|Natural History, Oct 1997 v106 n9 p49(1)|
When to wean. Katherine A. Dettwyler.
Abstract: The natural minimum age to wean infants is suggested to be about more than two and a half years, while the maximum age is seven. The weaning ages are enough to allow human brain growth and maturation of the child's immune system.
Full Text: COPYRIGHT 1997 American Museum of Natural History
Breast-feeding in humans is a biological process grounded in our mammalian ancestry. It is also an activity modified by social and cultural constraints, including a mother's everyday work schedule and a variety of beliefs about personal autonomy, the proper relationship between mother and child (or between mother and father), and infant health and nutrition. The same may be said of the termination of breast-feeding, or weaning.
In the United States, children are commonly bottle-fed from birth or weaned within a few months. But in some societies, children as old as four or five years may still be nursed. The American Academy of Pediatrics currently advises breast-feeding for a minimum of one year (this may be revised upward), and the World Health Organization recommends two years or more. Amid conflicting advice, many wonder how long breast-feeding should last to provide an infant with optimal nutrition and health.
Nonhuman primates and other mammals give us some clues as to what the "natural" age of weaning would be if humans were less bound by cultural norms. Compared with most other orders of placental mammals, primates (including humans) have longer life spans and spend more time at each life stage, such as gestation, infant dependency, and puberty. Within the primate order itself, the trend in longevity increases from smaller-bodied, smaller-brained, often solitary prosimians through the larger-bodied, larger-brained, and usually social apes and humans. Gestation, for instance, is eighteen weeks in lemurs, twenty-four weeks in macaques, thirty-three weeks in chimpanzees, and thirty-eight weeks in humans.
Studies of nonhuman primates offer a number of different means of estimating the natural time for human weaning. First, large-bodied primates wean their offspring some months after the young have quadrupled their birth weight. In modern humans, this weight milestone is passed at about two and a half to three years of age. Second, like many other mammals, primate offspring tend to be weaned when they have attained about one third of their adult weight; humans reach this level between four and seven years of age. Third, in all species studied so far, primates also wean their offspring at the time the first permanent molars erupt; this occurs at five and a half to six years in modern humans. Fourth, in chimpanzees and gorillas, breast-feeding usually lasts about six times the duration of gestation. On this basis, a human breast-feeding would be projected to continue for four and a half years.
Taken together, these and other projections suggest that somewhat more than two and a half years is the natural minimum age of weaning for humans and seven years the maximum age, well into childhood. The high end of this range, six to seven years, closely matches both the completion of human brain growth and the maturation of the child's immune system.
In many non-Western cultures, children are routinely nursed for three to five years. Incidentally, this practice inhibits ovulation in the mother, providing a natural mechanism of family planning. Even in the United States, a significant number of children are breast-fed beyond three years of age. While not all women are able or willing to nurse each of their children for many years, those who do should be encouraged and supported. Health care professionals, family, friends, and nosy neighbors should be reassured that "extended" breast-feeding, for as long as seven years, appears physiologically normal and natural.
Substantial evidence is already available to suggest that curtailing the duration of breast-feeding far below two and a half years -- when the human child has evolved to expect more -- can be deleterious. Every study that includes the duration of breast-feeding as a variable shows that, on average, the longer a baby is nursed, the better its health and cognitive development. For example, breast-fed children have fewer allergies, fewer ear infections, and less diarrhea, and their risk for sudden infant death syndrome (a rare but devastating occurrence) is lower. Breast-fed children also have higher cognitive test scores and lower incidence of attention deficit hyperactivity disorder.
In many cases, specific biochemical constituents of breast milk have been identified that either protect directly against disease or help the child's body develop its own defense system. For example, in the case of many viral diseases, the baby brings the virus to the mother, and her gut-wall cells manufacture specific antibodies against the virus, which then travel to the mammary glands and go back to the baby. The docosahexanoic acid in breast milk may be responsible for improved cognitive and attention functions. And the infant's exposure to the hormones and cholesterol in the milk appears to condition the body, reducing the risk of heart disease and breast cancer in later years. These and other discoveries show that breast-feeding serves functions for which no simple substitute is available.