What is lactose?
The lactose is the main sugar (or carbohydrate) naturally found, in various amount, in milk and dairy products:
- 1 cup of regular milk (250 ml) brinks 12 g of lactose
- 1 portion of yogurt (125 g or 4.4 oz) contains 5 g of lactose
- cheeses contain only traces of lactose.
An enzyme present in the small intestine, the lactase, is necessary to digest the lactose and split it into two simples sugars, glucose and galactose.
Glucose is the body’s main source of energy. It can be found in several types of foods. Conversely, lactose is the only source of galactose among life. Galactose is a component of several molecules (cerebrosides, gangliosides and mucoproteins), has various biological functions and serves in neural and immunological processes. It is also a component of the molecules that determine the ABO blood types in blood cells.
Lactose is an essential nutrient during childhood
Lactose is a necessary substrate for infants and child. Indeed, human milk contains 7.2% of lactose and provides up to 50% of an infant’s energy needs, while cow’s milk contains only 4.7% of lactose (and only provides up to 30% of an infant’s energy needs).
In order to digest lactose properly, in the normal population, lactase activity reaches a maximum at birth and starts declining after weaning to reach less than 10% of the pre-weaning level. This normal decline is called lactase non-persistence. It is more common in people of Asian, African, South American, Southern European and Australian Aboriginal heritage. However, in some populations of Northern European descent (Scandinavia, the British Isles and Germany) who continue to consume dairy products during adulthood, lactase activity remains in most people.
What about not digested lactose ?
In case of reduced lactase activity, some lactose is not digested. Non-digested lactose enters the colon where it is digested by the gut microbiota through a bacterial fermentation. For most individuals, this lactose fermentation produces few or no symptoms. However, for others individuals, the bacterial fermentation of lactose produces gas and increases gut transit time and intracolonic pressure, resulting on one or many symptoms such as bloating, diarrhea or flatulence. This is lactose intolerance. It is a lactose maldigestion that results in one or many digestive symptoms.
When lactose is not digested in the small intestine, it may be used as a nutrient by the gut microbiota (the microorganism population that lives in the digestive tract). Bacteria produce their own lactase, digesting lactose and resulting in the production of short chain fatty acids (acetate, propionate, butyrate) and gases (hydrogen, carbon dioxide, methane). Short chain fatty acids serve as energy locally for the gut microbiota and systemically after their absorption and their transport to the liver.
Undigested lactose and other milk sugars contribute also to promote the growth of bifidobacteria, a health-positive genus of bacteria.
With aging, there is a reduction in bifidobacteria, as well as markers of the immune function. In that case, lactose can then be considered as a prebiotic and play a life-long role in countering the aging-associated decline of some immune functions.