Lactose intolerance

See Lactase

Lactose is a type of sugar found in dairy products, and intolerance occurs in people who don’t produce lactase, the enzyme that digests lactose.

Defined by the NIH as the onset of gastrointestinal symptoms following a single dose challenge of ingested lactose by an individual with lactose maldigestion, which are not observed with the person ingests an indistinguishable placebo.

Undigested lactose is fermented in the colon, leading to gas, bloating, abdominal discomfort and cramps, and borborygmi

Symptoms usually occur 30 minutes to a few hours after eating or drinking the lactose containing food.
Symptoms of lactose intolerance depends on how and how much lactose is consumed as there is a dose dependent effect.
Consumption of 50 g of lactose in a single setting is sufficient to induce symptoms in a majority of individuals with lactase deficiency.
Delusion of other foods with lactose can decrease symptom burden.
Variables determining who maldigests  Lactose includes: dose, dilution, lactase expression, gut transit, colonic flora, and sensitivity to fermentation products.

Lactose malabsorption may be the result of congenital lactase deficiency, secondary lactose intolerance, and most commonly from acquired primary lactase deficiency.

Lactase expression is genetically driven but patients  have  variable enzyme expressions which is quantifiable and can impact the capacity for digestion and tolerance.

The colon microbiome determines how maldigested lactose is fermented, and the byproducts of this breakdown causes variation of symptoms.

Infants of all races and ethnicity produce lactase to.digest human milk or lactose present in standard infant formulas.
After weaning there is a programed reduction in lactase production.
Expression of lactase can be detected during gestation, peaks at birth, and begins to decline after a few months of life.
Lactose cannot be absorbed via the small intestine epithelium without first being broken down by lactase.
Decreased lactase enzyme in the small bowel brush border can reduce the ability to cleave lactose into monosaccharides galactose and glucose which can be easily absorbed in the bloodstream.
If there is low enzyme activity lactose is not digested in the small bowel and enters the lumen of the colon were bacterial fermentation occurs, forming lactic acid, freeing hydrogen, and also producing gastrointestinal symptoms.

Congenital lactase deficiency is a rare autosomal recessive process with alterations in the coding regions of the lactase gene and presents as diarrhea within days of birth.

Acquired primary lactase deficiency affects 65-74% of the worldwide population, with widespread regional and ethnic variations.

Hypolactasia refers to non-persistence or deficiency of the lactase enzyme.
 Maldigestion refers to the inefficient breakdown of lactose.

Persistence and non-persistence of lactase activity is attributed at least import to genetics.

The 13910C>T single nucleotide polymorphism is associated with lactase persistence and non-persistence in European populations.

The most common genetic Polymorphism that causes lactase persistence is a single nucleotide substitution young chromosome to involving the lactase gene, often seen in whites.
The most affected populations in North America are Asians with a 15-100 percent non-persistence depending on the population with the highest in southeast Asians, native Americans have a 79% non-persistence, African-Americans 75% non-persistence and Hispanics 51% non-persistence, whereas whites have 21% non-persistence.
Scandinavians and northern Europeans have a 5-20% likelihood of non-persistence, and are the least affected.

Lactose is never absorbed as a disaccharide.

Lactase persistence is attributed to the 13910*T allele, which has a high frequency of homozygosity in Northern European populations accounting for 70-90% of individuals, and is less frequent across southern Europe and the Middle East where there is 30-60% persistence of lactase, and is notably low in Asian and African populations of 0-5%.

Secondary lactose deficiency describes the process of the loss of brush border in the setting of other G.I. issues such as viral gastroenteritis, inflammatory bowel disease, celiac disease, surgery or other pathologic processes affecting the small intestine mucosa.

Lactase non-persistence occurs because of suppressive epigenetic changes at the 13010*C allele.

A minority adults retain persistently high levels of lactase in the small intestine into adulthood.

Some people have less lactase produced as they get older.

Processes that affect the stomach lining, small intestine infections, and bowel surgery can affect how much lactase is available to digest lactose.

Some people with lactose intolerance can tolerate eating small amounts of dairy.

Symptoms associated with lactose intolerance include:

Stomach cramps, pain, gas, diarrhea, and nausea, which occur 30 minutes to two hours after eating dairy products.

Symptoms range from mild to severe depending on how much lactose is in the food and how well the body digests it.

Lactase deficiency is common among African-Americans, Latinos, Asians, and American Indians.

Diagnosed by medical history and testing that may include a stool acidity test, a lactose tolerance blood test, and hydrogen breath test.

No clear consensus or criterion standard for diagnosis exists.

The hydrogen breath test, measures the level of hydrogen gas in a patient’s breath as a sign of bacterial breakdown of undigested lactose.

The lactose breath test is noninvasive assessment for lactose absorption using a 25 g lactose challenge as a measurement of breath hydrogen excretion over 3 to 4 hours.

The result is positive for lactose intolerance when breath hydrogen excretion exceeds 20 ppm resulting from intraluminal bacterial fermentation of unabsorbed and undigested lactose.

The hydrogen breath test measures a breakdown product produced by bacterial fermentation of undigested lactose that has reached the colon.

The lactose breath test is 78% sensitive and 98% specific for the diagnosis of lactose malabsorption.

