Most organisms have lactose as their main sugar in their milk.
What advantage does lactose give have over sucrose (Which is a common sugar in the plants, so it makes sense for it to be present in milk)
Is lactose synthesis easy? Or is the enzyme B-galactosidase easy to make?
Or is it more easily digestible, allowing mammalian infants to get energy faster?
Yes it is thought to provide safer milk, less vulnerable to every bacteria that respires and multiplies on sucrose, and which lets the baby grow faster due to higher sugar content.
Milk has evolved from pouch mucus, antimicrobal secretions of the immune system, Lysozyme in mucus is a glycoside hydrolase which ruptures bacteria cell walls. So the origins of milk are very different than for plants.
Lactose is a larger molecule than sucrose, so it exerts less osmotic effect per unit mass, allowing more carbohydrate to be included in am isosmotic secretion such as milk.
for example: marsupials have mostly longer oligosaccharides and their milk contains 11-14% sugar, zebras and and horses with only lactose achieve 7% humans and other primates achieve 8-9% using milk and polysaccharides.
Another advantage is that it's a molecule rarely produced in nature, and fewer bacteria have evolved the enzymes required to digest it, upgrading the lac-operon to produce galactose-permease, save for beneficial lactobacilli, e-coli, and other mostly mutualist species.
It is thought that milk evolved from secretory glands in the synapsid egg pouch which helped to protect parchment eggs, with moisture and antimicrobial secretions. In fact, lactose is thought to have come from mucus sugars, which are used to encapsulate and neutralize microbes in the body, "Because α-lactalbumin evolved from lysozyme before the division of amniotes into synapsids and sauropsids (see Figure Figure1),1), the capacity to produce lactose was an ancient trait that preceded its utility in milk synthesis"
One of the advantages of lactose in breast milk is that it is digested slower than sucrose and its ingestion results in lower fluctuations of blood glucose levels thus being a stable source of energy.
Lactose, a disaccharide of glucose and galactose, is uniquely present in mammalian milk. Human milk provides the infant with about 40% energy as lactose and 50% from fat. With 6-7 g/dL lactose, the breast fed infant consumes high carbohydrate/kg body wgt, suggesting carbohydrate induced hepatic lipogenesis would be problematic with a high fat milk diet. However, galactose, unlike glucose is non-insulinogenic and shows high hepatic clearance. We propose lactose in human milk confers unique metabolic advantage to the infant by delivering non-insulinogenic carbohydrate which can be slowly metabolized in the liver to glucose or to support fatty acid oxidation. We determined insulin, glucose, triglyceride, free fatty acid and amino acid responses in 16 healthy adult men given in random order 650 mL carbohydrate-free infant formula reconstituted to contain 50g lactose, glucose, sucrose or corn syrup solids. The milk formula was given 3 hr after a lactose and sucrose-free meal, then again after 3 hr, to model the carbohydrate load and continuous feeding of breast-fed infants. Blood was drawn every 15 min, and total and incremental area-under-curve change in insulin and glucose determined. Lactose gave a significantly lower rise in plasma insulin and glucose, in the first and particularly second 3 hr following intake, compared to glucose or corn syrup solids (P <0.001). Individual variability in triglyceride responses was high, with no difference due to carbohydrate in this 6 hr study. We show lactose enables maintenance of glucose and insulin equilibrium, avoiding high postprandial peaks achieved with glucose or corn syrups in milk fed at 3 hr intervals. We propose an important metabolic homeostatic role for lactose as carbohydrate source in human milk.
The lower glycemic index of lactose (46) compared to sucrose (65) also reflects slower conversion of lactose to blood glucose (gisymbol.com)
