|Activity, Energy Expenditure and Energy Requirements of Infants and Children (International Dietary Energy Consultative Group - IDECG, 1989, 412 pages)|
|Short- and long-term effects of low or restricted energy intakes on the activity of infants and children|
|5. Short- and long-term effects|
The studies that have looked at the effects of low or restricted energy intakes on the activity of infants and children, although small in number, coincide in that reduced intake leads to shorter duration or lower intensity of physical activity. This occurs within a few days after dietary energy decreases VITERI and TORUN, 1981). Initially, it seems to be a compensatory response to maintain energy balance without comprising growth. But if the dietary restriction is large enough, the decrease in activity and overall energy expenditure is not sufficient to preserve growth (TORUN and VITERI, 1981a).
Children with a chronically low energy intake that is not so extreme as to produce the clinical signs and metabolic changes of severe malnutrition, maintain a state of energy balance. In discussing the implications of this balance, it is necessary to differentiate between what may be called adaptation and accomodation.
We speak of adaptation when metabolic and behavioral modifications allow a more efficient use of the energy available without producing undesirable effects (e.g., reducing BMR or performing mechanical work with less costly movements). This has a different connotation from metabolic and behavioral modifications that permit survival and function at the expense of actually or potentially undesirable effects (e.g., increased risk of diseases and severe malnutrition, or limitations to perform activities that are healthy or economically and socially desirable). This can be referred to as accomodation (SRIMSHAW and YOUNG, 1989; BENGOA et al., 1989).
A decrease in BMR in the absence of reduced growth rate or an improvement in mechanical efficiency has not been demonstrated in undernourished children to explain the reduction in total energy expenditure shown in the studies discussed above. Even if these phenomena had occured without being detected, they could not be the sole explanation for the reduction in energy expenditure. Infants, preschool children and school-age children did become less active and engaged less or no longer in certain activities (CHAVEZ et al., 1972; CHAVEZ and MARTINEZ, 1979; RUTISHAUSER and WHITEHEAD, 1972; VITERI and TORUN, 1981; TORUN and CHEW, unpublished; SPURR and REINA, 1989a).
This was more evident in younger children, probably because they were subject to less social constraints and peer pressure. In the case of children who go to school, it seems that the school routine, which occupies a large proportion of daytime and restricts free physical activity, tends to mask the potential differences in activity related to nutritional status. The differences may become evident when children are given the opportunity and are encouraged to be more physically active, as seen in the summer-camp studies of SPURR and REINA (1988c).
CHAVEZ and MARTINEZ (1979) showed enhanced behavioral and exploratory activities associated with increased physical activity in the better nourished young preschool children. From the studies of preschoolers in Uganda and Guatemala it can be inferred that children who had adequate energy intakes and spent less time sitting or lying down and more time walking and moving around, had more frequent interactions with their peers, adults and their physical environment. This was, in fact, the subjective appraisal of the investigators and staff who participated in the clinical and community studies in Guatemala. Other investigators have also suggested that low energy intake and expenditure are likely to decrease the interactions between children and their immediate environment (GRAVES, 1976, 1978; RICCIUTI, 1981; BEATON, 1983).
POLLITT (1987) has pointed out the conceptual and methodological problems that do not allow making definitive statements about the influence of dietary energy deficiency on cognitive and socioemotional development. One of the major obstacles is the difficulty - if not impossibility - of isolating the nutritional components from other factors that affect the behavior and development of children. Nevertheless, the evidence that better nutrition allows more activity supports the suggestion of a sequential cause-effect linkage between adequate energy intake ® enhanced physical activity ® more interaction with people and the environment ® better social performance and cognitive development.
Physical activity is markedly reduced in malnourished children. VITERI (1973) used an animal model to demonstrate that this reduced activity contributed, by itself, to growth impairment. Weanling rats were fed either 50 or 73% of the food normally eaten by rats of the same age. When they were inactive, living in small metabolic cages, growth in length and weight gain were significantly less (p < 0.01) than in pair-fed animals who lived in larger cages and were forced to run in a revolving drum twice daily.
