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Whereas body weight and height have long been used to define the nutritional status of children, anthropometric studies of the nutritional status of adults have been accorded low priority. Serious attempts to distinguish between normal and undernourished or energy deficient adults have not been made largely because of the difficulties in establishing satisfactory reference standards which are applicable worldwide. This book calls attention to the need for more objective measures for the assessment of chronically undernourished adults since the prevalence of adult undernutrition is likely to be a better indicator of and reflect more truly the nutritional status of the community than estimates of childhood undernutrition alone.

The need for a method of diagnosing chronic energy deficiency in adults was a major issue which emerged at the first meeting of the International Dietary Energy Consultancy Group (IDECG), held in Guatemala in 1987. Further efforts to examine this problem were recommended and a report representing the first attempt at the international level to devise an operational definition of chronic energy deficiency (CED) in adults was made (James et al., 1988). Soon after the IDECG's Working Party completed its work, FAO began to explore the possibility of using the Body Mass Index (BMI) of adults as an indicator of the food situation and nutritional well-being of a community. Initially, the functional significance and behaviour of various weight/height indices were explored in depth; later, the BMI became the focus of attention. In this text, evidence provided by FAO and others is assessed and the case for using the BMI as an indicator for chronic energy deficiency is presented.

Throughout this publication, BMI data from various countries is presented and summarized. However, the detailed examples were provided from seven basic data sets. The reasons for the selection of these data sets are straightforward: they were complete and robust, they were accessible, and each had a large number of variables from which analyses could be drawn. It should be noted that all were collected for broader purposes than the examination of the BMI.

In Chapter 1, "Defining chronic energy deficiency", the distinction between acute energy deficiency and chronic energy deficiency is made. Particular attention is given to the level of normal fluctuations in body weight and energy expenditure. The advantages of adopting nutritional anthropometric indices such as height and weight as measures in adults are reviewed, followed by a review of the characteristics which constitute a good index. After a careful review of the weight/height ratio taken to various powers, the BMI (weight/height2) is proposed as the preferable index. The rationale for the use of the BMI as an objective measure of chronic energy deficiency in adults in a community is provided and the suitability of cut-off points related to CED is discussed.

In Chapter 2, "BMI as a reflection of body energy stores", the BMI as an index of both energy supply to the body and the variations in body composition, i.e., body fat and muscle mass that can occur at similar BMI values, are examined. In addition to describing these phenomena, the possible reasons for these variations are discussed.

In Chapter 3, "BMI as an indicator of CED", a background for the use of the BMI as an indicator of CED is given. Starting with the traditional classification of obesity according to the BMI, reasons are presented for adopting the BMI to define CED. The issues surrounding the choice of one cutoff point or several as well as the problem of causality are discussed.

In Chapter 4, "BMI and factors affecting access to food", an examination of the changes in BMI with variations in socio-economic status and seasonal patterns of household food availability is made. The conclusion is that the BMI is a responsive index, sensitive to changes in nutritional status which are influenced by socio-economic status, seasonal changes in food availability and demands on activity.

In Chapter 5, "Functional consequences of low BMI in adults", the implications of CED for physiological work performance are discussed and where data allow, the influence of low BMI on work performance and productivity is explored. In addition to work performance, behavioural changes as a consequence of low BMI are examined.

In Chapter 6, "Health and BMI", consideration is given to low BMI and pregnancy and lactation, women's nutritional vulnerability, and the potentially detrimental effects of low maternal BMI for the newborn child. Data from several studies are presented which demonstrate the association between low maternal BMI and low birth weight. Although limited data are available for studying the health consequences of low BMI, possible implications are drawn from morbidity and mortality data.

In Chapter 7, "BMI distribution in developed and developing countries", an extensive array of data sets are drawn upon to display and compare the distribution of the BMI from various regions. Differences are highlighted and possible reasons for the differences observed are presented.

The BMI is a simple but objective anthropometric indicator of the nutritional status of the adult population and seems to be closely related to their food consumption levels. It is relatively inexpensive, easy to collect and to analyse. Collection of data on weight and height from which BMI is easily derived can readily be incorporated into regional and national surveys that are presently being conducted. It could be used for the purpose of nutritional surveillance or for the purposes of monitoring since this allows for interregional or inter-country comparisons as well as longitudinal comparisons within the same region or country. The BMI is sensitive to socio-economic status and to seasonal fluctuations in food consumption relative to the level of physical activity. The BMI is a reasonably sensitive index of function and physical performance and may be useful if development projects depend on the physical activity of the community. The deleterious consequences of a low BMI status in an adult are only now being recognised; there is considerable need to evaluate immune function, proneness to illness, morbidity and mortality in low BMI adults. There is also scope for evaluation of intervention strategies in a community using the BMI as the parameter of choice to identify individuals at risk. Further epidemiological research on anthropometric data and individual food consumption measurements are still necessary, especially in different socioeconomic contexts. The percentage of false positives and false negatives needs to be assessed. However, there is reason to believe the BMI is a simple, responsive and useful index of nutritional status of the adult in a community and may indeed be the method of choice to assess the numbers of people who are undernourished world-wide.

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