Nutritional demands and enteral formulas for adult surgical patients

INTRODUCTION — Nutrition support represents one of the most important cornerstones in the management of patients undergoing surgery, although the nutritional needs of surgical patients vary widely. Those undergoing minor procedures may not require much more than their standard nutritional needs, while critically ill surgical patients and burn patients require significantly more support [1]. For all patients who do not have a contraindication, the preferred method of nutrition support is the enteral route. This topic covers determination of caloric requirements and selection of enteral formulas. The target goal of avoiding the extremes of starvation and overfeeding is defined by estimating energy requirements and is subject to continued monitoring and revision based upon the metabolic characteristics of patients and their tolerance of enteral nutrition [2,3].

Patient selection and timing of nutrition support and evaluating nutrition support is discussed separately. (See "Overview of perioperative nutrition support" and "Clinical assessment and monitoring of nutrition support in adult surgical patients".)

Parenteral nutrition support may be required when enteral nutrition support is not tolerated or contraindicated and is discussed separately. (See "Postoperative parenteral nutrition in adults".)

DETERMINING CALORIC REQUIREMENTS — The fundamental goal of nutrition support in those who are undergoing surgical procedures is to meet but not exceed the nutrition requirements. Assessment of nutrition support falls into two distinct categories: the initial requirements and the ongoing requirements. The assessment of caloric demands within the first 24 hours generates an initial goal so that nutrition support can be initiated. Equally important is the ongoing adjustment during what can often be a prolonged recovery.

For certain surgical subpopulations, such as in patients with burns or trauma, prolonged recovery can be associated with a prolonged course of hypermetabolism [2,3]. In burn-injured patients, the hypermetabolic response can be more prolonged than previously thought, possibly extending for up to three years postinjury [4]. (See "Hypermetabolic response to moderate-to-severe burn injury and management".)

Initial nutrition support goal — The target goal of avoiding the under- or overfeeding is determined by estimating energy requirements, and this goal is subject to ongoing revision based upon the metabolic characteristics of the patient and their tolerance of enteral nutrition [5,6]. Guidelines for initial nutritional support for critically ill patients from The American Society for Parenteral and Enteral Nutrition (ASPEN) suggest that there is no difference in providing 12 kcal/kg and 25 kcal/kg, and clinical judgment should be used to determine caloric intake [7]. Assessment of the energy requirements is an ongoing process and modified according to the progress of the patient [8,9].

Calculating energy requirements — The assessment of the energy requirements represents a composite of the following factors [10,11]:

●Basal metabolic rate (BMR)

●Hypermetabolism

●Ventilatory support

●Infections

●Sepsis

●Multiple organ failure

●Level of physical activity

●Thermic effect of food

Indirect calorimetry (IDC) estimation is the most accurate tool available to assess caloric requirements, but it requires specialized equipment [12-15]. With this limitation, many mathematical equations have been devised to estimate the caloric requirements of surgical patients [11,16]. No one formula accurately assesses the true caloric needs, and the available formulas may underestimate or overestimate the number of calories, depending on the clinical situation [17-20]. Avoidance of overfeeding minimizes the risks of hyperglycemia, fat accretion, and infections [5,6,21]. Continued vigilance is required to avoid complications associated with overfeeding or underfeeding [17-20,22-24].

Indirect calorimetry — IDC is considered the "gold standard" for assessing energy expenditure; however, it is easiest to perform on mechanically ventilated patients and therefore may not be easily applicable to all surgical patients. The IDC method uses respiratory gas exchange to estimate fuel consumption. IDC provides a measurement of both resting energy expenditure (REE) and respiratory quotient. The results of IDC are affected by oxygen therapy, hemodynamic instability, fever, and any ongoing procedures [25]. IDC has been found to be more useful for estimating caloric requirements of the surgical patient compared with predictive formulas in some populations; however, the equipment necessary to perform the tests is not widely available [12-14,20,26].

