Milk composition

According to Elsie Widdowson in 1984, of the 4,300 species of mammals, only the milks of 176 have been analyzed for protein, fat, and carbohydrate. Of these analyses, she said, only the figures for 48 of those species are considered to be reliable. The difficulty in the analyses lies with the fact that milk composition changes rather markedly during a lactation cycle.

The first milk, or "colostrum," contains a high concentration of maternal antibodies, or immunoglobins, active phago-cytic cells, and bacteriocidal enzymes. While neonatal primates, guinea pigs, and rabbits acquire their circulating maternal immunoglobins in utero, other mammals such as ungulates, marsupials, and mink depend on the colostrum as their sole source of a passive immune system. Yet another group, intermediate to these two, acquire maternal immuno-globins, both in utero and from colostrum. Among these animals are rats, cats, and dogs. The differences in in-utero transfer of immunoglobins are determined by the number of cellular layers in the placenta that separates fetal and maternal circulation.

In order for the secretion of colostral immunoglobins to be effective, the neonatal gut needs to remain permeable to their absorption and minimize any upper-tract digestion of these proteins. The time the mammalian intestine remains permeable to the intact immunoglobins varies between species: 24-36 hours after birth in the case of ungulates; 16-20 days in mice and rats; eight days in mink; and 100-200 days in large marsupials. The marsupial's prolonged absorption capabilities relate to the time the young reside in the mother's pouch.

Other types of immunoglobins that occur in milk, after absorption of the first wave of intact molecules has ceased, protect the neonatal gut from infection.

The major constituents of milk are water, minerals, proteins (such as casein), fat, and carbohydrates. Protein concentration ranges from under 3.5 oz/qt (10 g/l) in some primates to more than 3.5 oz/qt (100 g/l) in hares, rabbits, and some carnivores. Fat concentrations vary from small amounts in the milk of rhinoceroses and horses to more than 17.6 oz/qt (500 g/l) in some seals and whales. The main carbohydrate of placental mammal milk is the disaccharide lactose, a polymer of glucose and galactose. Lactose content also ranges from trace amounts in the milk of some marine mam mals and marsupials to more than 3.5 oz/qt (100 g/l) in some primates. Marsupial milk, nevertheless, can be very high in carbohydrates, but the sugars are primarily oligo- and poly-saccharides-rich in galactose.

Compromises have to be made between the physiological constraints to milk synthesis and selective pressures to maximize offspring survival. The variation in milk composition between species is one of them. Most aquatic mammals produce highly concentrated milks. The reduction in milk water content in aquatic mammals provides a high-energy, low-bulk diet that is useful in offsetting neonatal heat loss in cold environments. It also conserves water in the mothers of species (such as the northern elephant seal) that abstain entirely from eating or drinking during a relatively short, but intense, lactation. Similarly, seals that give birth on pack ice and have a short lactation period (e.g., hooded seals—four-day lactation) or those that leave the neonate for feeding trips lasting several days produce more concentrated, higher fat milks than do other seals.

For terrestrial mammals, the largest changes in milk composition over time occur in marsupials. Some marsupials can produce several kinds of milk simultaneously since they may have young of different ages. For the embryonic marsupial confined to the pouch, dilute, high-sugar milk provides nourishment similar to that occurring in the uterus of a placen-tal mammal during its longer gestation. Once the young leave the pouch, the milk becomes more concentrated with more fat and protein and less sugar. Most terrestrial placentals produce milks that are intermediate in concentration to the nutrient-rich milk of aquatic mammals and the very dilute milks of primates and perissodactyls. The milks of domestic cattle, goats, and camels contain about one-half the protein and energy per unit volume that occurs in the milks of wild eventoed hoofed mammals (also known as artiodactyls). The dilute milks from domestic artiodactyls are more similar to that produced by humans than they are to wild artiodactyls. Because sugars, particularly lactose, and some minerals such as sodium and potassium are important regulators of the osmotic potential or water content of milk in the mammary gland, concentrated milks have either a low sugar (such as marine mammals) or mineral content, while dilute milks have a higher sugar (such as primates and perissodactyls) or mineral content.

Picture And Information Mammals
Koalas (Phascolarctos cinereus) have developed bacteria in their stomachs to break down the toxins found in the eucalyptus leaves they eat. (Photo by © L. Clarke/Corbis. Reproduced by permission.)

The actual composition of the milk fat, protein, and sugar also differs between animals. For example, the fatty acids of most milk are dominated by palmitic and oleic acids. However, the main fatty acid of lagomorph and elephant milk is capric acid, which is synthesized in the mammary gland. Seal milk is composed of long-chain unsaturated fatty acids that are probably derived directly from the diet.

In carnivores, the amino acid, taurine, appears to be essential for the diets of most of their neonates. The taurine content of colostrum is usually higher than in mature milk. However, carnivores have a much higher concentration of taurine in the mature milk than do herbivores.

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  • pansy
    What disaccharide provides energy to young mammals?
    4 years ago
  • sami
    What disaccharide do mammals produce?
    3 years ago
  • kaiju
    What disaccharide provides energy to young mammals before they are weaned?
    2 years ago
  • bisrat
    Do mammals produce sugar?
    2 years ago

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