What macronutrient is usually the main source of energy in parenteral nutrition?

Definition

Carbohydrates are a large group of organic compounds ranging from simple sugars to complex starches and fibres. They contain carbon, and hydrogen and oxygen in the same ratio as water (2:1) and can usually be broken down to release energy.

Function

Carbohydrates are the major energy source in human nutrition, providing approximately 45 to 65% of daily energy requirements. Most consumable carbohydrates are broken down into glucose before entering the bloodstream.

Carbohydrates are the primary source of energy for the central nervous system, brain and red blood cells: the brain has no ability to store energy and therefore requires a constant supply of glucose to function properly.1

Carbohydrate deficiency or overload

When energy supply is limited, due to inadequate intake, or inadequate digestion or absorption, dietary protein is used as an energy source rather than carbohydrate. Inadequate protein intake results in the breakdown of lean body mass to provide an adequate supply of amino acids within the bloodstream to support the synthesis of acute phase proteins, production of glucose, healing of wounds, etc.2

Excess glucose and fructose may exacerbate metabolic complications in skeletal muscle, adipose tissue, and liver,3 and can increase complications such as severe infections, multiple organ failure, polyneuropathy and mortality.4,5

Role of carbohydrate during illness

Currently, there are no specific guidelines for carbohydrate intake in the acute or chronically ill population, therefore the DRI recommendations for healthy adults are commonly referred to. For adults, the recommended carbohydrate intake is at least 130g/day based on the average minimum amount of glucose utilised by the brain.1

The metabolic response to illness and injury is characterised by hypermetabolism, (catabolic response) including negative nitrogen balance, insulin resistance, hyperglycaemia, and fatty acid oxidation.6 These changes lead to an increase in hormones such as epinephrine, cortisol, catecholamines, and glucagon that in turn cause significant metabolic alterations,7 which continue to promote catabolism and oxidative stress, ultimately leading to malnutrition and organ damage.

Carbohydrate in Enteral Nutrition

Providing energy as carbohydrates or fat is essential to minimise protein degradation in all illness and injury.10 The clearance rate of circulating lipid is decreased during illness, which may lead to hypertriglyceridemia and hepatic fat accumulation. Thus, carbohydrate should be the primary energy source for patients experiencing changes in metabolism due to illness or injury.6

In most enteral formulas, carbohydrate is the primary macronutrient and principal energy source. However, poorly controlled glucose levels in critically ill patients can lead to glucose variability with hyper- and hypoglycaemia, conditions that can impede recovery and can be fatal.3

Low-glycaemic index enteral nutrition may provide metabolic and clinical benefits in critical illness. Lowering glycaemic index (GI) and glycaemic load (GL) can improve metabolic control.11 Furthermore, increasing the protein-to-carbohydrate ratio can reduce glycaemia.12

Key points

• •

  Parenteral nutrition is a medical intervention intended to prevent adverse outcomes associated with nutrient depletion.

• •

  “If the gut works, use it!” Parenteral nutrition is reserved for those with intestinal dysfunction.

As gastroenterology fellows, we are often called upon to make decisions regarding nutritional support. Our training usually provides us ample experience in establishing routes for enteral feeding, but training in the management of parenteral nutrition is often lacking. In this month's Fellows' Corner, Drs Schwartz and Semrad, along with Gilbert Cusson, a parenteral nutrition pharmacist, provide us with a practical approach to initiating parenteral nutrition. In a subsequent issue, these authors will follow-up with a discussion of the monitoring and complications of parenteral nutrition.

Juan Carlos Bucobo, MD

Fellows' Corner Editor

Senior Gastroenterology Fellow

State University of New York at Stony Brook

Stony Brook University Medical Center

Stony Brook, New York, USA

Parenteral nutrition (PN) (intravenous feeding) is a valuable tool for providing nutrients to patients who are unable to meet their needs via the GI tract. It is a medical intervention intended to prevent the adverse clinical outcomes associated with nutrient depletion. As such, physicians who use PN (particularly gastroenterologists and surgeons) must know the indications, principles of formulation, complications, and monitoring guidelines. This article provides a practical approach to PN in clinical practice.

