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How To Test for Lactose Intolerance

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How To Test for Lactose Intolerance

Lactose is the primary carbohydrate in milk and dairy products. The digestion of lactose into its building blocks, glucose and galactose, depends on the digestive enzyme lactase. Small intestinal cells produce lactase in a healthy digestive tract. Lactose digestion occurs at the surface of the small intestinal cells, called the "brush border." After weaning from breast milk/formula, humans naturally decrease lactase expression, causing many to become lactose intolerant.

Lactose intolerance, the presence of digestive symptoms due to the poor digestion of lactose, is estimated to affect 68% of the world's population and more commonly affects people of African and Asian descent. Digestive symptoms like gas, bloating, and diarrhea accompanying lactose intolerance can be uncomfortable and interfere with daily activities.


What is Lactose Intolerance?

Lactose intolerance is the inability to digest lactose due to a deficiency in the lactase digestive enzyme. Lactose intolerance is characterized by digestive symptoms resulting from consuming lactose-containing food products.

It is important to recognize that lactose intolerance is different from milk allergy. Milk allergy is an immunological response to at least one protein in milk products and causes a potentially life-threatening allergic reaction. While the symptoms of lactose intolerance can be very uncomfortable, they are not life-threatening.

Lactose Intolerance Symptoms

Digestive symptoms after lactose ingestion may include abdominal pain and cramping, nausea, gas, bloating, and diarrhea. The effects of lactose are dose-dependent, and many with lactose intolerance can tolerate small amounts of the milk sugar. If symptoms occur, they generally appear 30 minutes to 2 hours after lactose consumption. (1)

What Causes Lactose Intolerance?

Lactose intolerance is caused by a deficiency of lactase, the small intestinal brush border enzyme that breaks down lactose. There are three main types of lactose intolerance. The reason for enzyme deficiency differentiates between the three subtypes.

Congenital lactose intolerance is an extremely rare genetic condition that presents in newborns with first milk intake (5).

Primary lactose intolerance is the most common cause of lactose malabsorption in adults worldwide. It is characterized by the loss of lactase activity due to the inactivation of lactase genes with aging. Gene inactivation often begins after age two but is usually only present clinically later in life. Some sources estimate that one-half of American adults suffer from some degree of lactose intolerance due to aging's effects on the small intestine. (4, 5)

Secondary (Acquired) lactose intolerance can develop after acute mucosal damage of the small intestine and impairs the cells' lactase secretion. Secondary lactose intolerance is typically reversible after the assault on the small intestinal cells is removed, and the small intestine can heal (5). Some of the more common causes of secondary lactose intolerance include (4):

  • Acute gastrointestinal infection. Rotavirus and Giardia are two pathogenic organisms commonly implicated in lactose intolerance.
  • Small intestinal bacterial overgrowth (SIBO)
  • Celiac disease
  • Crohn's disease

Functional Medicine Labs to Test for Lactose Intolerance

A clinical diagnosis of lactose intolerance can be made based on the presence of digestive symptoms after ingesting lactose-containing foods that resolve with a dietary elimination of lactose products and return with reintroduction. (3)

There are several testing options to confirm a diagnosis of lactose intolerance. The most accurate and commonly utilized test in the clinical setting is the lactose hydrogen breath test. After an oral dose of lactose is administered to the patient, hydrogen expired in the breath is measured every 30 minutes for 4 hours. The test is positive if the breath hydrogen concentration exceeds 20 ppm over baseline within 3 hours. Factors that may cause inaccurate test results include rapid gastrointestinal transit, SIBO or other abnormal intestinal microbiota growth patterns, recent use of antibiotics, and smoking. (2)

The lactose tolerance test consists of administering an oral dose of lactose and obtaining multiple blood samples at timed intervals to measure blood glucose levels. The test is positive if digestive symptoms occur during the test and blood glucose levels increase less than 20 mg/dL above the fasting level. False-positive and false-negative test results occur in 20% of patients. (2)

A stool acidity test can be ordered for young children and infants. Bacteria in the large intestine ferment unabsorbed lactose into lactic acid, making the stool more acidic. Measuring stool pH can confirm stool acidity and help diagnose lactose intolerance. (1, 3)

An intestinal biopsy to measure small intestinal lactase concentrations is considered the gold standard for lactose intolerance but is rarely used due to the invasive and expensive nature of the procedure.

Your doctor may also recommend additional tests to rule out the causes of secondary lactose intolerance. These may include:

  • SIBO breath test
  • Comprehensive stool test to screen for intestinal infections
  • Definitive diagnosis of Celiac disease and Crohn's disease must be made by intestinal biopsy, but a CICA Assay, C-reactive protein (CRP), and fecal calprotectin can be ordered to aid in the diagnostic process.

