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Warunya Teokul
National Economic and
Social Development Board, Bangkok, Thailand
Philip Payne
London School of Hygiene
and Tropical Medicine, London, United Kingdom
Alan Dugdale
Department of Child Health,
University of Queensland, Australia
INTRODUCTION
In developing countries, most people make their living directly from the land. In those regions where there is one main crop a year, there are commonly wide variations in seasonal food availability and in physical economic activity. Some reports [18, 39] suggest that people tend to eat more when food is readily available, even if this does not coincide with heavy physical work, and this pattern may be the most efficient way of using the food that is available [16] . The result is that nutritional status, as measured by body weight and other anthropometric indicators, will vary throughout the year. This can have significant effects on the health and particularly the work capacity of adults.
This review deals with the seasonal variations in food intake, energy output, and nutritional status of adult males and non-pregnant adult females, as these groups carry the main economic activity of the community. The papers reviewed vary widely in their content. Some report only one of the factors, for example the level of economic activity throughout the year. There have been few reports on the economic activities of farmers practicing shifting (swidden) cultivation. (Where quantitative data are available on two or more of the factors at different seasons, the data have been tabulated and will be furnished on request by the authors. See also references 8, 25 and 27, as well as those cited throughout the text.)
SEASONAL VARIATIONS IN ECONOMIC WORK ACTIVITY
Although different methods have been used to describe the variations in economic activity throughout the year, the pattern is remarkably constant. In Thailand, the field labour force is 33 per cent higher in the peak season than in the slack season [32] . In Bangladesh, labourers work 10 to 12 hours a day in the fields in the harvest season and 5 hours in the slack season [11]. In the Philippines, the labour input into the dry-season rice crop is 71 man-hours per hectare, but 102 man-hours per hectare for the wet-season crop [2] . The output per hectare is the same for both crops.
There are very few data on groups practicing shifting agriculture. It is considerably difficult to estimate works loads, as the pattern of work varies from year to year with the cycles of clearing and planting. Four sets of data have been found. Three [12, 19, 22] were collated and recalculated by Bayliss-Smith [2] and a further study by Strickland shows the variation over the clearing-planting cycle [37] . The seasonal work outputs for these groups can be found in several sources [2, 12, 19, 33, 37]. These data must be taken as approximations only, and there are inconsistencies in the outputs of rice per hectare between Strickland [37] and those calculated by Bayliss-Smith [2]. Work allocations often refer to duration and not necessarily the intensity of work. It is also important to remember that shifting agriculture is not entirely dependent on rice; there are always other staples, as well as hunting, fishing, and collecting fruit. Nevertheless, the long duration of harvesting is consistent and must come at a time when stocks of grain from the last season are at their lowest.
In Africa the patterns of seasonal workload among farmers are the same as in Asia. In Mali, adult women work 10 to 12 hours per day in the rainy season, including 5 hours in the fields; but in the dry season they do not go to the fields [28] . Men average 8 hours a day in the fields in the wet season, but much less during the dry season. In Upper Volta, women work 3.7 hours per day in the fields in the wet season, but spend no time there in the dry season [9] . Men in this community spend more time doing heavy work in the wet season, but rest more in the dry season. Pastoralists in Mali work hardest during the dry season, when they have to carry water to their stock [28]. Brun [10] reviewed the current methods of measuring work expenditure of energy and results in some African countries.
Reports on seasonal variations in total energy expenditure are available in several sources [1,3,5,6,9,22-24,39].
SEASONAL VARIATIONS IN FOOD INTAKE
In agricultural societies where there is one main crop a year, food is freely available after the harvest, but with storage losses and use there may be little left in the growing season prior to the next harvest. Such communities tend to eat large amounts after the harvest even though their work output is low at this time [18, 39] . Prompt eating of the food in store maximizes the amount available, as there are inevitable storage losses [7]. Where climatic conditions or irrigation permits more than one crop a year, there is less need for seasonal variation in food intake as there is much less long-term storage of food. In Papua New Guinea and the South Pacific, where root crops are staples and seasonal variations in the climate or workload are not great, food intake is relatively constant [29].
