Special Formula Diets

The large number of specialized diets commercially available provides the clinical dietitian with a diverse number of alternative regimens from which to choose. Although the use of prepared formulas has many advantages, it is necessary to be careful and rigorous in assessing new as well as reassessing established commercial formulas. Products with which the team may have significant familiarity may undergo changes in their formulations without the patient or practitioner being made fully aware of the changes. It is therefore inappropriate to select a formula by “name”; instead, it is preferable to consider the pertinent components of the diet first. Then those components should be sought in a particular formula. As a rule, the least synthetic and least “predigested” or “elemental” formula that meets the patient’s expected nutritional needs will be the safest and best formula. This generic approach is much more rational and appropriate than considering brand name. With regard to selecting the generic formula, an assessment of the integrity of the patient’s gastrointestinal tract, which is part of a thorough nutritional assessment, should reveal the known or suspected absorptive defects or other problems that might impede normal nutrition. The assessment of the gastrointestinal tract includes an assessment of the adequacy of absorption of carbohydrates, fats, proteins, vitamins, and minerals, as well as consideration of the effect of damaged or nonfunctional bowel on specific nutrients. For general reference, a list of the commonly used formulas is provided in Table 89-3 .

The variable contents from which the nutrition support team has to choose must be determined on a rational basis. Each advantage cited for any particular defined formula diet implies potential problems. For example, one problem that tends to limit nutrient density is osmolality. Our experience suggests that even the child without intrinsic gastrointestinal disease may begin to complain of gastrointestinal symptoms when the osmolality of the formula approaches 600 mOsm/L. This experience is consistent with that of investigators who have reported delayed gastric emptying when the osmolality of the duodenal contents is 560 mOsm/L.

In general, the specific therapeutic benefits of specialized formula diets are those that accrue from improved or good nutrition. As more data support special therapeutic effects of special enteral formula (as is occurring in Crohn’s disease), we should expect expanding claims of efficacy for many diseases, especially diseases of the gastrointestinal tract and diseases with gastrointestinal symptoms. Because of the general population’s interest in nutrition (and especially older patients and parents of sick children), it is incumbent on the nutritionally focused clinician to critically evaluate research and therapeutic claims. It will continue to be important to distinguish therapeutic benefits that derive from improved nutrition from those that directly affect the disease process.

Formulas can be designed with the physiology of the gastrointestinal tract in mind. For example, the protein content of the formula has been demonstrated to be directly proportional to its acid-secreting potential. Gastrin release itself is related to the presence of specific amino acids and small peptides. The administration of intragastric amino acids results in significant pancreatic stimulation, which can be minimized if the infusion is intrajejunal and at neutral pH. The decreased pancreatic output that results may be effective for the treatment of pancreatitis and high-output intestinal fistulas.

Other considerations regarding the fat and carbohydrate contents of the diets and routes of delivery may also be pertinent. Overall, it must be emphasized that the comparison of studies using different formulas under nonstandard conditions is usually not valid. It is evident that careful and critical evaluation of each apparently relevant report is essential to place it in its proper therapeutic role.

The use of commercially prepared formulas of a known composition facilitates the accurate measurement of nutrient intake. They are therefore of particular value in metabolic studies and in critical care situations, in which accurate intake measurements are necessary.

Because of the flexibility of these formulas, it is common to have constituents vary in relation to one another. Although most formulas are designed to provide for an apparently well-balanced nutritional regimen, many modulars can be added to change the balance and content of nutrients. The use of these additional modulars requires additional attention and monitoring. Munro has pointed out in a review of oral versus parenteral nutrient metabolism that major deviations that occur in parenterally nourished patients are often the result not of the route of delivery of the nutrient but rather of the unusual pattern of nutrients administered. The clear implication is that similar aberrations are to be expected through the injudicious use of the enteral pathway, as can be expected with total parenteral nutrition (TPN). Despite shortcomings of commercially available formulas for the child with special nutritional needs, they offer a standardized nutrient regimen that may be superior to complex prescriptions that require intense labor to create or to imaginative nonrigorously derived concoctions. In cases where modular additives are required to augment caloric density or protein content, specific attention should be paid to avoid possible mixing errors. Erroneously mixed formulas can result in hypocaloric or hyperosmotic products. Such errors represent the potential risk of feeding intolerance, metabolic derangements and, in rare cases, infectious complications due to poor sanitary conditions.