Functional Medicine Treatment for Lactose Intolerance


A low-lactose diet is the therapeutic diet recommended for people with lactose intolerance to resolve digestive symptoms. The amount of lactose that can be safely consumed widely varies between individuals, but most people with lactose intolerance do not require a completely lactose-free diet. Research finds that most people can tolerate 12-15 grams of lactose daily. Working with a functional nutritionist can be extremely helpful when adapting to new dietary restrictions.

Milk and cream contain high amounts of lactose; however, many lactose- and dairy-free options are widely available. Most cheeses have less lactose than milk, especially hard, aged cheeses  (e.g., Swiss, parmesan, and cheddar), cottage cheese, and feta cheese. Yogurt has intermediate levels of lactose, but probiotic yogurts with live bacterial cultures are typically well tolerated by people with lactose intolerance because the probiotics help with lactose digestion. (6)

Lactose is often added to prepared and packaged foods, so those avoiding lactose should read food labels carefully. Common sources of hidden lactose include (2):

  • Bread and other baked goods
  • Breakfast cereals
  • Pancake, biscuit, and cookie mixes
  • Instant potatoes, soups, and breakfast drinks
  • Margarine
  • Non-kosher lunch meats
  • Salad dressings
  • Candy

Address the Gut

Secondary lactose intolerance is generally reversible when the underlying cause is treated. Lactose tolerance typically returns 3-4 weeks after the small intestinal lining is healed. (4)

Herbal antimicrobials like berberine and garlic show strong antimicrobial effects against major gastrointestinal pathogens and bacterial overgrowth. Some clinical studies have shown that berberine is more effective than conventional therapies.

Patients with Celiac disease must follow a strict gluten-free diet to achieve disease remission.

Various dietary modifications and stress management techniques can manage Crohn's disease successfully.


Anti-inflammatory herbs and supplements are commonly recommended by functional doctors to speed along the gut-healing process and restore a healthy small intestinal brush border. These may include, but are limited to, L-glutamine, zinc carnosine, licorice, aloe vera, and marshmallow root.

Supplemental lactase digestive enzymes can be taken before lactose-containing meals to enhance lactose digestion and decrease related digestive symptoms.

A 2020 systematic review found that oral probiotics ranging in doses from 10 million to 10 billion CFU improve hydrogen breath tests and clinical symptoms in lactose intolerance patients. Probiotics containing L. acidophilus, L. reuteri, L. rhamnosus, L. bulgaricus, S. thermophilus, and B. longum are most commonly implemented in lactose intolerance treatment protocols. It is proposed that probiotics increase lactose tolerance by acting as a source of lactase, competing with opportunistic microbes and secreting antibiotic-like substances to promote a healthy intestinal microbiome, and modulating intestinal permeability.

Calcium and vitamin D status should be assessed in patients who avoid milk products, and supplementation should be recommended when necessary (4). Non-dairy dietary sources of calcium include tofu, bone-in sardines, broccoli, kale, and cabbage. Few foods naturally contain vitamin D. Fatty fish (e.g., trout, salmon, tuna, mackerel) and fish liver oils are the best dietary sources. Beef liver, egg yolks, and mushrooms contain variable amounts of vitamin D.


Lactose intolerance is a digestive condition characterized by poor digestion and malabsorption of the milk sugar lactose due to a deficiency in the enzyme that digests it. Lactase deficiency is common as we age and results in digestive symptoms like abdominal pain, bloating, and diarrhea after consuming lactose-containing foods. Most lactose intolerance is treated by avoiding lactose-containing foods, supplemental lactase enzymes, and probiotics. However, in some cases, lactose intolerance may be secondary to another condition causing damage to the cells lining the small intestine. For these patients, functional testing can help to diagnose the underlying condition so that natural therapies can be implemented to restore lactose tolerance.

The information provided is not intended to be a substitute for professional medical advice. Always consult with your doctor or other qualified healthcare provider before taking any dietary supplement or making any changes to your diet or exercise routine.
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Lab Tests in This Article

1. Lactose Intolerance. Johns Hopkins Medicine.

2. Swagerty, D. L., Walling, A. D., & Klein, R. M. (2002). Lactose Intolerance. American Family Physician, 65(9), 1845–1851.

3. Malik, T.F., & Panuganti, K.K. (2022). Lactose Intolerance. StatPearls Publishing.

4. Lactose Intolerance. (2021, October 25). American College of Gastroenterology.

5. Roy, P. K. (2019, December 4). Lactose Intolerance. Medscape.

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