Is Lactose a Prebiotic?
Lactose is a disaccharide, a short carbohydrate chain made of one glucose molecule linked to one galactose molecule. Naturally found in milk, this is the major source of energy for nursing mammals – including humans. For lactose to be used as energy, it first needs to be cut into its glucose and galactose subunits, which are then readily absorbed by the digestive tract. Lactase is the enzyme responsible for cutting lactose and making it digestible. In most mammals, the dietary transition from milk to solid food, also called weaning, involves the disappearance of lactase, rendering the lactose in milk indigestible. Some people can digest lactose thanks to a genetic mutation while the rest of us are lactose-intolerant.
Lactose intolerance represents a major dietary challenge for affected individuals for two reasons: First, lactose is a soluble molecule poorly absorbed by the human digestive system, which means it acts like a magnet, pulling water out of the body and loosening stools. Second, lactose is readily digested by microorganisms in the gut, which leads to rapid gas production. These features explain the symptoms that lactose-intolerant individuals face when they consume foods containing even modest levels of this sugar and highlight why lactose is the quintessential FODMAP.
If lactose is so problematic, why are there so many dairy foods? And why did people first start eating dairy thousands of years ago? Even countries in which virtually everyone is lactose-intolerant, such as Mongolia, dairy makes up a substantial part of the diet. Part of the reason may have to do with the fact that many dairy foods have been transformed by microbes. Yogurt, for example, is cultured with lactic acid bacteria, including probiotic Lactobacilli, which ferment lactose. This reduces the amount of lactose present in the final version of the product, making it easier to digest.
Consumption of fermented dairy products not only offered a source of high-protein, high-fat foods to early humans, it also provided a source of probiotics. Furthermore, the small amounts of unfermented lactose would go undigested and pass into the large intestine, providing a prebiotic source for fermentation by the same types probiotic bacteria used to create the fermented product. This combination of a prebiotic (lactose) and probiotic (lactic acid bacteria) is known as a symbiotic because the two gut-health promoting factors work synergistically to provide benefit.
Since lactose is undigested – at least in those of us who are lactose-intolerant – and it stimulates the growth of health bacteria, one could make the case that lactose is indeed a prebiotic. However, the gas, diarrhea, and bloating associated with lactose consumption may outweigh the benefits of consuming it. Unlike our ancestors, we have a vast array of options in selecting a prebiotic source that delivers important benefits without the nasty side effects.