Energy Requirements

In most clinical situations, energy requirements for children are estimated based on age. Actual energy requirements vary widely among healthy individuals and are even more variable in disease. For the healthy individual, most published charts report energy requirements from the guidelines provided by the National Academy of Sciences, National Research Council. Their published Dietary Reference Intakes (DRIs) for energy provides an “Estimated Energy Requirement” or “EER.” The EER is the average dietary energy intake that is predicted to maintain energy balance in a healthy adult of a defined age, gender, weight, height, and level of physical activity consistent with good health. In children and pregnant and lactating women, the EER is taken to include the needs associated with the deposition of tissues or the secretion of milk at rates consistent with good health. It is evident that these can only be rough estimates for healthy individuals and that for sick children they can at best be viewed as crude guesses of the energy requirement.

In the nutritionally stable, generally healthy child, the simple application of DRIs as a screening evaluation of caloric requirements may be sufficiently informative. They are not, and were never intended to be, an adequate tool for estimating the energy needs for sick or malnourished children, such as patients with the conditions listed in Box 89-1 .

When the determination of a sick patient’s energy requirement is crucial to medical management, it cannot be accurately determined by charts or “rule-of-thumb” calculations. Instead, after an informed target is set, adjustments will be necessary according to the response to feeding over time as measured by growth, weight change, other anthropometrics, various laboratory param­eters such as serum proteins, wound healing, and general sense of well-being. These adjustments will determine the success of the feeding regimen. Occasionally estimation of energy requirements is complicated by the presence of multiple processes occurring simultaneously, for example, when increased needs caused by infection complicate postoperative wound healing, or decreased needs result from inactivity or increased body fat. Extenuating circumstances such as these may warrant special assessment techniques such as indirect calorimetry. Indirect calorimetry may have a limited clinical roll in general, but may be particularly helpful in certain clinical situations. Nonetheless, the procedure needs to be performed under carefully controlled circumstances and interpreted with caution by experienced personnel. Even when appropriately performed, there are differing opinions about how to use and interpret the results. Activity and injury “factors” have been suggested for calculating the increase in energy expenditure with various types of injury in the patient population. Although these values have not been validated in the pediatric population, they have been used in the pediatric clinical setting to account for observed differences in energy needs during illness and stress.

Fat

Fat has the highest caloric density of the nutrients. The intake of fat is important because our experience suggests that for most pediatric patients the limiting nutrient is energy (or caloric) intake. Patients who have unusual losses of protein-rich fluid will require increased amounts of protein, but even in those patients, providing adequate energy intake will ultimately be most challenging. Triglycerides, composed of glycerol esterified to long-chain fatty acids, yield approximately 9 calories per gram when oxidized. They are, therefore, highly desirable nutrients for any nutritionally depleted patient. The major considerations with regard to choosing the amount or type of dietary fat include the following: (1) the use and absorption of long-chain triglycerides; (2) the use and absorption of synthetic medium-chain triglycerides; and (3) the requirements for essential fatty acids (all of which are long-chain fatty acids.)

Dietary fat consists predominantly of long-chain triglycerides. The process of digestion and absorption of long-chain triglycerides is reviewed in Chapter 31 on fat absorption. Briefly, within the lumen of the gastrointestinal tract, the long-chain fat must be emulsified and acted on by gastrointestinal (predominantly pancreatic) lipases. The lipases break down the triglyceride to free fatty acids and glycerol. Bile salts from the liver are necessary to form micelles to solubilize the free fatty acids for transport in the intestinal lumen to the brush border of the intestinal mucosa. Once within the enterocyte, fatty acids are re-esterified with glycerol to form a triglyceride again. With the addition of apolipoproteins, phospholipid, cholesterol ester, and cholesterol, chylomicrons are formed which are then released into the lymphatics. The lymphatics in turn empty into the thoracic duct and then the left subclavian vein.

