Causes
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 increased production of gas, short-chain fatty acids and availability of osmotically active compounds due to malabsorption give rise to the symptoms of lactose intolerance. 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). The secondary form of intolerance is often reversible with mucosal healing.
Lactose intolerance must be distinguished from the less common cow’s milk protein allergy, which is an immune response to protein in milk (see Food allergies). Milk protein allergy has a symptom onset of between minutes to days, is frequently accompanied by skin reactions, and more often occurs in children. In milk protein allergy even small amounts of milk cause reactions, whereas most lactose intolerant individuals tolerant some lactose. Many individuals with lactose intolerance can tolerate up to 10g of lactose (i.e. one glass of milk) without significant symptoms.
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 Food tables). Sauces, soups and salad dressing often contain milk powder or products. Lactose is also 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 milk consumed with other products (such as chocolate, cereal) may greatly improve 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 supermarkets now offer lactose-free (i.e. reduced lactose content) dairy products, simplifying adequate intake of calcium, which is especially important in lactose intolerance.
Frequency in population and natural history
- 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, thyroid or sex hormone abnormalities and possibly, stress.
Symptoms
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 radiotherapy for cancer) and hormonal imbalances must be excluded using appropriate tests, such as endoscopy with biopsies, blood and stool samples (See Celiac’s diseaseCrohn’s disease). Once excluded, primary lactose intolerance can investigated using simple tests.
See Tests.
Management
A reduction in the dietary intake of lactose to individually tolerated levels will rapidly 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 lists). 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 generally contain little lactose, due to the manufacturing process.
Many supermarkets now offer lactose-free (i.e. reduced lactose content) dairy products, simplifying adequate intake of calcium, which is especially important in lactose intolerance.
Lactase, the missing enzyme, is available in tablet and liquid form and can be taken with meals. However, optimal quantity and timing of dosing is difficult to achieve and hence avoidance of lactose consumption remains the best form of symptom prevention.
It is important to maintain adequate calcium intake. The following are good dietary sources of calcium, calcium and vitamin D supplements are also useful: Broccoli, collard greens, salmon, sardines, oranges and calcium-fortified orange juice, almonds, soy milk and some types of bread.
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 expected according to the FODMAP theory of symptom generation.
- Treatment of proven small intestinal disease (inflammation, infection) may reverse a secondary lactose intolerance.
- Lactose intolerance may persist for a period following bacterial or viral gastroenteritis
- 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 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.
Links to literature
- Medical links:
http://www.gastro.org/patient-center/diet-medications/food-allergies-fructose-intolerance-and-lactose-intolerance#Lactose%20Intolerance
http://digestive.niddk.nih.gov/ddiseases/pubs/lactoseintolerance/
National Institutes of Health (USA) Consensus Statement on Lactose Intolerance and Treatment (click to download pdf).
- Comprehensive lay information, links and advice:
http://www.stevecarper.com/li/dairy_facts.htm
http://www.lactofree.co.uk/
- Medical review publications:
Matthews SB, Waud JP, Roberts AG, Campbell AK. Systemic lactose intolerance: a new perspective on an old problem: Postgrad Med J 2005;81:167–173 (click to download pdf).
Montalto M, Curigliano V, Santoro L, Vastola M, Cammarota G, Manna R, Gasbarrini A, Gasbarrini G. Management and treatment of lactose malabsorption. World J Gastroenterol 2006;12:187-191 (click to download pdf).
- Pediatric review publication:
Heyman MB. Lactose intolerance in infants, children and adolescents. Pediatrics 2006;118:1279-1286 (click to download pdf).