Studies on adipose tissue, body fat, body water and energy expenditure during the first four months of infancy using magnetic resonance imaging, skinfold measurements and the doubly labelled water method

Sammanfattning: During recent years much interest has been focused on the relationship between early nutrition and the future health of humans. Poor growth during early life has been associated with impaired glucose tolerance and coronary heart disease later in life, while rapid weight gain in infancy has been associated with an increased risk of obesity during childhood. Access to appropriate methods that can provide information about adipose tissue (AT), total body fat (TBF), and total energy expenditure (TEE) during infancy is important in order to conduct successful research in this area. The magnetic resonance imaging (MRI) technique based on the combination of a magnetic field and radiowaves can provide images of the body by which total adipose tissue volume (ATV) as well as its distribution between subcutaneous and non-subcutaneous ATV can be assessed. The doubly labelled water (DLW) method can assess total body water (TBW) and TEE. Estimates ofTBW can be used to calculate TBF. Measurements ofskinfold thickness (SFT) represent a traditional technique for assessing body fatness in humans, but even though it is simple in concept and easily applied, it has so far not been evaluated in infants under four months of age. The combination of the MRI and DLW methods represents a new possibility-to evaluate estimates of TBF based on measurements of SFT and to study various aspects of body composition during infancy. A method based on MRI which can assess total, subcutaneous and non-subcutaneous ATV was therefore developed, evaluated and applied in studies of infants.Total, subcutaneous and non-subcutaneous ATV was assessed in 46 fullterm infants between four and 131 days old (25 boys and 21 girls), and in eight moderately preterm infants (six boys and two girls) with a postnatal age of 44-75 days. The infants were also studied by means of the DLW method to assess TBW and TEE, while TBF and the degree of hydration in fat free weight (FFW) and AT fat content were calculated. TBF based on SFT measurements was also assessed in 43 of these infants.The MRI method can assess total ATV with good precision and with relative validity. During the first four months of life a considerable gain in ATV was found in both boys and girls. About 90% of the adipose tissue was located subcutaneously. Based on estimates of ATV and TBW, changes in body fat, FFW and in the degree of hydration in FFW were found to be similar to classical reference data for early infancy. During the first four months of life a significant increase in TEE (kJ/kg/24h) with age was found, probably an effect of a gradual increase in physical activity. In preterm infants at term postconceptional age and in fullterm newboms, the ATV (ml/kg body weight), TBW (%), the fat content in AT as well as the proportion of total ATV present as subcutaneous ATV were similar, while TEE (kJ/kg/24h) was 20% higher in the preterm infants than in the fullterm newboms. This difference could be explained by more rapid growth and also by higher physical activity in the preterm infants. Estimates of TBF based on SFT produces biased estimates of body fat in fullterm infants under four months of age. This bias can to a large extent be explained by a considerable variation in the fat content of AT. However, estimates of TBF obtained by means of the SFT method can be used to estimate body fatness in groups of infants with similar AT fat content.In conclusion, estimates of ATV obtained by means of MRI contributed to new knowledge regarding various aspects of body composition during infancy.