Baby mammals drink milk
**Lactose intolerance** is when a person cannot digest milk or milk products (such as cheese or yogurt). All mammals start off on mother's milk, and almost all change to a non-milk diet, a process we call *weaning*.

Lactose is a disaccharide sugar (double sugar molecule). To be digested it must be split (digested) into two simple sugars (monosaccharides), glucose and galactose. 

An enzyme called lactase is needed to break it down. People with lactose intolerance do not have this enzyme or make only small quantities. Because of this they are unable to digest lactose.

Up to 75% of people worldwide begin to lose the ability to digest lactose as they grow into adults. This loss of ability ranges from 5% in northern Europe, to over 71% in Sicily and to over 90% in parts of Africa and Asia. There is no cure for lactose intolerance. Those people need to change their diet to eat and drink substances with very little lactose. There are many milk substitutes. 

Milk is not a fermented product. If is not digested, then it may ferment in the small intestine which can cause a problem called pseudoallergy. Amino acids are changed to other substances, which may act in much the same way as histamine in a true allergy.

Lactose intolerance is not the same as milk allergy, which is an immune reaction to some of the proteins in milk.

Signs and symptoms
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Lactose intolerance can cause symptoms such as farting, diarrhea, bloating, stomach rumble and stomachache. The undigested lactose travels through the digestive system, causing various symptoms. 

One of the most common symptoms of lactose intolerance is bloating and gas. When lactose is not properly broken down, it ferments in the large intestine, causing gas to build up, leading to abdominal bloating, discomfort, and pain.

Diarrhea is another common symptom of lactose intolerance. The unabsorbed lactose draws water into the colon, leading to loose, watery stools. While diarrhea is a common symptom of lactose intolerance, some individuals may experience constipation instead. This occurs when the undigested lactose slows down the movement of stool through the digestive tract. Nausea and vomiting may occur in individuals who are lactose intolerant. When lactose remains undigested, it can cause irritation to the lining of the stomach, leading to various symptoms of lactose intolerance. Abdominal pain and cramps are also the symptoms of lactose intolerance. These may range from mild discomfort to severe pain. In some cases, lactose intolerance can lead to headaches. The reason for this is not clear, but it may be due to the inflammation caused by undigested lactose in the gut. Fatigue is another symptom that may occur in individuals with lactose intolerance. This is due to the inflammation caused by undigested lactose, which can lead to a general feeling of tiredness. In rare cases, lactose intolerance may cause skin problems such as eczema and acne. This is thought to be due to the inflammation caused by undigested lactose in the body. Anemia is a medical condition characterized by an insufficient number of red blood cells in the body. In some cases, lactose intolerance may lead to anemia due to the malabsorption of nutrients caused by undigested lactose. Weight loss is a rare symptom of lactose intolerance. This occurs when the individual avoids dairy products altogether, leading to a decrease in calorie intake.

The signs and symptoms of lactose intolerance may vary from person to person. However, if you experience any of the symptoms mentioned above after consuming dairy products, you may have lactose intolerance. It is essential to consult a healthcare professional to confirm the diagnosis and receive appropriate treatment.

Lactase evolution in humans
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Normally infant mammals drink their mother's milk. Then the gene which causes the body to make lactase is switched off, and the young move on to adult food. They can no longer digest milk.

Humans are somewhat different from other mammals. Some humans continue to produce lactase, and some do not. It is a type of genetic polymorphism.

The ability to digest lactose into adulthood ('lactase persistence') was useful to humans after the invention of animal husbandry. People kept animals which could provide milk.

Hunter-gatherer people before the Neolithic revolution were mostly lactose intolerant. So are modern hunter-gatherers. Many modern populations are lactose-intolerant, but European populations are tolerant. That may be because milk has long been in the European diet, and so people who can take milk products have a wider potential diet. Hence in hard times they will survive better.

### Genetics

A mutation on chromosome 2 stops the shutdown in lactase production. This makes it possible for those with the mutation to continue drinking fresh milk (and eating other dairy products) throughout their lives.

This appears to be a recent adaptation to dairy products. It occurred in both northern Europe and east Africa in people with a historically pastoral lifestyle. Lactase persistence, allowing lactose digestion to continue into adulthood, is a dominant allele, making lactose intolerance a recessive trait. 

Genetic studies suggest that the oldest mutations associated with lactase persistence have only become common in human populations in the last 10,000 years. Because of this, lactase persistence is often given as an example of recent human evolution. As lactase persistence is genetic, but animal husbandry a cultural trait, this is gene–culture coevolution.

Related pages
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* Lactase
* Lactic acid bacteria