Energy Metabolism
Utilization of energy-yielding
nutrients for fuel
Metabolism of Nutrients

Once absorbed, energy-yielding nutrients
are transported to cells
– Further metabolism (processing) occurs

Metabolic pathways: series of reactions that
transform nutrients into final product that can
be used by the body
 Catabolic pathways release energy trapped
in chemical bonds
– Energy converted into ATP

ATP is the energy currency of the cell
ATP:
Adenosine
triphosphate
Elimination
of Metabolic
Wastes:
Body Organ
Systems
The Carbohydrates:
Sugars, Starches, and
Fibers
Chapter 4
Introduction

Carbohydrates=energy fuel
 For the brain: glucose
 For muscles: glucose, glycogen, fat

Sources of carbohydrates?
 “Fattening” – overgeneralization
Carbohydrate Family


Abbreviation: CHO
Sugars (simple CHO)
 Monosaccharides
• Chemical shorthand: C6H12O6
• Glucose, fructose, galactose
 Disaccharides
• Maltose, sucrose, lactose

Polysaccharides (complex CHO)
Chemist’s View of
Monosaccharides
Fructose
Monosaccharides

Same numbers and kinds of atoms
 Differing structure and sweetness

Glucose – blood sugar
 Part of every disaccharide

Fructose
 Sweetest of the sugars

Galactose
 Only in a few foods
Disaccharides

Pairs of three monosaccharides
 Maltose – two glucose units
 Sucrose – glucose and fructose
 Lactose – galactose and glucose

Condensation
 Links two monosaccharides together

Hydrolysis
 Breaks a disaccharide in two
Polysaccharides

Glycogen
 Storage form of energy in muscle and
liver
 Glucose units

Starch
 Storage form of energy in plants
 Glucose units

Fibers
 Not digestible, but very beneficial
Fibers

Differ from starches
 Soluble fibers
 Mix with water - form a gel
 Slow transit time in colon

Insoluble fibers
 Do not mix with water or form a gel
 Speed transit time in colon

Both forms of fiber slow gastric
emptying
Whole
Grains



Bran layers: good
source of fiber and
vitamins
Germ: good source of
vegetable oils and
vitamin E
Endosperm: contains
starch and some
protein
• Carbohydrates in whole
foods, such as fresh
fruit, dairy or whole
grains, are in their
natural state
• Refining CHO
separates the CHO
from vitamins, minerals,
fiber, and
phytochemicals
• Whole: all edible parts
present
• Refined: some edible
parts removed
Whole vs. Refined
Foods
Digestion and
Absorption of
CHO
Absorption of CHO
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.
Fiber slows
nutrient
absorption
More Health Benefits of Fiber

Diets high in fiber, or indigestible
carbohydrates, can relieve or prevent
certain bowel disorders:
•
•
•
•
Hemorrhoids
Diverticulosis
Diverticulitis
Constipation
Epidemiological studies have
shown that the incidence of colon
cancer is lower in populations
that consume diets high in fiber.
Sources of Fiber
Carbohydrate Metabolism

Primary role of CHO: glucose for energy
 Fuels most of body’s cells
 Preferred source for brain, nerve cells,
and developing red blood cells

We need a constant supply!
The Constancy of Blood
Glucose

Steady supply in blood stream
 Intestines – food
 Liver – glycogen

Blood glucose homeostasis
 Insulin
 Glucose from blood into cells
 Glucagon & epinephrine
 Brings glucose out from storage
Carbohydrate Stores

We store glucose as glycogen in times of
abundance
 Liver storage
 Condensation into glycogen
 Hydrolysis for release of glucose when needed
 Muscle
 Hoards glycogen stores for use during exercise
 Muscle glycogen not available to supplement
blood glucose

If diet is insufficient, body converts other
compounds
 Making glucose from protein
 Amino acid conversion
 Gluconeogenesis
CHO Intake and Ketogenesis

When CHO is inadequate, fat metabolism
shifts
 We make ketone bodies from fat fragments
 Ketone body formation – starvation
 Dulls hunger
 Ketosis – acid-base balance
 Acidifies blood

Minimum carbohydrate needs for protein
sparing and prevention of ketosis: 50-100g/day
 Varies based on metabolic rate and activity

In times of excess: glucose used to make fat
Availability of
Carbohydrates
Determines
Fatty Acid
Metabolism
The Constancy of Blood
Glucose

Glycemic response
 Speed of glucose absorption, level of
blood glucose, and return to normal
glucose levels
 Low glycemic response
 Desired
 High glycemic response
 Glycemic index and glycemic load
 Way of measuring effects on blood sugar
of various foods
The Constancy of Blood
Glucose

