Lactose intolerance

Lactose (milk sugar) intolerance

Lactose intolerance


Incompletely absorbed (malabsorbed) dietary lactose is fermented by the flora in the lower bowel resulting in the formation of gas, principally H2, CO2 and methane, and chemical substances, such as short-chain fatty acids.

Malabsorption is due to decreased availability of the lactose-splitting enzyme, lactase, in the small intestine.

The threshold of lactose malabsorption varies individually and widely. In most populations of the world lactase production, and therefore the ability to digest dairy products, declines during childhood. However, in populations of Europeans, especially those of Northern European descent, a high percentage of individuals retain the ability to digest large amounts of lactose as adults. The genetically programmed decline in lactase with resulting symptoms is termed primary lactose intolerance. The symptoms of lactose intolerance are caused by increased production of gas and substances such as short-chain fatty acids and osmotically active compounds. Secondary lactose intolerance refers to decreased lactase activity due to inflammation (e.g. celiac’s disease, Crohn’s disease), infection or other reasons for loss of functioning small intestinal mucosa (e.g. abdominal radiotherapy, cancer chemotherapy). The secondary forms of intolerance are often reversible with mucosal healing.

Lactose is present in all mammals’ milk, except some sea mammals. Dairy products contain varying amounts of lactose, depending on processing methods. Besides the evident sources of lactose, the latter is often found in hidden form. Whey, for example, is commonly used in the food industry as a browning or binding agent, or to increase the smoothness and sweetness of food and beverages.

See lactose food tables

Sauces, soups and salad dressing often contain milk powder or products. Lactose is used as a filler substance in at least 20% of drugs. The lactose content of milk is not greatly affected by heat, fat content (skimmed vs. full fat), but consumption with other products (such as chocolate, cereal) may greatly improve milk tolerability. Yoghurt may not cause symptoms, as many of the bacterial strains in yoghurt produce lactase, aiding in the breakdown of lactose. Consequently, it is worth comparing the tolerability of different yoghurt brands. Many individuals with lactose intolerance can tolerate up to 10g of lactose (i.e. one glass of milk) without significant symptoms.

Intolerance of dairy products can have reasons other than lactose intolerance.

Cow’s milk protein allergy, which is an immune response to protein in milk (see Food allergies) is much more common in children than in adults. The onset of symptoms occurs between minutes to days, skin reactions are frequent and even small amounts of milk can cause reactions. Milk fat (or general fat) intolerance is further condition with bloating, nausea and stool changes, often secondary to bile acid malabsorption. There is some evidence that A1 beta-casein, a milk protein produced by many cattle of European origin, is associated with cows’ milk intolerance by releasing beta-casomorphin-7, which activates µ-opioid (“morphine”) receptors.

Frequency in population

  • White northern Europeans or Americans: 10-20%
  • South Americans or Europeans, Hispanics approximately 50%
  • Black Africans or Americans: 70-80%
  • Asians: 80-100%
  • Irritable Bowel Syndrome: approximately 50%

Commonly appears in adolescents or adults and may be triggered by gut infection or inflammation, after chemo- or radiotherapy, with thyroid or sex hormone abnormalities and possibly, stress.


Common symptoms are bloating, abdominal cramps and pain, diarrhea or constipation, increased intestinal sounds and gas production, reflux (e.g. acid taste in mouth, heartburn), nausea and vomiting. These symptoms resemble those of functional bowel disease and approximately 50% of patients with Irritable Bowel Syndrome are lactose intolerant.

Other symptoms outside of the gastrointestinal tract possibly associated with lactose intolerance are headache and light headedness, loss of concentration, poor short term memory, long term severe tiredness, muscle pain, joint pain, irregular heartbeat, and diverse complaints generally associated with allergies (skin rashes and itching, runny nose or sinusitis, asthma). The relevance of the latter is unclear and may have to do with the coincidence of intolerances and allergies in some individuals.

Testing and diagnosis

Distinguishing between primary (genetic) and secondary (small intestinal damage) forms of lactose intolerance is important, as treatments are completely different. Small intestinal damage (inflammation, e.g. celiac’s or Crohn’s disease, infections, e.g. giardia / lambliasis, damage following chemo- or radiotherapy for cancer) and hormonal imbalances must be excluded using appropriate tests, such as endoscopy with biopsies, blood and stool samples. 

See Celiac’s disease

Once excluded, primary lactose intolerance can be investigated using simple tests. Careful history taking and observation of food-related reactions may indicate the source of food intolerance.

Breath tests

Breath tests are the most useful and non-invasive tests for determining the various sugar (e.g. fructose, lactose, sucrose) and sugar alcohol (e.g. sorbitol, xylitol) intolerances. They are well validated, widely used, but some discussion still exists about the ideal test conditions. These tests should not be used in infants and reduced doses of test compound are used in children. The tests procedures are identical for all the intolerances.