Patient selection

Patient selection is guided by 2 key questions:

• 1.

  Is nutrition support needed and appropriate?

• 2.

  Is enteral nutrition (EN) via tube feeding possible?

Nutrition support should be considered in patients with malnutrition or those at risk due to prolonged impaired dietary intake. To assess for malnutrition, a nutrition scoring system can be used (eg, subjective global assessment, nutrition risk index), or more simply, the percentage of unintentional weight loss can be calculated.

Weight loss of >5% in 1 month or >10% in 6 months is clinically significant, predisposing patients to greater morbidity and mortality from underlying illness, trauma, or surgery. Weight loss of >20% within 6 months results in severe physiologic dysfunction.

The latter warrants nutrition support early, often within 48 hours of hospitalization. Well nourished or mildly malnourished patients tolerate longer periods of nutritional deprivation. Data on the optimal timing of nutrition support are lacking. Common practice is to start PN after 10 to 14 days of inadequate dietary intake.

3

ASPEN Board of Directors and Clinical Guidelines Task Force
Guidelines for the use of parenteral and enteral nutrition in adult and pediatric patients.

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The decision to use EN or PN depends on the functional status of the GI tract. “If the gut works, use it!” EN is more physiologic and costs less; however, safety depends on clinical prediction of feeding tolerance. PN is reserved for those with intestinal dysfunction (obstruction, prolonged ileus, enterocutaneous fistula, severe intestinal inflammation, malabsorption, or ischemia). Severely malnourished patients with EN intolerance or enteral access problems benefit from PN, particularly in the preoperative setting. Seven to 10 days of PN before surgery reduces the risk of major noninfectious complications.

Prescribing PN

PN consists of a complex mixture of macronutrients (carbohydrates, proteins, lipids), micronutrients (vitamins, minerals, trace elements), fluid and electrolytes. Carbohydrates are supplied as dextrose, proteins as amino acids, and lipids as intravenous fat emulsions (IVFEs). When all 3 nutrients are combined in a single solution, it is called a total nutrient admixture or a 3 in 1 admixture. IVFE can also be administered separately, piggybacked into the PN solution, in which case it is called a 2 in 1 solution.

PN is administered via a peripheral or central venous catheter and termed peripheral PN (PPN) or central PN (CPN), accordingly. Central and peripheral formulas differ dramatically in their osmolality. Concentrated PN solutions with high osmolality should never be given peripherally because doing so could cause a severe chemical phlebitis. In general, PPN solutions should not exceed 900 mOsm/L and should always be used with IVFEs, which decreases osmolality by a direct dilutional effect. If IVFE is contraindicated (hypertriglyceridemia, soy or egg allergy), then the osmolality should be further decreased to improve patient tolerance. CPN is usually hyperosmolar but can be infused safely into a large central vein with high flow rates. Concentrated CPN formulas can provide increased calories in a smaller volume.

Formulating a PN solution

Step 1: Energy and protein requirements

Energy needs are estimated by using the Harris-Benedict equations (Fig. 1), a weight-based calculation (resting energy expenditure = weight in kg × 25 kcal/kg), or indirect calorimetry. Adjustments are made for fluid weight, obesity, metabolic stress, and activity. Protein and calorie requirements increase with increasing metabolic stress, except in the critically ill. Severe stress metabolism, often found in the critically ill, causes hyperglycemia and hyperlipidemia. Therefore, permissive underfeeding with a hypocaloric (20 kcal/kg) and high protein (2 g/kg) formula is usually best tolerated initially.

, 

6

• Malone A.M.

Permissive underfeeding: its appropriateness in patients with obesity, patients on parenteral nutrition, and non-obese patients receiving eneral nutrition.

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Permissive underfeeding has also been used successfully in the morbidly obese. For obese patients whose actual body weight is >130% ideal body weight, energy and protein needs should be estimated using ideal or adjusted body weight (Fig. 1).

