Unless a more specific usage is indicated, malnutrition generally refers to protein-energy malnutrition (PEM). The term was introduced because the two deficits tend to occur together; an isolated protein deficiency is rare. Children are particularly vulnerable to malnutrition, and severe malnutrition is associated with high mortality. But many children live through chronic mild to moderate malnutrition, and there is reasonably good evidence that it adversely affects their cognitive development. Micronutrient deficiencies are also widespread. Two micronutrient deficiencies that also affect cognitive development are iodine and iron deficiency.
As well as the type of malnutrition, its timing, duration, and severity are important. Malnutrition may be prenatal or postnatal, and after birth it may be found in early or late infancy, childhood, or adolescence. Its origins can be equally varied. They can include contributions from the availability of food and its composition, the care provided by parents and others, the child’s appetite and food preferences, and the child’s illnesses and disabilities. The ability to eat is often taken for granted, like the ability to talk, but like talking, eating also involves very complex motor skills, and where they are compromised, as they are, for example, in children with cerebral palsy, serious malnutrition can result.
For obvious reasons, children have not been deliberately malnourished, so research in this area has principally used observational studies. But the families of malnourished children tend to differ from the families of well-fed children in many other ways that also affect their development, so the inferences that can be drawn from observational studies depend on the extent to which these can also be measured and taken into account. A valuable additional source of information comes from studies in which additional nutrients have been provided for children in trials that allow a later comparison with unsupplemented control children.
Protein-energy malnutrition is usually identified by its effects on growth, and the most commonly used measures of growth are height, weight, and weight relative to height. These measures can be interpreted only in relation to norms or standards, which are summaries of the distribution of the measured values in populations, often in the form of a growth chart. In general, the growth of well-nourished children in different populations is sufficiently similar for it to be possible to use a single set of standards internationally, as recommended by the World Health Organization (WHO). The United States National Center for Health Statistics/WHO international reference standard has mostly been used for this purpose. Criteria for malnutrition can be expressed in centiles (e.g., a height below the third centile of the reference population) or in standard deviation scores (e.g., a height more than two standard deviations below the average). Using this criterion, about a third of the children in developing countries were malnourished in 2000 (183 million). This proportion declined between 1980 and 2000 in most areas of the world (but not in Eastern Africa). There is good evidence that proteinenergy malnutrition in the childhood years is associated with slower development, as measured using well-standardized scales such as the Bayley Scales. There is also evidence that it is associated with lower intellectual ability at school age. This association has been found with intelligence tests and with more specific tests of psychological functions, such as attention and working memory, and of educational attainment, for example, in reading. The mechanism by which malnutrition has these effects is still uncertain. It could involve structural changes in the brain, or changes in the behavior of the child leading to reduced interaction with the environment, which might additionally result in lower levels of stimulation from adults or other children caring for them (this is the functional isolation hypothesis). It is difficult to make useful generalizations concerning the size of the effect of protein-energy malnutrition on intellectual development internationally. It tends to be associated with other kinds of malnutrition and with other adverse circumstances. There is reasonably good evidence that the size of the effect depends on the timing of the malnutrition, with poor nutrition in the earlier stages of life having a bigger effect, and on the duration of follow-up, with effects that are present in early childhood tending to ‘‘wash out’’ later (although there is some evidence that they may become more important again when general cognitive abilities decline in old age).
In industrialized countries, most of the available work on cognitive outcomes of malnutrition has been carried out in relation to children who have ‘‘failed to thrive.’’ This term is used for infants or young children whose weight gain is poor. Criteria similar to those used to identify malnutrition (say, a weight gain in the lowest 5%) are used to identify it. Its immediate cause is likely to be poor nutrition, perhaps with some contribution from ill health, and it is therefore likely to have effects similar to those of malnutrition in other parts of the world. Well-documented sequelae include Malnutrition and developmental delay in infancy and some reduction in intellectual ability at school age (about three IQ points). There is less direct evidence at the moment that school performance is affected, although that may simply reflect a lack of sufficiently large studies.
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