Designed for Growth
When newborns most need immune protection, they receive more defenses from their mothers’ colostrum. When a week-old baby needs readily digestible protein, breast milk contains a high concentration of soluble whey proteins. When the baby’s brain is growing fastest, breast milk is full of key fatty acids such as DHA. The list goes on and on. Sometimes the timeframe is days, other times it is weeks or months, but breast milk delivers vital nutrients when the infant needs them most.
The equation also holds true in reverse. When a baby’s immune system is fully developed, the concentrations of antibodies and immune factors in breast milk level off. When a baby’s gastrointestinal tract is mature and flourishing with beneficial microflora, the level of oligosaccharides in breast milk subsides. When a baby’s pancreas starts making digestive enzymes, the temporary digestive enzymes supplied by breast milk taper off. The resources of breast milk reflect the developmental needs of the growing infant.
Overall growth is one obvious way in which parents and physicians track the health and development of a baby. An infant’s growth is fueled by the nutritional energy as well as the building blocks that are delivered in breast milk. The development of the baby’s digestive and immune systems, bones, muscles, and circulatory and respiratory systems may be less visible, but here, too, breast milk delivers nutrients to babies when they need them most.
A baby’s birth weight typically doubles by 4 months of age and triples by 1 year. A baby will increase in length by roughly 50% by his or her first birthday. Another key measure for infants is head circumference. At birth the average is just over 14 inches (36 cm), roughly two thirds the circumference of an adult’s head. At 2 months the infant’s head circumference is approximately 15-3/4 inches (40 cm), at 4 months about 17-3/8 inches (44 cm), and at 1 year a little over 18 inches (46 cm). All of these figures are averages, of course. It’s important to remember that healthy babies come in all shapes and sizes. In any population, there will always be individuals, whether babies or adults, who are shorter than average, taller than average, heavier or lighter than average. An average is just a number, and there is a wide range of normal variation within any population.
At the same time there is variation in weight and height, there is also variation in stages in the development, such as when a baby will hold up his head, utter her first word, or take her first step. Developmental milestones are averages.
Evolution and the Recipe for Human Milk
“When we look at human milk compared to other species,” says Donovan, “it seems kind of dilute; its protein content in particular is quite low compared to other species. But it turns out that you can almost draw a direct line correlation between the richness of the milk and the growth rate of the infant.” Another way to look at it is the thinner the milk, the longer lactation lasts.
We are used to thinking of babies as growing fast, and by some measures they do. But compared to other mammals, human infants are real slowpokes. Hooded seals, for example, according to Sarah Blaffer Hrdy, “have one of the shortest periods of lactation known, as brief as a week. Mothers stockpile blubber in advance, then give birth atop ice floes to pups who imbibe pure cream, a high-protein formula containing 60% fat, approximately 1,400 calories in an 8-oz glass.” The seal pup can gain 50 lbs in a matter of days.
Growing slowly has its advantages, particularly for highly social, language-using primates like ourselves. Growing slowly also represents, in evolutionary terms, an enormous reproductive investment on the part of the mother. “Our reproductive rate is slow compared to other species,” explains Donovan, “and we typically have one infant per birth, and so we put a lot of resources into raising that infant compared to species that can have several litters in a year.”
It might strike parents as odd or even off-putting to talk about babies in terms of resources and investment, but examples of a mother’s biological “investment” can be clearly seen in the composition of breast milk. It takes energy for the mother to assemble, transport, and manufacture many of the special components of breast milk, particularly those components that protect the infant from pathogens and disease.
“So while human milk doesn’t need to provide lots of protein for quick growth,” says Donovan, “it has proteins and other components that pack a lot of bioactivity and functionality. Much of that functionality is related to helping defend the infant against infections. That seems to be really a hallmark of many of the proteins, such as lactoferrin, lysozyme, and the immunoglobulins. And even when it comes to carbohydrates, we also have all of these oligosaccharides which again have a role in defending the infant against infection.” Breast milk protects our precious investments.
Mother's Milk (VIDEO)
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Designed for Growth
Recipe to Develop
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Related Health Centers:
Infant Nutrition Health Center, Mother-Baby Bond Health Center, Mother’s Milk Health Center, Monthly Infant Development Calendar Health Center,Weekly Pregnancy Calendar Health Center