Malnutrition and perioperative nutritional rehabilitation in major operations

Malnutrition is a common condition among patients undergoing major surgical procedures, with its prevalence varying significantly depending on the surgical specialty, patient case mix, and the diagnostic criteria employed in different studies. Table 2 summarizes the findings of representative studies in this review that used standardized definitions for diagnosing malnutrition. However, comparing these studies is challenging due to inconsistent definitions, which often show poor concordance. Despite these limitations, some general trends are evident. Patients undergoing pancreatic and esophagogastric surgeries exhibit the highest prevalence of malnutrition, with over half of those undergoing major organ resections for cancer being affected. Severe malnutrition rates in these groups typically range from 10 to 20%. Patients undergoing thoracic, colorectal, urological, and orthopedic surgeries are generally at a moderate nutritional risk, with overall malnutrition rates of 25–50% and severe malnutrition rates often exceeding 10%. In contrast, cardiac surgery patients are at the lowest nutritional risk, with overall malnutrition rates below 20% and minimal occurrences of severe malnutrition.

The prevalence of malnutrition is influenced by several factors beyond the anatomical site of surgery. These include advancing age; cancer diagnosis and staging; higher American Society of Anesthesiologists (ASA) classifications (> II); polypharmacy (use of > 10 medications); increased comorbidity burden; and underlying medical conditions such as severe, untreated, or poorly controlled bowel, liver, kidney, or metabolic diseases (e.g., diabetes) [27, 28]. Geriatric frailty is inherently associated with malnutrition in surgical patients. Although age alone is not always confirmed as a risk factor, the progressive decline in health and functional status associated with aging is a significant determinant of malnutrition in older individuals [29]. While cognitive decline, dementia, and psychiatric disorders are not consistently identified as risk factors for malnutrition in surgical populations, they have been extensively studied in non-surgical populations and should be considered when evaluating these patients [29].

Malnutrition affects various organs and systems, leading to structural changes and functional impairments. Muscle atrophy impairs mobility, functional independence, and cardiorespiratory reserves [30]. It suppresses the immune system, particularly cell-mediated immunity, thus increasing susceptibility to infections [31]. Atrophy of the bowel mucosa compromises the intestinal barrier, heightening the risk of enteric infections, while reduced pancreatic secretion exacerbates nutritional deficiencies [32]. Anemia, commonly associated with deficiencies in iron, vitamin B12, or folate, further compromises wound healing. Additionally, the liver’s reduced capacity to synthesize proteins, including clotting factors, aggravates clinical outcomes [33]. Malnutrition is an unequivocal risk factor for poor postoperative outcomes across surgical disciplines, settings, and populations undergoing major operations [8, 13, 22, 34‐37]. Malnourished patients face 2–3-times higher rates of postoperative complications, including life-threatening events, resulting in a similar increase in postoperative mortality [38, 39]. These outcomes are independent of other risk factors such as the type of surgery, patient performance status, presence of malignancy, and comorbidities [25, 40‐47].

Preoperative malnutrition is a strong independent predictor of not only short-term postoperative survival but also of long-term survival in both benign and malignant conditions [39, 47‐51]. It increases the incidence of surgical site infections [10, 52‐54] and remote infections [15, 18, 54]. Furthermore, malnutrition is associated with hospital stays prolonged by 6 days on average [10, 22, 27, 37, 38, 47, 51, 55, 56], extended intensive care unit (ICU) admissions [18, 22, 42, 51, 57, 58], increased rates of reoperations and readmissions [18, 37, 47, 50, 59], reduced quality of life [37], impaired functional status [56, 58, 60‐62], higher healthcare costs [55], and a greater incidence of postoperative delirium [24, 63, 64].

Given the high volume of surgical patients and the time and resource constraints in surgical settings, applying comprehensive malnutrition assessment methods and diagnostic criteria universally is impractical, unfeasible, and cost ineffective. Screening tools offer a rational approach by stratifying patients into risk categories, ensuring that only those at a higher risk undergo detailed nutritional assessments and interventions. This strategy can significantly reduce the need for comprehensive assessments, as up to 80% of patients undergoing major surgery are typically well nourished [4, 39]. Screening enables the early identification of malnutrition risk, ideally before clinical manifestations such as organ dysfunction or complications arise. This proactive approach facilitates the timely implementation of interventions, preventing the onset of malnutrition during the perioperative period. In contrast, diagnostic methods and criteria often identify malnutrition only after it has become clinically evident, advanced, and more challenging to manage effectively.