Block error: "Call to a member function type() on null" in block type: "linkfield"

Contact us if you have problems accessing tests locally.

The following are important details regarding the test procedure.

  • Dietary restrictions, no smoking or excessive exercise from the day before testing and no antibiotics or colonoscopy in the last week pre-test.
  • Ingestion of a specific amount of the sugar or sugar alcohol to be tested for intolerance.
  • Regular breath samples taken for a standardized period.
  • Diary of symptoms during the next day for calculation of the symptom index.
  • Measurement of specific gases, e.g. hydrogen and methane, in the breath samples.
An intolerance is diagnosed based on the symptoms following the test and the gas concentrations in the breath samples.

The diagnosis is confirmed by a significant decrease in symptoms whilst on a diet low in the poorly tolerated sugar or sugar alcohol. Expert dietary advice is very helpful for the identification of food and drink containing the difficult-to-spot offending ingredient. Examples of such ingredients are so-called hidden lactose or fructans in vegetables or whole-meal products.

See Food tables

Direct enzyme quantification in tissue biopsies

Direct enzyme levels can be determined in biopsies of the small intestine, but lactase levels differ throughout the intestine, so tests can be inaccurate.

Genetic tests

The severest genetic form of intolerance (CC13910 / GG22018 i.e. homozygotes) can be very reliably diagnosed using a simple genetic test with cells taken, for example, from the inside of the cheek. The breath test is abnormal in all these individuals. Those with an intermediate genetic form of intolerance (e.g. CT13910 / GA22018 i.e. heterozygotes), which are the majority of individuals, have a wide range of intolerance severity to lactose, varying between asymptomatic and very severe symptoms. In summary, breath testing assesses the actual lactose digestion and is therefore the most useful form of testing. Genetic testing reliably identifies a proportion of severely affected individuals and is also useful in screening larger numbers of individuals or relatives of patients.

Other tests: stool, blood

Various stool tests, including pH tests, have been used, but they should no longer be advocated except for testing in infants. Measurement of blood glucose following ingestion of lactose (lactose tolerance test) is less specific than breath testing and is susceptible to more interference. Use of this test should be discouraged.


A reduction in the dietary intake of lactose to individually tolerated levels will within a few days lead to symptom relief in most individuals. Although many dairy products are easily recognized, lactose in different forms is added to a wide range of foods, including baked, processed and ready-made products, beverages, medicines and health supplements

See Food tables

Food labels must be read carefully. The following terms indicate the presence of lactose: buttermilk, casein, condensed milk, cream, curds, lactalbumin, malt, milk solids, non-fat dry milk powder, non-fat milk solids and all forms of whey. Yoghurt may be tolerated, as many of the bacterial strains in yoghurt produce lactase. Consequently, it is worth comparing the tolerability of different yoghurt brands. Hard cheeses and butter generally contain little lactose, due to the manufacturing process.

Many supermarkets offer lactose-free (i.e. reduced lactose content) dairy products, simplifying adequate intake of calcium. The following are good dietary sources of calcium: Broccoli, collard greens, salmon, sardines, oranges and calcium-fortified orange juice, almonds, soy milk and some types of bread.

Lactase, the missing enzyme, is available in tablet and liquid form and can be taken with meals. Reduction of lactose consumption remains the best form of symptom prevention. However, complete strict avoidance of lactose may increase sensitivity to lactose long-term.

See Food tables for a listing of lactose and calcium food content.

Information nuggets

Food allergies may co-exist with sugar intolerances. Generally tell-tale signs of possible additional allergies are skin rashes and itching, sinusitis or asthma and hay fever. 

Lactose and fructose intolerance co-exists in approximately 20-30% of individuals. This is postulated by the FODMAP theory.

Lactose intolerance may persist for a period following bacterial or viral gastroenteritis (post-enteritic IBS).

More frequent, smaller portions of dairy products, eaten with or after other food, are less likely to lead to symptoms than larger portions ingested on an empty stomach.

Find out which shops offer lactose-reduced or lactose-free products. Coconut, soy and rice milk are naturally lactose free.

Check all medications and health supplements for hidden lactose (ask pharmacy or manufacturer). You may be able to tolerate some of these products due to the very low amounts present.

Evidence on the usefulness of probiotics (suspensions of beneficial bacteria) is mixed. Some individuals with lactose intolerance or irritable bowel syndrome derive benefit from probiotics, such as Lactobacillus or Bifidobacterium bacteria.

Individuals adhering to a low milk diet because of lactose intolerance appear to have a lower risk for several cancers and also require different doses of thyroid replacement hormone in case of thyroid disease.

Further reading

Comprehensive lay information, links and advice

Wilder-Smith CH et al.

Fructose and lactose intolerance and malabsorption testing: the relationship with symptoms in functional gastrointestinal disorders.

Aliment Pharmacol Ther 2013

PDF Download