Most people are born with the ability to digest lactose, which is the main carbohydrate in milk. However, a large part of the population loses this ability at some point in their lives. It is estimated that 70% of the world population suffers from lactose intolerance, which is one of the main gastrointestinal problems today.

What is lactose?

Lactose is a disaccharide sugar found in mammalian milk, which makes up about 2 to 8% of milk, but this amount can vary among species and individuals.

Lactose is a large sugar molecule made up of two smaller sugar molecules, glucose and galactose. Lactose is first broken down into glucose (used as an energy source) and later into galactose (it becomes a component of glycolipids and glycoproteins) by the lactase enzyme, and then absorbed by intestinal enterocytes into bloodstream. The digestion and absorption of lactose takes place in the small intestine.

What is the role of the lactase enzyme?

The lactase enzyme is encoded by the LCT gene (2q21), and is responsible for the hydrolysis (breaking down), or we can say even for the digestion, of lactose into glucose and galactose. The expression control of the LCT gene is regulated by the MCM6 gene. Some individuals have variants in this gene that maintain the production of intestinal lactase, allowing these individuals to digest milk properly throughout their lives.

If the lactase enzyme is absent (alactasia) or deficient (hypolactasia), unabsorbed lactose molecules draw fluid into the intestinal lumen, leading to an increase in volume and fluidity of the intestinal contents. 

In addition, unabsorbed lactose passes into the colon, where it is fermented by bacteria that produce short-chain fatty acids and gases, possibly causing various gastrointestinal symptoms.

Did you know that lactase starts to be produced during pregnancy?

Lactase activity can be detected on the surface of the intestinal mucosa from the eighth week of gestation, peaking at the 34th week. However, from the first month of life, lactase activity starts to decline (lactase is not persistent), and its decline is highly variable from weaning to undetectable levels as a normal consequence of down-regulation of lactase activity.

In humans, about 30% of the population has continuous lactase activity after weaning in adulthood, thus enabling the consumption of large amounts of lactose.

What is lactose intolerance?

It is a gastrointestinal condition characterized by the individual’s inability to digest lactose, the sugar present in milk and dairy products, usually caused by a deficiency of the enzyme lactase.

Hypolactasia or lactase deficiency, is the “failure” to express the lactase enzyme in the small intestine; which results in a condition that determines lactose malabsorption, that usually occurs after 6 to 7 years of age, and also occurs much later, showing a steady increase in the prevalence of this condition, even in the age groups above 65 years old. 

The process of reduction and the amount of residual lactase have considerable variability between different ethnic groups and even among individuals. However, 50% of the lactase enzyme is sufficient to ensure effective lactose digestion.

Understand the types of Lactose Intolerance:

• Congenital Intolerance: it is an autosomal recessive condition in which the child has genetic variants in both copies of the LCT gene, being characterized by severe diarrhea from the child’s first milk intake, which prevents breastfeeding (only for lactose-free formulas); it persists lifelong and requires the complete exclusion of lactose sources. This form of lactose intolerance is extremely rare. Nutrient deficiency causes growth retardation, with rapid onset of dehydration and alkalosis.
• Primary Intolerance: (non-persistence of lactase) is a condition resulting from the physiological decline of lactase activity in intestinal cells. It affects a large number of individuals. It is a known cause of abdominal discomfort: such as diarrhea, abdominal bloating and flatulence. Its incidence varies widely among ethnicities.

Lactase non-persistence is a normal condition represented by the loss of lactase expression and is associated with a specific variant (SNV) in the LCT gene, in which the nitrogenous base cytosine is replaced by a thymine (C>T). The CC genotype is associated with hypolactasia (residual lactase is approximately 10% compared to levels at birth), whereas the TT genotype is associated with persistence of lactase activity. The presence of a CT genotype predisposes to the presence of intermediate lactase expression levels.

• Secondary Intolerance: It occurs when damage to the intestine mucosal lining causes a temporary deficiency of lactase. Typically, all small bowel diseases (such as celiac or Crohn’s disease, ulcerative colitis, irritable bowel syndrome, radiation, and immune deficiencies) are capable of causing a secondary lactase deficiency, but more commonly bacterial, viral, or parasitic infections (as a giardiasis, it induces a transient loss of enzyme in the mucosa affected by the inflammatory or infectious process. It is a reversible condition that requires a lactose-free diet until normal intestine mucosal lining is restored.

Lactose malabsorption refers to any cause of failure to digest and/or absorb lactose in the small intestine and is a necessary precondition for lactose intolerance. However, the two should not be confused, as many individuals with malabsorption have no symptoms after ingesting dairy products.