Step 2: Macronutrients

Caloric intake must be divided among the different macronutrients. Each gram of amino acid, fat, and dextrose yields 4 kcal, 10 kcal (9 kcal from fat, 1 kcal from emulsifier), and 3.4 kcal of energy, respectively. The decision of whether to count amino acids as a source of calories is controversial. In the hospitalized patient, amino acids are given in higher amounts and may be used as an energy source. Therefore, they are often tallied into total caloric intake. In outpatients with active protein synthesis, amino acids are preferentially used as a synthetic substrate, and caloric needs are largely met by dextrose and fat. In this population, protein calories are not included in the calculation of total calories supplied.

Protein needs vary according to the level of metabolic stress: 1.0 to 2.0 g/kg/d (Fig. 2). Protein restriction is rarely indicated. Transient exceptions are severe hepatic encephalopathy and azotemia requiring imminent hemodialysis. Fat is provided at 1 g/kg/d. Fat is restricted in the morbidly obese (minimal level to prevent essential fatty acid deficiency [ie, 10% of estimated fat calories] is supplied) and those with hypertriglyceridemia at levels >400 mg/dL.

Dextrose calories supply the remaining daily energy requirements and are calculated by subtracting lipid and protein calories from total daily calories. To avoid starvation ketosis, the amount of dextrose required is approximately 1.4 g/kg actual body weight. In hospitalized patients, the initial dextrose content in PN should not exceed 200 g in order to avoid iatrogenic hyperglycemia and the refeeding syndrome. Gradual titration to target amount is pursued over several days to a week.

Step 3: Fluid, electrolytes, and minerals

The volume of PN required is based on weight and fluid status. Euvolemic patients require 30 mL/kg per day. Under conditions of fluid overload, cardiopulmonary disease, and renal dialysis, PN volume is restricted. The minimal amount of fluid in which PN contents can be formulated is best guided by the PN pharmacist, but it is usually 1 to 1.2 L. Volume requirements of each component, osmolality, and solubility issues must be considered.

The electrolyte content of PN is guided by laboratory values, fluid losses, and medications (eg, diuretics, steroids, chemotherapy). Sodium content is most easily estimated by determining whether normal saline solution (154 mEq/L), half-normal saline solution (75 mEq/L), or another concentration is desired. Ranges for other electrolytes are listed in Table 1. Special caution is needed with the addition of potassium, calcium, and phosphorus. Potassium in PN solutions should not exceed 10 mEq/h for patients in an unmonitored, non-intensive care unit setting. Calcium and phosphorous content should be carefully assessed, as calcium-phosphate crystals can precipitate from solution. To avoid this problem, the calcium-phosphorus sum per liter solution ([mEq calcium + mmol phosphorus]/L) should be <45.

7

Baumgartner TG. Clinical guide to parenteral micronutrition. 3rd edition. Deerfield, Illinois; Fujisawa, USA; 1997.

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Consultation with the PN pharmacist is advised for assistance with complex formulas (eg, restricted volume, high calcium and phosphorus requirements).

Table 1Daily electrolyte requirements in PN

∗

Determined by electrolyte status and losses and renal function.

∗ Determined by electrolyte status and losses and renal function.

† All commercial parenteral amino acids contain acetate as a constituent. Supplemental acetate is added when metabolic acidosis is present as acetate is converted to bicarbonate in vivo.

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Step 4: Micronutrients and additives

Vitamin and trace element content is supplied by using preset solutions. Additional supplementation may be warranted based on documented serum levels or excessive GI losses. Copper and manganese are held from PN in patients with hyperbilirubinemia because they are excreted in the bile. Selenium should be decreased in patients with renal insufficiency. Zinc content is increased in patients with diarrhea, enterocutaneous/pancreatic fistulas, or large wounds (eg, burns).

The most common medications added to PN formulas are insulin and histamine-2 receptor antagonists. When hyperglycemia is present, an insulin additive of 0.1 units/g dextrose is usually safe (be cautious in insulin-naïve patients and in patients without diabetes whose hyperglycemia is disease or drug related). Sliding scale insulin coverage improves glycemic control and guides titration of PN insulin content. Hypoglycemia may occur in those with resolving physiologic stress or who are tapering steroids.

Disclosure

All authors disclosed no financial relationships relevant to this publication.

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