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Chapter 4:
The Carbohydrates: Sugars,
Starches, and Fibers
Key points
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Identification of carbohydrates
Types of carbs
Digestion and absorption of carbs
Roles of carbs in the body
Blood glucose regulation
Glycemic response
Recommendations
Types of sweeteners
Guidelines to groceries
What is a carbohydrate?
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Contains C, H, O (C6H12O6)
Provides 4 kcal/g
Polyhydroxyaldehyde
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Six carbon backbone—hexose
OH groups—alcohols
Aldehyde group
Dietary carbs come from plants
Representations of Glucose
Types of Carbohydrates
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Simple sugars
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Monosaccharides
Disaccharides
Oligosaccharides
Complex carbohydrates or polysaccharides
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Starch
Glycogen
Fibers
Simple Sugars (1)
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Monosaccharides—composed of one sugar
unit (C6H12O6)
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Glucose—blood sugar
Fructose—fruit, honey—most sweet
Galactose—milk—least sweet
After absorption, monosaccharides are
converted into glucose in liver
Simple Sugars (2)
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Disaccharides—made of two sugar units
linked together
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Sucrose = glucose + fructose
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Maltose = glucose + glucose
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table sugar
sweetest
Malt
Metabolism byproduct
Lactose = glucose + galactose
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Milk
least sweet
How are the sugars linked?
Condensation reactions
How are they broken apart?
Hydrolysis reactions
Simple Sugars (3)
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Oligosaccharides
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3-10 sugar units
Beans, legumes
Not digested very well in some people
Beano
Complex Carbohydrates
(polysaccharides)
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Most made of at
least 100 sugar units
linked together
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Starch
Glycogen
Fibers
Starch
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Composed of glucoses linked together
Found in plants
Chains of sugars fairly straight
Glycogen
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Composed of repeating glucose units
Made by animals
Not found much in food
Highly branched for rapid hydrolysis
Figure 5.4
Fibers
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Indigestible
polysaccharides
Beta linkage
History
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Sylvester Graham
J. H. Kellogg
William Kellogg
C.W. Post
Comparing alpha and beta linkages
Types of Fibers
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Water soluble (viscous, fermentable)
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Water insoluble (nonfermentable)
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Oats, barley, legumes, fruit
Protective against heart disease
Grains, bran, veggies
Promote G.I. tract health
May reduce cancer risk
Functional fibers—added to foods
Table 5.1
Carbohydrate Digestion
Figure 5.6
Carb digestion—step by step
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Mouth: starches polysaccharides (alpha
amylase)
Small Intestine: polysaccharides
disaccharides (pancreatic enzymes)
On villi: disaccharidesmonosaccharides
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small intestinal enzymes
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Maltase
Lactase
sucrase
Lactose Intolerance
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Deficiency of lactase
Only about 30% of people are
not lactose intolerant
Kids have 5-10% of the lactase
activity of infants
Carbohydrate absorption
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Monosaccharides villi blood->
liverall converted to glucose
Functions of glucose in the
body
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Energy
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Immediate
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Stored
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Glycogen (1/3 in liver)
Spare protein
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Blood glucose
Inhibit gluconeogenesis
Prevent ketosis
Recommended Intakes
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Total Carbs: 45-65% total kcals
Complex: 35-55% total kcals
Simple: 10% or less total kcals
Fibers: 30-35 grams per day
Too much carbohydrate
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Fiber: phytobezoars
Simple sugars: malnutrition, dental
caries, possible increase in CVD risk,
not hyperactivity
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Average American eats 150 lbs sugar/year,
or 30 teaspoons/day
Complex: possibly obesity from too
many kcals
Too little carb
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Fiber--Diverticulitis
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Increased risk heart disease
Increased risk cancer
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35g/day lessens cancer risk by 40%)
Obesity
Simple sugars—none
Complex
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muscle wasting
ketosis
Regulation of blood glucose
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Normal range 70-120 mg/dl
Hormones
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Insulin—shuttles sugars into cells, lowering
blood glucose levels
Glucagon—releases glucose from glycogen
in liver, raising blood glucose
Epinephrine—same as glucagon--faster
Disorders of glucose
regulation
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Too high—diabetes (types I, II)
Too low--hypoglycemia
Glycemic response
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How quickly glucose is absorbed
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Low means slow (good)
Glycemic index
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Blood glucose response to a given food
compared to a standard like glucose.
Low < 55
Intermediate 55-70
High > 70
Relative Glycemic Indices of Selected
Foods
Types of sweeteners
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Nutritive
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Sugars
Sugar alcohols
Nonnutritive
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Aspartame (nutrisweet--blue)
Saccharin (pink)
Acesulfame-K (sunnette, Pepsi 1)
Sucralose (splenda—yellow)
Stevia
Figure 5.13
Guidelines to groceries
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1 serving of grain = 15 g carb (starch)
1 serving starch veggie = 15 g carb
1 serving nonstarchy veggie = 5 g carb
1 serving fruit = 15 g carb (sugars)
1 serving milk = 12 g carb (cheeses have
no carbs mostly)
1 serving meat = no carbs
1 serving legumes (meat alternative) =
20 g carb (starch) including 7 g fiber