People with lactose intolerance may experience bloating, stomach pain, diarrhea, constipation, increased gas, and headaches.

Lactose intolerance is a condition where people cannot fully digest sugar (lactose) in milk and milk products.  Lactose is an organic sugar that is found in dairy products that break down lactose. A lactase enzyme deficiency often causes lactose intolerance since lactase breaks down lactose into two sugars, glucose, and galactose, for digestion. Interestingly studies show that almost all people are born with lactase and the capability to digest lactose. Lactose intolerance often occurs after childhood. Since children produce less lactase as they grow older or due to diseases in the gastrointestinal lining such as bacterial or viral infections, which destroy lactase. Usually, when the small intestines produce less lactase, lactose moves to the large intestines undigested, which combines with bacteria here to produce uncomfortable side effects related to lactose intolerance. It results in gastrointestinal symptoms like diarrhea, bloating, passing gas, and abdominal pain upon drinking or eating dairy products.

Signs And Symptoms of Lactose Intolerance

Bloating and Stomach Pain

This occurs in both adults and children. It usually occurs when undigested lactose in the body is fermented by the bacteria in the colon, resulting in excess gases, water, and acids, causing stomach pain in the lower tummy.  Studies have shown that in some people increased water and gas in the colon causes the gut wall to stretch, leading to a swollen tummy and bloating, which may cause nausea or vomiting.


Diarrhea is a disease in which faeces are excreted from the bowels frequently in a liquid form. When somebody has diarrhea, they pass 7 ounces (200grams) of stool within a period of 24hours. Lactose intolerance causes diarrhea when the undigested lactose ferments in the colon, increasing the volume of water and thus increasing the liquid content of the stool, especially in babies and children. According to one study, for diarrhea to occur, one must have 45grams of carbohydrates in their colon, similar to consuming 3-4 cups of milk, making a huge amount of lactose remain undigested before reaching the colon. Bacteria found in the colon ferment undigested lactose, producing short-chain fatty acids and gasses as the excess acids and lactose increase water in the digestive tract. Interestingly not all carbohydrates that trigger diarrhea contain lactose. Some of the carbs consumed often arrive in the colon undigested.

Increased Gas

Some people feel bloated and gassy upon consuming milk or milk products because their bodies never digest lactose.  Lactose ferments in the colon increase methane, hydrogen, and carbon dioxide gases. Bacteria in the colon called microflora ferment lactose exacerbates the acids and gases, causing increased lactose fermentation.  Gases produced vary from individuals depending on the reabsorption of gas in the colon and the effectiveness of the bacteria microflora.  Normally people who are lactose intolerant pass odourless gases due to protein breakdown inside the gut.


It is a condition that occurs when bowel movements are less frequent, and stools become hard to pass. Lactose fermentation produces methane gas, which slows down digestion, causing constipation, though it is rare compared to diarrhea. Studies have shown that the pasteurization process of milk removes essential enzymes that aid in its digestion, which worsens for people who are lactose intolerant. One study also found that modern farming methods in which cows are fed grain rather than grass potentially cause digestive problems and trigger constipation among lactose-intolerant individuals.  Even though constipation is reported as a symptom of lactose intolerance, it’s not commonly connected to it, especially if small amounts of dairy products are consumed. Additional causes of constipation compose eating foods low in fiber, not drinking enough water, certain medication, irritable bowel syndrome, hemorrhoids, and diabetes.

Other Symptoms

Studies have shown that gastrointestinal problems are the primary cause of lactose intolerant. Other symptoms are loss of concentration, headaches, fatigue, eczema, problems urinating, and mouth ulcers. People with milk allergies sometimes attribute these allergies to lactose intolerance, even though these symptoms have not been verified to cause lactose intolerance. Milk allergy and lactose intolerance may occur simultaneously, even though they are not related; thus, knowing the cause of the effect becomes difficult.  Milk allergy is common in children and is life-threatening and fatal, so it’s important to identify the symptoms early and not confuse it with lactose intolerance. Some milk allergy symptoms include asthma, vomiting, eczema, rash, diarrhea, and stomach pain. It’s important if you are lactose intolerant before eliminating dairy products from your meals. One study conducted using a hydrogen breath test that gave participants 50grams of lactose to test the intolerance found that 20% of the participants with abnormal lactose absorption tested negative and had elevated levels of hydrogen in their breath.  Not all people with abnormal lactose absorption have lactose intolerance, and even those who test positive might not have any symptoms. Therefore, the amount of lactose in our diet, availability of symptoms, and absorption rate can determine if a person is lactose intolerant.  By avoiding high-lactose foods such as cheese and milk, one can treat lactose intolerant conditions.  In addition, people with lactose intolerance can consume and tolerate small amounts of dairy products like fermented yogurt and cheese by spreading them throughout the day without much effect.

The Bottom Line

Lactose intolerance affects 70% of the world’s population. It includes gas, bloating, stomach pain, nausea, diarrhea, and vomiting as the major symptoms. Others have reported headaches, eczema, and fatigue as its effects. To accurately confirm if you have abnormal lactose absorption, you have to undertake a hydrogen breath test. Removing high sources of lactose like ice cream, milk, and cream from your diet can treat lactose intolerance, though if taken in small amounts, one might not experience any effect.  All the symptoms differ on each individual; hence it’s important to verify which dairy products trigger lactose intolerant symptoms before eliminating all dairy products from your diet.

Crystal Kadir

MS, Durham University

The work of a family doctor includes a wide range of clinical diversity, which requires extensive knowledge and erudition from a specialist. However, I believe that the most important thing for a family doctor is to be human because the cooperation and understanding between the doctor and the patient are crucial in ensuring successful health care. On my days off, I love being in nature. Since childhood, I have been passionate about playing chess and tennis. Whenever I have time off, I enjoy traveling around the world.

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