The negative effect of inactivity on the growth of malnourished animals and the positive effect of exercise can be reversed, as shown in another experiment with a cross-over design with rats fed 60% of the normal food intake VITERI and TORUN, 1981). Whether the animals were inactive from the beginning of the study or became inactive after an initial period of forced activity, growth rate decreased. Conversely, an increment in activity produced better growth in length and weight.
The positive role of activity was further confirmed in the course of the nutritional rehabilitation of malnourished 2- to 4-year-old children (TORUN et al., 1976, 1979). When a group of patients were encouraged to participate in games that involved running after a ball, walking up a slope, climbing stairs and rolling and tumbling, they grew more in length and lean body mass than a control group of patients who continued with the usual, limited activities customary in most nutritional rehabilitation centers. Based on individual weekly calibrations of heart rate to oxygen consumption and heart rate monitoring, mean energy expenditure during the daytime was calculated as 1.97 X BMR in the more active children, compared with 1.70 X BMR in the control group.
In a more recent study using a similar program of physical activity in a hospital for malnourished children in Guatemala, but with food intake ad libitum, preliminary results indicated that the more active patients reached normal weight-for-height earlier than their less active counterparts (URIZAR and TORUN, unpublished).
It has not been shown whether physical activity has similar effects on the growth of children with mild and moderate dietary energy deficiency, but there are no reasons to believe that they would differ in this respect from the more severly malnourished children. As to the question of "How important is big?", we can answer that larger muscle (protein) and energy (fat) reserves might give poor, underprivileged children better protection against a recurrence of protein-energy malnutrition. Furthermore, as discussed below, small body size can limit maximal work output.
Physical fitness may influence the type of activities performed by children and the time allocated to those that demand most energy. Only a few studies have been done on the overall physical fitness of undernourished children or on its modification with changes in nutritional conditions. These have been based on aerobic capacity as a function of heart rate, on submaximal aerobic tests and on maximal oxygen consumption.
Physical fitness is reduced in children with severe protein-energy malnutrition. When adequate treatment is given, physical fitness increases with improvement of nutrition conditions, as shown by TORUN et al. (1976, 1979) in children between 2 and 4 years old. Figure 8 shows that, as nutritional rehabilitation progressed, there was a gradual weekly increment in the regression coefficients of oxygen consumption on heart rate, which indicates an increase in aerobic capacity.
Submaximal exercise tests have been done using a treadmill or bicycle ergometer in 6-year-old Colombian children (SPURR et al., 1978) and in children of school-age in Ethiopia (ARESKOG et al., 1969), Tanzania (DAVIES, 1973a, b), India (SATYANARAYANA et al., 1979), and Brazil (DESAI et al., 1983). Maximal oxygen consumption while walking on a treadmill, has been measured in Colombian boys and girls between 6 and 16 years old (SPURR and REINA, 1989b; other studies summarized by Spurr, 1983, 1987). The results obtained in most of those studies indicate that undernourished children had lower maximal oxygen consumption values, compared to children with better nutritional status or background from their own country, or from the United States and Europe. This is illustrated in Figure 9. However, the effect of nutritional status disappears when aerobic capacity is expressed per unit of body weight or lean body mass. Thus, it seems that the physiological potential to perform physical work is maintained in children with mild or moderate malnutrition, but their smaller size limits the maximal effort that they are capable of. This is consistent with the observations and conclusions of VITERI (1971) and SPURR (1983, 1987) in undernourished adults with reduced maximal oxygen consumption, largely due to their decreased muscle mass.
The decrease in maximal aerobic capacity may have a negative impact on the children's ability to do physical work that demands high energy expenditure. In rural areas of the developing world, culture and economics often demand that children of school age and adolescents engage in heavy physical work from an early age. Even in societies where child labor is not customary, the small size of undernourished children may have important consequences on physical activity in the long term. It is conceivable that these undernourished children will become small adults with depressed physical work capacity and reduced productivity in heavy work (DESAI et al., 1984; IMMINK et al., 1984; SATYANARAYANA et al., 1979; SPURR, 1983; VITERI, 1971).