Nutrition Overview

Basic Nutritional Needs

• The human body requires a few essential nutrients to maintain body composition, mass, and function. These nutrients are derived from the consumption of carbohydrates, fats, and proteins from dietary or endogenous sources.
• Several of these nutrients are compounds the body is unable to synthesize from other ingredients, and thus their supply is considered “essential” for the maintenance of body composition and function. They consist of eight to ten amino acids, two fatty acids, thirteen vitamins, and approximately sixteen minerals.
• The body breaks down carbohydrates, fats, and proteins into adenosine triphosphate, which is the basic energy unit of cells. The majority of energy metabolism is from dietary carbohydrates and fats. Breakdown of dietary proteins provides amino acids for the body’s protein synthesis. However, excess protein consumption can also serve as an energy substrate. Ultimately, metabolic pathways for carbohydrates, fats, and proteins overlap.
• Excess carbohydrates can be stored as glycogen in the liver and skeletal muscle, while excess protein may be converted into fatty acid precursors or glycogen. Excess fats may be stored as triglycerides in adipose tissue.
• During periods of fasting, insulin secretion decreases in response to a fall in blood glucose. Glucagon secretion increases, triggering the breakdown of liver glycogen stores. Glycogen stores are typically depleted within twenty-four hours. Should the fasting period continue, amino acids are broken down by the body for gluconeogenesis—an important pathway for the function of erythrocytes, neural tissue, and renal medullary cells, as glucose is the primary source of energy.
• Fatty acids become the principal energy source after glycogen stores are depleted. Triglycerides enter the glycolytic pathway, producing acetylcoenzyme A and ketone bodies. If fasting is prolonged, tissues in the kidneys, muscle, and brain can utilize ketone bodies as an energy source.
• A well-nourished patient undergoing surgery can tolerate fasting periods of up to a week if fluid and electrolyte needs are met. However, acute illness, surgical stress, and injury place additional stressors on the patient, which may result in differing nutritional needs. The optimal nutritional support in these settings is still not well understood.¹

Nitrogen Balance

• The body is dependent on nitrogen protein synthesis and production of nitrogen-containing compounds involved in a range of necessary body functions (hormones, neurotransmitters, and immunologic function).
• Nitrogen homeostasis in the human body is a complex process involving the balance of protein turnover, amino acid oxidation, and urea production, leading to nitrogen excretion in response to periods of fasting and prandial periods.²
• During periods of critical illness, such as sepsis, polytrauma, and burns, patients may enter a hypermetabolic state and resort to the catabolism of proteins. The excess protein breakdown in these patients may result in increased morbidity and mortality.
• Studies have shown that a high-protein diet in nutritional supplementation in the intensive care setting may improve prognosis. Recent guidelines for nutritional therapy recommend the maintenance of nitrogen balance for high-risk patients.³

