|Chronic Energy Deficiency : Consequences and Related Issues (International Dietary Energy Consultative Group - IDECG, 1987, 201 pages)|
|Seasonality in energy metabolism|
|3. Seasonal body weight fluctuations|
Since children are the most vulnerable and sensitive members of the community, it is obvious to look at them in search of early symptoms of seasonal impact. Evidently, crude weight changes can not be a suitable indicator during childhood, but growth rates can represent an acceptable model for investigating seasonal pressures on the energy metabolism of the child, while birth weight can be an indicator of maternal and foetal energy status combined.
Seasonal differences in mean birth weight, ranging from 150 to 450 9, have been reported in several countries where marked seasonality in food availability and/or in work demand exists, such as rural Taiwan (ADAGE, 1984) and rural Gambia (PRENTICE et al., 1983) (Figure 2). The picture is complicated, however, by the unexpected fact that seasonal differences in birth weight have been reported also in areas where climatic and agricultural seasonality are small, if at all present (e.g., in New Guinea; CRITTENDEN and BAINES, 1986). To explain this observation, special external pressures were thought to operate, disrupting the fragile New Guinean ecosystem, and the precarious agricultural and socioeconomic system. This disruption supposedly resulted in a great amplification of the originally very modest seasonal cycling.
Marked seasonal differences in weight and in skinfold thick-nesses have been described in children of Sahelian pastoralist populations, the Wodaabe (LOUTAN and LAMOTTE, 1984) and the Ferlo (BENEFICE, CHEVASSUS-AGNES and BARRAL, 1984). AS can be seen in Figure 3, the impact on Ferlo children depends on age. Note that the growth disturbances occurred at different times of the year in the two ethnic groups, despite their shared geographic and climatic ecozone. This lack of synchronism highlights the complexity of the interaction between environmental seasonality and sociocultural, physiological and pathological variables. Upper Volta children living in a country with high seasonality index did not show the expected seasonal changes in their growth rates (SERRE, 1955). New Guinean children from the Nembi plateau (CRITTENDEN and BAINES, 1986) were also peculiar as they exhibited seasonal patterns of growth, in spite of the absence of climatic and agricultural seasonality (Figure 4). The picture in this case was further complicated by the observation that the children born during one part of the year with low birth weights, had a better postnatal growth performance than the children born in the "better" part of the year with higher birth weights. Seasonal patterns in growth performance were found to occur also in urban areas (TOMKINS et al., 1986), closely reproducing the rural pattern (ROWLAND, COLE and WHITEHEAD, 1977) with lowest increments during the rainy season, and compensatory catch-up growth during the dry months (Figure 5). These findings were unexpected because of the cash economy of the urban environment and the steady demand for women labour throughout the year.
In trying to understand these findings, several ideas have been put forward. Infectious diseases may play a major role in the urban Gambian environment (TOMKINS et al., 1986). For the New Guinean children of the Nembi plateau, the initial benefit of a higher birth weight during the "good" part of the year may be counterbalanced by an earlier exposure to weaning stress and the accompanying loss of passive immunity, because mothers resumed their field work when prevalence of infectious diseases was the highest of the year (CRITTENDEN and BAINES. 1986).
- Wodaabe (LOUTAN and LAMOTTE, 1984): weight changes (in absolute values) of children aged 1 to 5 years.
- Ferlo (BENEFICE, CHEVASSUS AGNES and BARRAL, 1984): mean weight-for-height changes in different age groups.
In conclusion, seasonal fluctuations in birth weight and in infant and child growth performances have been reported from various parts of the world; they have been considered to reflect the seasonal occurrence of a powerful synergism between mother's workload and food shortages. However, the occurrence of fluctuations in growth velocities in areas of the world without seasonality suggests that factors other than primary energy imbalances may play an important role, namely the close interrelation between environmental seasonality, infectious diseases and sociocultural characteristics. Furthermore, it is impossible to evaluate the extent to which the observed growth retardation represents the direct expression of a primary perturbation of the energy metabolism of the child, rather than being an unspecific reactive body response to a multiplicity of environmental stress-ors. It is important to remember that seasonal fluctuations in weight and height gains have been described also in children of affluent societies, even when controlling for infectious diseases. This should be kept in mind when examining seasonal growth disturbances in developing countries, as a significant component of it "may be found to be due to factors other than lack of food or incidence of disease" (PAYNE, in press).