Re: What is the benefit of having the disaccharide, Lactose, in milk?

Hi David,

Lactose is an interesting sugar. As you said, it is a disaccharide, composed of two six-carbon sugars, glucose and galactose. Lactose is only made by mammals, and it is only found in milk. In fact, the word "lactose" means 'milk sugar', and lactose is the primary saccharide found in milk. For the most part, mammals are the only animals that can digest lactose, because we can produce the enzyme lactase, which breaks lactose into glucose and galactose. Some bacteria can also digest lactose, because their enzymatic repertoire is much greater than those of animals, and they can hydrolyze the beta glysoside linkage between the 4 carbon of glucose and the 1 carbon of galactose.

In almost all mammals (with one major exception) lactase is only expressed in newborns, so that only newborn mammals can digest lactose. Once they are able to feed themselves, they lose the ability to express lactase and cannot digest lactose. Animals that do not produce lactase are said to be lactose intolerant. If a lactose intolerant human drinks milk (or some other food rich in lactose), they can experience nausea, gas, cramps, diarrhea, bloating and other similar gastrointestinally related discomforts. I know from experience that adult ferrets, along with many domesticated dogs and cat breeds are lactose intolerant, and suffer similar symptoms, and I assume that most adult mammals are.

So, it seems likely to me that the benefit of having lactose in milk is that it makes milk only drinkable by newborn mammals, eliminating competition for milk between newborns and their older relatives (or strangers). Making milk requires a lot of energy. A mammal that is nursing her young would want all of that energy directed to her newborn, and not to older offspring or unrelated animals, who should be able to feed themselves. Older offspring would be stronger and able to displace newborns from the teat, essentially starving them. The nursing mother's own tissues will also be unable to metabolize lactose. So, very early in the evolution of mammals, a new dissaccharide (lactose) was recruited for milk production, and a new enzyme (lactase), which could be regulated without affecting other enzyme pathways, permitted the restriction of lactose digestion to newborns.

Maltose and sucrose would not be suitable disaccharides for milk, as the enzymes that digest them are part of other pathways, and because those sugars are found in other food sources; mammals of all ages might need to be able to digest those. Note that in plants, maltose and sucrose are used in a manner similar to lactose in mammals; only certain tissues can digest them, and they serve as tissue specific energy transporters.

The odd exception to the rule that only newborn mammals can digest lactose is of course, humans. Even odder is the observation that many adult humans drink milk produced by another species. The reasons for this are described in other answers in our archives. Take a look at this answer (1110993926.Ev), for example. It seems likely that lactose tolerant domestic animal (e.g., dog and cat) breeds, are the result of a selection akin to that which occured in humans.

You can find copious information about lactose and milk, via a simple google search. For example, this Introduction to Lactation Biology page covers many of the basics.

If you want more information about the evolution of lactation, I suggest reading this answer from our archives (1008253501.An) and perhaps even contacting the Blackburn lab.

Keep asking questions!

Added by Mike Onken:
There are also biochemical restrictions that make lactose the most readily available disaccharide. As an excreted material, milk is produced in the ER and Golgi of mammary epithelial cells, such that any sugars in milk must be in these compartments to be secreted. Since the Golgi is the major site of glycosylation, it is full of monosaccharides; however, only a handful of sugar moieties are available - mostly derivatives of mannose, galactose, and N-acetyl glucosamine - not including glucose or fructose. In lactating mammary cells, the GLUT1 glucose transporter changes its subcellular localization to the Golgi trafficking vesicles to pump glucose into the Golgi. The ?-1,4-galactosyltransferase 1 enzyme is then directed by ?-lactalbumin to use glucose preferentially over N-acetylglucosamine as a substrate to produce lactose. To make maltose, a novel Golgi ?-1,4-glucosyltransferase enzyme—something like a glycogen synthase with a signal peptide and a Golgi retention domain—would have to be evolved. To make sucrose, all of the above would have to hold, and the fructose transporter, GLUT5, would also have to be expressed in mammary cells and localized to the Golgi to pump fructose in. Much easier to just make lactose.