Lactose Intolerance and/or Irritable Bowel Syndrome (IBS)?

Recently, the following paraphrased exchange between two participants was posted on another public IBS forum: “Does lactose intolerance cause IBS?” with the response, “Yes, it did for me.”

As useful as “been there, done that” experience from peers with IBS can be in certain contexts, this is a clear example of basic scientific misunderstanding by both individuals involved, and points out the risks of relying too heavily on random, unsourced information found online.

Lactose intolerance and irritable bowel syndrome can both cause abdominal pain or discomfort, diarrhea, bloating and/or excessive gas that may have a strong odor. The severity and symptom pattern may vary among different individuals and may appear to fluctuate, possibly requiring some trial and error with eating habits. Some people who actually have IBS may mistake it for lactose intolerance. Others who actually have lactose intolerance may mistake it for IBS. Both conditions are very common. A publication for health providers from the U.S. National Institute of Child Health and Human Development (NICHD) estimates the prevalence of lactose intolerance in the U.S. alone as 30-50 million adults, which is similar to widely quoted ranges for the number of American adults and children with IBS. Thus, logically, many people with IBS also have or may develop lactose intolerance later, but that’s where the similarity ends. The two conditions involve different parts of the digestive system and have different causes and interventions that help them.

Lactose intolerance is the reduced ability or, much more rarely, the complete inability, to digest lactose, the type of sugar found naturally in any animal or human milk, as well as dairy products and foods with dairy ingredients. Lactose is also sometimes added as a filler to certain medications and other products. The intolerance results from absent or reduced levels of lactase, the enzyme that breaks down lactose. Lactase is produced or not in the small intestine, while IBS affects the function of the colon, also known as the large intestine.

While complex genetic influences in IBS are still being researched, scientists have established that for the vast majority of people with lactose intolerance, it is an autosomal recessive trait, with one copy of the gene in question inherited from each biological parent. In a small subset of this group, often found in Finland, children are born with the inability to produce lactase enzyme at all. This is a serious risk for malnutrition if not diagnosed relatively quickly, as they cannot digest either human breast milk or formulas containing lactose. More commonly, a person gradually loses the ability to process lactose over time, as lactase enzyme production decreases beginning in early childhood and adolescence, though many people will not notice actual symptoms until well into adulthood or even the senior years. This type can be inconvenient but is more easily accommodated than congenital intolerance in babies. While the corresponding gene for lifelong lactose tolerance is dominant, meaning that if  a person inherits one gene for tolerance, and one for intolerance, he or she will be able to tolerate lactose, this gradual (“primary”) lactose intolerance is actually very common in most ethnic groups, including people with genetic origins in the Mediterranean, Asia, Africa, and Ashkenazi Jewish, Hispanic and Native American communities. The prevalence is lowest in those of northern and western European ancestry, with percentages in the single digits, while in most of the other mentioned groups, statistically, a majority of adults will eventually experience some degree of lactose intolerance as part of the natural aging process that can’t be reversed. The overall global prevalence is estimated to be about two-thirds of adults worldwide

In contrast, some people develop what is called secondary lactose intolerance, which may be the result of delayed lactase enzyme production in premature infants, celiac disease, Crohn’s disease affecting the small intestine, chemotherapy, radiation, or other gastrointestinal illnesses or medical procedures that damage the small intestine. If the underlying problem can be dealt with and improved, sometimes this type of lactose intolerance is temporary, but again, these causes have no relationship to the onset of IBS.

Lactose intolerance is rarely an all-or-nothing intolerance. Many people with lactose intolerance can comfortably eat or drink small amounts of dairy products, larger amounts spread out over a longer period of time, or types of dairy products that have relatively lower levels of lactose such as cheese or yogurt.The ability of the digestive system to handle lactose at any given time also depends on what else is being eaten at the same time, which may affect the speed at which lactose or lactase enzyme supplements move through the small intestine and may account for sometimes variable symptoms in the same person.

Unlike with IBS, there are blood, breath or stool tests that physicians can do for lactose intolerance. According to Genetics Home Reference from the National Library of Medicine, also a unit of the National Institutes of Health, genetic tests are also available in several countries in Europe. However, many doctors don’t find any formal tests necessary if a patient’s history or a food diary clearly indicates that avoiding lactose or using lactase enzyme supplements or special reduced or lactose-free food products help consistently. If lactose intolerance is the only gastrointestinal issue, these relatively simple interventions should greatly reduce or eliminate symptoms, but they won’t have any effect on someone who only has IBS. While many people with IBS do report dairy products as a trigger for symptoms, perhaps because of the relatively high fat content, avoiding dairy usually won’t resolve symptoms as readily and consistently as it does with lactose intolerance. For those who have both lactose intolerance and IBS, separating out the influence of each may be more complicated, but it’s undisputed scientific fact that one condition does not cause the other.

For more detailed sources on lactose intolerance, please see the additional reputable links below.

“Lactose Intolerance”, a brief page from IFFGD’s About Kids GI website.

Lactose Intolerance,” a brief page from the U.S. National Digestive Diseases Information Clearinghouse

“Lactose Intolerance” from, a detailed discussion in several online sections.

IFFGD Fact Sheet #122 “Lactose Intolerance: Definition, Symptoms and Treatment”, a free, somewhat more technical article, in downloadable PDF format by Eli D. Ehrenpreis, MD of the University of Chicago and Benjamin Z. Ehrenpreis of Bradley University in Peoria, Illinois.

IFFGD Publication #218, “Why Does Milk Bother Me?” a free article on management of lactose intolerance in downloadable PDF format, adapted from the National Digestive Diseases Information Clearinghouse.

IBS Impact and this blog focus on accurate awareness and advocacy so that people with IBS can make informed decisions. For specific treatment advice  or concerns,  please consult your own health care providers.

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