From this brief description of the assimilation of long-chain fat, many of the pathologic processes that will require special attention from the nutrition team are evident. For example, pancreatic insufficiency results in decreased lipases; bile duct obstruction or hepatitis results in decreased bile flow to the intestine and thereby decreased micelle formation; gastrointestinal mucosal damage results in decreased surface area for absorption of fatty acids; metabolic derangements inhibit the formation of chylomicrons; and diseases or obstruction of the lymphatics inhibit the transport of lymph back into the circulation. All of these represent specific situations requiring special nutritional considerations. In some circumstances, it is acceptable to allow the increased fat malabsorption that results from increased fat intake to take advantage of the increased caloric density of the fat that does get absorbed. In other circumstances, long-chain fat must be largely excluded from the diet.

Medium-chain triglycerides are synthetic fats that contain a glycerol backbone esterified to fatty acids that are 8 to 10 carbons in length. Medium-chain triglycerides yield 8.3 calories per gram on oxidation and are therefore essentially as useful as long-chain fat from an energy point of view. Medium-chain triglycerides are hydrolyzed more rapidly than long-chain fats in the small intestine when pancreatic lipases are present and are then converted almost exclusively into free fatty acids and glycerol. If no pancreatic lipase is present, the medium-chain triglyceride can be found in the intestinal mucosa mainly as unhydrolyzed triglyceride. Thus, even in the absence of pancreatic lipase and bile salts, medium-chain triglycerides are transported intact through the intestinal lumen and through the brush border into the enterocyte. The unusual behavior of medium-chain triglyceride in the intestinal lumen and its absorption characteristics are due to the greater water solubility of medium-chain triglyceride and its products. The lower interfacial tension of the smaller triglyceride molecules with water facilitates its emulsification, so the formation of micelles is not necessary.

The exact mechanism and the rate-limiting step in the absorption of these different fats are unknown. The relatively high solubility of intact medium-chain triglycerides enhances their diffusion to the brush border of the healthy enterocyte. The triglyceride and the medium-chain free fatty acids are readily taken up into the enterocyte. Within the enterocyte, the triglyceride is acted on by intracellular lipases, and the resulting free fatty acids are released directly into the portal circulation, bypassing the lymphatic channels. The clinical indications for the use of medium-chain triglycerides are based on these characteristics. For example, patients with cholestatic liver disease and disrupted enterohepatic circulation often require a formula high in medium-chain triglycerides as they are otherwise unable to process and assimilate long-chain fats.

There are circumstances under which one, the other, or both types of fat become highly desirable as constituents of the prescribed diet. It has been demonstrated that the maximum absorption of long-chain fat is decreased when medium-chain triglycerides are added. The total amount of triglycerides absorbed when both are present, however, is greater than the maximum absorption of either alone. Absorption of medium-chain triglycerides from the rat cecum and the dog colon has been reported. The importance of this phenomenon and whether it has any implications for humans is unclear.

The true capacity of the gastrointestinal tract to absorb fat has not been precisely measured. It is generally accepted that the rate of fat absorption is proportional to the amount of fat ingested, and the more fat ingested the more absorbed.

If there are no specific defects in fat absorption demonstrated or anticipated, the fat of choice for a special formula diet is regular, long-chain (dietary) fat. The use of medium-chain triglycerides in patients who have intact mechanisms for the normal digestion of fat can lead to osmotic diarrhea resulting from the rapid lipolysis of the medium-chain triglycerides to glycerol (and medium-chain free fatty acids). This diarrhea may exacerbate the symptoms for which the formula was originally prescribed resulting in unforeseen iatrogenic consequences. In addition, there are a few patients who seem to have an unexplained or idiosyncratic reaction resulting in severe diarrhea when they ingest even a small amount of medium-chain triglycerides. A mixture of both medium-chain triglycerides and long-chain triglycerides may conceivably help patients who require increased calories. Ideally, fecal fat determination can be used as a guide to determine the optimal combination of medium-chain triglyceride, long-chain triglyceride, or both, to be given to the patient. In lieu of fecal fat determinations, careful clinical monitoring of nutritional repletion may suffice.

Essential fatty acids are all long-chain fatty acids and therefore absent in medium-chain triglycerides. Because the administration of medium-chain triglycerides may decrease the absorption of long-chain triglycerides, the addition of a small amount of linoleic acid to a medium-chain triglyceride formula may be insufficient to prevent essential fatty acid insufficiency. An adequate intake (AI) was established for essential fatty acids based on the median intake of healthy individuals. The AI for infants is based on the total fat from human milk until 6 months old and fat from human milk and complementary foods from 7 to 12 months of age. Clinicians must be cautious to guard against the development of essential fatty acid deficiency. If long-chain fat absorption is severely compromised, the parenteral administration of lipid emulsions containing essential fats must be considered.

