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Short- and long-term effects of low or restricted energy intakes on the activity of infants and children

Abstract
1. Introduction
2. Infants and children under two years of age
3. Preschool children

3.1. Short-term study in a clinical setting
3.2. Longer-term study in a clinical setting
3.3. Community-based studies

4. School-age children
5. Short- and long-term effects

5.1. Adaptation and accomodation
5.2. Reduction in energy expenditure
5.3. Behavior and social performance
5.4. Low physical activity and growth
5.5. Reduction in physical fitness

6. Conclusions
Acknowledgements
References
Discussion (summarized by N. Solomons)

B. TORUN *

* Institute of Nutrition of Central America and Panama (INCAP), Apartado Postal 1188, 01901 Guatemala City, Guatemala.

Abstract

Low or restricted energy intakes reduce the amount or intensity of physical activity of infants, preschool and school children. This can occur within a few days as a compensatory response to maintain energy balance while preserving growth. If energy intake becomes adequate again after a short time, it may have no important morphologic, functional or behavioral consequences. Conversely, a prolonged dietary deficit may result in a sustained decrease in physical activity that can limit or reduce the child's physical fitness, social interactions and exploration of the immediate environment. This is more evident in younger children who are less influenced by social constraints and peer pressure, and among children who are encouraged to participate in physically demanding games and sports, or who must work or perform chores that result in relatively high energy expenditures. A reduced physical activity can also contribute to a decrease in longitudinal and lean body mass growth which, together with a decrease in maximal oxygen consumption, may limit maximal work output. All this may hinder the child's potential for biological, behavioral and social development. Therefore, the reduction in physical activity due to low dietary intakes may be an adequate compensatory response in the short term, but it cannot be considered a desirable adaptation because of its actual or potential consequences in the long term.

1. Introduction

Apart from the energy needs for growth, energy expenditure determines the dietary energy requirements of children. Within limits that allow physiological accommodation or metabolic adaptation, there tends to be a balance, such that expenditure influences intake and vice versa. In order to maintain that balance, a decrease in energy intake would have to reduce some or all the components of energy expenditure and/or reduce growth.

The effect of low energy intake on the growth of children is widely recognized. However, it only becomes evident after a period of several weeks or months, depending on the child's age. Furthermore, this reduction in growth is one of the deleterious end-results of low intake and, as such, cannot be considered a desirable adaptive response.

A reduction in energy expenditure might be an adequate metabolic response, as long as it does not limit the child's function or behavior. This could be manifested by a decrease in basal metabolic rate (BMR) while the child is growing adequately, or by a greater efficiency in the performance of energy-demanding activities. The latter has not been adequately studied and the former is not known to occur, as a decrease in BMR has been demonstrated only after growth is already impaired.

Physical activity becomes a major component of energy expenditure after early infancy. From that age onward, a decrease in energy intake could be expected to induce a compensatory reduction in activity. This paper will review the evidence that such reduction occurs, and it will analyze whether this is a suitable response or an undesirable effect with negative consequences for the child.

2. Infants and children under two years of age

CHAVEZ and collaborators (1979) conducted a longitudinal study in a rural Mexican community of about 300 families to assess the influence of nutritional conditions on various physical and behavioral functions. Forty-one women received a daily supplement of milk, vitamins and minerals, beginning on the second month of pregnancy and continuing through lactation until they became pregnant again. Their babies were also supplemented with milk and strained foods.

These children grew better than their non-supplemented counterparts and they were significantly heavier by 11 and taller by 17 months of age. The differences continued increasing gradually with age.

Physical activity was assessed for one year at 2- or 3-month intervals in 19 supplemented and 17 non-supplemented children, whose ages ranged between 4 and 12 months at the beginning of the study (CHAVEZ et al., 1972). They were observed on one day for 10 minutes every 2 hours, between 8 a.m. and 8 p.m. The observer recorded the number of steps taken by the child during that period and, in babies who did not walk or were lying down, the number of kicks or other foot contacts with the floor, mattress, sides of the crib, and walls. Physical activity was quantified in terms of 'foot contacts' in 60 minutes. Figure 1 shows the results of 47 sixty-minute observations in each group. Between 13 and 24 months of age, the supplemented children were 3 to 6 times more active that the controls (p < 0.01).

Other observations in the children's homes at 2- to 6-month intervals also suggested that the better-nourished were more active (CHAVEZ and MARTINEZ, 1979):

- After 40 weeks of age, the supplemented group tended to sleep less during the day. By 1 year of age, they slept approximately 25% less and were out of their cribs more often that the non-supplemented controls.

- At around 8 months of age, the supplemented children already tended to spend more time outdoors. According to Chavez and Martinez, this did not depend only on the mothers' attitudes but also on the children's independent movements and demands.

- During the second semester of life, the supplemented infants were carried, wrapped in a shawl on their mother's backs, 30% less time than the controls. This seemed partly due to increased restlessness of the children, and partly to the greater burden imposed on the mothers by their heavier body weights.

