Lecture 3

Question 1

Colon physiology intro

Answer 1

• once lactose is in colon, only thing we can do is microbial fermentation
• amount is what determines severity (more fermentation = worse symptoms)

fermentation bacteria/ by products:
- lactic acid (3 carbon)
- acetic acid (2 carbon) (SCFA, can be good)
these produce symptoms therefore the composition of the bacteria defines how well you digest lactose

• some bacteria express the enzyme beta-galactosidase which breaks the glycosidic bond in lactose into free glucose and galactose
• bacteria either ferment intact lactose or the monosaccharide components

Question 2

lactose in the colon lumen

Answer 2

• beta galactosidase breaks lactose down
• lactose and galactose produce symptoms
• glucose doesnt
• acids (good when not in lumen) and gases produced by bacterial fermentation
• acids: decrease colon lumen ph, water reabsorption dilutes = diarrhea
• gas: bloating and flatulence

Question 3

Colon and symptom variability

Answer 3

high individual variability in symptoms due to:
- variation in microbial composition and activity in colon
- transit time in GI tract (faster time = less time to ferment)
- genetics and lactase enzyme expression and activity in SI

SAME AS MIDTERM QUESTION IN LEC 2 DECK?

Question 4

Colon physiology summary and review

Answer 4

colonic bacteria ferment undigested lactose in the colon lumen

• produces SCFA which cause bloating, flatulence, pain
• undigested lactose acidifies the colon and increases osmotic load: loose stools and diarrhea
• symptoms not restricted to GI tract: headache, lethargy etc
• possibly due to toxic metabolites generated during fermentation (acetylaldehyde, acetoin, ethanol)

Question 5

ways to diagnose lactose intolerance

Answer 5

jejenum biopsy to measure lactase expression/activity: INVASIVE, EXPENSIVE

genotyping to look for SNPs: EXPENSIVE, BUT SIMPLE

blood glucose levels after a lactose load: LONG AND EXPENSIVE

try gluten free diet: COMMON, LONG, NOT FUN

MOST COMMON TEST:

breath hydrogen test (BHT):
- cheap, non invasive, reliable
- ingest lactose, measure exhaled breath hydrogen (normal lactose assimilation should go straight to liver therefore wouldnt see it in breath)
- 20% false negatives because it only tests for hydrogen but methane can also be produced by methanogens

Question 6

lactose intolerance treatment

Answer 6

success dependent on degree and severity of lactose intolerance AND amount of lactose consumed

• avoiding lactose foods
• calcium and vit D supplements
• may consume milk with meal
• yogurt is normally okay BECAUSE co consuming live bacteria and fat (high fat slows digestion and therefore lactose digestion)

Question 7

lactose maldigestion totality

Answer 7

lactase deficiency is seldom total - patients retain some ability to digest lactose causing variation in symptoms

symptoms based on
- levels of remaining lactase enzyme activity
- amount of lactose consumed at once
- GI transit time
- colon fermentation ability and composition

Question 8

ideal transit time in LI

Answer 8

• want to maximize lactose breakdown in SI before it gets to colon (slow SI transit time)
• slower gastric emptying, more opportunity for lactase activity (however much there is)
• want to minimize lactose fermentation in the colon
• less gastric symptoms

*“worst case scenario: fast gastric emptying and SI transit, slow colon transit”