Subjects and methods

The study was carried out on 17 free living infants of low socioeconomic status in the south east of Santiago, Chile. The criteria for their selection were: age 1 month at the beginning of the study; birth weight more than 2900 g; exclusively breast fed; no congenital pathologies; and an APGAR score of 9 at five minutes. When the mothers had been informed about the experimental protocol and had agreed to follow it, they were asked to give written consent. The nutritional review board of the Institute of Nutrition and Food Technology and the ethical committee of the Sotero Del Rio Hospital approved the study. Table 1 gives the characteristics of the 17 infants studied.

When the infants were 1 month old, a nutritionist and a nurse visited their homes and, after confirming that the infants were exclusively breast fed, asked the mother to collect a sample of the infant's urine (spot sample). Infant's weight, height, tricipital skinfold, and arm and head circumferences, and mother's weight were recorded. Infants were weighed again at the end of the seven day measurement period. A record was kept of the small amounts of extra fluids ingested by the infant during the follow up visits for the sampling collection.

The day after baseline samples had been collected, a doubly labelled water dose (0.2 g/kg deuterium oxide (99.9%) and 2.0 g/kg 10% H₂ ¹⁸O) was given to the baby to assess milk intake, body composition, and energy expenditure. For seven consecutive days after this dose, a nurse visited the house every other day to supervise the mother's collection of the daily urine spot sample and the recording of the time of collection. The procedure was conducted according to the multipoint protocol described by Coward,9and, to ensure linearity of the data, the first and last samples were collected by the nurse herself, who also checked on extra fluid intake and the infant's health.

Samples were stored in refrigerators or kept in a cold place in the house, and all urine samples were frozen thereafter at −20°C until analysis.

Results

Table 1 shows the general data for the infants. It can be seen that they have normal birth weight and height, and their mean growth in weight and height in the first month followed the 50th percentile (National Center for Health Statistics). A mean (SD) weight gain of 34.0 (7.5) g/day corresponds to a milk intake of 728 (101) g/day, which was sufficient to provide for normal growth during this first month of life.

Table 2 gives data on body composition in the infants. The percentage body water in relation to weight is 64.9, consistent with other studies and lower than values suggested by Fomon et al.21 The percentage body fat and rate of weight gain (34.0 (7.5) g/day) are in line with changes in the reference child21 during the first month of life.

Table 3 shows the measured daily energy expenditure: 1205 (312) kJ. The energy stored in new tissue was calculated using values of 38.7 kJ/g fat and 23.6 kJ/g protein22, giving a total of energy stored of 607 (130) kJ/day. Daily energy requirements were obtained by adding together daily energy expenditure plus energy stored (1812 (351) kJ/day). Total energy requirement was 391 (60) kJ/kg/day at 1 month of age.

Energy supplied by milk (728 g) was converted into metabolisable energy content assuming that 92% of milk is used in growth. This was compared with the amount of energy needed by infants at this age, which gave an energy content of milk of 2.71 kJ/g (0.65 kcal/g).

In table 4, the present results are compared with the energy requirements found for similar infants in the United Kingdom and United States, with the same type of measurements.

Discussion

Stable isotopes have proved extremely useful for field studies on breast milk intake and body composition.8 ,15 It is also possible to measure energy expenditure in infants. The evaluation of energy metabolism in free living unrestricted infants is of particular importance for setting future energy recommendations and to study the effect of diet on energy metabolism, energy balance, and growth.23 Establishing energy requirements by the doubly labelled water method for energy expenditure and calculating the energy needed for adequate growth (metabolisable energy) are of particular relevance in breast fed infants.

The advantages of the doubly labelled water method are the non-invasive character of the measurements, the relative ease with which it can be applied to infants in field conditions, and the possibility of combining this type of evaluation with others, such as anthropometry, milk intake, and body composition, providing valuable information on energy balance and growth. The main disadvantages are the high cost of the isotopes, in particular ¹⁸O, and the complexity of the laboratory measurements, which require a mass spectrometer and trained personnel.

To our knowledge, this is the first energy expenditure study carried out in 1 month old infants in Latin America. Previous studies were conducted in protein malnourished 3–18 month old children in an inpatient metabolic ward in Lima, Peru24, the above mentioned study in 4 and 6 month old Mexican infants,4 and also similar studies carried out in Gambia.25

The infants in this study were comparable with those in previous studies in the United Kingdom7 ,13 and United States16 with respect to birth weight, healthy nutritional status at the time of evaluation, breast feeding status, and adequate rate of growth. In this study, daily total energy expenditure in infants is in close agreement with previous results in 1 month old infants from developed countries, as shown in table 4.

Estimates of energy requirement have been based on energy intake,1 and the recommendations for this age group are 485 kJ/kg/day. The energy requirements determined for these Chilean infants (391 kJ/kg/day) are 19.4% lower than the FAO/WHO/UNU recommendations, which is also in agreement with incoming results from different groups of researchers, suggesting that adequate growth may be sustained on a smaller supply of energy than recommended at present. In this study, it is also clear that milk intake is sufficient to provide the necessary energy for normal growth during the first month of life. This fact is relevant for many developing countries, stressing the necessity to continue to promote exclusive breast feeding during the first months of life. For many developing countries, and most developed countries, where obesity is an important public health and nutrition problem, it is also important to establish the real energy requirements in order to avoid excess intake, and thereby prevent obesity and chronic diseases from early childhood.