The literature describes over 50 nutritional screening tools developed for surgical, non-surgical, and mixed populations, many of which have been individually and comparatively evaluated in various surgical settings [4, 38, 65‐69]. Although there is no consensus on the optimal tool for surgery [2], the available evidence supports the Malnutrition Universal Screening Tool (MUST) as the most accurate for preoperative malnutrition screening [4]. The MUST incorporates body mass index (BMI), unintentional weight loss, and the presence of acute disease, categorizing patients into low, intermediate, and high nutritional risk levels ([70]; Table 3). Developed by the British Association for Parenteral and Enteral Nutrition (BAPEN) in 2003, the MUST was designed to be valid, reliable, and easy to use across all adult care settings. For patients identified as having a medium risk, BAPEN recommends documenting dietary intake for 3 days, with reassessment in 7 days if intake is adequate. For those at a high risk, referral to a nutritional specialist or implementation of interventions based on local policies and protocols is advised.

The next step in nutritional management after screening is to confirm the diagnosis of malnutrition. Screening tools are designed to identify individuals at risk; however, their diagnostic accuracy compared to diagnostic methods is typically around 80%. This means that many patients who test positive in screening may not actually have malnutrition, leading to unnecessary interventions which may be both costly and harmful, such as establishment of a parenteral nutrition route [4, 65, 68]. Accurate diagnosis is essential for targeted, effective interventions. This approach is superior to applying generic nutritional strategies, as it enhances the evaluation of treatment outcomes and enables meaningful comparisons across different patient populations. Consequently, accurate diagnosis improves research quality, care delivery, and the rational allocation of healthcare resources.

Internationally recommended GLIM diagnostic criteria for malnutrition, as outlined in the introduction, should be employed to standardize diagnosis and facilitate comparisons of treatment outcomes and healthcare policies [2, 7]. The Subjective Global Assessment (SGA) is a diagnostic tool that evaluates 10 features derived from a patient’s history and physical examination. It has been extensively validated in nutritional studies across both surgical and non-surgical populations [71]. SGA is recognized as a valid, comprehensive method and a strong predictor of clinical outcomes in various settings. It is considered the gold standard for evaluating the diagnostic accuracy of most nutritional screening tools and has been used in validation studies for the GLIM diagnostic criteria [72‐74]. Furthermore, the use of SGA allows for valid comparisons with historical cohorts when necessary.

Both the GLIM criteria and the SGA tool can be applied by non-nutritional specialists. However, if a diagnosis of malnutrition is confirmed, a detailed nutritional assessment by a specialist dietitian is recommended. Such an evaluation encompasses the patient’s nutritional history, clinical presentation, physical status, somatometry, biochemistry, and body composition analysis [2, 73]. Accurate diagnosis of malnutrition identifies the specific type of condition, such as micronutrient deficiency, protein–energy malnutrition, marasmus, overnutrition etc., and facilitates targeted, effective interventions [75]. This holistic approach addresses the multifaceted nature of malnutrition, which includes medical, social, and psychological parameters, and tailors interventions to the patient’s health status, age, activity level, and cultural preferences. A significant limitation of this strategy is the unavailability of dietitians in many healthcare settings. In such cases, protocolized general assessments and interventions provide a reasonable alternative.

The goals of preoperative nutritional therapy are to prevent the occurrence of malnutrition and its associated complications in at-risk patients and to treat malnutrition in diagnosed cases. This approach aims to prevent further deterioration and mitigate the impact of malnutrition on surgical outcomes. Current screening tools, however, were not designed to identify individuals at risk of developing malnutrition in the near perioperative period, as they were validated against simultaneously performed diagnostic methods. Developing tools specifically to identify patients in whom early intervention could prevent the onset of malnutrition should be a focus of future studies in surgical patients.

