Lecture 4: Carbohydrates- Sugars, Starches, and Fiber Flashcards
Carbs
- The brain and body’s best
fuel source - Provide 4 kcal per gram
- Broken down into glucose
molecules in the body - Made through
photosynthesis
Classifications of Carbohydrates
Based on # of units linked together
Can be simple or complex
(Simple carbs: mono or disaccharides, oligosaccharides, complex carbs: polysaccharides)
Simple Carbs
Monosaccharides
Disaccharides
Perceived as sweeter than complex carbohydrates
* Mix with saliva and react with taste buds
Basic unit of carbohydrates: monosaccharide
* Glucose
* Fructose
* Galactose
Disaccharides: 2 monosaccharides linked together through condensation
* Maltose
* Sucrose
* Lactose
Complex Carbs
Polysaccharides
Condensation Reactions
Links monosaccharides to form disaccharides
Glucose + Glucose = Maltose
Glucose + Fructose = Sucrose
Glucose + Galactose = Lactose
Complex Carbs
Made up of many monosaccharides linked together in
chains
* Oligosaccharides: 3-10 monosaccharides.
~In foods: Legumes, beans, cabbage, brussels sprouts,
broccoli
~Found in breast milk
* Polysaccharides
~Glycogen (animals)
~Starch, fiber (plants)
Polysaccharides
Starch
~Plant glucose storage in two forms:
-Amylose
-Amylopectin
Amylose
Straight chain of polysaccharides found in starch
- More resistant to digestion than amylopectin – “resistant
starch” is not digested in the GI tract.
~May improve health of digestive tract
~May improve glucose tolerance
~May stimulate growth of beneficial intestinal bacteria
Amylopectin
Branched chains of polysaccharides found in starch
* Easier to digest
Where to get starch from in diet?
- Toots and tubers
- Grain seeds
- Legumes
- Cornstarch
- Many processed foods
Glycogen
- Storage form of carbohydrates in animals
- Stored in liver and muscles in limited amounts
- Source of glucose for body
- Dissipates with rigor mortis, therefore is not present in
animal products that humans consume
Fiber
- Most forms of fiber are nondigestible polysaccharides and occur naturally as a structural component called cellulose.
- We cannot digest or absorb fiber
- Only found naturally in plant foods, not foods of animal origin
Dietary Fiber
Natural Fiber
Functional Fiber
Added Fiber
Total Fiber
Dietary + Functional Fiber
Listed as dietary fiber on food labels
Soluble Fiber
- Dissolves in water to form viscous solutions
- Can be fermented by intestinal bacteria
- Includes: pectins, beta-glucans, mucilages
- Food sources: oats, apples, beans, seaweed
Benefits:
* Helps lower cholesterol
* Slows gastric emptying
* Control appetite
* Normalizes blood sugars
Insoluble Fiber
- Cannot be fermented by bacteria in the large intestine
- Does not dissolve in water
- Derived from structural parts of plants such as cell walls
- Includes: cellulose, lignin, some hemicelluloses
- Food sources: wheat bran, rye bran, fruit & vegetable peels
Benefits:
* Increases bulk of stool
* Helps relieve constipation
Apple
Skin= Cellulose: insoluble fiber
Inside apple= Pectin: soluble fiber
Why is dietary fiber so important?
Benefits:
* Helps lower risk of
~Bowel irregularity
~Constipation and diverticulitis
~Obesity
~Heart disease
~Cancer
~Diabetes
What can fiber help prevent?
Heart Disease
Diabetes
Cancer
How does fiber prevent heart disease?
- Soluble fiber helps decrease blood cholesterol levels.
- Soluble fiber may reduce the rate at which fat and
carbohydrate are absorbed. - Insoluble fiber has been shown to promote heart health
How does fiber prevent diabetes?
- Soluble fiber helps slow digestion and absorption of glucose,
helping control blood glucose levels. - Higher consumption of fibers from cereals has been shown to reduce the risk of developing diabetes
How does fiber prevent cancer?
- Fiber helps reduce the incidence of several cancers.
~Decreases the amount of time cancer-promoting substances spend in contact with the intestinal lining
~Encourages the growth of colon-friendly bacteria and their
fermentation by-products
~Reduces concentration of bile acids in the colon
How does fiber prevent obesity?
- Increases satiety, resulting in reduced food intake
- Some cautions about fiber
~Introduce fiber into the diet slowly.
~Initially, a high-fiber diet can cause flatulence and bloating.
~It may reduce the absorption of some minerals
How are carbs digested and absorbed?
- Disaccharides and starch are digested into monosaccharides.
- Monosaccharides are easily absorbed by the small intestine.
- Fiber passes through the GI tract undigested.
Where are carbs digested?
Mouth:
~Saliva contains amylase enzyme, which starts breaking down amylose and amylopectin into smaller starch units and maltose
~Mechanical digestion of fiber
* Stomach:
~HCl inactivates salivary amylase
~Fiber delays gastric emptying
* Small intestine
~Pancreatic amylase breaks down remaining starch into disaccharides (maltose)
~Maltose and other disaccharides are broken down to monosaccharides and absorbed into blood
* Large intestine
~Fiber continues to the large intestine, where some is metabolized by bacteria in the colon and the majority eliminated in your stool
~Production of short-chain fatty acids by fiber metabolizing bacteria
How are carbs absorbed?
