Bioc L08 Carbs Flashcards
What does “sugar” refer to w/ dietary carbs?
What are added sugars?
Sugar refers to mono and disaccharides.
- *Monosaccharides** include Ribose, Glucose, Galactose, and Fructose.
- *Disaccharides** include Lactose (galactose-glucose), Maltose (a-1,4-Glucose-glucose), Isomaltose (a-1,6-glucose-glucose), Sucrose (glucose-fructose).
Added sugars are sugars or syrups added during preparation such as high fructose corn syrup (hfcs). HFCS is associated with heart disease, American Heart Association recommends consuming only 5% of calories from added sugar.
What does starch consist of?
Starch is a combination of amylose and amylopectin.
Generally grain-derived starch (wheat/maize) have a higher amylose % and level of retrogadation compared to tubers (potato/tapioca).
Why is raw starch difficult to digest?
The starch molecules are encapsulated in plant cells.
What is gelatinized starch?
What is Starch Retrogradation?
What’s the problem with eating retrograde starch?
Moist heat (cooking) causes starch to absorb water, swell and burst out of the plant cell resulting in gelatinized starch. The gelatinized starch is easier to digest.
When starch gel cools, linear portions of amylose line up (plus small amounts of linear portion of amylopectin). They remain together due to H-bonding and expel water. This crystallization process of starch is called retrogradation.
Retrograde starch is less digestible because of the reduced water content and limited surface for digestion. As a result retrograde is fermented by intestinal bacteria leading to increased gas formation. Baked Beans have a high retrograde (resistant) starch.
What is dietary fiber? What are the two types?
Dietary fiber resists human digestion and is made up of complex polysaccharides.
Insoluble fiber can’t be digested by humans or colonic bacteria. Found mainly in vegetabls, wheat bran, whole grain-breads and cereals.
Soluble fiber can’t be digested by humans but can be fermented by colonic bacteria. Found in oats, legumes and fruits.
How is soluble fiber beneficial?
In the stomach the water holding capacity of fiber changes the viscosity, delays gastric emptying for slower delivery of nutrients.
In the small intestine, fiber lowers bile acid resorption resulting in lower blood cholesterol, digestion and absorption of fat and carbs is slowed due to viscosity.
In the large intestine the bulk and transit time are reduced, increasing stool weight and concentration Microbial growth is also promoted, increasing energy usage from fermented metaboilites (short chain fatty acids).
What are the main locations of starch digestion?
The main sites are in the mouth and intestinal lumen.
The digestion is rapid and mostly complete by the time the stomach contents reach the junction of the duodenum and jejunum.
Hydrolysis of glycosidic bonds is catalyzed by glycosidases (usually very specific)
What is the mechanism and location of digestion for different sugars?
Mouth: a-amylase digests starch, no a-amylase for lactose, sucrose of cellulose in the mouth.
Stomach: low pH stops activity of a-amylase, no gastric enzymes for carbs so no digetstion here.
Duodenum: Pancreatic a-amylase and HCO3- digests starch dextrins, mucosal membrane bound enzymes digest isomaltose, maltose, lactose and sucrose into glucose, fructose and galactose which are reabsorbed and enter portal circulation to the liver. Cellulose can’t be digested.
What results from abnormal degradation of disaccharides? (e.g. lactase deficiency)
What molecule is in the breath that correlates with fermentation?
With a lactase deficiency, lactose won’t be digested and will be fermented back bacteria, producing 2-C reactions, CO2, 3-C reactions and H2, causing more water to enter the lumen than exits.
The fermentation products result in diarrhea and dehydration from the excess water entering the lumen and not getting reabsorbed, and bloating from the gasses produced.
H2 can be measured in the breath and correlates with fermentation in the intestine.
What foods should be avoided by people with an isomaltase-sucrase defiency?
How can carbohydrate intolerance be used to determine any glycolytic enzyme deficiency?
Starch (creates isomaltos) and Sucrose.
An oral sugar tolerance test (measurement of H2 gas in the breath). Eat a certain disaccharide and see if H2 in breath increases to determine which dissacharide enzyme is deficient.
Besides poor digestion, what can cause osmotic diarrhea?
What is an example?
Malabsorption of monosaccharides because of a transporter deficiency.
Example: there are two transporters for fructose, glucose-inderpendent fructose transporter and Glucose-dependent fructose co-transporter (main transporter). If the main transporter is difficient, fructose could accumulate in the intestines and cause osmotic diarrhea.
Fructose malabsorption in mostly seen when fructose exceeds glucose (fructose mainly needs co-transporter). Also, if fructose and sorbitol are ingested together, malabsorption is more than additive, suggesting they compete for the same transporter.
What might be the cause of non-specific diarrhea in children?
Fruit juice consumption. Orange juice is the only common one that doesn’t consist of more fructose than glucose.
Are carbohydrates essential nutrients?
No because the carbon skeletons of amino acids can be converted into glucose. However, lack of dietary carbohydrates leads to ketone body production and degradation of body protein for gluconeogenesis.
Recommended amout of carbs to eat per day is 130g (amount needed by brain and arythrocytes each day). Adults should consume 45-65% of their total calories from carbohydrates.
What determines the glycemic index?
Glycemic index (GI) is defined by the area under the “two-hour blood glucose response” curve (AUC).
The AUC of the test food is divided by the AUC of the standard and multiplied by 100.
Higher index indicates blood glucose will rise more quickly after ingestion. Cooked carbs have a higher glycemic index than raw carbs. Higher fiber content and high fat content both lower GI.