Predictive equation: Harris-Benedict — When IDC is not available, predictive equations are relied upon. While IDC provides a measure of REE, the predictive equations were designed to provide an estimate of basal metabolic rate (BMR). BMR is often used interchangeably with REE. While BMR is a minimum number of calories required for basic functions at rest, REE is the number of calories that the body burns while at rest. The Harris-Benedict equation is an accepted standard for estimating BMR and is the most widely used formula for estimating caloric requirements in adult surgical patients (calculator 1). An arbitrary stress or exercise factor can be used in certain patient populations, such as burn patients [9,17]. The accuracy of the Harris-Benedict equation is variable in surgical and intensive care unit (ICU) populations ranging from 17 to 67 percent [13,27-29].

●The equation used for men is:

●The equation used for women is:

Other general predictive equations

American College of Chest Physicians calories-per-kilogram equation — The American College of Chest Physicians (ACCP) published guidelines for nutritional management in intensive care unit patients in 1997 [30]. They recommended 25 kcal/kg of usual body weight for most patients but made alterations for both patients who are overweight or underweight. For patients with a body mass index (BMI) >25 kg/m2, the recommendation was that the calculation should be made with the ideal body weight. For underweight patients (BMI <16 kg/m2), the calculation used the patient's existing body weight for the first seven to ten days because of the risk of refeeding syndrome, then the calculation was based on ideal body weight. Unfortunately, validation studies of this equation have found that it only has an accuracy of 12 percent [31], and a coefficient of determination (R2) value of only 0.24 [32]. The use of this equation can lead to either under- or over-feeding, particularly in a critically ill population.

Mifflin equation — The Mifflin equation was developed using data from 498 health subjects [33]. It may be useful for healthy surgical patients, but validation studies have reported poor accuracy in critically ill patients [13,31]. Accuracy in predicting energy needs was only 35 percent when used alone, and even when a correction factor of 1.1 was applied, accuracy was 58 percent when compared with IDC [13].

●The equation for men is:

●The equation for women is:

Swinamer equation — The Swinamer equation was based on data from ventilated critically ill patients with trauma, surgical, and medical diagnoses. This equation includes variables that were determined to contribute greater than 3 percent to the variance in energy expenditure [34]. This equation has proved difficult to use in a clinical setting due to difficulty obtaining all of the included variables reliably. The validation studies that have been performed have found an accuracies of 45 and 55 percent when compared with IDC [13,29]. This equation is only applicable to ventilated patients, and it requires a determination of tidal volume.

●The equation is:

Ireton-Jones equation — The Ireton-Jones equation was developed in 1992 using data from 200 critically ill patients. The accuracy of the formula was 28 to 83 percent in various validation studies [29,35]. In 1997, the data used to derive the original equation was re-analyzed with the objective of improving its accuracy [36]. The revised equation improved accuracy to 58 percent in an initial study, but a later validation study found its accuracy to only be 36 percent [35].

●The 1992 equation is:

●The 1997 equation is:

Penn State equation — The Penn State equation was originally developed in 1998 based on data from 169 mechanically ventilated surgical and medical patients. In the original equation, the adjusted body weight was used for patients with obesity and was validated to have an accuracy rate of 68 percent in one study [35] and 39 percent in another [29]. The Penn State equation was modified in 2003 to use actual body weight in subjects with obesity [35]. The modified equation had an accuracy rate of 43 percent [13], and 72 percent [35] in two separate validation studies. As with many of the other equations, the use of this equation outside of a critically ill population has not been studied.

●The 1998 equation is:

Use adjusted body weight for patients with obesity (add 25 percent of excess body weight to ideal body weight) for HBE.

●The 2003 equation is:

Use actual body weight with patients with obesity for HBE.