For patients diagnosed with malnutrition, preoperative nutritional therapy is reasonable and justified, and its omission would be negligent given the substantial body of evidence linking preoperative malnutrition to postoperative complications [76]. Furthermore, evidence suggests that preoperative nutritional supplementation reduces the risk of postoperative complications. However, not all clinical trials investigating preoperative nutritional therapy have exclusively included malnourished patients. It is possible that some well-nourished patients would also benefit from such interventions, as they may be protected from developing postoperative malnutrition [77, 78]. Currently, there are insufficient data to determine which well-nourished patients are most likely to develop postoperative nutritional deficiencies and whether preoperative nutritional intervention could prevent postoperative malnutrition and its sequelae. These questions present compelling areas for future research.

A particularly important subset of patients includes oncology patients who are well nourished at the time of their initial diagnosis but undergo preoperative oncological treatments which often lead to gastrointestinal symptoms as side effects, thus increasing the risk of malnutrition [79]. According to international guidelines, these patients should be screened for malnutrition at diagnosis, during treatment, and after completing treatment [80]. Patients initially identified as being at a low risk for malnutrition but scheduled for preoperative oncological therapy should undergo regular nutritional screening. While there is no definitive evidence regarding the optimal frequency of these assessments, it is reasonable to conduct them every 1–2 weeks, as a reduction in dietary intake lasting more than 1 week is a diagnostic criterion for malnutrition [2, 70, 80].

Prevention and early diagnosis of malnutrition are crucial for enabling timely nutritional interventions that optimize patients’ physical status. These interventions are most effective when integrated into an enhanced recovery after surgery (ERAS) program or a prehabilitation framework [81‐83]. This comprehensive approach has been shown to reduce postoperative complications and shorten hospital stays, as recommended by international guidelines [76, 78, 83‐85]. Meta-analyses of published studies demonstrate a pooled reduction of approximately 20% in the risk of postoperative complications among patients undergoing gastrointestinal cancer surgery who receive preoperative nutritional interventions. Effective nutritional interventions require supervision by dietitians as part of a specialized multidisciplinary team (MDT). MDTs should systematically monitor malnutrition rates in newly diagnosed patients and assess the outcomes of nutritional therapies as part of ongoing quality-improvement initiatives. In settings where MDT support is unavailable, specialists in nutrition should establish protocolized, generic nutritional interventions. These protocols should be designed for implementation by non-specialists and include mechanisms for monitoring their effectiveness in addressing perioperative malnutrition and improving postoperative outcomes. Prescribing nutritional supplements without individualizing them to patients’ needs or monitoring compliance and effectiveness is suboptimal and should be avoided.

Nutritional interventions should begin as early as possible in the disease trajectory, ideally during the patient’s first consultation with a surgeon or oncologist [85, 86]. The duration of nutritional therapy should be tailored to the surgical indication. For benign and symptomatically controlled diseases, such as large abdominal wall hernias or elective aortic and orthopedic surgery, operations may be postponed until malnutrition is adequately addressed, which may take several weeks. Cancer patients undergoing preoperative therapy should receive nutritional support during their oncological treatments, which can last several months for conditions such as esophagogastric, head and neck, or rectal cancers. For other elective surgeries, nutritional optimization typically spans 2–6 weeks [78]. Patients presenting with malnutrition and low muscle mass may require longer interventions, often 4–5 weeks, supplemented by physical exercise, which has a synergistic effect on muscle mass restoration [87]. For patients with severe malnutrition, at least 7–10 days of nutritional support is essential, even in the presence of symptomatic or advanced cancer and conditions such as achalasia, gastric outlet obstruction, or subacute bowel obstruction. This approach has been shown to reduce surgical site infection rates as much as threefold [88, 89]. The optimal duration of preoperative nutritional interventions remains unclear due to a lack of high-level evidence, underscoring the need for further research in this area. For patients who are unstable, unwell, or septic while awaiting surgery, nutritional therapy is not a priority. Instead, the focus should be on resuscitation with crystalloid solutions and blood products where necessary, along with treatment of the underlying cause. Parenteral nutrition solutions are not resuscitation fluids, have not been tested for safety in this context, and should not be used (Fig. 2).

