Lactose Intolerance

in lactose intolerance, what happens to lactose?

lactose isn’t broken down in the small intestine so it reaches the colon intact and is fermented by bacteria (cannot be digested because there is no lactase enzyme expression in the colon)

• it cannot be broken into its constituents (glucose and galactose) but instead is broken into gases and acids (clinical symptoms - abdominal pain, gas, diarrhea)

on average, how much CHO do we ingest daily and how much of this is lactose?

~ 400 g daily

up to 10% in lactose (40 g)

in regular functioning, where is lactase expressed?

in the jejunum and ileum

where is the brush border found?

the microvilli at the apical membrane of the intestinal epithelial cells

sucrose, lactose and maltose get broken down into which constituents?

sucrose → glucose + fructose

lactose → glucose + galactose

maltose → glucose + glucose

which borders must be crossed to enter the bloodstream?

the apical and basolateral borders

describe glucose and galactose transport across the borders

glucose and galactose cross the apical border via SGLT1 by co-transport with Na+

a sodium potassium pump maintains the Na+ concentration gradient

GLUT2 facilitates movement through the basolateral border via facilitated diffusion

describe fructose transport across the borders

fructose crosses the apical border via GLUT5 by facilitated diffusion (independent of Na+)

GLUT2 facilitates movement through the basolateral border via facilitated diffusion

what concentration gradient needs to be maintained for glucose and galactose to be able to move to the blood?

high [Na+] outside the cell and low [Na+] inside the cell

what could be the consequences of SGLT1 or Na/K pump failure?

manifests the same way as lactose intolerance because the monosaccharides making up lactose cannot make their way into the blood

what gene encodes the GLUT5 protein?

SLC2A5 gene

what is fructose malabsorption caused by? what is the consequence?

deficient or low expression of GLUT5 in the small intestine

if the protein is not present on the apical membrane, fructose cannot pass and then there will be the same clinical symptoms as lactose intolerance and IBS

why is fructose malabsorption clinically relevant?

due to use of sucrose and high fructose corn syrup in commercial food products

what is the difference between fructose malabsorption and hereditary fructose intolerance?

hereditary fructose intolerance is a potentially fatal condition in which liver aldolase B is not expressed which leads to the accumulation of fructose-1-phosphate (cannot be broken down without the aldolase B enzyme) which can lead to the death of liver cells over time

what are the different types of lactose intolerance?

1. primary lactase deficiency (inherited)

2. secondary lactase deficiency (acquired)

describe primary lactase deficiency

it is irreversible

genetically programmed progressive loss of lactase enzyme expression and/or activity in the small intestine (this is wild-type and what should be happening to all of us)

describe secondary lactase deficiency

it is acquired

loss of lactase expression due to GI mucosal injury damaging the brush border of the small intestine (reversible)

what are the major causes of mucosal injury?

infections, allergies, inflammation due to other GI diseases (celiac, inflammatory bowel disease)

with regards to lactose intolerance, what is common in children?

gastroenteritis - infectious diarrhea where sufficient degrees of mucosal damage reduces the quantity of lactase in the damaged areas of the small intestine

when is intestinal lactase activity maximal?

during the perinatal period (time immediately before and after birth)

what occurs after 2-12 years of age?

two distinct groups emerge (can develop either childhood onset or adult onset of deficiency)

describe the lactase wildtype

lactase non-persistence (lactase intolerance) → group of individuals with low lactase activity (aka hypolactasia)

what can lactase non-persistence be attributed to?

any of the following:

1. low lactase gene expression → low lactase protein expression → L.I. symptoms

2. low lactase protein expression → SOME protein produced, not all gene expression (mRNA) is translated to protein → L.I. symptoms

3. competent expression levels of lactase gene expression and protein but enzymatic activity of the protein is low → L.I. symptoms

describe the lactase mutant

group of individuals who retain their neonatal level of lactase activity into adulthood

describe what occurs in the lactase enzyme activity graph for lactase persistence and lactase non-persistence

• all groups begin with low lactase enzymatic activity in gestation

• close to birth, lactase levels and enzymatic activity peaks

• in people with lactase persistence (30%), lactase enzymatic activity stays consistent at that peak

• in people with lactase non-persistence (70%), the activity starts to taper off after birth and continues to decrease throughout weaning and adulthood

• in people with intermediate levels (lactase persistence with reduced enzyme activity), lactase activity tapers off after weaning

SNPs are associated with the _____ phenotype

with the lactase persistence phenotype (mutant)

with regards to the lactase (LCT) gene, where are SNPs located?

located in a neighbouring gene (MCM6), in a regulatory region that controls the expression of the lactase gene (LCT)

what is MCM6 and what does it do?

MCM6 is upstream of the lactase gene

MCM6 influences the lactase promoter and controls lactase transcription

what are SNPs in the MCM6 gene responsible for?

cause lactase persistence

which SNP is common among European and North American Caucasian populations? Asian and African populations?