Enteral vs. Parenteral Nutrition

• Nutritional support, especially in the perioperative setting, has gained increasing attention in the past decade as there is increasing recognition of the association between malnutrition and postoperative complications. Surgical stress produces a catabolic response in the body caused by the release of inflammatory mediators and stress hormones. Patients with poor nutritional reserves struggle to meet these metabolic demands and are more likely to experience poor surgical outcomes.^4,5
• The prevalence of malnutrition in the surgical population is likely underestimated. Malnutrition is commonly seen in surgical patients with comorbid, chronic illnesses such as malignancy, inflammatory bowel disease, and chronic organ failure. Malnutrition can cause a host of poor surgical outcomes, including impaired wound healing, hospital-acquired infection, postoperative complications, prolonged hospital stay, and increased mortality.^6,7
• A nutritional assessment should be performed in the preoperative period, and nutrition should be optimized throughout the perioperative period. The goal preoperatively is to identify and treat any preexisting malnutrition. According to the American Society for Parenteral and Enteral Nutrition (ASPEN), several clinical markers can be used to assess nutritional status. A body mass index (BMI) less than 18.5 kg/m2 is considered underweight and may be a sign of poor nutrition, but even patients with higher BMI who report involuntary weight loss of more than 10% may be undernourished.⁴
• Imaging studies, such as DEXA scans, magnetic resonance imaging, and computed tomography scans, can be used to analyze body composition and determine the proportion of fat and lean body mass. Subjective signs of malnutrition include decreased appetite and reduced food intake. There are several serum biomarkers of nutrition, including low serum albumin, low prealbumin, high c-reactive protein (CRP), elevated white blood count, and high blood glucose. Serum albumin has fallen slightly out of favor because of its long half-life. Further, serum albumin can be elevated in states of chronic inflammation, making it less sensitive as a marker for malnutrition.^6,8
• Chronic inflammation is linked to malnutrition. Chronic inflammatory conditions can cause increased consumption of vital nutrients in the preoperative period. Clinical signs of chronic inflammation include fever, hypothermia, tachycardia, etc. The presence of any of these in the preoperative period may indicate a state of undernutrition.
• Any patient identified as malnourished should receive oral supplementation. Oral supplementation can take many forms, and a nutrition or dietetics consultation may be beneficial in cases of severe malnutrition. The Enhanced Recovery After Surgery Society guidelines recommend rapid consumption of a 100g carbohydrate-rich drink the night before and 2-3 hours prior to scheduled surgery. However, caution should be taken in those patients with documented or suspected delayed gastric emptying.⁸
• For patients who are unable to optimize their nutrition enterally, parenteral nutrition (PN) prior to surgery is recommended. This has been shown to optimize outcomes in patients who are severely malnourished. However, caution must be taken to avoid hyperglycemia in this patient population. The majority of published studies initiated parenteral nutrition 7-14 days prior to surgery, but there are no clear guidelines on the optimal duration of preoperative PN.
• In the postoperative period, current guidelines prioritize early enteral nutrition (EN) via oral route, nasogastric tube, nasoenteric tube, or feeding jejunostomy on or before postoperative day 1 when feasible. Studies suggest that early enteral feeding decreases the risk of postoperative complications, including a decreased intensive care unit length of stay, decreased incidence of mechanical ventilation, and reduced mortality. However, nutrition strategies must be tailored to the patient’s physical status and individual comorbidities.
• Postoperative PN is utilized for patients whose enteral intake is insufficient to meet their nutritional goals (e.g., gastrointestinal malabsorption syndromes, chronic gastroparesis, anastomotic leak, feeding tube complications, etc.). However, patients who can receive enteral feeds can receive supplemental EN via a temporary feeding tube if their oral intake is insufficient.⁶
• Absolute contraindications for PN in the postoperative period are patients who can receive enteral feeding, anticipated duration of fasting fewer than 5 days, terminal illness with short life expectancy, patient refusal, or advanced directive contraindication.^6,9,10
• The average nutritional requirement per ASPEN is 25kcal per kilogram of ideal body weight per day. This can be adjusted upwards for patients who have undergone major surgery. This rate is adjusted based on clinical findings and lab values (e.g., CRP, 24-hour urine nitrogen excretion). PN is ordered every 24 hours to allow for adjustment based on clinic and laboratory values.
• The PN is typically divided as follows: 20-25% protein, 10-30% lipid emulsion, and 50-60% carbohydrates. Additional electrolytes, including sodium, chloride, potassium, magnesium, and calcium, may be included, and phosphorus acetate can be added instead of sodium chloride in patients with elevated chloride levels.
• It is important to monitor caloric needs, as underfeeding and overfeeding can happen with PN. Additional complications include hyperglycemia, hypertriglyceridemia, and dyslipidemia. PN-dependent patients require close monitoring and tailoring of their nutritional therapy. The timing of PN is typically delayed for patients whose period of fasting is expected to last fewer than 5 days, and intravenous fluids containing dextrose are often sufficient in the short term to prevent catabolism. Finally, EN should be reestablished as soon as possible if it is a feasible option.^9,10

Summary

• Preoperative parenteral nutrition may be considered for a short duration in patients who are unable to meet their nutritional requirements via the enteral route.
• Patients who are fasting produce energy first from liver glycogen stores, next from excess fat in adipose tissue, or, after prolonged starvation, from amino acid breakdown.
• It is important to resume enteral feeding as early as possible in the postoperative period when no contraindication exists.
• Parenteral nutrition should be limited to patients who are unable to receive EN within 5 days of surgery or for patients who have a contraindication to enteral feeds.