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Dietetic consultations, including follow-up sessions and the provision of oral nutritional supplements when appropriate, represent the rational approach to managing malnutrition in the preoperative setting. However, the current level of evidence supporting this practice remains limited, highlighting the need for further clinical research [77]. During the initial consultation, patients receive tailored advice regarding their nutritional status, its potential impact on surgical outcomes, and their dietary habits. This counseling is highly individualized, taking into account personal habits, preferences, comorbidities (e.g., diabetes, food allergies, bowel diseases), ongoing treatments (e.g., oncological therapies, hormone treatments), social determinants (e.g., access to and availability of high-quality food), and the underlying surgical condition. Such an expert-driven approach is inherently complex and challenging to standardize into protocols for use by non-specialists. Furthermore, no established guidelines currently exist to provide a structured framework for dietetic consultations in the preoperative context. Developing such guidelines would facilitate a common language among healthcare professionals, standardize care delivery, enable comparison of interventions across populations, and promote global standards in preoperative nutritional management. Follow-up sessions, conducted either in person or via telephone, play a critical role in reinforcing patient adherence to dietary modifications. Dietitians provide positive feedback on progress such as weight changes, enhance compliance, and monitor outcomes by setting achievable weekly goals. This practice with or without oral nutritional supplements has been linked to improvements in nutrition intake, less deterioration of nutritional status than controls, and improved clinical outcomes in randomized trials with limited numbers of patients [77, 78].

Oral nutritional supplements should be provided to all patients whose nutritional requirements cannot be met through regular food intake. However, in unselected or well-nourished patients, the administration of oral supplements during the preoperative period has not been demonstrated to improve surgical outcomes in randomized studies [90‐94]. The results of controlled studies reporting reductions in postoperative infections in the treatment arms as well as of meta-analyses of studies consistently showing no significant positive effects should be interpreted with caution [76, 95]. When oral nutritional supplements are indicated, immune-modulating formulas should be prioritized. These formulas are enriched with arginine, nucleotides, omega‑3 fatty acids, glutamine, and antioxidants, and their use should be extended into the postoperative period. Evidence suggests that perioperative administration of these specialized supplements reduces the risk of postoperative complications and shortens hospital stays [34, 76, 96]. If nutritional requirements cannot be fulfilled through regular food and oral nutritional supplements, enteral nutrition (e.g., nasogastric, gastric, nasojejunal, or jejunal feeding) should be implemented. If neither oral nor enteral nutrition is feasible, parenteral nutrition should be introduced as a complementary measure. In malnourished patients in whom oral or enteral nutrition is contraindicated, complete parenteral nutrition should be administered for 1–2 weeks prior to surgery, as previously discussed (see Sect. “Indications”).

The most accurate method for calculating energy requirements in surgical patients is indirect calorimetry, which involves calculations based on oxygen consumption and carbon dioxide consumption. This approach is relatively straightforward for intubated patients, as it involves the application of Weir’s equation:

Resting energy expenditure (kcal/day) = [(VO₂ × 3.941) + (VCO₂ × 1.11)] × 1440, where VO₂ is the oxygen consumption and VCO₂ is the CO₂ production. However, in non-intubated patients, indirect calorimetry necessitates specialized equipment, which is both expensive and not widely available in most surgical departments. Consequently, various equations have been proposed to estimate caloric needs in surgical patients, with the Harris–Benedict equation being among the most well known [97]. Despite their popularity, none of these formulas have demonstrated adequate accuracy when compared to indirect calorimetry, mainly because they do not correctly account for the hypermetabolic state or surgical stress-induced energy demands [98]. In clinical practice, a simplified weight-based calculation is often employed, using the rule of 25–30 kcal/kg of ideal body weight. This method assumes that surgical patients and healthy individuals have similar total resting energy expenditure. However, this approach is suboptimal, as it overlooks important considerations: for example, approximately half of cancer patients who experience weight loss are known to be hypermetabolic and patients with postoperative complications, systemic inflammation, and organ dysfunction have variable energy requirements [99, 100]. Given these limitations, research efforts should prioritize the development of non-invasive, accurate, and practical technologies or methods, such as wearable metabolic monitors, for determining energy requirements in non-critically ill surgical patients.

