Carbohydrates Flashcards
Carbohydrate
Functions
-
Energy
- largest source of dietary calories (4 kcal/g)
- calories from carbs ↑ over past 30 years
- mostly due to caloric sweetners
-
Contributes to taste, texture, and preservation of food
- very high sugar content is bacteriostatic
Carbohydrate Sources
Obtained from both plant and animal sources.
-
Plant sources (main source)
- Polysaccharides (aka complex carbs)
- linear chains ⇒ amylose
- branched chains ⇒ amylopectin
- Disaccharides and simple carbohydrates (trisaccharides)
- Sugar alcohols
- Monosaccharides
- Fiber
- Polysaccharides (aka complex carbs)
-
Animal sources (minor)
- glycogen
- lactose
Carbohydrate
Dietary Requirements
Carbohydrates are not an essential nutrient.
Most amino acids → glucose → other sugars.
Dietary recommendations:
-
AMDR: Carbs should supply 45-65% of energy intake.
- Majority should come from complex carbs
- In reality, ~50% is refined/processed carbs
-
RDA is 130 g/day.
- Based on minimal glucose needed for the brain.
- EAR: Median intake 200-300 g/day.
Starch
Glucose polymer.
Mixture of amylose (linear) and amylopectin (branched) chains in 20:80 ratio.
Most common source of carbs in the diet.
High content in potatoes, wheat, corn, and cassava.
Disaccharides
-
Sucrose ⇒ glucose and fructose
- common additive to processed food
-
Lactose ⇒ glucose and galactose
- only found in milk
- Maltose ⇒ glucose dimer
Sugar Alcohols
Produced by the reduction of the keto or aldo group in a monosaccharide.
Can occur naturally (apples & pears) or industrially (most common).
Poorly absorbed and metabolized (1-2.5 kcal/g).
Sorbitol is the most commonly encountered.
Found in gums and candies designed for caloric restriction.
Monosaccharides
Not naturally present in foods in significant amounts.
Fructose a minor exception.
High-fructose corn syrup (HFCS) produced by enzymatically treating corn syrup to convert glucose ⇒ fructose then mixing with pure corn syrup.
HFCS 55 most common.
Digestion
Mouth and small intestine are the principal sites of digestion.
Monosaccharide end-products absorbed in the small intestine.
-
Salivary 𝛼-amylase
- endoglycosidase which hydrolyzes random 𝛼-1→4 bonds in starch and glycogen
- cannot break 𝛼-1→6 bonds in amylopectin and glycogen
- forms mix of short branched and unbranched oligosaccharides called dextrins
- inactivated by low pH of stomach
-
Pancreatic 𝛼-amylase
- secreted by the pancreas along with bicarb to raise pH
- forms mixture of
- maltotriose (𝛼-1→4 glucose trisaccharide)
- isomaltose (𝛼-1→4 glucose disaccharide)
- maltose (𝛼-1→4 glucose disaccharide)
- other dissacharides
-
Intestinal disaccharidases
- located on the brush border of enterocytes in upper jejunum
- isomaltase, maltase, sucrase, lactase
- forms monosaccharides which are absorbed
- Carbs which cannot be digested (e.g. cellulose) passed into large intestine and excreted ⇒ “unavailable carbohydrates”
Lactose Intolerance
Caused by a deficiency in lactase which breaks down lactose into glucose and galactose.
~ 70% of world population lactose intolerant.
-
Primary lactose intolerance
- age-dependent loss of lactase starting around 2 y/o
-
Secondary lactose intolerance
- due to disease or disorder of the digestive tract
-
Tertiary lactose intolerance
- complete absence of lactase
Intestinal bacteria digests lactose to 2 and 3 carbon metabolites, lactic acid, CO2, and H2.
Causes bloating, cramping, flatulence, and diarrhea.
Absorption of Monosaccharides
Glucose, galactose, and fructose are the major monosaccharides produced by digestion.
Absorbed by enterocytes in the jejunum.
- Glucose and galactose absorbed via Na-dependent glucose transporter (SGLT1)
- Fructose absorbed via the facilitated transporter GLUT5.
- All three enter blood via the facilitated transporter GLUT2.
- Sugar alcohols enter and leave via passive diffusion.
Fiber
Classifications
Non-starch plant polysaccharides and lignin.
Nondigestible.
-
Insoluble fiber sources
- From cellulose: bran, root vegetables, legumes, cabbage family, fruit skin
- From lignin: wheat, carrots, broccoli, fruit with edible seeds (e.g., strawberries)
-
Soluble fiber sources (can be metabolized by gut bacteria)
- Hemicellulose: whole grains
- Pectins: apples, strawberries, citrus fruits gums: oatmeal, barley, legumes
Alternate classification:
Dietary fiber (naturally-occurring)
Functional fiber (extracted and added to processed foods)
Functions of Fiber
-
Fiber binds H2O and swells
-
Soluble fiber causes an increase in viscosity
- decreases speed of digestion and absorption
- delays gastric emptying
- increases satiation and satiety
-
Insoluble fiber enlarges and softens stools
- eases elimination
- shortens fecal transit time
- can be used in constipation
-
Soluble fiber causes an increase in viscosity
- Blunts the glycemic response ⇒ hypoglycemic effect
-
Binds bile acids and increases excretion
- reduces plasma cholesterol levels ⇒ hypocholesterolemic effect
-
Soluble fibers fermented in colon by anaerobic bacteria
- produces gases and short-chain FAs
- products promote uptake of H2O and Na+ in the colon
- promotes normal laxation
Fiber
Dietary Recommendations
DRI for fiber
25 g/day for women
38 g/day for men
Up to 50 g/day recommended for diabetes or hyperlipidemia.
Deficiency linked to constipation, hemorrhoids, and diverticula.
Over ingestion can cause constipation and intestinal blockage if insufficient amounts of water ingested, early satiation, decreased absorption of mineral cations like Ca2+ because they bind to acidic components of soluble fiber.
Carbohydrate Additives
- Complex carbohydrates added as thickeners and stabilizers
-
Sweeteners are the #1 food additive in the US
- Sucrose
- High-fructose corn syrup
- Perservatives
- Coloring agents
High-fructose Corn Syrup
Mixture of glucose and fructose.
-
HFCS-55
- used to sweeten carbonated soft drinks, other beverages, and desserts
-
HFCS-42
- used in baked goods, canned fruits, and condiments
Rise in HFCS parallels rise in obesity but not causitive.
Artificial Sweeteners
Are not carbohydrates and provide little to no energy ⇒ non-nutritive.
- Saccharin (Sweet N Low) - 300x sweeter than sugar
-
Aspartame (Nutrasweet, Equal) - 200x sweeter than sugar
- dipeptide (asp + phe)
- contraindicated with defects in phenylalanine metabolism (PKU)
-
Sucralose (Splenda) - 500-600x sweeter than sugar
- chlorinated sucrose
Glycemic Index
(GI)
Ranking of foods based on rise in BGL caused by carbs in the food compared to same amount of carbs in 50g of glucose or white bread.
Factors that influence GI:
- rate of ingestion
- form of the food
- method of cooking
In general, the easier a food is to digest, the higher the GI.
