolized differently in the body• low glycaemic response• side effects include GI discomfort.

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