A genotype determination is available for the 13910 position that leads to persistence of lactase and non persistence.
Stool pH  testing can reveal an osmotic diarrhea with a pH less than 6. relating to carbohydrate malabsorption.
Unabsorbed carbohydrates increase osmolality of the intestinal lumen pulling in fluid resulting in diarrhea and lowers pH reflecting hydrogen and short chain fatty acid release as carbohydrates are fermented in the colon.
Self reported intolerance to milk is not sensitive nor specific for diagnosis: correlation between intolerance and maldigestion is poor.
Lactase activity can be quantified at the intestinal brush border from duodenal biopsy specimens, which is conducted more in pediatric populations.

Small bowel bacterial overgrowth testing precedes lactose breath testing because of its ability to result in false positive lactose breath test results.False negative test results can occur in the presence of bacteria that produce methane, which can use excreted hydrogen to reduce carbon dioxide to methane.

A lactose tolerance test measures a patient’s blood sugar level after they are given a dose of lactose: a patient who does not properly absorb lactose will not have as much of an increase in glucose, one of the byproducts of lactose breakdown.

The lactose tolerance test involves the collection of serial blood samples following ingestion of a 50 gram load of lactose: a rise in blood glucose of 20 mg/deciliter or higher from baseline values results from adequate digestion the lactose and absorption of the individual monosaccharides.

A lower level indicates maldigestion as lactose fails to be split into glucose and galactose, thus glucose is not detected at a high-level in peripheral blood.

Inaccurate testing results may occur if small bowel bacterial overgrowth is present because small bowel bacteria break down the lactose disaccharide.


Avoiding dairy products.

Using lactase supplements.

Drinking lactose-free milk.

Majority of population has a gradual loss of lactase activity.

Lactase in the intestine required for normal digestion and absorption of lactose.

Exact prevalence is unknown.

Most patients with lactase deficiency do not have lactose intolerance.

Malabsorption of lactose does not necessarily produce gastrointestinal symptoms.

Symptoms related to the amount of lactose consumed,amount of lactase activity in the small bowel,gastric emptying times, colon mucosa absortion capabilities, colon bacterial metabolites and intestinal transit time.

Symptoms that arise from lactose malabsorption result from fluid shifts into the intestinal lumen driven by osmotic gradients and subsequent fermentation of lactose by bacteria that reside in the gastrointestinal tract.

Lactose is a disaccharide composed of galactose and glucose link via beta-1,4 glycosidic bond that cannot be absorbed via transported across the small bowel epithelium.

Lactase is an intestinal brush border enzyme present on intestinal villous tips, arising from the lactase-phlorizin hydrolase gene.

Lactase is capable of hydrolyzing the beta-glycosidic bond in lactose.

The activity of lactase is highest immediately before infant weaning, and expression declines to 5-10% in infant levels in adulthood.

By age 3, about 2/3 of the world’s population stops making high levels of lactase, leading to various degrees of poor lactose absorption.

Some populations, including those of Northern European descent continue to make enough lactase throughout their lives, while others, such as Asians and certain African populations, do not.

Secondary lactose intolerance can result from gastrointestinal conditions that damage the absorptive surface of the small bowel, including celiac disease, Crohn’s disease, small bowel bacterial overgrowth, intestinal infections, and radiation/chemo therapy enteritis.

Many patients can consume 12g of lactose (1 cup of milk or yogurt) without minimal or no symptoms.

If typical symptoms appear about an hour or two after eating dairy products the diagnosis can be confirmed that the symptoms are caused by dairy rather than other foods, lactose intolerance can be presumed.

The major treatment process for lactose intolerance is reducing lactose containing foods in the diet.

12-15 g of lactose, about the amount in 1-2 cups of milk, is generally tolerated by people with lactose intolerance.

Dairy products with the highest levels of lactose include milk and ice cream.

Yogurt and cheeses, especially hard cheeses contain much lower amounts of lactose.

Reducing the quantity of products containing lactose can reduce symptoms.

Lactose reduced products such as lactose-reduced milk are available.

Patients are advised not to completely eliminate dairy products as they are important sources of calcium and vitamin D.

Lactase replacement pills can reduce symptomatology by helping the gut breakdown lactose, allowing for better absorption.

The prolonged ingestion of lactose nor exclusion of lactose from the diet can influence the capacity of the small intestine to absorb lactose.

Individuals with IBS have increased symptoms at lower levels of lactose consumption.

Symptom development in individuals with lactose intolerance is related to intestinal transient time of lactose, the amount of ingested lactose, and interindividual variability in intestinal hypersensitivity.

Patients with lactose intolerance are usually able to consume 240 mL of fat containing milk with 12 g of lactose without an increase in symptoms.

Lactose avoidance Causes patients to avoid milk and dairy products which are common source of calcium and vitamin D, Potassium, magnesium, riboflavin, proteins, and other nutrients.
Lactose avoidance may predispose deficient bone acquisition in children and loss of bone mineral density in adults.
Yet, meta-analysis have shown conflicting results on the impact of dairy consumption on osteoporotic fracture risk.
Lactose and calcium absorption are not correlated, so lactose does not decrease the efficiency of calcium transport across the intestine and into the bloodstream.
Individuals with lactose intolerance have poorer quality of life scores compared with controls.
Because of lactase deficiency individuals restrict the intake of other food items especially legumes and fruit, which puts them at risk for additional nutrient deficiencies.

Avoiding lactose can have both physical and psychological impact.

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