|Food and Nutrition Bulletin Volume 13, Number 1, 1991 (UNU Food and Nutrition Bulletin, 1991, 82 pages)|
Susan Randolph, Richard D. Ely, Lindsay H. Allen, Adolfo Chávez, and Gretel H. Pelto
The assessment of caloric adequacy on the basis of caloric shortfalls from nominal needs misclassifies individuals (undernourished as well-nourished, and vice versa) unless important inter-individual variations in energy needs are controlled for. This paper explores the extent of misclassification resulting from inter-individual differences in activity choice.
Although many factors influence health and nutrition status, nutrient intake has tended to occupy centre stage as the principal one. It is common practice to assess nutrition status on the basis of whether caloric intake meets or falls short of some nominal energy requirement. In this assessment, controlling for differences in energy needs between individuals in a meaningful way is essential. Failure to do so results in the misclassification of individuals and impairs our understanding of the extent and causes of malnutrition and who is most at risk. Despite agreement on this issue in principle, considerable controversy surrounds the question of which of the many possible factors influencing need can safely be ignored and which must be addressed.
Several articles have suggested that controlling for inter-individual differences in activity levels may be a matter of some importance. After reviewing several time-allocation studies from Indonesia, New Guinea, Côte d'Ivoire, and India, Edmundson and Sukhatme  concluded that the poor spend more time on economically productive work and are more likely to be engaged in heavy physical labour than are the wealthy. Strauss  also pointed out that higher earnings are likely to be associated with increased leisure consumption. Both factors imply that the poor are likely to be misclassified as well-nourished and the wealthy as poorly nourished. Failure to control for activity may bias results on nutritional gains during the course of national development, given changing occupational structures and employment opportunities . However, the direction of the bias is not obvious.
There seems little question but that estimates of the extent of undernutrition will vary depending on whether inter-individual variations in activity level are accounted for. From the policy perspective, however, the critical question is whether ignoring these differences leads one significantly astray. What proportion of the population is likely to be misclassified if they are ignored? Is there a real danger of improperly identifying target groups? We addressed these questions in the context of four subsistence farming communities in the valley of Solis, about 75 miles northwest of Mexico City.
Methods and data
This study estimated energy adequacy on the basis of the ratio of observed intake to nominal need. If energy intake falls short of (exceeds) this level, the ratio will be less than (greater than) 1.0. Nominal need is assessed in two ways: on the basis of published WHO norms for activity levels in subsistence farming communities, and on the basis of individual-level activity data. To isolate the influence of inter-individual differences in activity mix, any variation between individuals in basal metabolic weight, observed weight change, and energy need for lactation is controlled for. The resultant energy adequacy measures are as follows:
Here Ii is the energy intake of individual i measured in kilocalories per 24 hours; R is the basal metabolic rate (BMR); W is the kilocalorie equivalent of daily weight change (8.27 kcal per gram), including the observed weight changes accompanying childbearing; and L is the kilocalorie expenditure for lactation. Nominal energy adequacy (NEA) uses a BMR multiplier, Si, of 1.78 for men and 1.76 for women; these are the WHO norms for activity levels in subsistence farming communities . Corrected energy adequacy (CEA) uses (1/1,440) SjAijNj in place of Si in the estimate of energy needs. Here the variable Aij is the time (in minutes) individual i expends in each of the activities j over the 24 hours; Nj is the WHO multiplier of BMR for activity j ; and 1,440 is the number of minutes in 24 hours.
Basal metabolic rates were estimated on the basis of age, sex, weight. and height using WHO norms . No further adjustment was made for pregnancy, in line with the findings of a study that concluded that pregnancy does not increase BMR beyond that accounted for by increasing weight during pregnancy  Data from a study on the average milk production of mothers by infants' weight  were combined with data on the average monthly weight of nursing children in the study population to estimate the average millilitres of milk produced during each month of lactation. The estimated requirement was converted to kilocalories at the rate of 0.7 kcal per millilitre using an 80% conversion rate.
An NEA value of 1.0 means that intake was sufficient to enable the individual to perform the activities typical in subsistence communities at nominal intensity levels. Individuals devoting less time to energy-intensive productive activities and more time to leisure activities than is typical in subsistence communities will have NEA values of less than 1.0, as will those in less strenuous occupations such as the village shopkeeper and schoolteacher. Such individuals are misclassified as malnourished on the basis of NEA. Similarly, persons engaging in more hours of physically strenuous activity than is typical in subsistence communities may have NEA values greater than 1.0 even if their energy intake is inadequate to enable them to engage in these activities at nominal intensity levels. Thus, they may be misclassified as well-nourished. The CEA measure avoids such misclassification by ensuring that the need assessment reflects the observed activity mix. A CEA value less than 1.0 implies that energy intake was inadequate to perform the activities undertaken if performed at nominal intensity levels.