Although evidence for overfeeding (exceeding 110% of energy requirements) in non-critically ill malnourished patients is limited, current guidelines advise against it due to the associated risks, which have been clearly demonstrated in critically ill patients (see Sect. “Nutritional support for the critically ill after major surgery”) [101]. Nutrition therapy should be initiated cautiously in malnourished patients who have consumed little to no food for more than 5 days, as they are at risk of refeeding syndrome, a potentially life-threatening condition. It is recommended to start at less than 50% of the nutritional requirements and progressively increase to 100% over 2–3 days. International guidelines suggest a protein intake of 1.2–2.0 g/kg/day from high-quality protein sources, distributed throughout the day (e.g., 20–40 g of protein per meal), to ensure protein needs are met, particularly in preparation for surgery ([102]; Table 4). The highest-quality proteins are derived from animal products, such as chicken, eggs, beef, fish, and milk, as these provide all essential amino acids and exhibit superior bioavailability. In contrast, plant-based protein sources typically lack one or more essential amino acid; therefore, combinations of plant proteins are necessary to meet protein requirements in healthy individuals [103]. A balanced diet incorporating both animal and plant-based foods ensures adequate intake of essential micronutrients, including vitamins, minerals, fiber, phenols, and antioxidants. There is no evidence for any benefits from specific supplementation of micronutrients including vitamins and minerals in the preoperative period.

International guidelines recommend that prolonged overnight fasting before surgery to reduce the risk of aspiration is unnecessary and may even be harmful [104, 105]. Patients without conditions such as gastroesophageal reflux, gastroparesis, or poorly controlled diabetes should be permitted to consume clear fluids up to 2 h before an elective operation. Clear fluids include water, clear tea, black coffee, fruit juice without pulp, carbonated beverages, and carbohydrate-rich drinks. Light solid foods, including milk, are permissible up to 6 h before elective procedures, while heavier meals, such as fried or fatty foods, or larger portions may require longer fasting periods. The consumption of a carbohydrate-rich drink—800 mL the night before surgery and 400 mL 2 h before the operation—has been associated with benefits such as reduced postoperative insulin resistance, decreased protein breakdown, preservation of muscle strength, and reduced myocardial injury [82]. These volumes correspond to typical carbohydrate solutions (e.g., maltodextrin) with concentrations ranging from 5 to 12.5%. For patients unable to consume fluids orally, international guidelines recommend glucose infusion at a rate of 5 mg per kilogram of ideal body weight per minute as an alternative. This should be administered using a 20% glucose solution to deliver an adequate quantity in a low volume, ensuring a sufficient insulin response [106]. It is important to note that carbohydrate loading has not been studied extensively and is not expected to provide benefits for patients with type 1 diabetes, as their condition is characterized by insulin deficiency rather than insulin resistance.

The preferred route for nutritional support in malnourished patients is oral intake through a balanced diet, guided by dietitian counseling [77, 101, 107]. If nutritional requirements cannot be met through regular food, oral nutritional supplements should be introduced. For patients unable to use the oral route or where oral intake is insufficient due to swallowing difficulties or a dysfunctional/inaccessible gastrointestinal tract, nasogastric tube feeding should be considered as a primary or complementary method until the oral route is viable [101]. Examples include patients with swallowing impairments caused by central nervous system trauma or disease and those with uncontrollable oropharyngeal or esophageal dysphagia, such as individuals with head and neck cancer, achalasia, or esophagogastric cancer. Tube feeding is also an option for patients experiencing severe symptoms—such as abdominal pain, nausea, vomiting, or anorexia—that limit oral intake. This approach requires functional and structurally intact lower gastrointestinal (GI) tracts. Gastric tube feeding can involve either bolus administration of blended food or specialized nutritional formulas. In hospitalized or critically ill patients, continuous feeding over 16–24 h is often preferred (see section “Timing and route of nutritional support in critically ill postoperative patients in the ICU”).

