Chapter 3: Carbohydrates

Carbohydrates
33
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© Cengage Learning 2017
Introduction
• Carbohydrates are preferred energy
source for many of the body’s functions
• When available, used exclusively by brain
as an energy source
• Should not be avoided when trying to lose
weight
– Physical activity, portion size, fiber-rich eating
pattern are important

The Chemist’s View of Carbohydrates
• Monosaccharides (single sugars)
– Glucose: blood sugar
– Fructose: sweetest; found in fruits and honey
– Galactose: occurs mostly as part of lactose

Chemical Structure of Glucose

(cont’d.)
• Disaccharides (double sugars)
– Sucrose: glucose and fructose; table sugar
– Lactose: glucose and galactose; milk sugar
– Maltose: two glucose units; starch breakdown
product

(cont’d.)
• Polysaccharides (monosaccharide chains)
– Glycogen: polysaccharide composed of
glucose and stored in liver and muscle
– Starch: plant polysaccharide composed of
glucose molecules in occasionally branched
chains or straight chains
– Fibers: found in plant-derived foods;
non-digestible by human digestive enzymes

Glycogen and Starch Compared

The Bonds of Starch and Cellulose Molecules
Compared (Small Segments)

The Chemist’s View of Carbohydrates:
Polysaccharides (cont’d.)
• Fibers
– Resistant starches are classified as fibers
– Some fibers are digested (fermented) by
bacteria in the human digestive tract
– Soluble fibers dissolve in water, are viscous,
are easily fermented
– Insoluble fibers do not dissolve in water, are
non-viscous, are less readily fermented

Digestion and Absorption of
Carbohydrates
• Goal: break sugars and starches into
small molecules (mostly glucose) the body
can absorb and use
• Many soluble fibers digested by bacteria
• Details in Table 3-2
• Which sugars can be absorbed by
intestinal cells?

Regulation of Blood Glucose
• Blood glucose homeostasis maintained
primarily by insulin and glucagon
(hormones made by the pancreas)
• Insulin
– Facilitates blood glucose uptake by the
muscles and adipose tissue
– Stimulates glycogen synthesis in the liver
– Reduces blood glucose

Regulation of Blood Glucose (cont’d.)
• The release of glucose from glycogen
– Glucagon triggers the breakdown of liver
glycogen to single glucose molecules
– Raises blood glucose when it falls too low

Health Effects of Sugars and Alternative
Sweeteners
• Naturally occurring vs. added sugars
• Sugars
– Dramatic upward trend in added sugars
consumption in recent decades
– Major sources: sweetened beverages and
processed foods
– Average U.S. intake is excessive

Sweeteners: Sugars (cont’d.)
• Sugar and obesity
– Excess sugars + excessive energy intake
increases risk of weight gain, excess body fat
– Limiting added sugars effective for weight loss
• Sugar and heart disease
– Excess added sugars may alter blood lipids in
ways that increase risk

• Sugar and type 2 diabetes
– Excess body fatness is more closely related
to type 2 diabetes than diet composition
– Increased sugar-sweetened beverage intake
linked to greater risk
• Nutrient deficiencies
– Added sugars in empty-kcalorie foods
displace nutrients

• Sugar and dental caries
– Carbohydrates support bacterial growth
– Greater risk with greater length of time food
stays in mouth
– Prevention: good oral hygiene, fluoridated
water, limiting time sugars and starches are in
the mouth

• Recommended sugar intakes
– Dietary Guidelines for Americans
• Reduce the intake of kcalories from added sugars
– The USDA Food Intake Patterns
• Maximum of 8 teaspoons for 2000-kcal pattern
– DRI committee
• Maximum of 25% of kcalories

• Recognizing sugars
– Sugar takes many forms (Box 3-2)
– Honey very similar to sugar nutritionally
– Naturally occurring sugars in fruits,
vegetables, and nonfat milk (nutrient-dense
foods) are acceptable

Sweeteners (cont’d.)
• Alternative sweeteners: sugar alcohols
(nutritive sweeteners)
– Trigger a lower glycemic response
– 2 to 3 kcalories per gram
– What are some side effects of sugar
alcohols?
– Advantage: do not promote dental caries

Sugar Alternatives on Food Labels

• Alternative sweeteners: nonnutritive
sweeteners
– Minimal or no carbohydrate or energy
– FDA endorsement: Safe over a lifetime within
Acceptable Daily Intake (ADI) levels
– Do not cause tooth decay
– Table 3-4 describes U.S.-approved
sweeteners

• Alternative sweeteners: nonnutritive
sweeteners
– Nonnutritive sweeteners and weight
management
• Unknown whether they promote weight loss or
reduce health by reducing total energy intakes
– Safety of nonnutritive sweeteners
• Moderate intakes not hazardous
• Aspartame a danger with phenylketonuria (PKU)

Health Effects of Starch and Dietary
Fibers
• Carbohydrates: disease prevention and
recommendations
– Fiber-rich carbohydrate foods benefit health
– Heart disease
– Heart-healthy: whole grains, legumes, vegetables
– Gel-forming soluble fibers lower blood cholesterol
– Diabetes
– High-fiber foods risk type 2 diabetes risk

Fibers: Disease Prevention (cont’d.)
• GI health
– Dietary fibers enhance large intestine health
• Cancer
– High fiber intakes from foods linked to low
colon/ rectal cancer risk
• Weight management
– High-fiber foods low in kcal help increase
fullness, delay hunger, and reduce intake

• Harmful effects of excessive fiber intake
– Binds with minerals so they are excreted
– May limit kcal intake in individuals with
marginal food intakes (due to bulkiness)
– Dehydration risk with low fluid intakes

• Recommended intakes of starches and
fibers
– Total carbohydrate
• DRI: 45% to 65% of kcal
• Daily Value: 60% of kcal
– Fiber
• FDA: 28 grams per day for 2000-kcal intake
• Adequate Intake (AI): 14 g/1000 kcal/day

Fiber in Selected Foods

Fiber in Selected Foods (cont’d.)

Fibers (cont’d.)
• Carbohydrates: food sources
– Grains
– Vegetables
– Fruits
– Milk and milk products
– Protein foods: limited to nuts and dry beans

Fibers (cont’d.)
• Carbohydrates: food labels and health
claims
– Labels must list:
• Total carbohydrate (grams, %DV)
• Fiber (grams, %DV) and sugar (grams)
• Proposed label: added sugars (grams)
– FDA-authorized health claims related to
cancer and coronary heart disease

Nutrition in Practice:
The Glycemic Index in Nutrition Practice
• Measurement of glycemic response
• Factors that influence glycemic effect
• Mixed evidence regarding role of low-GI
diet in chronic disease prevention
• More research needed
• Plant-based diet of minimally processed
foods recommended

Chapter 3: Carbohydrates

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