The study's data were drawn from the Mexico-CRSP data base, a full description of which is available . Marginal malnutrition was the basis of community selection and is evident in anthropometry; children tend to be stunted but not wasted. Households with children in any of three target groups and both parents present were invited to participate in the study; nearly 98% did so. Activity data were collected on the parents of the target children for two consecutive days out of each month. Interviewers asked the adults to recall how they spent their time during the previous day to the nearest minute, on two consecutive different days of the week each month. Individual intake data were collected for two consecutive 24-hour periods each month using a combined record-recall-weighing method. Food composition data specific to Mexico  were used to calculate the caloric content of foods consumed. Adults' heights and weights were measured every three months; for pregnant and lactating women, weights were recorded monthly.
Data collected during the calendar year 1984 were used for this study, as the activity data were most complete for that period. In the analyses, each intake observation day for each individual was initially treated as a single observation. Data on activity, weight, and lactation were interpolated for all days during the study period and matched to each intake observation day. A quasi-hermetic spline technique was used to interpolate weight and a cubic spline function to interpolate the activity data. The median (non-pregnant) height value was used to estimate BMR. These procedures resulted in an average of 14 observations on each of the 222 women and 11 observations on each of the 179 men in the analysis data set. The mean values of each energy adequacy measure were used in the analyses that follow to control for intra-individual variation in intake and activity.
Table 1 shows the extent and pattern of misclassification both for the population as a whole and separately for men and women. Overall, 13% of adults in the valley of Solis were misclassified on the basis of NEA, with 10% erroneously classified as well-nourished. These people tended to have a more energy-intensive activity mix than is typical of subsistence communities. This was particularly true for men, 19% of whom were misclassified as well-nourished on the basis of NEA, compared to 3% of women. In fact, NEA is more likely to misclassify women as undernourished than well-nourished. The gender disparity in misclassification is serious enough to result in the improper identification of target groups. Women were identified as facing a greater risk of malnutrition on the basis of NEA, whereas, once activity levels were controlled for, men were found to he at greater risk. If one chooses a lower cutoff value to differentiate between the well- and poorly nourished, the proportion of the population misclassified on the basis of NEA increases. A cutoff value of 0.9 resulted in the misclassification of 16"/o of the population (23"/o of the men and 11% of the women). A cutoff value of 0.8 resulted in the misclassification of 18% of the population 28% of the men and 10% of the women). As in the case of choosing a cutoff value of 1.0, when cutoff values of 0.9 or 0.8 were employed, the gender facing the greatest risk of undernutrition was misidentified on the basis of NEA.
TABLE 1. Extent and pattern of misclassification for the total population, men, and women percentages)
|Nominal energy adequacy||Corrected energy adequacy|
|Undernourished (CEA < 1.0)||Well nourished (CEA >= 1.0)||Total|
|Undernourished (NEA < 1.0)||77.6||2.7||80.3|
|Well-nourished (NEA >= 1.0)||10.0||9.7||19.7|
|Total (N= 401)||87.5||12.5||100|
|Men (N= 179)||92.7||7.3||100|
|Women (N=222)||83 3||16.7||100|
Table 2 provides an alternative way of looking at the effect of inter-individual variation in activity mix on the assessment Of nutritional status. This table compares individuals' decile ranks as assessed by NEA with those for CEA. Positive entries imply that, on average, individuals classified in a given NEA decile rank improved their rank when reclassified on the basis of CEA. Negative entries imply that, on average, individuals lost rank - that is, were seen to be less well nourished - when reclassified on the basis Of CEA.
TABLE 2. Summary measures of decile rank change after controlling for activity, by sex
|NEA decile(a)||Percentage unchanged||Mean absolute change||Mean algebraic change|
a.The first dccilc is thc most undernourished
b. Figures in parcnthcscs represent SD
On average, only 28% of men and 35% of women maintained their decile ranking after accounting for inter-individual differences in activity mix. Misclassification was especially pronounced for individuals who did not fall in the extreme NEA decile positions. Controlling for activity mix was of somewhat more importance for men than for women. Men moved an average of 13 percentiles and women an average of 10 percentiles after accounting for activity. If needs are not adjusted for intra-individual variation in activity, the nutritional well-being of men tends to he overstated and that of women understated; on average, the decile rank of men decreased by 0.8 and that of women increased by 0.7. These findings reinforce those presented in table 1 concerning the misidentification of target groups when interindividual variations in activity are ignored.
Failure to control for differences in energy needs arising from differences in the activity mix of individuals resulted in a substantial underestimation of the number of individuals with energy shortfalls in the Solis valley. The study population was relatively homogeneous; the extent of misclassification is likely to be even greater for more heterogeneous populations. The energy intensity of men's activities exceeded that of women's by a considerable margin. and this difference could lead to the improper identification of target groups if ignored. In other settings, the energy intensity of women's activities may exceed men's. While it may not be practical in most circumstances to control for differences in energy needs between individuals on the basis of activity-level data, the use of community-specific, or at least region-specific, norms in the assessment of energy adequacy would appear essential.