Special surgical populations

Transplant patients — Malnutrition is a major factor influencing outcomes following transplantation, leading to the recommendation that nutritional status should be regularly monitored in those on the transplant waiting list as well as those who have received organ transplant. Those with malnutrition should be treated with oral nutritional supplements or tube feedings [1]. No specific adjustments are required in calculating nutrition support goals for transplant patients beyond those required to account for the stress of a surgical procedure [1]. The fact that most transplant are at major institutions may make the use of IDC more feasible in this population.

Undernutrition leads to impaired functional status, which influences outcomes following transplant while also worsening the chronic medical conditions that lead to the need for transplantation [37-40]. Given that there are often long preoperative waiting periods, this time can be used to influence the nutritional status of patients while on the waiting list [41-43]. Despite this, in these studies, there has been no difference in mortality between patients on the waiting list and patients after transplantation based on nutritional intervention. Some studies suggest using immune-modulating formulas during the waiting period and five days after liver transplantation leads to beneficial long-term outcomes on total-body protein and a reduction of infectious complications [43]. (See "Overview of perioperative nutrition support", section on 'Immune-enhancing nutritional supplements'.)

Following heart, lung, liver, pancreas, and kidney transplantation, early intake of normal food or enteral nutrition is recommended within 24 hours of surgery [44-46]. Even after transplantation of the small intestine, enteral nutrition can be initiated early, but should be increased very carefully within the first week following transplantation. With respect to which enteral nutrition formula to use in transplant patients, the use of a high-fiber formula with probiotic bacteria (Lactobacillus plantarum) has been shown to reduce infections when compared with formulas that contain fiber alone [47,48]. If necessary, enteral and parenteral nutrition can be combined. If parenteral nutrition is being used, there have been benefits reported with administration of a combination of medium-chain triglycerides and long-chain triglyceride lipid emulsions compared with long-chain triglyceride-alone emulsions [49]. In particular, following liver transplant, use of combination emulsions showed benefits with regard to ischemia-reperfusion graft injury, infectious morbidity, and post-transplant hospital stay [50,51].

Long-term nutritional monitoring and dietary counseling are reasonable because many patients undergoing transplantation show inadequate body composition. Increased fat and reduced lean body mass (LBM) were observed in 145 patients undergoing renal transplantation, and patients with a normal body mass index had better renal graft function than those with obesity [52].

Patients with obesity

Predictive equation for high body mass index: Cunningham — The validity of the more commonly used predictive equations in patients with obesity (relative to IDC) has been questioned. (See 'Predictive equation: Harris-Benedict' above.)

An equation that appears to be useful for at least some patients with obesity is the Cunningham prediction equation, which uses fat-free body mass (FFM) in calculating REE [53].The Cunningham prediction equation uses FFM in calculating REE [53,54]. FFM is calculated by defining the ideal body mass index as 25, back-calculating ideal weight using the patient's height and defining excess weight as the difference between actual weight and ideal weight.

An injury factor of 20 percent is used to calculate the increase in energy expenditure in burned patients.

A retrospective review of 28 burn and trauma patients with obesity found that the Harris-Benedict equation and the Cunningham prediction equation underpredicted the BMR/REE when compared with IDC [54]. When an injury factor of 20 percent is included in the calculations, the Cunningham equation was more accurate for predicting REE. The Cunningham equation potentially prevents overfeeding by using lean body weight rather than current weight [53,54].

Another study evaluating the best predictive equation for hospitalized patients with obesity found that the equations are only accurate in approximately 50 percent of patients, but in their study, the most accurate equation was the classic Harris-Benedict equation [55].

Role of permissive underfeeding — LBM is the site of metabolism, and it is just slightly higher in individuals with obesity compared with normal-weight individuals [54]. Permissive underfeeding, which uses ideal body weight to estimate caloric need, counters hyperglycemia and weight gain that result from overfeeding patients with obesity [56].

