Lactose Intolerance

Lactose is a disaccharide, which means it is made up of two monosaccharides, or simple sugars. Lactose is the primary carbohydrate, a compound sugar that is found in the milk produced by mammals. Lactose is a vital source of energy for infant and young offspring for most species but has a different level in different types of milk. Cow's milk, for example, contains 4-5% lactose whereas human milk contains almost twice that. But in order for these compound sugars to be converted into energy they need to be broken into their simple forms, glucose and galactose.

This is where the enzyme lactase comes in.

Breakdown of Lactose by Lactase

As the image shows, lactase combined with water breaks lactose into the two simple sugars. Lactase is produced in the brush border membrane of the small intestine.

Small Intestine Brush Border

The term "lactose intolerance" is technically not the correct term for the condition related to the inability to digest lactose. What happens is that the body produces reduced amounts of lactase as a normal part of aging. Lactase activity is high during infancy, but in most mammals, including most humans, the amount of lactase produced declines after the weaning phase. The result is more correctly called "lactose malabsorption" or "lactase deficiency". In other healthy people, lactase production persists at a high level throughout their adult life, allowing them to digest lactose as adults. This is known as lactase persistence and is a dominantly inherited genetic trait.

In a person with sufficient lactase activity, lactose consumed in the diet is broken into its component sugars and then absorbed through the walls of the intestine. In cases where there is insufficient lactase the lactose transits into the large intestine where the bacteria which are normally present act upon the lactose which results in fermentation, much like in the process of making wine. The result is large amounts of gas and can include other symptoms such as cramps, bloating and diarrhea.

There are three major types of lactose intolerance:

- Primary lactose intolerance - this is developed as a result of the practice of weaning in non dairy consuming societies such as most African and Asian societies where mother's milk is the only available milk and therefore drinking milk beyond infancy is uncommon.

- Secondary lactose intolerance - this is a result of an injury or illness that damages the brush border. Examples are certain gastrointestinal diseases, including parasites such as giardia. It can also result from conditions like sprue, cystic fibrosis, severe gastroenteritis, as well as chemotherapy and radiation treatments. Secondary lactose intolerance may be temporary or permanent.

- Congenital lactase deficiency from birth.

The incidence of lactose intolerance varies by group. Scandinavians tend to have the lowest and Asians and indigenous North Americans tend to be the highest.

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