Lactose is the simple little sugar that defines all mammals. It's in basically all milks from mammals, but doesn't show up in large amounts anywhere else in nature. Why? We're going to explore that question as we take a deep-dive into what lactose really is. Lactose is made up of two very simple sugars, glucose and galactose. These two monosaccarides (or single sugar molecules) are connected together into what is called a disaccharide, by a specific glycosidic bond (beta 1,4). What this means for the lactose in food is that if the glycosidic bond between these two sugars molecules is not broken during digestion, lactose can't be absorbed by babies or by adults. Not surprisingly, all infants that are mammals produce the enzyme lactase, which breaks down lactose during digestion for easy absorption. We still don't know everything that lactose can do, because it hasn't been widely studied scientifically. That's partly because it's not specifically an "essential nutrient" (AKA we can live without it). So, if lactose is not essential, what is it good for? While babies can survive without lactose, it has a ton of benefits. First, as energy support, lactose provides its two sugars in high concentrations. Babies need lots of available, soluble glucose for growth and the lactose in milk provides that glucose. Both Glucose and Galactose are key building blocks for many critical cell structures in all tissues of developing babies. An obvious question is: why doesn’t milk just contain glucose? Interestingly, if glucose is absorbed very, very quickly (as it would be if milk contained just glucose) then babies would experience blood sugar spikes, which would lead to a rise in insulin, and finally a dip in glucose. This rapid variation in blood sugar is what happens in adults when consuming too much simple sugar in their diet. Milk apparently solves this problem of fuel supply speed by combining glucose with galactose so that the glucose is released by the baby’s lactase enzyme at the speed that the baby can manage it. Genius! Second, as a small molecule, lactose controls the osmotic strength of milk. Osmotic strength is, essentially, how much water is in our biological fluid. An example of osmotic strength would be when we stay in water a long time and our fingers and toes go "pruny". That's our tissues working to balance osmotic strength in the face of pure water. Our cells and fluids are not pure water, they're concentrated solutions. Milk has the same osmotic strength as a baby’s other fluids and this means that it is a very gentle form of liquid food for a baby. Lactose is the main controller of milk’s osmotic strength. Isn't that amazing? Lactose also fuels the bacteria in a baby’s gut. When it's undigested, it can serve as fuel for some bacteria in the intestine of some infants, but not all. So, lactose that is undigested while in the upper intestine of the infant continues down the intestine to the lower gut that is populated by anaerobic bacteria. This undigested lactose would provide a fuel supply for those bacteria that can break it down and use it. If infants have been successfully colonized by specific Bifidobacteria, then lactose would fuel them. One of the consequences of that fermentation process is the production of short chain fatty acids that lower the pH of the colon and as an interesting benefit increase the absorption of minerals, especially calcium. So, it's increasingly clear from still growing scientific evidence that lactose is a unique and remarkable component of milk and as many other components provides multiple benefits to infants.