Carbohydrates

Complex carbohydrates such as starch are initially broken down by salivary and pancreatic amylases. The contribution of the various amylases in health and disease to carbohydrate digestion remains a subject of study. Once carbohydrates are reduced to disaccharides, they are further hydrolyzed to individual monosaccharides by the brush border oligosaccharidases. The individual monosaccharides are then absorbed into the enterocytes via specific transport systems. There are four distinct brush border oligosaccharidases: maltase (glucoamylase); sucrase-isomaltase (sucrase α-dextrinase); and lactase. The fourth, trehalase (acts on the sugar trehalose, which is found only in young mushrooms and insects), is of little clinical significance.

Maltase (glucoamylase) is the most abundant oligosaccharidase in the gastrointestinal tract. Sucrase is present in intermediate amounts, and lactase, which splits lactose into glucose and galactose, is present in the least quantities.

All the disaccharidases may be expected to be depressed when there is mucosal disease of the small intestine. Lactase appears to be the most sensitive to injury and the last of the disaccharidases to recover full activity after mucosal damage. Because disaccharidases are present in older enterocytes, any process that causes rapid intestinal regeneration and repopulation of the intestinal villi with young cells (i.e., recovery from acute gastroenteritis) will represent a period of relative disaccharidase deficiency for the patient. In addition, inflammatory conditions such as celiac disease and small bowel Crohn’s disease can result in villous blunting/destruction. Starvation alone may be sufficient to lower disaccharidase activity, as it results in decreased cell turnover, mucosal hypotrophy, and therefore decreased surface area. It may therefore be important to transiently limit dietary sources of lactose in the malnourished child or in the child recuperating from gastrointestinal insult for a short period given the risk of developing secondary lactose intolerance. Restriction beyond a brief period necessary to demonstrate mucosal recovery is typically not warranted. Breath hydrogen testing (when clinically indicated) is a noninvasive method to determine the status of carbohydrate absorption (i.e., lactose, sucrose), so resumption of acceptable amounts of dietary carbohydrates is not delayed.

In selecting a carbohydrate for a special (defined) formula diet, it is important to note that the use of glucose significantly increases the osmotic concentration of a formula. A disaccharide will provide, on a gram basis, one-half the osmotic load of a monosaccharide. The use of synthetic glucose polymers may contribute as little as one fifth to the total osmotic load of a similar amount of glucose. Many specialized formula diets, therefore, use glucose polymers and long-chain starches as their carbohydrate source. The assimilation of these long-chain carbohydrates appears to proceed so that the osmolarity of the intestinal contents is not adversely affected. Although there is evidence that sucrose or fructose feedings may cause increases in the intestinal level of sucrase and maltase, no such dietary effect on lactase activity has ever been demonstrated.

It is important to document the existence of carbohydrate malabsorption when it is suspected and a formula change is contemplated. This may be done by testing for the presence of hydrogen in breath samples or by searching for reducing substances in fresh stool. If the patient’s formula contains a nonreducing sugar (i.e., sucrose), it must first be hydrolyzed or the stool may be misleadingly reported as not having reducing substances present. Generally, if carbohydrate fermentation products are present in the stool of an infant with diarrhea, the stool has an acidic pH and it seems to “burn” the infant’s diaper area regardless of how meticulous the caretaker is. This finding is not invariably present.

Protein

Protein requirements, as with energy requirements, must be approximated for any specific patient. Protein requirements will vary with energy intake. If energy intake is inadequate, the amino acids supplied from the protein will be deaminated, with the resulting carbon skeletons burned for energy. If energy intake is adequate, protein intake can be estimated according to the patient’s age and gender. For patients who might be pregnant or lactating, their protein requirements should be further adjusted. Protein requirements in the first 6 months of life have been estimated based on the amount of protein ingested from mother’s milk by healthy growing infants. Estimates beyond 6 months are similarly based on intake of normal infants, toddlers, and children, with few high-quality experimental data available. Overall, the range of estimates for protein requirements in healthy infants and children varies between 0.8 and 1.5 g/kg per day. For critically ill children in the intensive care unit, it has been shown that the daily protein requirement is approximately 1.5 g/kg per day, on average. The ultimate value, however, should be determined by the medical team based on the clinical status of the child. Some patients will have markedly elevated protein losses (i.e., patients with significant burn injury) and thus require increased protein intake. For all ill patients in whom there is any question of adequacy or excess protein intake, additional monitoring of serum proteins and serum urea will be necessary to ensure appropriate intake.