In Bangladesh, where there are two rice crops each year, the variations in food intake are much less. Studies based on household intakes showed no difference in caloric intake between the peak period after the first (aus) harvest (1,744 kcal per day) and the lean period before the second (amen) harvest (1,749 kcal per day) [20]. After the amen, the daily intake rose to 1,975 kcal and then fell to 1,806 kcal before the next aus harvest.
Fewer data are available on the food intakes of pastoralists. Corkill [13,14] gave details of the diets of pastoralists in the Sudan in different seasons. His data were for families and calculated back to individuals. In the early dry period (December) the daily intake was recorded as 4,872 kcal; in the hot, dry season (July) it was 4,926 kcal; and in the rainy season (September) it was 3,733 kcal. Dietary intakes for pastoralists are also given by Benefice et al. [3] .
SEASONAL WEIGHT CHANGES
Seasonal weight changes are the rule among adults in the rural areas of developing countries. In most agricultural groups, body weight is maximal shortly after the end of the harvest, while the minimum is 2 to 5 kg less during pre. harvest cultivation. Among pastoralists, weight is lowest during the hot, dry season when the milk supply from animals is lowest and activity is high [38].
Body weight and body composition are indicators of nutritional status, but, more important, are related to physical work capacity. Several studies showed that work capacity is related to fat-free body mass, which is closely related to body muscle mass [15,17,34,35]. Spurr et al. [36] also showed that maximal oxygen consumption was related to lean body mass and productivity. However, Immink et al. [26] found no relationship between mid-arm muscle circumference and productivity in sugar-cane cutters.
The data from an Indian village in Andhra Pradesh [4] are not given quantitatively, but the authors indicate that only a small and non-significant decrease in body weight and other anthropometric indicators occurred during the rainy season. The number of days worked was higher at this time and food intake did not vary throughout the year.
DISCUSSION
Seasonal variations in adult nutrition have not been widely reported, although the importance of season on the health and growth of children is well known [30,31] . The communities reported here are not a random sample of rural groups in developing countries, but are widely separated geographically and culturally. Most groups, particularly those who had a single staple crop a year with little opportunity for other work, showed significant seasonal variations in body weight and other anthropometric measurements. The range and timing of the weight changes are similar in men and women. In the Papua New Guinea study, the authors [29] commented that there were no seasonal changes but that these communities ate largely root crops that were harvested throughout much of the year. In the Andhra Pradesh study [4] the seasonal changes were not statistically significant. This Indian study shows the levelling effects of a complex and sophisticated interaction between a single major crop and a system of wage payments, credits, and price-support schemes.
In only three studies were energy balances measured. The Papua New Guinea study by Norgon et al. [29] showed no significant seasonal differences and the authors reported no major changes in crop productivity. Both the Gambian and the Burmese studies [18,39] showed similar patterns of energy intake and expenditure. These patterns also explained the observed changes in body weight. Both groups had a single annual cereal crop. Immediately after the harvest when the work load was lighter but the food stores full, food intake was high. Eating food soon after harvest decreased storage losses but increased maintenance metabolic energy. Several months later when the rains started, the preparation of the ground for the next crop demanded heavy work, but at this time food stocks and food intake were lower. Harvesting was the period of heaviest work, but food intake at this time did not rise to cover energy output. A computer simulation used to model the effects of different patterns of food consumption [16] showed that the observed pattern is more efficient than the one classically advocated by nutritionists, that is, that food should be taken to cover energy needs.
Pastoralists also have seasonal weight changes. Immediately after the rainy season there is plenty of water and feed for animals. Milk and other products are plentiful and labour needs low. Later in the dry season food and water are less plentiful, so productivity is down and considerable effort is needed to ensure that the stock receive enough food and water to survive. The situation is complicated by the farming that some pastoralist groups do during the wet season.
Data from groups practicing shifting cultivation show widely different levels of activity, but these people eat a wide range of foods in addition to their staple crop.
The periodic increases in body weight are unlikely to lead to the diseases of affluence, as the body mass index (BMI) is still near the bottom of the normal range. However, the decreases in weight could have an effect on the work capacity of both men and women. Evidence exists [15,17, 34,35] that maximal work capacity is related to lean body mass, which reflects largely muscle tissue. Farmers must do heavy work when their BMI is low, and muscle mass is likely to be an important determinant in their ability to cultivate the fields for next year's crop. In no group does the mean BMI fall below 17, and this might be regarded as a minimal level for adults who have to do heavy work.
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