Europe and N. America = intron 13 of MCM6 C→T

Asia and Africa = intron 13 of MCM6 C→G

which state (persistence or non-persistence) is the dominant allele?

the persistence allele is dominant

describe hypolactasia

wild-type

2 C>C alleles (CC)

low mRNA expression (zero or little)

describe the heterozygous form of lactase persistence

1 C>C allele

1 C>T allele

CT

a mutation on 1 allele is enough for a person to be lactase persistent

this person can still digest lactose (or to some extent for certain foods)

describe the homozygous form of lactase persistence

2 C>T alleles

high mRNA expression

TT

mutant

describe the additional level of regulation of the lactase (LCT) gene expression (the transcription factors)

Cdx2 and HNF-1a are 2 transcription factors that activate LCT transcription; they fine tune lactase gene expression and bind to the LCT gene to cause lactase transcription and translation (lactase persistence)

what is the role of PDX-1?

PDX-1 is a transcriptional repressor that blocks and inhibits the function of Cdk2 and HNF-1a

result is no or reduced LCT transcription leading to low lactase protein expression which may result in lactose intolerance symptoms

what happens if an individual has low expression of PDX-1?

will have LCT gene expression and lactase persistence

what happens if an individual has high PDK-1 expression?

strong repression of LCT gene expression = low lactase persistence or lactase non-persistence

what happens if an individual has high expression of Cdx2 or HNF-1a?

LCT transcription activated and gene expression → lactase persistence

what happens if an individual has low expression of Cdx2 or HNF-1a?

transcription factors will barely activate LCT transcription, so low gene expression → low lactase persistence or lactose intolerance

what happens in cases where individuals overload on the amount of lactose the body can digest?

some lactose may go to the colon (or its monosaccharides) for fermentation and result in same clinical symptoms as lactose intolerance

in the colon, what occurs when there is no or very little lactase expression?

bacterial fermentation

gases: H2, CO2, CH4 (methane)

acids: lactic acid (3C), acetic acid (4C) are the two major types of SCFA produced

may also produce propionic acid (3C) and butyric acid (4C)

some bacteria may express a particular enzyme which helps ferment lactose. what is the name of this enzyme and what does it do specifically?

some bacteria may express beta-galactosidase, which breaks the glycosidic bond in lactose into free glucose + galactose

which bacteria types are capable of fermenting intact lactose OR monosaccharide components

• enterobacter species

• citrobacter sp.

• klebsiella sp.

• escherichia coli

• serratia sp.

• streptococcus sp.

if there is a high composition of bacteria capable of fermenting lactose or its monosaccharides, what can result from this?

high composition = more lactose fermentation = more clinical symptoms

which bacteria are non-lactose fermenting bacteria?

• salmonella sp.

• shigella sp.

if there is a high composition of bacteria incapable of fermenting lactose or its monosaccharides, what can result from this?

high composition = less lactose fermentation = less clinical symptoms (doesn’t respond to lactose and it goes straight to the feces)

why is diarrhea one of the clinical symptoms of lactose intolerance?

the bacterial fermentation results in the production of acids which lower the pH of the colon lumen → more water brought in to the lumen to dilute the acidity of the lumen (increased osmotic load) → more liquidy environment → diarrhea

why are bloating and flatulence clinical symptoms of lactose intolerance?

due to the production of gases (COS2, H2, CH4)

CH4 responsible for flatulence

why is there high variability in the symptoms associated with lactose intolerance?

variation in microbial composition (which bacteria are present and what is their effectiveness?) and activity in the colon

transit time in the GI tract: faster transit time = less water re-absorption, less diarrhea and less time to ferment lactose = fewer clinical symptoms

not all patients with lactose maldigestion (lactose intolerance) experience symptoms because there is high individual variability influenced by…

• genetics and lactase enzyme expression, functionality and activity in the small intestine

• rate of gastric emptying (want a slow release and more time spent in the small intestine)

• GI transit time

• large intestine microbial composition and activity

what are some ways to diagnose lactose intolerance, but aren’t the two main methods for doing so? (also explain what the pros and cons are)

1. jejunum biopsies and measure the expression and activity of lactase - invasive and expensive, can be used in severe cases

2. genotyping individuals to determine if they carry the lactose persistence associated SNPs in chromosome 2 - expensive but simple and non-invasive; also lactose intolerance is not exclusively explained by SNPs

3. measuring blood glucose/galactose levels after a lactose load - time-consuming and expensive (galactose is converted to glucose and is used in metabolism)

what are the two main methods of diagnosing lactose intolerance? what are the cons of each method?