While permissive underfeeding is a reasonable strategy for critically ill nonsurgical patients with obesity, it is not recommended patients with obesity who have undergone surgery (nonbariatric surgery; specific recommendations for bariatric surgery patients are provided below). Permissive underfeeding relies on fat oxidation to mobilize peripheral stores as an energy source, which is a process that is impaired following surgery or in patients with significant traumatic injuries or burns [57-59]. Patients who have not undergone surgery have less metabolic derangement and less need for short-term support compared with surgical patients.

Recommendations following bariatric surgery — The preoperative assessment of a patient with obesity prior to bariatric surgery should include screening for malnutrition and deficiency in vitamins and trace elements. Early oral intake can be recommended after bariatric surgery, and these patients should be no different in their management compared with patients undergoing other upper gastrointestinal surgical procedures [60-62]. (See "Clinical assessment and monitoring of nutrition support in adult surgical patients", section on 'Patients with obesity'.)

However, due to the changes to their intestinal anatomy, nutritional care in patients undergoing bariatric surgery extends beyond the perioperative period. Following bariatric surgery, patients must have follow-up with a dedicated team so that they can be monitored for weight loss, dietary counseling, and to check nutritional labs to prevent deficiencies of micronutrients, with special emphasis on bone health (vitamin D3, calcium). Clinical practice guidelines were first developed in 2008 and have been updated regularly [63]. In the early postoperative period, bariatric patients may require supplementation with protein powders so that patients can meet their daily protein requirements (60 g protein/day), but care must be taken in the selection of supplement, because supplements with high glucose content can cause dumping syndrome. (See "Bariatric surgery: Postoperative nutritional management".)

Cancer patients — In general, the caloric requirements for patients with cancer should be assumed to be similar to that of other surgical patients [5]. However, patients with cancer are prone to weight loss and may not have adequate calorie consumption due to their disease process as well as the side effects of any cancer treatments [64]. If the patient is a candidate for surgical resection, malnutrition can cause significant complications and increase the risk of perioperative mortality [65]. As such, perioperative nutrition support, as indicated, has been recommended as an essential part of preoperative care in malnourished cancer patients, as it reduces the rate of postoperative complications [5,66]. (See "The role of parenteral and enteral/oral nutritional support in patients with cancer", section on 'The perioperative setting'.)

Burn patients — Initial estimates of energy expenditure in burn patients provide a particular challenge because of the variability of the hypermetabolic response and the factors that result in heterogeneity of energy expenditure in burn patients. Initial BMR/REE measurements are often done using the various formulas, but in most burn centers, IDC is used to follow nutrition needs over time after the initial estimates are made. (See "Clinical assessment and monitoring of nutrition support in adult surgical patients", section on 'Indirect calorimetry'.)

Predictive equations in burn patients

Harris-Benedict equation in burns — When using the Harris-Benedict equation for burn patients, the BMR is multiplied by an arbitrary activity or stress factor ranging from 1.2 to 2. The acceptable stress factor is between 1.2 and 1.5 for all but the most extensive burn injuries. While useful for initial estimate of energy demands, the Harris-Benedict equation overestimates the caloric requirements of burn patients.

Curreri formula — The Curreri formula provides for maintenance needs plus the additional caloric requirements for the burn injury. The two factors used in this formula, (ie, percent total body surface area [TBSA], body weight prior to the burn) estimate the energy requirements by linear regression analysis based on the number of calories required to prevent weight loss during the first few weeks postburn. This formula was based on nine adult patients. It does not consider sex, age, activity, or ventilatory status.

●The Curreri formula for adults age 16 to 59 years is:

●The Curreri formula for adults over age 60 years is:

While the Curreri formula has been accepted worldwide, it overestimates the caloric needs of burn patients when compared with metabolic expenditure requirements as measured by IDC. The overestimation may be related to the improvements in burn care since the time that formula was originally developed. Early wound closure, higher ambient temperature, improvements in infection control, and pain management have all reduced the hypermetabolic response to the burn injury. (See "Hypermetabolic response to moderate-to-severe burn injury and management", section on 'Attenuation of the hypermetabolic response'.)

