Growth in the young and maintenance in the adult are complex, genetically
orchestrated, metabolic processes that require adequate intakes of substrates and
The importance of adequate nutritional support as a component in the comprehensive
clinical management of patients is now widely appreciated, and over
the past two decades advances have been made in enteral feeding techniques.
Dr. Young: It is presumptuous of me and certainly very difficult to try to
overview and integrate the information we’ve been presented with during the
course of the last two days.
The intestinal tissues, especially the small intestinal mucosa, have a high
rate of protein synthesis and energy expenditure. Because a substantial proportion
of mucosal protein synthesis is probably devoted to secretion into the
lumen, the intestine is in a permanent state of positive protein balance.
the adult organism, maintenance of body protein stores is accomplished
through a diurnal rhythm of catabolic and anabolic phases . Under physiological
conditions, a net loss of body proteins occurs in the interprandial periods,
particularly during the night, as well as during exercise .
The history of research into mammalian peptide transport has, until recently,
been one of frustration as the main features, but not the detail, of a transport
process that had first been suggested over 100 years ago were apparent by
Normal adults maintain nitrogen balance over long periods, and during growth
in childhood there is modest positive nitrogen balance. Nitrogen is consumed
mainly as protein in the diet, and is made available to metabolism as amino acids,
following digestion and absorption.
Protein nutrition has so far been mainly focused on the amino acid composition
of the dietary proteins necessary to meet the body’s requirements.
The problem of their bioavailability and stability during processing and storage
has been the principal concern of the food industry.
Several factors are known to regulate postprandial protein deposition. Some
depend on nutrients other than proteins (energy intake, for example), others
depend on the pathophysiological condition (aging, stress, and so on), and some
depend on the dietary protein itself.
The importance of enteral nutrition, especially for critically ill patients, is now
widely recognized. There are clear benefits compared with parenteral nutrition
in terms of reduced septic morbidity resulting from preservation of gut barrier
function, not to mention reduced costs.
Critical injury and illness induce a number of metabolic, immunologic, and
functional responses which influence clinical outcome and normal defenses. Raised
levels of counterregulatory hormones, immobilization, respiratory failure, and
violation of host defenses with intravenous catheters and endotracheal intubation
alter the balance between the bacterial assault and immunologic host defenses.
For many years there has been a focus on indispensable (essential) amino
acids in both enteral and parenteral nutrition. These are considered to be those the
body cannot synthesize because it lacks the necessary enzymes.
There is a long list of so called essential and conditionally indispensable
nutritional substrates, mostly old or old but newly packaged. Certain amino
acids, short-chain fatty acids, nucleotides and so on have been known to be
essential for a very long time.
In humans, the gastrointestinal tract has evolved to absorb whole proteins. In
abnormal states, nutritional support in the form of particles smaller than protein
size compensates for abnormalities in the intestinal mucosa and for pancreatic
Multiple factors have been implicated in the pathogenesis of chronic pancreatitis
. Alcohol is the cause in 60–70% of patients. Dietary factors such as
high protein or either very high or very low fat content may potentiate the injurious
effects of alcohol [2, 3].
Malnutrition is a common problem in patients with advanced renal failure,
regardless of the cause of reduced renal function and the underlying kidney
The normal daily ingestion of dietary protein is usually in the region of
70–100 g but, in addition, endogenous proteins are secreted into the gut lumen
which require assimilation.