1. exclusion diagnosis based on clinical symptoms → remove lactose-containing foods from the diet and see if symptoms improve; can be followed with re-introducing lactose-containing foods and should see symptoms reappear (not fun for the patient, can take several weeks and success is dependent on patient compliance and having no other GI diseases)

2. breath hydrogen test - oral ingestion of 50g of lactose (~1L milk) and exhaled hydrogen from breath measured 3-6 hours after lactose ingestion; if H2 > 20 ppm suggests L.I., < 20 ppm sometimes referred to as “lactose maldigesters”

what is the principle behind the breath hydrogen test?

in normal lactose assimilation, lactose → glucose and galactose → liver

in lactose intolerance, lactase is not present or is present in small amounts, so lactose enters the colon to be fermented to gases (hence H2 → liver → lungs) and acids

BHT = 20% false negatives, why?

15-30% of people have Methanobrevibacter smithii (aka methanogens) in the microbiota which converts hydrogen and carbon to methane which is excreted as flatus and thus H2 not detected in the breath and the 20% of people may not meet the cutoff even if they are lactose intolerant

what is successful treatment dependent on?

dependent on the degree or severity of the individuals lactose intolerance AND the amount of lactose consumed

which foods are richest in lactose content?

cows milk (whole & skimmed): 4.6 - 4.8% weight

yogurt: 4.0 - 4.7

ice cream: 4.8 - 5.2

why do people with lactose intolerance usually have a better time with yogurt than milk and ice cream despite the similar lactose contents?

many yogurts have live bacteria and fat

live bacteria - if it contains beta-galactosidase, the lactose is cleaved into its monosaccharides before it is consumed, thus resulting in fewer clinical symptoms

fat - causes slow gastric emptying → slower entrance to the small intestine → giving the lactase enzymes the best possible opportunity for digestion of lactose and maximum time to digest

give examples of some cow’s milk alternatives and their pros and cons

animal (e.g. goat) and plant (e.g. oat, almond, cashew, LactAid) milk sources

• will be fortified with calcium and vitamin D

• may have higher sugar content (dependent on type of milk)

• may contain phytate - binds minerals like calcium that will decrease its bioavailability (not 100% absorbed)

**describe the study and its results: self-described “lactose intolerant” individuals and lactose maldigesters can tolerate the amount of lactose in a serving or more of milk and other dairy foods

• 3 different subjects all consumed 15 g lactose

• first 2 of self described LI racially mixed adults, 3rd of female racially mixed adults

• first group given 1 cup of milk with breakfast = 30% LT, 70% LI

what do lactose intolerance symptoms persist based on? (5)

1. level of remaining lactase activity (low activity → less digested and absorbed lactose in SI → greater intolerance symptoms)

2. amount of lactose consumed at a time (alone or with other foods/fats)

3. GI transit time - slower transit time through SI increases amount of time in the SI to digest lactose VERSUS slower transit time in the colon (more time to ferment lactose)

4. ability of colon microbiota to metabolize/ferment lactose → each person has different microbial composition which impacts severity of LI symptoms

5. composition of colon microbiota determines how much lactose is fermented

what would be the best case scenario for transit times through the GI tract in a lactose intolerant patient?

• to maximize lactose breakdown in the SI → slow transit time through SI → slower gastric emptying → food bolus/chyme gradually enters SI → more opportunity for lactase activity to break down lactose

• to minimize lactose fermentation in the colon → faster transit time through the colon → minimize symptoms → less time in colon = less opportunity for lactose fermentation by colonic bacteria → results in less gas production, bloating and diarrhea

what would be the worst case scenario for transit times through the GI tract in a lactose intolerant patient?

to minimize lactose breakdown in the SI → fast transit time through the SI → quick gastric emptying → food bolus/chyme enters SI quickly → less opportunity for lactase activity to break down lactose

to maximize lactose fermentation in the colon → slow transit time through the colon → maximize symptoms → more time in colon = more opportunity for lactose fermentation by colonic bacteria → results in more gas production, bloating and diarrhea

what are some strategies to improve tolerance to dairy foods?

1. adjust amount of lactose consumed at a time

2. drink milk with a meal or snack - slows gastric emptying (if higher fat content) and/or delivery of lactose to the colon

3. choose the dairy food to consume wisely (e.g. yogurts with live bacteria cultures, whole milk vs. lower fat milk, aged cheeses)

4. try lactose-free or lactose reduced products

describe unpasteurized dairy

has “lactase” activity (beta-galactosidase activity) which survives the low pH of the stomach and is active in the SI and colon during digestion

what does pasteurization do to unpasteurized dairy?

destroys most beta-galactosidase activity

what do cultures milk products contain? what may be the benefit of these

contain Streptococcus lactis, Streptococcus cremoris (lactose fermenters) but may not survive the acidity of the stomach

describe sweet acidophilus milk

non-fermented milk supplemented with Lactobacillus acidophilus culture - a probiotic treatment to help reduce lactose intolerance symptoms