Toronto formula — Another complicated formula, the Toronto formula, can be used to predict the energy requirements of the ventilated burn patient [9,67,68]. The formula requires an ongoing collection of patient data. The formula was developed by using multiple regression analyses to determine the factors that best approximated the measured energy expenditure (MEE). The factors included in the equation are a factor of activity, TBSA, caloric intake (CI), BMR estimated by the Harris-Benedict formula, and number of postburn days (PBD).

ENTERAL NUTRITION FORMULA SELECTION — Many commercial preparations are available for providing enteral nutrition support. These formulas come in a variety of compositions, and the "standard" adult nutritional formulas can provide appropriate support for wound healing and maintenance of lean body mass (LBM) when they are used to provide adequate calorie and protein intake. Involvement of a dietician in the initial nutrition support of the surgical patient, as well as when changes in physiologic and clinical status occur, is desirable.

Formula composition — The selected formula should meet the required caloric needs of the patient (as described above) as well as the appropriate level of macronutrients and micronutrients desired to meet the needs of the individual patients. We recommend delivery of [69,70]:

Macronutrients — In addition to establishing initial caloric energy requirements, determination of the appropriate composition of protein, carbohydrates, and fats for a nutrition support regimen is an early priority. Carbohydrates stimulate protein synthesis and limit loss of LBM, thereby making a moderate-carbohydrate diet preferable in patients who will or have undergone surgical procedures.

●Protein – Administration of nutrition support with protein of 1.2 to 2.0 g/kg/day, approximately 25 to 30 percent of calories per day, will provide a balance between synthesis and breakdown [69]. This level of protein support was confirmed in the American Society for Parenteral and Enteral Nutrition (ASPEN) guidelines for critically ill patients [7]. (See "Nutrition support in intubated critically ill adult patients: Initial evaluation and prescription".)

●Glucose – Glucose is the preferred substrate for wound healing and should be the major source of calories in surgical patients. Ideally, carbohydrates administered for nutrition support should consist of 1 to 2 g/kg/day of glucose and represent approximately 50 percent of total calories provided [70].

●Lipids – Lipids are an important dietary component, as they contain the essential fatty acids and serve as carriers of lipid-soluble vitamins, but they should comprise no more than 15 percent of total calories, or about 1 to 1.2 g/kg/day.

Micronutrients — We agree with recommendations from the European Society for Clinical Nutrition and Metabolism (ESPEN) that suggest that a combination of antioxidant vitamins and trace minerals (table 1) in doses reported to be safe for critically ill patients should be provided for surgical patients who require specialized nutrition therapy [71]. (See "Nutrition support in intubated critically ill adult patients: Parenteral nutrition", section on 'Vitamins, minerals, and trace elements'.)

Immunonutrition — The use of immune-modulating supplements or formulas is particularly appealing for patients with profound hypermetabolic response or immune suppression, which are associated with a variety of surgical conditions (eg, burns, transplant, cancer, sepsis).

Guidelines recommend that perioperative administration of specific formula enriched with immunonutrients (arginine, omega-3 fatty acids, ribonucleotides) should be given in malnourished patients undergoing major cancer surgery; however, there is no clear evidence to support the use of formulae enriched with immunonutrients when used only in the preoperative period [1,72-75]. Meta-analyses of the use of immunonutrition in general surgical patients suggest that perioperative administration of immune-modulating nutritional formulae have contributed to a decreased rate of postoperative complications and consequently to a decreased length of stay in the hospital [76-80]. For undernourished patients with cancer undergoing surgery, the ASPEN guidelines from 2009 gave a strong recommendation for the use of immunonutrition; however, this issue was not directly addressed in a later 2021 guideline [6,7]. Additionally, the ASPEN/Society of Critical Care Medicine (SCCM) guidelines from 2016 also recommend the use of immune-modulating formulas for critically ill patients who had surgery [81]. Most of the studies using immunomodulating substrates were performed with arginine, omega-3 fatty acids, and ribonucleotides. In spite of the positive results in these meta-analyses, the included randomized controlled trials' concerns have been raised about the general use of immunomodulating formulae. This is due to the heterogeneity of the single studies, including different periods of application and the lack of homogenous criteria for the definition of complications and hospital discharge. Neither ESPEN nor ASPEN/SCCM guidelines recommend the use of enteral glutamine supplementation in surgical patients [45,81].

