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Carbohydrates
Chapter 2
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Carbohydrates
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More than just “sugar” and “starch”
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Provide the majority of calories in almost all human diets
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Carbohydrate classifications
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Carbohydrates
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Class of energy-yielding nutrients that contain only carbon, hydrogen, and
oxygen, hence the common abbreviation of CHO
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Simple carbohydrates contain only 1 or 2 sugar molecules
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Complex carbohydrates contain many sugar molecules
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Carbohydrates (cont’d)
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Carbohydrate classifications (cont’d)
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Monosaccharides
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Simplest of all sugars
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Most common
 Glucose
 Fructose
 Galactose
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Carbohydrates (cont’d)
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Carbohydrate classifications (cont’d)
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Monosaccharides (cont’d)
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Glucose
 Also known as dextrose
 Component of all disaccharides
 Sugar into which the body converts all other digestible carbohydrates
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Carbohydrates (cont’d)
Carbohydrate classifications (cont’d)
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Monosaccharides (cont’d)
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Fructose or “fruit sugar”
 Sweetest of all natural sugars
 High-fructose corn syrup (HFCS)
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Galactose
 Does not occur in appreciable amounts in foods
 Combines with glucose to form lactose
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Carbohydrates (cont’d)
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Carbohydrate classifications (cont’d)
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Disaccharides
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Made of two linked monosaccharides
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Sucrose, maltose, and lactose are disaccharides
 Sucrose or “sugar”
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Composed of glucose and fructose
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Differences among sugars is degree of refining
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Occurs naturally in some fruits and vegetables
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Carbohydrates (cont’d)
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Carbohydrate classifications (cont’d)
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Disaccharides (cont’d)
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Maltose
 Composed of two joined glucose molecules
 Not found naturally in foods
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Lactose or “milk sugar”
 Composed of glucose and galactose
 Found naturally in milk
 What does it do?
 Least sweet of all sugars
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Carbohydrate classifications (cont’d)
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Carbohydrates (cont’d)
Polysaccharides
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Complex carbohydrates
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Do not taste sweet
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Starch, glycogen, and fiber are types of polysaccharides
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Carbohydrates (cont’d)
Carbohydrate classifications (cont’d)
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Polysaccharides (cont’d)
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Starch
 Glucose not used by plants stored as starch
 Grains
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World’s major food crop
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Foundation of all diets
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Potatoes, dried peas and beans, and other starches
Carbohydrates (cont’d)
Carbohydrate classifications (cont’d)
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Polysaccharides (cont’d)
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Glycogen
 Animal (including human) version of starch
 Liver glycogen releases glucose into the bloodstream to maintain blood
sugar between meals
 No dietary source of glycogen
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Carbohydrates (cont’d)
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Carbohydrate classifications (cont’d)
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Polysaccharides (cont’d)
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Fiber
 Generally considered a group name for polysaccharides that cannot be
digested by human enzymes
 Commonly referred to as “roughage”
 Categorized as insoluble or soluble
 “Dietary” fiber
 “Functional” fiber
 Commonly assumed that fiber does not provide any calories
 Source of energy for colon lining
 Carbohydrates (cont’d)
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Sources of carbohydrates
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Grains, Vegetables, Fruits, and Milk are the MyPyramid groups that provide the
majority of carbohydrates
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Nuts and dried peas and beans
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Oils group
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Grains
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Synonymous with “carbs”
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Classified as “whole” or “refined”
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Carbohydrates (cont’d)
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Sources of carbohydrates (cont’d)
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Grains (cont.)
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Whole grains
 Consist of the entire kernel of a grain
 Can be cracked, ground, or milled into flour
 Composed of 3 parts:
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The bran
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The endosperm
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The germ (embryo)
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Carbohydrates (cont’d)
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Sources of carbohydrates (cont’d)
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Grains (cont’d)
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“Refined” grains
 Rich in starch, but lack the fiber, vitamins, trace minerals, fat, and
phytochemicals found in whole grains
 Enrichment
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Required to be fortified
 Examples include white flour, white bread, white rice, and refined
cornmeal
 Carbohydrates (cont’d)
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Sources of carbohydrates (cont’d)
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Vegetables
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Starch and some sugars provide the majority of calories in vegetables
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“Starchy” vegetables provides 3 times more carbohydrate than the same
amount of “watery” vegetables
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Carbohydrates (cont’d)
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Sources of carbohydrates (cont’d)
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Fruits
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Almost all of the calories in fruit come from sugar (mostly fructose), with small
amounts of starch and minute quantities of protein providing negligible calories
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Exceptions to this are avocado, olives, and coconut, which get the majority of
their calories from fat
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Fiber is located in the skin of fruits
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Carbohydrates (cont’d)
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Sources of carbohydrates (cont’d)
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Milk
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One cup of milk provides 12 g of carbohydrate in the form of lactose
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Cottage cheese, which has about 6 g of carbohydrate per cup
Discretionary calories
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“Extras”
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Carbohydrate content varies
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Carbohydrates (cont’d)
How the body handles carbohydrates
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Digestion
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Cooked starch begins to undergo digestion in the mouth by the action of
salivary amylase
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Most carbohydrate digestion occurs in the small intestine
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Disaccharidase enzymes split disaccharides into monosaccharides
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Monosaccharides are the only form of carbohydrates the body is able to absorb
intact
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95% of starch is digested usually within 1 to 4 hours after eating
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Carbohydrates (cont’d)
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How the body handles carbohydrates (cont’d)
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Absorption
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Sugars absorbed through intestinal mucosa cells
