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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 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 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 at 22:44
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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 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 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 at 17:39
  • $\begingroup$ The two factors convey a huge gargantuan advantage compared to a banana/fig. If common bacterial resistance is not an advantage, then what is it? A baby and udder without botulin issues sounds like a good idea to me. Ease of containment and transport of sugars due to lower osmotic pressure is also an advantage, because it allows for more energetic milk than with sucrose, and therefore allows the child to put on weight faster, and more safely. $\endgroup$ – com.prehensible Nov 4 at 18:26
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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 at 22:36

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