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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?

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    $\begingroup$ Possible duplicate of What selective factors drove the evolution of lactose in lactation? $\endgroup$ – tyersome Nov 2 '19 at 18:21
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  • $\begingroup$ It doesn't make sense to have sucrose in milk if mammals are unable to produce it. $\endgroup$ – anongoodnurse Nov 4 '19 at 17:35
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    $\begingroup$ As @anongoodnurse states elesewhere, the comparison with sucrose is arbitrary. The question is better stated in terms of lactose v. other disaccharides, but unfortunately has been asked before. I would mention that ideas like "synthesis being easy", "enzymes being easier to make" and "get energy faster" are biochemically naive. (And most organisms do not produce milk. Try "mammals".) $\endgroup$ – David Nov 5 '19 at 22:44
  • $\begingroup$ If you feel the existing question and answer do not answer your question please explain why or expect the question to be closed. $\endgroup$ – John Jan 14 at 6:34
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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.

full text.

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"

Intro to synapsid and sauropsid precursors of lactose.

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    $\begingroup$ "and not many bacteria have evolved the enzymes required to digest it" There is clearly no problem for bacteria in evolving lactase/beta-galactosidase — the whole dairy industry depends on that — so this argument clearly does not hold. For the rest your answer does not explain why lactose is preferred to sucrose — the poster's primary concern. The complex argument proposed in the question that this is a duplicate of, implies that there is no obvious easy answer. $\endgroup$ – David Nov 4 '19 at 12:00
  • $\begingroup$ If you want to contradict what a professor of dairy chemistry states, the author of the stated arguements, it would be wise to have some fact to support your arguement. For the rest my answer explains that sucrose tends to osmotically travel through the membranes into teh cell walls and presumably to the capillaries, and lactose doesn't because it's too big thereby increasing the energy content of the milk, and also that botulin and every bacterial that respires and grows on petri dishes can process glucose and infect udders adn babies, but not lactose. $\endgroup$ – com.prehensible Nov 4 '19 at 17:31
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    $\begingroup$ This answers a question, but not the OP's question. The OP is comparing mammals to grapefruit, then asking why. $\endgroup$ – anongoodnurse Nov 4 '19 at 17:39
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    $\begingroup$ The validity or not of arguments depends on their logic, not on who makes them. The one at issue is rehearsed in an answer to an earlier version of the question. I have not commented on that answer, only on the defects in yours. In particular I criticized the original statement about the lack of bacteria that can metabolize lactose, which now seems to have morphed into an equally contentious argument about the bacteria with sucrase v. those with beta-galactosidase. To this I would query the statement that "lactose is a larger molecule". Lactose and sucrose have the same molecular weight. $\endgroup$ – David Nov 4 '19 at 18:36
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    $\begingroup$ The original version of your answer — to which my criticism was directed — contained no mention of the hypothesis of Urashima et al., quoted by Alan Boyd in his answer to a similar question. It did, and still does, contain the false assertion that “lactose is a larger molecule than sucrose” — an assertion Urashima et al. certainly did not make. In fact both lactose and sucrose have molecular weights of 342.3! As you now seem to be leaning on the hypothesis of Urashima et al., would it not be better to vote to close this question as a duplicate, rather than rehashing Alan Boyd’s answer? $\endgroup$ – David Nov 5 '19 at 22:36
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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)

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