Search

The influence of nutrient intake on body composition in growing low birthweight infants is essential knowledge in order to optimize nutritional management for the support of growth and development. Until recently, indirect methods of measuring body composition were inappropriate; but the latest generation of whole body densitometers, dual-energy X-ray absorptiometry (DXA) may be a useful tool for this age group. The application of DXA for quantitative assessment of fat, lean and bone mass should provide measures with precision and accuracy while being safe and noninvasive for use in infants and small children. Our objective was to establish the precision and accuracy of DXA measurements of whole body bone mineral content (BMC), fat and lean tissue in young piglets which are similar in body composition to infants1,2.

There has long been controversy over the optimal form of enteral feeding for the premature infant. It is generally accepted that human milk is the desired form of nutrition for the term infant, but it has been proposed that human milk, especially banked mature human milk, is not suitable for the premature infant1,5. Currently, attention has been focused on assessing the adequacy of feeding the preterm infant milk from his/her own mother2–8. Atkinson et al.5 have suggested that the use of preterm milk supports significantly better nitrogen retention and protein nutritional status than pooled mature milk, especially during the first two weeks of life, due to its higher protein content. Therefore, it is thought that the use of “preterm milk” would more closely fulfill the needs of the preterm infant without causing the metabolic abnormalities that have been observed in infants fed formula with higher protein content9. Previous studies have all measured only total nitrogen of milk and excreta when determining nitrogen balance in premature infants. However, when assessing nitrogen utilization and protein requirements in infants fed human milk one has to consider the possible contribution of the non-protein nitrogen (NPN) fraction of human milk to the utilizable N pool provided to the recipient infant.

The central theme of this workshop was the utilization of specific minerals and vitamins from human milk and an evaluation of the adequacy of these nutrients in milk to support growth and development of normal term and premature infants. The trace elements iron, zinc, copper, manganese and selenium; the minerals calcium and phosphorus; and a water soluble vitamin, folic acid were discussed.

The dynamic changes that occur in milk composition associated with duration of lactation have been recognized for over a century as documented in several early publications some dating back to 1874 (1–3). In the mid 1900s Macy (4) reviewed 1500 publications on milk composition from many countries and selected 268 papers based on “the characteristics of subjects, methods of collecting samples, analytical techniques, diet and environmental factors”. From the data surveyed, they compiled a table of composite values for more than 50 components in human milk at three lactational stages: colostrum, representing one to five days postpartum; transitional milk, representing days six to ten postpartum; and mature milk, representing day 15 to 15 months postpartum. Since that time and particularly since the mid 1970s, a plethora of information on milk composition has emerged providing further characterization of important biological components in human milk and knowledge of those factors which are associated with alterations in the composition and volume of human milk produced by the average healthy lactating woman (Table 1).

Acrodermatitis, diarrhea, hyperirritability and delayed growth in association with low plasma zinc levels have been documented in case reports of premature infants exclusively fed human milk (1–3). In all cases the etiology of the nutritional zinc deficiency was attributed to levels of zinc in the infants‘ mothers’ milk that were below the usual range of values reported for milk at similar lactational stages. However, there has only been speculation as to whether this is caused by defective mammary gland zinc secretion or whether it simply represents the lower end of a normal spectrum (3). In the present report, we describe the quantitation of citrate, a major zinc binding ligand in human milk (4), and the distribution of zinc, copper and iron in the casein, whey, fat and whole milk fractions of milk which we determined to be “zinc deficient.”

The objectives of this workshop were two fold: first, to consider the effects on growth and body composition of feeding human milk or proprietary formulas in both term and premature infants. Secondly, to examine the interrelationships between energy intakes and utilization of the major energy source in milk (i.e., fat) during early developmental stages in infants.

Human milk whey contains proteins thought to have functional and nutritional roles for the infant. These proteins include lysozyme(Lys), lactoferrin(Lf), secretory IgA(sIgA), serum albumin(SA) and α-lactalbumin(α-LA). SA and α-LA primarily have nutritional importance as sources of amino acids. Lys, Lf and slgA may have both nutritional and physiological functions. Lys and Lf have been suggested to have bacteriostatic effects within the infant’s gastrointestinal (GI) tract. SlgA acts immunologically within the gut of the infant. For these proteins to exert their functions, they must remain largely intact during passage through the upper GI tract. We have demonstrated (AJCN 41: 852(1985)) that LF and slgA are detected in feces of exclusively breast-fed term infants. We were interest in quantitating these proteins in feces of preterm infants for two reasons. First, preterm milk contains higher quantities of these whey proteins. Secondly, the proteins may serve a greater protective role for preterm infants, due to immature immune function in such infants.

Determination of the biochemical composition of milk from mothers giving birth prematurely (preterm milk) by Atkinson et al, (1–3) and subsequently by others (4–9) demonstrated that preterm milk had greater concentrations of nitrogen, lipid, medium chain fatty acids, energy and sane minerals when compared to term milk at similar lactational stages. Evidence for these compositional differences is not universal and such lack of agreement amongst investigators may be attributed to differences in sample collection methodology, the inclusion of wide ranges of gestational stage and the greater degree of inter-individual variability in milk composition in preterm compared to term milk. For a more complete discussion of these areas, see chapter by Atkinson and Kaufman in this book.