Weight Gain Composition during the First Year of Life

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Evaluation of weight gain composition is of major importance in the assessment of the nutritional requirements of preterm and term infants. Up to now, duplicate analyses of 3-day metabolic and energy balances have been performed to estimate energy expenditure in association with nitrogen and energy retention in order to estimate FM deposition, LBM gain and protein accretion in preterm and term infants according to the feeding regimen. In addition, measurement of calcium and phosphorus retention enabled estimation of the adequacy of mineral deposition compared to reference values. Those data were considered as representative of growth quality over a larger period.

Analysis of the data presently available for preterm infants made it possible to evaluate the main determinants of weight gain, nitrogen retention and FM deposition [27]. Protein intake and the protein energy ratio are the main

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Fig. 3. Evolution of fat mass (FM) and fat-free mass (FFM) related to age, weight and length. From De Bruin et al. [13], Olhager et al. [2], Rigo et al. [5], Fomon and Nelson [4] and Butte et al. [7].

determinants of weight gain. Protein intake is the only determinant of LBM gain in contrast to FM gain that is positively related to energy intake and negatively to protein energy ratio. Thus, protein and energy needs are reciprocally limiting. If there is a surfeit, one affecting the ability of the infant to assimilate b a d c c

Fig. 4. Weight gain (WG) composition evaluated from successive DEXA scans in preterm infants fed fortified human milk (n = 50) and preterm formula (n = 100; ap < 0.05 vs. PT HMF), in preterm infants fed formula after discharge (n = 26) and in breast-fed (n = 16) and formula-fed (n = 47) term infants (bp < 0.05 vs. T BF). BF = Breast-fed; FF = formula-fed; FFM = fat-free mass; FM = fat mass; HMF = human milk fortified; PD = postdischarge; PT = preterm; PTF = preterm formula; T = term.

Fig. 4. Weight gain (WG) composition evaluated from successive DEXA scans in preterm infants fed fortified human milk (n = 50) and preterm formula (n = 100; ap < 0.05 vs. PT HMF), in preterm infants fed formula after discharge (n = 26) and in breast-fed (n = 16) and formula-fed (n = 47) term infants (bp < 0.05 vs. T BF). BF = Breast-fed; FF = formula-fed; FFM = fat-free mass; FM = fat mass; HMF = human milk fortified; PD = postdischarge; PT = preterm; PTF = preterm formula; T = term.

the other, and if energy intake is insufficient, protein is used as an energy source and nitrogen balance becomes less positive. Increasing the caloric intake will spare protein loss and improve nitrogen retention, but with limited protein intake, protein retention reaches a plateau and the energy excess is used only for fat deposition. Nevertheless, when protein supply is in the range of the protein requirement, the effect of the energy increase on protein retention appears to be minimal. Therefore, with a view to increasing the LBM accretion and limiting the FM deposition in the premature infant, an increase in the protein energy ratio is mandatory.

Unfortunately energy and metabolic balances, requiring the use of a metabolic bed, infant relative contention and a reduction in nursing, are relatively limited. Therefore, we recently suggested that DEXA might also be used to analyze weight gain composition over a longer period of time and in a less invasive fashion [28-30].

DEXA body composition was measured at the beginning of the study and then 3 weeks later. Lean, fat and bone mass gain was determined by subtracting the second from the first determination. In preterm infants (birth weight < 1,750 g) fed either fortified human milk or preterm infant formula, the weight gain composition evaluated between a mean of 34 and 37 weeks of post-conceptional age was in the range of the data obtained previously using energy and nutrient balance (fig. 4) [30]. Formula-fed infants showed a greater weight gain, FM deposition, BMC gain and bone area increase compared with the fortified human milk group. In contrast, calcium retention estimated from bone mineral gain was greater than that determined with nutrient balance techniques. The reasons for the latter observation are not entirely clear but might be explained by the low threshold level of bone mineral detection necessary to estimate bone mass in preterm infants [5].

In term infants, the growth pattern of breast-fed infants is known to deviate from that of formula-fed infants. Mainly after 3 months of life, breast-fed infants grow more slowly than formula-fed infants even if a relative catch-up growth tends to occur during the 2nd year of life. Evaluating body composition, Butte et al. [31] recently suggested that FFM was lower in breast-fed infants compared to formula-fed infants at 3, 6 and 9 months of age. In addition, a gender effect was also reported. Similarly, weight gain composition was also estimated in term infants breast-fed and formula-fed during the first 2 months of life.

The use of DEXA allows evaluation of the weight gain composition in term infants. From birth to 2 months of age, we compared body composition in a limited number of breast-fed (n = 16) and formula-fed (n = 47) infants. From birth up to 2 months, weight gain (31.3 ± 6.9 vs. 35.8 ± 7.4g/day; p = 0.04) and FFM gain (19.7 ± 3.9 vs. 22.7 ± 4.5g/day) were significantly lower in breast-fed than formula-fed infants. The percent FM increased in the 2 groups from 15.1 to 23.8% in breast-fed infants and from 14.8 to 24.0% in formula-fed infants. During the study period, weight gain corresponded to about 75% of the birth weight and was similar in the 2 groups. FM deposition accounted for 36.3 and 35.8% of the weight gain in breast-fed and formula-fed infants, respectively [29].

During these studies [29, 30], it was also possible to determine the minimal detectable changes in weight gain composition according to time in preterm and term infants in relation to the size of the population. In 3 weeks, between 1,500 and 2,200g body weight at the time of discharge, the minimal detectable differences in body weight gain were 2.3 g for weight, 2.1 g for LBM, 1.2 g for FM, and 76 mg/kg for BMC between the 2 groups of 20 very low birth weight infants. Considering the relatively low FM content at the time of discharge, this sensitivity (±30%) appears to be better than could be obtained by other indirect methods in preterm infants [30].

In comparison, in term infants from birth to 2 months of age, the minimal detectable differences in body weight gain were 1.9 g for weight, 0.8 g for LBM, 1.5 g for FM and 50 mg/kg for BMC between the 2 groups of 15 infants. In our study, the highest differences were for weight gain (0.7g/kg/day; p = 0.37) and FM gain (0.5g/kg/day; p = 0.39). These values, corresponding to 27 and 33% of the minimal significant differences, should be reevaluated in a larger cohort of infants [29].

In conclusion, determination of whole body and weight gain composition is one of the major keys to the evaluation of the nutritional requirements of preterm and term infants, whereas the dynamic aspect of body composition during the first years of life plays a fundamental role in the nutritional programming of adult morbidity. In preterm and term infants, the only practical means of measuring body composition is by noninvasive and indirect methods. Of the various techniques presently available, anthropometry and impedancemetry have not yet been appropriately validated in young infants. TOBEC and stable isotope tracer dilution techniques appear more appropriate but difficult to adapt for widespread use in infants. In contrast, although in investigation, ADP seems easy to use, comfortable, reliable, and accurate in infants, but further investigations are needed to establish normative values and to validate weight gain determination. DXA, a widely available technique using the three-compartment model, is now considered by some as the reference technique for determining body and weight gain composition. The technical procedure for DXA scan acquisition and analysis is quite simple. Radiation exposure to the infant is minimal. DXA techniques have been validated for measuring body composition in infants. Unfortunately, commercial DXA instruments have major differences, and software especially adapted to preterm and term infants is not always available.

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