Balancing within the normal range
 Balanced meals at regular intervals

Diabetes mellitus
 Insulin is either inadequate or ineffective
 Type 1 diabetes
 Type 2 diabetes

Hypoglycemia
 Occurs most frequently in poorly
managed diabetes
Abnormal Blood Glucose:
Diabetes Mellitus
Leading cause of blindness in the US
 Accounts for 44% of all new cases of
kidney failure
 Gestational diabetes can occur in
women during pregnancy

– May increased risk of developing type 2
diabetes later in life
Diabetes Symptoms and
Complications

Immediate Symptoms: excessive thirst,
frequent urination, excessive hunger

Long Term Complications: damage to
the heart, blood vessels, kidneys, eyes
and nervous system.
– Infections are more common in patients
with diabetes, and amputations may be
necessary
Diabetes Treatment

Goal: maintain blood glucose within
normal range
 Treatment involves diet, exercise,
and sometimes meds
 Refined CHO increasingly
implicated in type 2 DM risk
 Best way to reduce risk for type 2
DM: maintain healthy body weight
Lactose Intolerance
The enzyme lactase is needed to digest
lactose, the sugar found in milk.
 If lactose is not digested in the small
intestine, it passes through to the large
intestine.
 Undigested lactose in the small intestine
can cause cramping, abdominal
distention and diarrhea.

Lactose Intolerance

Lactase activity
 Highest immediately after birth
 Declines with age


Symptoms of intolerance
Prevalence
 Genetically determined
 Worldwide ~70%
 US ~25%
Lactose Intolerance

Dietary adjustments
 Manage dairy consumption rather than
restriction (for some people)
 Probiotic bacteria can ease symptoms,
aid in digestion
 Fermented milk products often better
tolerated
 Response is highly individual
 Potential nutrient deficiencies?
 Riboflavin, vitamin D, and calcium
 We can get these from other foods
Health Effects of Sugars


Pleasure in moderate amounts
In excess: potential nutrient deficiencies
and increased risk of chronic disease
 Energy with few other nutrients
 Best limited to discretionary kcal
 Honey
 More energy per spoonful than table
sugar, but twice as sweet
 Health benefits

Sugar sources: concentrated vs. dilute
Recommended Intakes of
Sugars

Dietary Guidelines
 Choose and prepare foods with little
added sugar

DRI (from USDA)
 Added sugars
 No more than 25% of day’s total energy
 Impact on other food groups

WHO recommendations: no more than
10%!
Alternative Sweeteners

Artificial sweeteners
 Non-nutritive sweeteners
 Large doses and adverse effects

Stevia – an herbal product
 Generally recognized as safe (GRAS)

Sugar alcohols
 Provide kcalories
 Benefits and side effects
Health Effects of CHO

Heart disease
 Whole grains
 Slow release of sugars into blood
 Phytochemicals - antioxidant effects
 Soluble fibers
 Help to eliminate cholesterol
 Improving heart disease risk factors
Health Effects of CHO

Diabetes
 High-fiber foods
 Quantity and quality of CHO foods

GI health
 High-fiber foods
 Ample fluids

Weight management
 High-fiber foods and whole grains
 Feeling of fullness
Health Effects of CHO

Cancer
 Dietary fiber and colon cancer
 Fiber supplements vs. food sources
 Sources of dietary fiber - what are they?
 Phytochemicals
 Preventing colon cancer
 Diluting, binding, and removing cancercausing agents
 Bacterial fermentation—SCFA
Health Effects of CHO

Excessive fiber - usually due to
supplementation
 Insufficient energy or nutrient needs
(fullness)
 Abdominal discomfort, gas, diarrhea
 GI obstruction
 Impaired nutrient absorption

Dietary goals for CHO
 Balance, moderation, variety
From Guidelines to Groceries

Read food labels
 Total carbohydrate
 Sugars
 Fiber
 Sugars
 Added vs. natural sugars - how can we
tell?
 Grain products (bread, crackers, etc.)
 Hidden refined ingredients
 What to look for
A sugar by any other name…

Agave nectar
 Brown sugar
 Cane crystals
 Cane sugar
 Corn sweetener
 Corn syrup
 Crystalline fructose
 Dextrose
 Evaporated cane juice
 Fructose
 Fruit juice concentrates
 Glucose

High-fructose corn
syrup
 Honey
 Invert sugar
 Lactose
 Maltose
 Malt syrup
 Molasses
 Raw sugar
 Sucrose
 Sugar
 Syrup