After carbohydrates are digested to monosaccharides they are:
* Absorbed through the intestinal cell mucosa
* Transported to the liver via the portal vein
* Metabolic needs direct the fate of the monosaccharides
Absorption of galactose and fructose
Used by the liver for energy
Converted to glucose
Absorption of Glucose
- Used for energy
- Converted to glycogen through glycogenesis
- Excess glucose is converted to glycerol and fatty acids for storage in adipocytes.
Functions of Carbs
- Provide energy
~4 kilocalories per gram - Maintain blood glucose levels
~Glucose is the primary fuel for the brain.
~Glycogenolysis occurs 4 hours after a meal.
~Muscle glycogen cannot be used to raise blood glucose levels. - Spare protein
~Prevents the need for gluconeogenesis
-Occurs mainly in the liver but can occur in the kidney - Prevents ketosis
~Ketosis is an elevated number of ketone bodies in the
blood that occurs after approximately 2 days of fasting
-Protein from muscle and organs broken down to make
glucose
-Brain switches to using ketone bodies for fuel to spare
protein-rich tissues
-If fasting continues, protein reserves are depleted and
death occurs
Ideal blood glucose level
The body wants to keep blood glucose between 70 milligrams/deciliter and 110 milligrams/deciliter to ensure availability to cells
Hormones responsible for maintaining blood glucose level
Insulin
Glucagon
Insulin
- Released from pancreas
- Lowers blood glucose level
~Needed for glucose to enter the cell from the bloodstream
~Liver, kidney, and brain cells can use glucose without insulin.
~Stimulates glycogenesis (liver and muscle)
~Stimulates lipogenesis (liver and adipose)
~Inhibits lipolysis
~Inhibits gluconeogenesis
Glucagon
- Released from pancreas
- Target organ is the liver
- Increases blood glucose level
~Stimulates the release of glucose into the blood
-Stimulates glycogenolysis
-Stimulates gluconeogenesis
Fed (postprandial) state
Blood glucose increases
* Pancreas releases insulin
* Insulin allows for the uptake of glucose by peripheral tissue (glucose travels from the blood into tissue = drop in blood glucose)
~Muscle stores glucose as glycogen (glycogenesis)
~Adipose stores glucose as fat (lipogenesis)
* Liver will store as glycogen and then fat lipogenesis; reduces glucose output
* Reduction in blood glucose signals `pancreas to reduce insulin secretion
Fasting State
- Blood glucose drops
- Pancreas releases glucagon
- Glucagon stimulates the breakdown of glycogen into glucose (glucose travels from the tissue into blood = increase in blood glucose)
- Stimulates gluconeogenesis in the liver; increases glucose output
- Increase in blood glucose signals pancreas to reduce glucagon secretion
Hypoglycemia
Low blood glucose below
- Occurs with excess insulin production
Symptoms:
Hunger, dizziness, feeling light-headed, confused, weak, or
beginning to sweat
Treatments:
* Eat or drinking
carbohydrate-rich foods
* Eating smaller, well-balanced meals throughout the day
Glycemic Response
Blood glucose response
- How quickly and how high blood glucose level rises after
consuming carbs - How quickly it returns to normal
Affected by:
* type and amount of carbohydrate consumed - refined sugars
and starches raise blood sugar levels faster than unrefined
carbohydrates and fiber
* amount of fat and protein consumed
* Genetics
* Microbiome
* Sex
* Anthropometrics
* Other environmental factors (sleep, physical activity, meal
timing)
Glycemic Index (GI)
A rating scale of the likelihood of foods to increase the levels of blood glucose and insulin.
Concerns:
* Does not account for the amount of carbohydrate
consumed.
Glycemic Load (GL)
The amount of carbohydrate in a food multiplied by the amount of the GI of that food.
- Adjusts the GI to account for the amount of carbohydrate
typically eaten
DRI Carb Recommendation
130 grams/day
MyPlate Carb Recommendation
- 6 servings of grain/day
- 3 servings of vegetables/day
- 3 servings of dairy/day
- 2 servings of fruit/day
AMDR carb recommendation
45-65% of total kilocalories/day
Overconsumption of carbs
Most adults in the United States consume well over the
minimum DRI: 180-330g/day (~50% kcal)
* Primarily from refined bread, soda, baked goods
Daily Fiber recommendations
DRI: 14 grams per 1,000 kilocalories consumed
AI: 25 to 38 grams per day
Best sources of carbs
Whole foods
Some sources are better than others such as grains, vegetables, fruits, and beans
Best choices should include:
* Nutrient-dense, low-saturated-fat foods, with low amounts of
simple carbohydrates
* Higher amounts of fiber and complex carbohydrates
Parts of a whole grains
Endosperm
Bran
Germ
Endosperm
Largest part of kernel.
Made up primarily of starch
Contains most of kernel’s protein + some vitamins and minerals
Bran
Outermost layer
Contains most of the fiber
Good source if many vitamins and minerals
Germ
Located at the base of kernel
Embryo where sprouting occurs
Source of oil
Rich in vitamin E
Whole-wheat flour
Missing folic acid
Uses who kernel
Enriched flour
Only uses Endosperm
Wheat flour (not enriched)
Uses only endosperm
No nutrients added back
Enrichment
Adding nutrients back
Riboflavin
Niacin
Thiamin
Iron
Fortification
Adding extra nutrients
Folate
Whole Grains
- Decrease risk of stroke
- Decrease risk of dying from cancer or heart disease
- Fiber linked to lower risk of diabetes
- More satiating