A child’s protein requirements may be met by using intact proteins, protein hydrolysates, individual amino acids, or a combination of these. Patients who have no known deficiency of the exocrine pancreatic function and no suspected major absorptive defect or protein allergy should receive high-quality intact protein. Intact protein is considerably less expensive than other dietary forms, is much more palatable, and does not contribute significantly to a formula’s osmolarity. Within a given volume and osmolarity, the formula containing intact protein will allow for significantly greater nutrient density. Patients who are unable to tolerate intact protein because of milk-protein allergy, exocrine pancreatic deficiency/insufficiency, or severe intestinal mucosal disease should receive a protein hydrolysate, a more readily absorbed form of protein.

There has been a significant increase in the number of infants demonstrating allergic reactions to the oligopeptides present in conventional formulas containing protein hydrolysate. Although it is not completely clear why this is occurring, it is clear that there are some infants who require their protein in the form of amino acids.

For many years, it was assumed that amino acids represented the optimal form of predigested protein for everyone and that a protein source containing only amino acids would be more easily absorbed. This was an oversimplified and incorrect view of the mechanism of protein absorption. The absorption of small peptides, especially dipeptides, has been shown to play a significant role in the digestion of protein. Perhaps one-fourth of the absorbed protein in health is absorbed in the form of oligopeptides. The oligopeptides, especially dipeptides, are absorbed and then hydrolyzed by brush border and cytoplasmic hydrolases to free amino acids. The mechanism of transport of the individual amino acids has been shown to be energy dependent and active and in many cases dependent on true carrier proteins with some degree of specificity for neutral, basic, and acidic amino acids. A lack of competition between amino acid uptake and the uptake of peptides suggests the separate carriers for absorption are present. It has thus become clear that in the severely damaged mucosa, the ideal form of protein for absorption will be found in a mixture of oligopeptides and individual free amino acids.

The clinical importance of peptide transport has been demonstrated in certain diseases; for example, in patients with Hartnup’s disease (in which there is a specific inability to transport some neutral amino acids such as tryptophan), normal development can occur, and this has been similarly demonstrated in cystinuria (another disorder involving amino acid transport). It is conceivable that in the damaged mucosa, similar other transport mechanisms are important. Some investigators have shown that in surgically shortened intestines, there are marked decreases in capacity to absorb free amino acids. Furthermore, there appears to be significantly less impairment in the absorption of dipeptides. Rats with exocrine pancreatic deficiency have been shown to use a diet containing 85% of the protein as oligopeptides significantly better than when fed a similar diet, as either intact protein or as free amino acids.

From a practical point of view, it is necessary first to determine the appropriate form of protein to be fed. When possible, this should be high-quality intact protein. There is no evidence that the use of formulas containing only synthetic amino acids offers any advantage to patients with compromised protein absorption. On the contrary, evidence suggests that such formulas may represent a significant problem for some patients. These formulas may be poorly tolerated because of either bad taste or high osmolar load. The use of oligopeptides (protein hydrolysates) and formulas that are mixtures of oligopeptides and free amino acids is usually adequate for the enteral support of most patients in need of the specialized defined formula diets who cannot tolerate intact proteins.

Vitamins and Minerals

Vitamin and mineral requirements for the pediatric population are listed in Tables 89-4 and 89-5 . In general, individuals who consume a varied diet that is adequate for the macronutrients (total energy, protein, and fat) are at low risk for vitamin and mineral deficiencies. Potential vitamin and mineral deficiencies in the pediatric population may occur even with adequate caloric intake, in the presence of fat malabsorption, drug–nutrient interactions, and with prolonged tube feeding with highly specialized diets.

TABLE 89-4

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