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Lactose malabsorption and intolerance


Lactose is the main source of sugar from milk and milk products from all mammals except the sea lion. Inadequate lactase activity allows lactose to reach the large intestine. There, the gut flora provides a salvage pathway for lactose digestion by cleaving lactose into short-chain fatty acids and gas, mainly hydrogen (H2), carbon dioxide (CO2), and methane (CH4). Non-digested lactose can cause osmotic diarrhoea; products of its bacterial digestion can lead to secretory diarrhoea and gas distending the intestines, events that are likely to lead to clinical symptoms. 

Lactose malabsorption actually refers to inefficient digestion of lactose due to reduced expression or impaired activity of the enzyme lactase. After ingestion, lactose passes into the small intestine where it comes into contact with lactase at the intestinal brush border where it is hydrolysed into the monosaccharides glucose and galactose, which can be readily absorbed.

Disaccharide digestion occurs on the brush border of the intestinal lining. The lactase enzyme is bound to the plasma membrane and catalyzes the cleavage of lactose into glucose and galactose. Lactase breaks down milk sugar into glucose and galactose so that these can be absorbed. The physical proximity of the lactose allows for the absorption of digested nutrients. Newborn babies have the highest concentration of lactase in their brush border; with aging, it decreases to a variable degree in different populations resulting in primary lactose malabsorption. In people with deficiency of brush border lactase, undigested lactose passes to the colon where it is fermented by the bacteria to produce gases such as hydrogen, methane and hydrogen sulphide causing osmotic diarrhea, bloating, flatulence, and pain. When lactose malabsorption is associated with symptoms, most of which mimic Irritable Bowel Syndrome (IBS), it is called lactose intolerance. 

Treatment of lactose intolerance should not be aimed at reducing malabsorption but rather at improving digestive symptoms. Reduction of lactose intake rather than exclusion is recommended because, according to the results of most of the studies the majority of patients with self-reported lactose intolerance can ingest at least 12 g lactose (equivalent to 250 ml milk) without experiencing symptoms and taken with other foods, up to 18 g lactose can often be tolerated. Lactase enzyme replacement is another option although this changes the taste of the food when mixed with the dairy products because glucose and galactose produced by lactose digestion are sweeter than the original sugar. Another strategy involves probiotics that alter the intestinal flora and may have beneficial effects in IBS patients that persist even after treatment.

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