BMS208 Human Nutrition

Topic 4: The Carbohydrates: Sugars, Starches, and Fibers

Chris Blanchard

Learning objectives• Classify carbohydrates as mono-, di- or polysaccharides. • Explain the two major types of dietary fibers and their

uses in the body.• Trace the digestion and absorption of carbohydrates.• List the enzymes used in the digestion of carbohydrates.• Describe lactose intolerance symptoms, prevalence and

treatment. • List the major uses of glucose in the body.• Discuss the hormones used by the body to maintain

normal blood glucose concentration.

Learning objectives (cont’)• List the major types of diabetes.• Explain glycemic load and the pros and cons of its use in

meal planning.• Identify the major sources of added sugars in the diet.• Describe the effect of added sugars on dental health.• Identify the health benefits of a diet rich in starch and

fiber. • Identify the Daily Value and DRI for fiber and methods to

increase fiber in the diet.• List the common alternatives to sugar in the diet.

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, 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.

Nomenclature

Monsaccharides

Disaccharides• 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.

Condensation and Hydrolysis

The Complex Carbohydrates• Few (oligosaccharides) or many

(polysaccharides) glucose units bound/linked together in

• May be straight or branched chains.• Examples: Glycogen, Starch, Dietary Fibre

Glycogen• Storage form of glucose in

the body• Provides a rapid release of

energy when needed

Glycogen

Starch• Storage form of glucose in plants• Found in grains, tubers, and legumes

Starch (amylopectin) Starch (amylose)

Dietary fiber• Provide structure in plants• Very diverse• Cannot be broken down by human enzymes.• Soluble fibers

– Viscous and can be digested by intestinal bacteria Found in fruits and vegetables.

• Insoluble fibers – Non-viscous and are not digested by intestinal bacteria. – Found in grains and vegetables.

Starch vs Cellulose

Fiber Sources• Dietary fibers are found in plant foods.• Functional fibers are health-benefiting

fibers that are added to foods or supplements.

• Total fiber considers both dietary and functional fibers.

Resistant starches• Escape digestion • Found in legumes, raw potatoes and

unripe bananas.

Phytic acid• Also known as phytate • Has a close association with fiber • Binds some minerals.

Carbohydrate Digestion• In the mouth,

– Salivary enzyme amylase begins to hydrolyze starch into short polysaccharides and maltose.

• In the stomach, – Acid continues to hydrolyze starch – Fiber 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, – Fibers remain 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.

Monosaccharides, the end products of carbohydrate digestion, enter the capillaries of the intestinal villi.

In the liver, galactose and fructose are converted to glucose.Small

intestine

Monosaccharides travel to the liver via the portal vein.

Carbohydrate Absorption

Lactose Intolerance• Symptoms include bloating, abdominal

discomfort, and diarrhea.• 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, yogurt, and kefir

(fermented products)• Use of enzymes• Individualization of diets• Must be careful that vitamin and mineral

deficiencies do not develop

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.

Carbohydrate Metabolism• Making ketone bodies 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.

Constancy of Blood Glucose• Need to maintain glucose homeostasis

– Low blood glucose may cause dizziness and weakness.

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

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.

Constancy of Blood Glucose• Glucose can be maintained 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.

Diabetes and Hypoglycemia • 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.

• Hypoglycemia is low blood glucose and can often be controlled by dietary changes.

Glycemic response• Glycemic response: how quickly the blood

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

potential for raising blood glucose.• Glycemic load: refers to a food’s glycemic index

and the amount of carbohydrate the food contains.

• The benefit of the glycemic index is controversial.

Glycemic Index

Health Effects 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 kcalories. – Just because a substance is natural does not

mean it is nutritious. (Example: honey)

Dental caries• May be caused by

bacteria residing in dental plaque fermenting sugars from our diet

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

misbehavior in children and criminal behavior in adults.

• There is a theory that sugar increases serotonin levels, which can lead to cravings and addictions. This seems unlikely.

Recommended Intakes of Sugars• Added sugars can be included in the diet

as part of discretionary kCalories. • Dietary Guidelines recommend limited

intake of foods and beverages that are high in added sugars.

• DRI suggest added sugars should contribute no more than 25% of a day’s total energy intake.

Health Effects of Starch and Fibers• Fiber may provide some protection from heart disease

and stroke– Soluble fibers bind with bile and thereby lower blood

cholesterol levels.– Fiber may also displace fat in the diet.

• Reduce 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 Fibers• Promote weight control because complex

carbohydrates provide less fat and added sugar.

• Harmful effects of excessive fiber intake– Displaces energy and nutrient-dense foods– Abdominal discomfort and distention– May interfere with nutrient absorption

Recommended Intakes of Carbohydrates• RDA for carbohydrate is 130 g per day, or 45-

65% of energy intake.• Daily Value is 300 grams per day.• Dietary Guidelines encourage a variety of whole

grains, vegetables, fruits and legumes daily.• Healthy People 2010 recommends six servings

of grains and five servings of fruits and vegetables.

Recommended Intakes of Fiber• FDA recommends 25 grams for a 2,000-kcalorie

diet.• DRI at 14 g per 1000 kcalorie intake (28 g for a

2,000 kcalorie diet)• American Dietetic Association recommends 20-

35 g per day.• World Health Organization suggests no more

than 40 g per day.

From Guidelines to Groceries• Grains – encourage whole grains• Vegetables – starchy and nonstarchy vegetables

differ in carbohydrate content• Fruits – vary in water, fiber and sugar content• Milks and Milk Products – contain carbohydrate;

cheese is low• Meat and Meat Alternates – meats are low but nuts

and legumes have some carbohydrate• Food labels list grams of carbohydrate, fiber and

sugar; starch grams can be calculated.

Alternatives to Sugar• Artificial sweeteners (nonnutritive

sweeteners)• Sugar replacers

Artificial Sweeteners• Saccharin• Aspartame• Acesulfame-K (acesulfame potassium)• Sucralose• Neotame• Tagatose• Alitame • Cyclamate

Saccharin• Trade names: Sweet ‘n Low• Relative sweetness: 450X• Used primarily in soft drinks and as a tabletop

sweetener• Rapidly excreted in the urine• Does not accumulate in the body• Initial fears that excessive amounts may be

linked to cancer appear to be unfounded.

Aspartame• Trade name: Nutrasweet,

Equal, NutraTaste• Relative sweetness: 200X• General purpose sweetener• Not suitable for for those with

phenylketonuria• Excessive intake should be

avoided by those with epilepsy

Acesulfame potassium• Trade name: Sunette, Sweet One, Sweet ‘n Safe• Relative sweetness: 200X• Organic salt• Passes through the body unchanged

Sucralose• Trade name: Splenda• Relative sweetness: 600X• Made from sugar: • 3 OH groups replaced by Cl• Passes through GI undigested and unabsorbed

Neotame• Contains phenylalanine, aspartic acid, a

methyl group plus an additional side chain• Not broken down so safe for PKU• Relative sweetness: 8000X

Tagatose• Trade name: nutralose• Relative of fructose• Relative sweetness: 0.8X• 80% remains unabsorbed until it reaches the

large intestine• Fermented by bacteria• Used for foods and beverages• Provides less kcalories than sugar• High doses can cause flatulence and loose stools.

Alitame• Relative sweetness: 2000X• Approved in some countries

Cyclamate• Relative sweetness: 30X• The role in promoting cancer development

being investigated• Approved in some countries

Stevia• Herbal Alternative• From leaves of a shrub which has been

used by people in South America to sweeten their beverages

• Lacks research• Classified as a dietary supplement

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 glycemic response• Side effects include GI discomfort