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Malnutrition and Development

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 in to 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

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 protein-energy 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).

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