When nasogastric tube feeding is required for more than 4 weeks, including the preoperative and postoperative periods in patients undergoing major surgery, international guidelines recommend the use of a percutaneous endoscopic gastrostomy (PEG) tube. PEG tubes are considered as safe as nasogastric tubes but offer several advantages, including reduced gastroesophageal reflux symptoms, improved effectiveness in treating malnutrition, and greater patient preference due to less inconvenience and fewer limitations to social activities [108]. If PEG tube insertion is not feasible due to technical reasons, such as upper gastrointestinal (GI) obstruction, a radiologically inserted gastrostomy (RIG) tube is a reasonable alternative. Although RIG tubes are equally safe, their smaller caliber makes them more prone to occlusion and dislodgement compared to PEG tubes [109]. Surgical gastrostomy is recommended when neither PEG or RIG insertion is viable due to technical constraints. However, this approach is associated with a higher risk of wound complications and typically requires general anesthesia. In cases where the stomach must be bypassed—such as after partial gastrectomy with delayed gastric emptying, incomplete duodenal or gastric outlet obstruction, or severe pancreatitis—a nasojejunal catheter is indicated for continuous feeding with an appropriate enteral formula. For long-term nutritional support exceeding 1 month, a needle jejunostomy catheter is recommended, similar to the PEG tube. Enteral nutrition is preferred over parenteral nutrition, as substantial evidence from meta-analyses of randomized trials indicates that it is associated with lower postoperative infection rates and shorter hospital stays [110, 111]. Additionally, enteral nutrition is more cost effective and offers easier access.

However, recent trials highlight that with an improved understanding of energy requirements, advancements in parenteral formulas, and better infection-prevention protocols, parenteral nutrition has become a viable alternative to enteral nutrition. The most recent American Society for Parenteral and Enteral Nutrition (ASPEN) guidelines explicitly recommend parenteral nutrition for critically ill patients when appropriate [112]. Parenteral nutrition is indicated when the enteral route is contraindicated or insufficient to meet nutritional needs in both the preoperative and postoperative periods. Contraindications for enteral nutrition include incomplete intestinal obstruction or ileus, intestinal bleeding or ischemia, severe inflammatory bowel disease, and high-output gastrointestinal fistulas, among others [85, 106, 113]. In these cases, parenteral nutrition can be employed either as a supplement to enteral nutrition, when partial enteral feeding is possible, or as the sole method of nutritional support. Preoperative administration of parenteral nutrition for 7–14 days in severely malnourished patients is particularly beneficial, reducing postoperative complications by one third and lowering postoperative mortality [114, 115]. However, it is important to note that parenteral nutrition has no demonstrated benefit in well-nourished or mildly malnourished patients, and its use in these populations is associated with increased morbidity [106].

There is limited evidence in the literature specifically addressing postoperative critically ill patients in the ICU. However, numerous studies focus on nutrition in mixed surgical and non-surgical ICU populations, with many cohorts derived from diverse surgical specialties [116, 117, 119‐123]. Thus, it is reasonable to extrapolate these findings to surgical patients. Patients who have not been screened or assessed for malnutrition preoperatively should undergo nutritional assessment upon ICU admission. Critically ill patients capable of oral intake should be encouraged to eat within the first 48 h of ICU admission. If they are able to meet at least 70% of their nutritional requirements during the first week, no additional nutritional interventions are necessary. If oral intake is not feasible, enteral nutrition (via gastric or jejunal routes) should be initiated within 48 h, as it is associated with a 50% reduction in infection rates and a shorter hospital stay compared to parenteral nutrition [124]. Early oral or enteral nutrition is recommended for unselected surgical ICU patients, including those undergoing trauma surgery, uncomplicated gastrointestinal surgery, aortic surgery, or laparostomy procedures, regardless of bowel sounds, bowel movements, or diarrhea. Contraindications to enteral nutrition include shock, severe hypoxemia, hypercapnia, acidosis, active upper gastrointestinal bleeding, bowel ischemia, abdominal compartment syndrome, high-output intestinal fistulas or gastric aspirates exceeding 500 mL over 6 h. Enteral nutrition may be cautiously administered to patients with abdominal hypertension (without abdominal compartment syndrome) and those with acute liver failure. For patients with delayed gastric emptying unresponsive to prokinetic agents or those at a high risk of aspiration, post-pyloric feeding via a nasojejunal tube should be considered. Risk factors for aspiration, as outlined in international guidelines, include an inability to protect the airway, mechanical ventilation, age over 70 years, reduced level of consciousness, poor oral care, nurse-to-patient ratios below 1:1, neurological deficits, supine positioning, gastroesophageal reflux disease, and bolus intermittent feeding [125, 126]. In other cases, the gastric route is preferred due to its simplicity, convenience, and physiological benefits over jejunal feeding.

