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Things that donors know: why is there lactose in milk?

"Banana" asks: How is lactose-free milk produced?  

Lactose-free dairy products. Image:
Lactose-free dairy products. Image:

Before we get to milk, let's start with lactose itself. Lactose is a disaccharide, meaning a combination of 2 sugar units. The primary sugar that plants produce is glucose ("grape sugar") a molecule of six carbon atoms, six oxygen atoms and 12 hydrogens. Small changes in the molecule create other "monosaccharides" such as fructose ("fruit sugar"). An interesting feature of sugars is their ability to form chains: 2 connected sugar units are not surprisingly called a disaccharide (the most common in this family is the sucrose in the supermarket that is simply called "sugar") Any chain of 3 or more sugars is called a polysaccharide. It doesn't matter how many units in the chain we put into the body - the blood will absorb from the intestine and transfer only monosaccharides to the body's cells. What is not broken down will continue to the large intestine where it will meet bacteria that will utilize it and produce gas as an embarrassing byproduct.

Lactose is a disaccharide unique to mammals, almost no plant or other living thing produces it. Binding the 2 monosaccharides glucose and galactose to form lactose is a complicated biochemical process and requires an investment of energy. It is not possible to "just" stick glucose to galactose and it is necessary to attach a phosphate group to galactose and remove it from the molecule after the formation of the disaccharide. Lactose is produced only in the cells of the mammary glands that pass on to the offspring where it is broken down again into monosaccharides in the intestine by a dedicated enzyme called, not surprisingly, lactase. It is interesting to note that a human baby is born without the breaking down enzyme and it appears in the digestive tract only two days after birth, until then the newborn produces energy from reserves it prepared for itself in the womb. Since lactose is only found in milk, there is no need for the breaking down enzyme after weaning, indeed in animals and in many humans the intestine stops processing lactose after infancy. Some see the cessation of lactase as a signal for the offspring to move to the next stage of nutrition and to wean, and weaning from breastfeeding is sometimes a signal for the mother to return to ovulation. A young mutation that originated in Europe only a few thousand years ago allowed our ancestors to use milk even in adulthood. Those who did not inherit these genes will discover "intolerance to lactose" which means that the lactose will pass through the small intestine unharmed and reach the bacteria of the large intestine. "Lactose-free milk" is milk in which the lactose is previously broken down into its components by a small addition of the enzyme.

Lactose-free milk is a product that is easy to understand, what is strange and difficult to explain is precisely the milk with lactose. Why would the mammary glands connect 2 monosaccharide units to lactose only for this pair to break down again into its parts in the mammal? Why not provide the monosaccharides directly in the milk and bypass two difficult processes: assembly and disassembly?

The key to the mystery lies in the evolution of milk - a murky field because this liquid leaves no fossils. We know what the cave bear looks like and can reconstruct how it lived, bred and became extinct but not taste its milk. And yet there were those who also studied the prehistory of milk.

Clues to the dairy ancestors can be found in the comparative analysis of the milk sugars in the 3 groups of mammals. The carbohydrate in the milk of placental mammals (a group that includes us and the vast majority of mammals around us) is, as mentioned, mainly lactose, but also some short polysaccharides. The milk of bib mammals: egg-laying mammals of which only 3 species have survived in Australia and New Guinea contains mainly the same polysaccharides and is almost lactose-free, while marsupials (the kangaroo, the opossum, the koala...) have milk whose main carbohydrates are their unique polysaccharides. Just before you look for koala milk or kangaroo cheese in the supermarket, we should point out that what these milk polysaccharides have in common is that the disaccharide lactose can be cut from one of the ends of the chain. In other words, it seems that mother evolution did not sew lactose from two monosaccharides but the opposite: shortened polysaccharides inherited from previous generations to the problematic disaccharide.

Although lactose is found in nature only in milk and only mammals produce milk, surprisingly, mammals did not invent it. The copyright for this invention is reserved to our and the kangaroo's ancestors: the synapsidsea. The first vertebrates to adapt to a semi-terrestrial lifestyle were amphibians. The amphibians can get by on land, but they must lay their eggs and go through the first stage of life in a freshwater mikvah. About 320 million years ago, creatures appeared that adapted to a complete life on land, meaning they laid their eggs outside the water. This group is the amniotes, named after the membrane that surrounds the embryo and is not found in fish and amphibian eggs. This membrane creates an aquatic environment for the embryo that is isolated from the environment and is what makes a completely terrestrial life possible.

To the extent that the milk originates in an oily secretion that the ancestral mother of mammals applied to the egg to preserve moisture and protect

The embryo growing in the swamp needs, of course, a moist environment and therefore a solution is required that prevents dehydration. Here the newcomers to the land split into two groups, one - the one that would later bring out the dinosaurs, reptiles and birds developed waterproof eggs. The second one we are dealing with brought out the first mammals. These cold-blooded creatures, whose ancestors are very difficult to identify, laid eggs covered in skin (similar to today's Australian duck). The origin of the milk is, so it is speculated, in an oily secretion with which the mother applied the egg to preserve moisture and protection. Evolution reduces the construction of new structures and usually modifies and remodels old structures and thus the female produced this thick secretion from glands in the skin from which the apocrine sweat glands developed: those that develop close to hair roots and secrete scented sweat in the armpits for example.   

It turns out that not only is the source of the milk in the secretion similar to earwax, but that the human "twins of the deer" are nothing more than sweat glands that have grown and taken on new roles. The fatty secretion also contained additives whose role was apparently to protect the sensitive egg from bacteria. These additives were the ancestors of the polysaccharides in milk and therefore the ancestors of lactose. 312 million years separate us from the earliest fossil of a synapsid. This is a long time, during which the ancient sweat-like secretion assumed a secondary role of transferring nutrients through the shell and as the first meal for the offspring hatching from the egg. The ancestors of mammals were small creatures (which necessitates small, yolk-poor eggs) and endothermics, meaning they have a constant body temperature that must be invested in maintaining it, and teeth that completed their development only after birth. Under such conditions, the milk supplanted the yolk as a main source of nutrition and it evolved to contain more essential ingredients. The transition from eggs to the placenta made the function of coating the eggshell redundant and left milk with a nutritional role only. Milk has come a long way since then and the polysaccharides have been cut down until only the pair at the end, which is lactose, remains. The bloating and gases following the consumption of dairy foods are the souvenir fossils that our ancestors left us a moment after they split from the ancestors of the dinosaurs.

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2 תגובות

  1. Why "eggs"? Where did the second Y after the B come from?!
    Not only does it not exist, but you shouldn't even hear the single Y () of the full axial.

  2. To Tommy, I thought that the role of lactose is to attract the placentas to suckle and eat..

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