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Undigested starch passes with stools
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Fibers may impair the absorption of some minerals—namely calcium, zinc, and
iron—by binding with them in the small intestine
Carbohydrates (cont’d)
How the body handles carbohydrates (cont’d)
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Metabolism
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Fructose and galactose are converted to glucose in the liver
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Liver releases glucose to maintain blood sugar level
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Rise in blood sugar causes pancreas to release insulin
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Postprandial state
 Blood glucose concentration begins to drop
 Pancreas releases glucagon
 Stimulates liver to release glucose
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Carbohydrates (cont’d)
How the body handles carbohydrates (cont’d)
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Glycemic response
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Increase in blood glucose levels
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Glycemic index
 Determined by comparing the impact on blood glucose after 50 g of a
food sample is eaten compared to the impact of 50 g of pure glucose or
white bread
 May help diabetics and athletes
 Carbohydrates (cont’d)
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How the body handles carbohydrates (cont’d)
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Glycemic response (cont’d)
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Glycemic load
 More accurately define a food’s impact on blood glucose levels
 Not reliable for choosing healthy diet
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Functions of carbohydrates
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Glucose metabolism = balance between catabolism and anabolism
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Carbohydrates (cont’d)
How the body handles carbohydrates (cont’d)
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Functions of carbohydrates (cont’d)
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Glucose for energy
 Primary function of carbohydrates is to provide energy for cells
 Brain is totally dependent on glucose for energy
 Spares protein and prevent ketosis
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Protein sparing
 Need to consume adequate carbohydrates
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Carbohydrates (cont’d)
How the body handles carbohydrates (cont’d)
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Functions of carbohydrates (cont’d)
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Preventing ketosis
 Without adequate glucose, fat oxidation prematurely stops at the
intermediate step of ketone body formation
 Increased production of ketones causes nausea, fatigue, loss of
appetite, and ketoacidosis
 Dehydration and sodium depletion may follow
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Carbohydrates (cont’d)
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Using glucose to make other compounds
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Glycogen
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Body’s backup supply of glucose
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One third of the body’s glycogen reserve is in the liver
Nonessential amino acids
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Carbohydrate-containing compounds
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Fat
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Carbohydrates (cont’d)
Dietary reference intakes
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Total carbohydrate
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The RDA for carbohydrates is set at 130 g for both adults and children
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Acceptable macronutrient distribution range
Fiber
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Adequate intake for total fiber is set at 14 g/ 1000
Sugar
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Maximal level of 25% of total calories or less from added sugars is
recommended
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Carbohydrates in Health Promotion
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Concentrate on fiber and whole grains
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Most consistent benefit of consuming adequate fiber is to relieve or prevent
constipation
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Linked to a decreased risk of heart disease, cancer, diabetes, and obesity
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Dietary guidelines recommend that adults and children consume at least one half of
their grain servings from whole grains
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Results in 20% to 30% lower risk of atherosclerotic cardiovascular disease
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Carbohydrates in Health Promotion (cont’d)
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Concentrate on fiber and whole grains (cont’d)
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Dietary guidelines (cont’d)
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Less likely to develop insulin resistance and metabolic syndrome, common
precursors of type 2 diabetes
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Associated with improved body weight management
Tips for choosing whole grains
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On average, Americans eat one serving per day of whole grains
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Factors contributing to the low intake of whole grains include consumers’
inability to identify whole grains, a lack of awareness of their health benefits,
and the cost, taste, and unfamiliarity with how to prepare whole grains
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Carbohydrates in Health Promotion (cont’d)
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How to increase dietary fiber:
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Replace refined grains with whole grains
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Choose a ready-to-eat cereal with 5 g of fiber or more per serving
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Eat dried peas and beans 2 to 3 times per week
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Eat at least 5 servings of fruits and vegetables daily
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Eat a variety of plant foods daily
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Increase fiber intake gradually to avoid GI intolerance
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Consume adequate fluid
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Carbohydrates in Health Promotion (cont’d)
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Limit added sugars
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Sugar adds flavor and interest
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Inaccurately blamed for a variety of health problems
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Limiting intake is prudent
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Sugar alternatives
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Sugar alcohols
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Nonnutritive sweeteners
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Carbohydrates in Health Promotion (cont’d)
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Limit added sugars (cont’d)
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Sugar alternatives (cont’d)
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Sugar alcohols
 Sorbitol, mannitol, and xylitol
 Natural sweeteners derived from monosaccharides
 Most are commercially synthesized
 Incompletely absorbed
 Side effects
 Offer sweetness without promoting cavities
 Carbohydrates in Health Promotion (cont’d)
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Limit added sugars (cont’d)
– Sugar alternatives (cont’d)
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Nonnutritive sweeteners
 Virtually calorie free
 Sweeter than sugar
 Do not raise blood glucose levels
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Risks and benefits of nonnutritive sweeteners
 Weight management
 Diabetes mellitus
 Safety
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Carbohydrates in Health Promotion (cont’d)
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Take steps to avoid dental caries
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Choose between-meal snacks that are healthy and teeth-friendly
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Limit between-meal carbohydrate
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Avoid high-sugar items
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Brush promptly after eating
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Chew gum sweetened with sugar alcohols
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Use fluoridated toothpaste
Carbohydrates in Health Promotion (cont’d)
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The most common sources of added sugars in the American diet:
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Soft drinks
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Cakes, cookies, and pies
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Fruit drinks and punches
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Dairy desserts such as ice cream
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Candy
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Carbohydrates in Health Promotion (cont’d)
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Sugar: too much of a good thing
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Behavioral problems in children
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Obesity
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Diabetes mellitus
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Heart disease