- 'Play' activities were recorded in several supplemented babies beginning at 6 months of age, whereas they began 12 weeks later in most of the control group.

CHAVEZ and MARTINEZ (1979) also showed exploratory and behavioral differences between the supplemented and non-supplemented children. A child was placed in the center of a square of 3 x 3 m, surrounded by a 90 cm fence and with perpendicular lines drawn as a grid on the floor at 30 cm intervals. The child's mother stood outside the fence in the middle of one side, toys were placed inside the fenced area on the opposite side, and two observers stood outside the fence in the middle of the other two sides of the square. The child's movements within the square and its behavior were recorded during 10 minutes by both observers. Figure 2 shows that a supplemented 2-year-old child moved around more, played with the toys, approached the unknown observers and did not cry, in contrast with a non-supplemented counterpart.

The increased activity seen in these studies among the supplemented children could be due to many factors, including the child's greater and more advanced motor development with better nutrition, the mother's attitude towards a healthier-looking baby and other behavioral modifications among the family members induced by the supplementation program. Nevertheless, a causal effect of the higher food (i.e., energy) intake cannot be ruled out.

3. Preschool children

3.1. Short-term study in a clinical setting
3.2. Longer-term study in a clinical setting
3.3. Community-based studies

3.1. Short-term study in a clinical setting

A direct effect of food intake on activity was clearly demonstrated in short-term studies of well-nourished Guatemalan children who had fully recovered from protein-energy malnutrition, before and after taking them off the high-energy, high-protein therapeutic diet (VITERI and TORUN, 1981). A minute-by-minute time-motion technique was used under controlled clinical conditions to record the activities of 5 boys, 1.5-4.5 years old, on 4 days prior to, and in the last 4 days of a week during which dietary intake was reduced from 120-150 kcal and 3-4 g protein/kg/d to the 70-90 kcal and 1.8-2.0 g protein/kg/d provided by the home diets of most children from poor Guatemalan families. Supplementation with vitamins and minerals did not change.

Table 1 and Figure 3 show that the time spent in the more energy-demanding activities was reduced, on the average, by 17 to 56%. In contrast, the time spent lying down in the games' room, either resting or in sedentary play, increased twofold.

Table 1. Changes in time allocated to different physical activities by 6 preschool children after dietary intake was reduced from 120-150 to 70-90 kcal/kg/d (mean of 4 days)

* n = 4
Source: VITERI and TORUN, 1981.

Basal metabolic rate remained constant at 55 ±5 and 53 ±4 kcal/kg/d (mean ±SD), before and one week after the dietary change, respectively. Using indirect calorimetric measurements for some activities and estimates of the energy cost of others, it was calculated that, on the average, the children were in energy balance during the two weeks with standard deviations of 19 and 27 kcal/kg/d, respectively. Mean weekly weight changes decreased from 2.5 ±1.2 to -0.5 ±0.6 g/kg/d. The mean weight gain expected in healthy children of the same ages and heights, eating home diets with about 100 kcal/kg/d, is approximately 0.6 g/kg/d (US-NCHS, 1976; FAO/WHO/UNU, 1985).

This study showed that changes in activity pattern occur within a few days of the reduction in dietary energy intake. It is unlikely that this was due to the concurrent decrease in protein intake, as even the lower diet contained more than the recommended safe levels of protein (FAO/WHO/UNU, 1985).

3.2. Longer-term study in a clinical setting

Five boys, 25 to 40 months old, participated in a longer-term study, also under strict supervision at INCAP's Clinical Center (TORUN and VITERI, 1981a). Dietary modifications only involved decreases in energy density, without changes in protein, vitamin and mineral contents, and the energy reductions were of a smaller magnitude than in the preceding study.

The children first ate a diet that provided net or metabolizable energy (i.e., food energy minus fecal energy, measured by bomb calorimetry) equivalent to 90 ±3 kcal/kg/d, which had been shown to be the requirement of similar children living at INCAP's Clinical Center (VITERI et al., 1981; TORUN and VITERI, 1981b). The energy content of the diet was then reduced twice at 40-day intervals. During these experimental periods the children ate the equivalent of 82 ±4 and 71 ±4 metabolizable kcal/kg/d. The lower of those levels of intake is often found in dietary surveys among low-income preschool children in Guatemala.

The children were encouraged, but not forced, to participate in active play several times each day. Their energy expenditure was calculated every 20 days from individual calibrations of heart rate to oxygen consumption relationship and heart rate monitoring during the day, and from their basal metabolic rates at night.

Table 2 shows that the reduction in energy intake from 90 to 82 kcal/kg/d was accompanied by a decrease in total daily energy expenditure (p < 0.025) without affecting weight gain. An additional reduction of intake to 71 kcal/kg/d did not significantly affect expenditure, but weight gain was markedly reduced (p < 0.01). Growth in height was not affected.

Table 2. Total energy expenditure, energy balance and weight gain of 5 children with successive reduction in energy intake (mean ±standard deviation) ^a