what percent of our energy intake comes from carbs
40-75%
what is the most important nutritional property of carbohydrates
digestibility in the small intestine
what are available carbohydrates
those that are hydrolyzed by enzymes of the GI tract to monosaccharides and are absorbed by the small intestine and enter pathways of carb metabolism
what are unavailable carbohydrates
those that are NOT hydrolyzed by endogenous human enzymes (may be fermented in the large intestine to varying extents)
examples: cellulose and some oligosaccharides
what are the most important carbs in food
monosaccharides (glucose and fructose) disaccharides (sucrose and lactose), oligosaccharides (raffinose, stachyose, fructo-oligosaccharides), polysaccharides (starch, cellulose, hemicellulose, pectins, fructans, gums, mucilages, algal polysaccharides), su
when does digestion of carbs start? where? by what enzyme?
as soon as meal is taken into the mouth
saliva in the mouth contains starch digesting enzyme (salivary amylase)
what does amylase do
break internal alpha 1-->4 bonds of amylose and amylopectin molecules
explain what salivary amylase can digest and how fast
starch lactose, sucrose, and cellulose taken in. Starch is digested into starch dextrins, isomaltose, maltose (lactose, sucrose, and cellulose are not digested)
salivary amylase is rapid but NONE of the products formed by its action are ready for absorpti
is salivary amylase essential for normal digestion? why?
no
pancreas contains enzyme similiar to salivary amylase known as pancreatic amylase that is in considerable excess of dietary requirements
when is salivary amylase important
in infants: developmental delay of pancreatic enzymes
backup for patients with pancreatic insufficiency: example: cystic fibrosis
are carbs digested in the stomach? Why?
not digested in stomach
acidic pH inactivates enzyme and no regular enzymes in stomach for carb digestion
what happens to carbs in the small intestine
site where most carbs are digested by cleaving alpha1-->4 of starch
pancreatic amylase is secreted into the intestinal lumen of the small intestine by the pancreas
the alkaline pH of the pancreatic secretions neutralizes the acidic nature of food that pas
describe amylase (another term to describe its action) what it cleaves
endoglycosidase
cleaves internal alpha 1-->4 bonds in starch (amylose, amylopectin) and glycogen
acts at random location along the starch chain
what are the end products of amylose digestion
maltotriose, maltose, gluco-oligosaccharides (may have small amounts of glucose)
what are the end products of amylopectin digestion
maltotriose, maltose, gluco-oligosaccharides, and LIMIT DEXTRIN and isomaltose and some glucose
what bonds can amylase not break? explain
cannot break terminal bonds and alpha 1-->6 at branch points of amylopectin
describe intestinal enzymes location and secretion
intestinal brush border synthesizes and secretes the enzymes that can completely digest the end products of starch hydrolysis that occurs in the mouth and intestine
what are the four different enzyme complexes that are attached to brush border
glucoamylase
sucrase-isomaltase complex
lactase
trehalase
explain function of glucoamylase (another term to describe it )
exooligoglucosidase
breaks alpha 1-->4 between glucose residues
BEGINS at NON-reducing end of an oligosaccharide, limit dextrins, or polysaccharides and sequentially releases glucoses
digests limit dextrins down to isomaltose
explain function of sucrase-isomaltase
3 activities:
1. sucrase digests sucrose to glucose and fructose (alpha 1--> beta 2)
2. maltase digests maltose to glucose (alpha 1-->4 (only the dissachharide)
3. isomaltase digests isomaltose to glucose (alpha 1-->6 in isomaltose and limit dextrins)
explain function of lactase
digests lactose to galactose and glucose (beta 1-->4)
explain function of trehalase
digests trehalose (not a major dietary disaccharide to glucose (alpha1 -->alpha 1 linkage)
describe the absorption and transport of monosaccharides
ONLY monosaccharides are absorbed by mucosal cells of human small intestine
different sugars have different mechanisms of absorption
D-Glucose and D-Galactose: secondary active transport-->sodium glucose co transporter (SGLT1) (can either do one or the ot
SGLT1
sodium glucose co transporter
GLUT-2
facilitative glucose transport for glucose (on luminal side) and both glucose and galactose (on serosal-->extracellular-->circulation side)
GLUT-5
facilitative diffusion of fructose on luminal side
how many glucose transport proteins exist in tissues
more than 10 (GLUT 1-12)
is insulin required for glucose uptake by intestinal mucosal cells
no because it is a spontaneous transport
how are monosaccharides transported once in blood
through hepatic portal circuit (loop)
how is glucose, fructose, and galactose finally broken down once they are absorbed
liver uses hormones insulin and glucagon
what happens when blood glucose levels are high
following digestion of a meal, pancreas secretes insulin causing the liver to transform glucose to glycogen and glycogen is stored in liver and in muscles
is cellulose digested
cellulose is not digested
what is the disorder associated with troubles with lactose absorption and how does it work
lactose intolerance
(africans and asians, genetic and age dependent loss of enzyme)
lactase enzyme deficient
less able to metabolize lactose so it builds up and moves to large intestine. in large intestine bacterial enzyme produce gases (CO2, H2, 2 and 3