Symptoms of Lactose Intolerance:

Symptoms, as well as their severity, can vary among individuals, depending on the amount of lactose ingested and the individual’s ability to digest it. The onset of these symptoms is strongly related to the appearance of hydrogen gas during breath tests. The most common intestinal symptoms include:

• Abdominal pain
• Abdominal bloating 
• Flatulence 
• Diarrhea 
• Borborygmus (bowel sound) 
• Nausea
• Vomiting

Some studies have reported that the clinical presentation of lactose intolerance is not restricted to intestinal symptoms. Neurological symptoms or cardiac arrhythmia may occur in about 20% of individuals.

Neurological symptoms:

• Headache
• Vertigo
• Memory loss
• Lethargy

One of the possible factors responsible for these systemic symptoms may be the production of toxic metabolites, generated by the fermentation of lactose in colonic bacteria, which can alter the cell signaling mechanism.

When there is the presence of systemic complaints, it is important to exclude cow’s milk protein allergy, which occurs in up to 20% of patients with symptoms of lactose intolerance. 

Difference between allergy and Lactose Intolerance

Gastrointestinal symptoms of intolerance can be similar to cow’s milk allergy and can be misjudged as “milk allergy” symptoms, leading to misdiagnosis and poor dietary management. The treatment of lactose intolerance involves the reduction, but not the complete elimination, of foods containing lactose. On the other hand, individuals with suspected milk protein allergy should undergo a restricted diet test with elimination of cow’s milk protein-based foods.

There are important differences between lactose intolerance and milk allergy, so a better understanding of these differences could limit misunderstandings in the diagnostic approach and management of these conditions.

• Learn more about the difference between allergy and intolerance in the text: Food Intolerance and Allergy exam: Understand the difference between them by clicking here.
• And also know everything about the A200 test, which examines food intolerance mediated by IgG against more than 200 foods.

Diagnosis of Lactose Intolerance

Some tests can be performed to diagnose lactose intolerance, the most common being the following:

• Hydrogen breath test: is the method of choice for assessing lactose malabsorption and intolerance. It measures abnormal excretion of hydrogen in exhaled air after the ingestion of a standard dose of lactose (diluted in water). As hydrogen is not produced by mammalian enzymes, its presence indicates contact of the sugar with bacteria, indicating malabsorption.
• Lactose Tolerance Test: It measures blood sugar increase at different time points (for example: 30, 60, 120 min) after ingestion of 50g of lactose.
• Gaxilose Test: The gaxilose test involves the administration of the lactase substrate gaxilose (4-galactosylxylose) with measurement of D-xylose in urine or blood. Gaxilose measurements is ideal for assessing intestinal lactase as enzyme activity is assessed over the entire small intestine.
• Genetic tests: evaluate the presence of variants in genes associated with intolerance. However, it is not suitable for secondary lactase deficiency.

The main limitation of genetic, enzymatic, and gaxilose testing is that malabsorption is common in healthy individuals, and a positive test does not confirm whether symptoms are caused by this condition. Breath testing is generally the method of choice, as reasonably reliable information is obtained about the patient’s digestive function and symptoms.

What exams does SYNLAB offer for lactose intolerance?

Synlab offers the following exams: 

Lactose Test, which consists of the genetic analysis of the patient’s DNA to determine the presence of the main variants (-13910C/T and -22018G/A) in the MCM6 gene, associated with the development of lactose intolerance in adulthood. 

The exam is performed using the Sanger sequencing technique, which consists of determining the exact sequence in which the nucleotides are found in the MCM6 gene for the studied variants. 

The Lactose Test is indicated in:

• The evaluation of hypolactasia in adults with clinical symptoms of lactose intolerance, such as: abdominal pain and distension, bloating, gas and diarrhea;
• Clinical suspicion of lactose intolerance.

INTOLERANCE2, an exam that allows you to know in a single analysis if there is a genetic predisposition to celiac disease and/or primary lactose intolerance. As an alternative to breath testing, the analysis is performed from a blood sample, avoiding direct patient exposure to substances.

In addition to the hydrogen breath test for the detection of lactose intolerance, through gas chromatography analysis.

Treatment of Lactose Intolerance

The treatment of lactose intolerance aims to improve patient’s symptoms and avoid the risk of long-term malnutrition. A low lactose diet is usually recommended. 

However, if lactose intolerance is present or diagnostic investigations are inconclusive, a detailed diet along with a history of symptoms should be undertaken and, if appropriate, foods containing lactose may be excluded from the diet until symptoms improve, usually for at least four weeks. 

Nutritional advice should be sought to ensure the nutritional adequacy of the diet. 

About the SYNLAB Group

The SYNLAB Group is a leader in providing medical diagnostic services in Europe, providing a full range of clinical laboratory analysis services to patients, healthcare professionals, clinics and the pharmaceutical industry. Resulting from the Labco and SYNLAB merger, the new SYNLAB Group is the undisputed European leader in medical laboratory services.