Carb Metabolism 1 Flashcards
sources of dietary carbs for pre-agricultural humans
- roots, tubers, and rhizomes
- seeds above ground
- certain fruits and nuts
- inner bark of some trees
low in starch, high in fiber
sources of dietary carbs for industrialized humans
starch and fibers: pasta, apples, potatoes, bread, rice
simple sugars: sucrose, glucose, fructose: juices, honey, candy, soda
glucose use: postprandial
mainly liver, gut, and skeletal muscle
glucose use: post-absorptive (in between meals)
brain!
can brain live without glucose?
NO!
it can survive without oxygen for a few minutes, but not at all without glucose
most abundant form of starch
pectin
benefits of branching starches
it is better to keep energy in a more restricted space (human adaptation)
main dietary carbohydrates: disaccharides
- maltose
- lactose sucrose
- trehalose (a1 –> 1 D-glucose)
lactose is made of
galactose + glucose joined by beta 1,4
sucrose is made of
glucose + fructose joined by alpha 1,2
salivary and pancreatic alpha-amylase break down what into what
starch into maltose, trisaccharides (smaller molecules, but still not absorbable)
2 main enzyme categories of carbohydrate digestion
- endoglucosidases
- small intestine disaccharidases
endoglucosidases mechanism
hydrolyze alpha 1,4 bonds (NOT alpha 1,6) between glucosyl residues
shortened polysaccharide chains that are formed are called alpha-dextrins
endoglucosidases examples
alpha-dextrins
salivary alpha-amylase
pancreatic alpha-amylase
small intestine disaccharidases mechanism
hydrolyze alpha 1,4, alpha 1,6, beta 1,2 (sucrose), and beta 1,4 (lactose) bonds in limit dextrins, oligo-, and disaccharides
small intestine disaccharides examples
sucrase-isomaltase
maltase-glucoamylase
lactase
sucrase isomaltase
(alpha-dextrinase) glycosidase activity: alpha-1,4, alpha-1,6, beta-1,2
what happens if someone has a sucrase mutation?
we can’t degrade sucrose so we create a malabsorption for fructose, promotes diarrhea (water is trapped, fructose is never degraded)
no lactase =
lactose remains in intestinal lumen, draws in water, leads to diarrhea
bacteria is waiting for lactose
lactase deficiency produces
flatulence and diarrhea when dairy sugars are consumed
3 types of lactase deficiency
- primary hypolactasia
- primary congenital hypolactasia (alactasia)
- secondary hypolactasis
primary hypolactasia
autosomal recessive; lactase activity declines over many years, usually beginning in late childhood
primary congenital hypolactasia (alactasia)
very rare genetic conditionl complete absence of lactase
primary congenital hypolactasia (alactasia)
very rare genetic conditionl complete absence of lactase
secondary hypolactasis
due to damage of bursh border of intestinal enterocytes due to intestinal disease
lactase cuts…
lactose into galatose and glucose
when does lacrase activity begin
in utero
when is lactase activity highest
during the last trimester
remains that high for amonth after birth
reduction of lactase activity
reduces considerably after 1 month after birth
by 8 years old it is very reduced
why did human adults start drinking milk?
it is nutrient dense
it has water –> cleanest source of water at the time!
Sweden: why are they so special
90%+ of population is lactose tolerant
bc they did not domesticate cattle
what type of groups are lactose intolerant
groups not dependent on cattle
EX: Southeast Asia, West Afrida, Native Americans in southwest of US
what type of groups are lactose tolerant
cattle herders
EX: Europe, the Middle East, and Africa
SGLT limitations
limited capacity as a transporter
only capable of sensing limited amounts of glucose in cytoplasm
GLUT5
specialized in fructose absorption
main function of insulin
put away glucose, so energy can be used at a later time
GLUT1 + GLUT3
incorporate glucose into important tissues
GLUT3 tissue of interest
brain
which transporter is stimulated by insulin?
GLUT4
Which transporter is an exclusive transporter of fructose?
GLUT5
regulation of insulin secretion by glucose steps
- GLUT2 brings glucose into cell
- ATP blocks potassium channel
- Massive release of insulin
More glucose in circulation = more glucose absorbed in first step = more release of insulin