For severely malnourished patients, early postoperative parenteral nutrition should be initiated within 48 h in hemodynamically stable individuals when the oral or enteral route is contraindicated or insufficient to meet at least 70% of nutritional requirements [124]. In such patients, a gradual introduction of oral, enteral, or parenteral nutrition is necessary to prevent refeeding syndrome. For well-nourished or mildly malnourished patients, parenteral nutrition should only be initiated after 7 days in the ICU if oral or enteral routes are impossible or inadequate [112, 124, 126]. Notably, the recommendation to prefer enteral nutrition over parenteral nutrition in critically ill patients is not based on high-quality evidence and perhaps warrants further investigation.

The calculation of nutritional requirements in critically ill surgical patients should ideally be based on indirect calorimetry [112, 126], as mentioned in Sect. “Nutritional requirements.” Formulas using only VCO₂ are less accurate than calorimetry [127]. In the absence of calorimetry and VCO₂, VO₂ measurement, considering energy requirements of 20–25 kcal/kg/day, is a common approach. Nonetheless, this calculation often over- or underestimates the actual energy requirements. During the first 3 days of critical illness, hypocaloric nutrition (providing 50–70% of the calculated energy requirements) has been associated with improved infection rates, reduced ventilation duration, and shorter hospital stays [118, 123]. After this initial period, energy delivery should progressively increase to meet 100% of the patient’s energy requirements. The current literature does not support protein intake higher than 1.3 g/kg of body weight for critically ill patients. In particular, patients with acute kidney injury or multiorgan failure have demonstrated improved survival with protein doses averaging around 0.9 g/kg/day [122]. The timing of protein intake may also play a significant role in optimizing ICU outcomes. Retrospective studies suggest that lower protein doses of < 0.8 g/kg/day during the first 5 days are associated with improved 6‑month mortality [116]. These findings align with physiological data showing that amino acid metabolism is severely disturbed during critical illness and improves progressively after the early phase of acute illness, which may last at least 10 days depending on disease severity [128]. In summary, a reasonable recommendation for protein intake in ICU surgical patients would be to start with less than 0.8 g/kg/day during the first 5 days, followed by a gradual increase to 1.3 g/kg/day thereafter.

The general recommendation is to allow patients to consume normal food, oral supplements, and clear fluids as early as possible after major surgery. Early oral feeding and the avoidance of nasogastric tubes within an ERAS protocol significantly reduces postoperative complications and the length of hospital stay, as demonstrated by meta-analyses of randomized trials [129]. In colorectal surgery, patients can safely begin drinking clear fluids within a few hours postoperatively (for both open and laparoscopic procedures) and progress as tolerated to normal food, oral supplements, or enteral nutrition within the first 48 h. Similar outcomes have been reported following upper gastrointestinal surgery, though oral intake is generally introduced more gradually than in colorectal cases [130]. For hepatobiliary and pancreatic surgery, international guidelines recommend early nutrition, although supporting evidence remains limited [81, 82, 85]. In non-gastrointestinal surgery patients, resumption of oral nutrition is typically straightforward. If patients fail to meet at least 50% of their nutritional requirements within the first 5–7 days, supplementation should be initiated in the following order: oral supplements, enteral nutrition, and, if necessary, parenteral nutrition.

There is no evidence supporting a more aggressive nutritional approach for malnourished patients. However, in cases where patients are unlikely to meet nutritional needs within the first postoperative week via the oral route—such as those undergoing upper gastrointestinal, pancreatic, head and neck, or neurosurgical procedures—intraoperative planning should ensure early enteral nutrition within 48 h. This may involve placing a nasogastric or nasojejunal feeding tube, a feeding gastrostomy, or a jejunostomy if not already available, as well as securing venous access for parenteral nutrition when enteral feeding is contraindicated or expected to be insufficient [85, 112, 113]. In such cases, clinical judgment is essential to assess tolerance to oral and enteral nutrition and to anticipate potential delays in resuming oral intake.

