Nutritional Adaptation of the Gastrointestinal Tract of the Newborn
Adequate nutrition, a critical factor to the health and well being of the
newborn infant, is dependent on the digestive and absorptive capacities of the
intestinal epithelial cells at birth. These cells must also function as a "barrier"
to the nonspecific uptake and transport of lumenal macromolecules.
The present chapter summarizes some recent and less recent work on the
positioning, anchoring and biosynthesis of the small-intestinal sucrase-isomaltase
(SI) complex, which is the most abundant integral protein of the brush
border membrane; it then discusses the implications of the results as to the
possible mechanisms underlying human sucrose-isomaltose malabsorption.
The problem of the regulation of activity of intestinal disaccharidases, especially
lactase, has attracted the attention of researchers for many years (6,8);
there are still many unresolved questions.
The surface membrane of the small intestine comes into contact with luminal
nutrients, organisms, drugs and foreign antigens. The natural frontier
between the body and the intestinal content is represented by the glycoproteins
of the brush border membrane, which act as receptors, carriers and hydrolytic
Ontogeny of the intestinal immune system is relevant to infant nutrition
in many ways. Not only are lymphocytes and lymphoid tissues integral components
of the stomach, small intestine, and colon, but immune responses in
these organs also influence the nature of the commensal gut flora, confer
protective immunity against pathogenic microorganisms and parasites, and
contribute, via hypersensitivity mechanisms, to intestinal diseases, including
malabsorption. Studies of human neonatal intestinal immunology have not
been performed, and, indeed, clinical investigation of the gastrointestinal lymphoid
apparatus in man has been confined almost exclusively to studies of
secretory antibodies and to counts of mucosal lymphoid cells.
About three-quarters of a century ago, Santayana wrote, "Those who cannot
remember the past are condemned to repeat it." This somber dictum is splendidly
illustrated by the history of the study of protein digestion and absorption
over the same three-quarters of a century (12,13). This chapter begins with a
summary of this history, since it provides a deeper understanding of the principles
involved than a mere outline of present views accompanied by an account
of the minutiae of recent advances in the field.
The study of gastrointestinal function during infancy has been impeded by
the invasiveness, impracticality, and imprecision of the conventional techniques
of internal medicine transposed to newborn and young infants.
Neonatal necrotizing enterocolitis (NEC) is a well-described and extensively
investigated affection of the high-risk newborn. It is a cosmopolitan disease,
the clinical course of which ranges from modest abdominal distention and
gastrointestinal dysfunction to a fulminant course of sepsis, disseminated intravascular
coagulation, cardiovascular collapse, and death.
The two major causes (48) of intrauterine human growth retardation (IUGR)
(63), which has become a major concern to neonatalogists (3,4), are a reduction
in the blood flow to the fetus and maternal malnutrition.
This chapter presents some preliminary experimental data which indicate
that maternal dietary amino acid imbalance affects fetal growth. As background,
among approximately 1,164 mother-baby pairs studied at midpregnancy,
we found that mothers who later had small babies had a pattern or
"profile" of amino acids and nutrient levels in their plasma which differed
from mothers who later had large babies (1,3) (Fig. 1).
The first intravenous fat (IVF) preparation used to any great extent in the
United States was made from cottonseed oil and marketed under the tradename
of Lipomul®. A great deal of experience was gained with the use of this
emulsion and, in fact, an entire symposium was published reviewing the data
regarding this agent (Metabolism, 6:591-831, 1957).
The growth curves of a fetus in utero (4) and of a preterm infant are shown
in Fig. 1. Following birth, weight is lost and there is a delay until growth is
resumed. Afterwards, there is usually little difference between the slope of both
curves, and it is only when there is a flattening out of the intrauterine (IU)
curve that our preterm infant curve catches up with it.
There are three major considerations affecting the quantity and quality of
protein to be given to a low-birth-weight baby. These are (a) requirements for
normal growth and body composition, (b) development of protein and amino
acid metabolism, and (c) renal function.
In preterm infants an optimal nutritional supply must be provided early
during the neonatal period. Indeed, undernutrition leads to growth retardation
which may be hazardous for brain development (18).
This chapter deals with the modifications of human milk composition during
the early stages of lactation and the possible implications of these modifications
as far as the needs of the newborn are concerned.
Weaning has been recognized as a dangerous process in many developing
countries since Ceciley Williams described kwashiorkor in 1935. The term
weaning has been described as ambiguous by David Morley and should be
avoided because "the following two meanings are possible: (a) It is equivalent
to the French 'sevrage' i.e. cessation of breast feeding; (b) the time or period
in which solid foods or food other than milk are introduced into the
Absorption, protein, see Protein
Alkaline phosphatase, 53
and intravenous fat emulsions, 167
and weaning foods, 222
Allograft rejection, and small intestinal
absorption, 85-88, 103; see also
concentration, in preterm infants,