Specialty formulas — Several specialty formulas are commonly used in an attempt to reduce renal failure, glucose intolerance, and pulmonary failure.

Commercially available formulas for renal failure patients are lower in protein and restrict electrolytes, particularly potassium and phosphate. Many of these formulas use additional carbohydrate calories to compensate for the protein calories that are restricted in renal patients, and some contain additional fat as the source of calories. Formulas that are higher in carbohydrates and lower in fat may be used appropriately in critically ill patients with renal failure and electrolyte abnormalities, recognizing the likely need to supplement protein in the intensive care unit (ICU) patient population.

Formulas available for patients with diabetes or those with stress glucose intolerance are lower in carbohydrates and higher in fat; which makes them an ineffective choice in ICU patients, since higher-carbohydrate, low-fat diets have been clearly associated with better immune function and lesser loss of LBM [24,82,83].

NUTRITION SUPPORT ADJUSTMENTS — The energy and nutrient needs of surgical patients are heterogeneous through the course of recovery. Nutritional needs fluctuate based upon wound healing (greater than 95 percent closure of open wounds) and physical activity and may be impacted by complications such as pneumonia, renal insufficiency, or sepsis.

Adjustments in nutrition support are best conducted under the guidance of an experienced dietician as a member of the care team and using a constellation of techniques that aid in monitoring the adequacy of nutrition support. (See "Clinical assessment and monitoring of nutrition support in adult surgical patients", section on 'Monitoring nutrition support'.)

SOCIETY GUIDELINE LINKS — Links to society and government-sponsored guidelines from selected countries and regions around the world are provided separately. (See "Society guideline links: Nutrition support (parenteral and enteral nutrition) in adults" and "Society guideline links: Care of the patient with burn injury".)

SUMMARY AND RECOMMENDATIONS

●Assessing energy requirements – The assessment of the energy requirements in surgical patients is a composite of the basal metabolic rate (BMR), hypermetabolism, ventilatory support, infections, sepsis, multiple organ failure, thermic effect of food, and level of physical activity. (See 'Determining caloric requirements' above.)

•Indirect calorimetry – The preferred method to estimate caloric requirements in surgical patients is by indirect calorimetry (IDC) when available; however, IDC requires specialized equipment that may not be readily available. (See 'Determining caloric requirements' above.)

•Estimations – Alternatives to IDC for estimating caloric requirements for adult surgical patients include the modified Harris-Benedict equation and the Cunningham equation for patients with obesity. Caution must be used with both equations to avoid overfeeding or underfeeding. Other predictive equations are available with variable accuracies reported compared with IDC. (See 'Calculating energy requirements' above.)

●Daily requirements – For most surgical patients, we recommend delivery of:

•1.2 to 2 g protein/kg/day

•1 to 2 g/kg/day of glucose, representing approximately 50 percent of total calories

•1 to 1.2 g/kg/day of lipid intake and not representing more than 15 percent of total calories

•Vitamins A, C, and D in standard multivitamin formulations

●Caloric mix – We use a nutritional formula that provides at least 50 percent of calories as carbohydrates, 35 percent as protein, and no more than 15 percent as fat, supplemented with micronutrients and macronutrients. (See 'Formula composition' above.)

●Supplements – In our practice, we supplement the enteral formula with a standard multivitamin that includes trace minerals for critically ill surgical patients. These include vitamins A, C, and D and trace minerals copper, zinc, and selenium. We also use a standard protocol for electrolyte replacement. (See 'Micronutrients' above.)