The energy requirements of patients following elective, uncomplicated major surgery appear to remain similar to those in the preoperative period. Although indirect calorimetry is infrequently employed postoperatively, formula-based estimations are comparably imprecise to the commonly used rule of 25–30 mL/kg body weight per day. Consequently, there is an urgent need for more practical and accurate methods of calorimetry in clinical practice. Protein requirements also remain consistent with the preoperative period, estimated at 1.2–2.0 g per kg of ideal body weight per day [85, 112, 113]. Clinicians should exercise caution regarding refeeding syndrome when reintroducing nutrition in patients who have experienced 5 or more days of insufficient caloric intake prior to surgery.

The role of postoperative oral immunonutrition after major abdominal surgery merits careful consideration. A comprehensive meta-analysis of randomized trials in patients undergoing major gastrointestinal surgery demonstrated a beneficial effect on postoperative complications and a reduction in the length of hospital stay. However, these positive outcomes were diminished when the analysis was limited to trials with a low risk of bias and those not funded by industry [131]. Moreover, the heterogeneity of patient populations, particularly regarding varying degrees of malnutrition, limits the generalizability of these results to patients with clearly defined nutritional deficits or those failing to meet their nutritional requirements. A more recent meta-analysis that included patients undergoing head and neck cancer surgery as well as gastrointestinal surgery reported that immunonutrition—administered primarily during the postoperative period for approximately 5–7 days—was associated with reductions in postoperative complications and hospital stay, without significant adverse events [34]. Nonetheless, there is no conclusive evidence that immunonutrition is superior to standard oral nutritional supplements, as meta-analyses of randomized trials have shown similar benefits with standard supplementation [76, 78]. In light of these findings, perioperative oral immunonutrition appears to be a reasonable recommendation, particularly for patients who are moderately to severely malnourished, are undergoing major cancer surgery, and are unable to meet their nutritional requirements through oral intake alone, provided that their diet is supervised by a qualified nutritional specialist.

Regarding the optimal route for nutritional delivery post-surgery, the principles outlined in Sect. “Types of preoperative nutritional interventions” (and illustrated in Fig. 3) remain applicable. When nutritional requirements cannot be met with normal food intake, oral nutritional supplements should be the first-line approach. If the oral route is contraindicated or insufficient, the enteral route is a reasonable alternative for either total or supplemental nutrition therapy, with a preference for the gastric route unless contraindicated, in which case the jejunal route should be considered. If these methods fail to meet nutritional needs, parenteral nutrition should be employed as either total or complementary nutritional support. Throughout the postoperative period, continuous clinical evaluation is imperative to manage symptoms, complications, and treatment-related adverse effects, thereby optimizing the use of the oral route for nutrition.

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There is limited scientific evidence regarding the optimal nutritional follow-up after major surgery, highlighting a significant area for further research. Given this gap, it is reasonable to recommend that patients who were malnourished during the perioperative period undergo regular nutritional assessments while hospitalized—potentially on a weekly basis. These evaluations should include assessments of protein and energy requirements as well as nutritional intake to ensure effective management of malnutrition. Once patients meet their nutritional needs and achieve a stable body weight, follow-up may be gradually reduced to once or twice per month for a period of 3–6 months. If further improvement is observed and patients no longer meet the diagnostic criteria for malnutrition, additional dietetic follow-up is unlikely to provide significant benefits. However, patients should be instructed to monitor their body weight and seek medical attention if they experience a significant decline.

Certain patient populations are at a particularly high risk for postoperative malnutrition and warrant continued monitoring in the late postoperative period. These include individuals undergoing esophagogastric, hepatopancreatobiliary, or head and neck surgery, as well as those receiving postoperative oncological treatments such as chemotherapy, radiation, or immunotherapy. Additionally, elderly frail patients, individuals with chronic diseases, and those who have undergone bariatric procedures should receive long-term nutritional follow-up. Although a detailed discussion of follow-up strategies for these groups is beyond the scope of this text, long-term monitoring by treating clinicians or primary care physicians, in collaboration with specialized healthcare providers, is strongly recommended. Conversely, well-nourished patients who successfully meet their nutritional requirements typically do not require structured nutritional follow-up. Nevertheless, they should be advised to adhere to a healthy balanced diet in accordance with World Health Organization (WHO) guidelines.