BASIC NUTRITIONAL NEEDS
Maintenance of normal body mass, composition, structure, and function requires the periodic intake of water, energy substrates, and specific nutrients. Nutri-ents that cannot be synthesized from other nutrients are characterized as “essential.” Remarkably, relatively few essential nutrients are required to form the thou-sands of compounds that make up the body. Known essential nutrients include 8–10 amino acids, 2 fatty acids, 13 vitamins, and approximately 16 minerals.
Energy is normally derived from dietary or endogenous carbohydrates, fats, and protein. Meta-bolic breakdown of these substrates yields the ade-nosine triphosphate required for normal cellular function. Dietary fats and carbohydrates normally supply most of the body’s energy requirements. Dietary proteins provide amino acids for protein synthesis; however, when their supply exceeds requirements, amino acids also function as energy substrates. The metabolic pathways of carbohydrate, fat, and amino acid substrates overlap, such that some interconversions can occur through metabolic intermediates (see Figure 32–4). Excess amino acids can therefore be converted to carbohydrate or fatty acid precursors. Excess carbohydrates are stored as glycogen in the liver and skeletal muscle. When glycogen stores are saturated (200–400 g in adults), excess carbohydrate is converted to fatty acids and stored as triglycerides, primarily in fat cells.
During starvation, the protein content of essen-tial tissues is spared. As blood glucose concentration begins to fall during fasting, insulin secretion decreases, and counterregulatory hormones, such as glucagon, increase. Hepatic and, to a lesser extent, renal glycogenolysis and gluconeogenesis are enhanced. As glycogen supplies are depleted (within 24 h), gluconeogenesis (from amino acids) becomes increasingly important. Only neural tissue, renal medullary cells, and erythrocytes continue to utilize glucose—in effect, sparing tissue proteins. Lipolysis is enhanced, and fats become the principal energy source. Glycerol from the triglycerides enters the gly-colytic pathway, and fatty acids are broken down to acetylcoenzyme A (acetyl-CoA). Excess acetyl-CoA results in the formation of ketone bodies (ketosis). Some fatty acids can contribute to gluconeogenesis. If starvation is prolonged, the brain, kidneys, and muscle also begin to utilize ketone bodies efficiently.The previously well-nourished patient under-going elective surgery could be fasted for upto a week postoperatively without apparent adverse effect on outcomes, provided fluid and electro-lyte needs are met. The usefulness of nutritional repletion in the immediate postoperative period is not well defined, but likely relates to the degree of malnutrition, number of nutrient deficiencies, and severity of the illness/injury. Moreover, the optimal timing and amount of nutrition support following acute illness remain unknown. On the other hand, malnourished patients may benefit from nutritional repletion prior to surgery.
Modern surgical practice has evolved to an expec-tation of an accelerated recovery. Accelerated recov-ery programs generally include early enteral feeding, even in patients undergoing surgery on the gastroin-testinal tract, so prolonged periods of postoperative starvation are no longer common practice. All well-nourished patients should receive nutritional support after 5 days of postsurgical starvation, and those with ongoing critical illness or severe malnutrition should be given nutritional support immediately. The mal-nourished patient presents a different set of issues, and such patients may benefit from both preoperative and early postoperative feeding. Clearly, the healing of wounds requires energy, protein, lipids, electro-lytes, trace elements, and vitamins. Depletion of any of these substrates may delay wound healing and pre-dispose to complications, such as infection. Nutrient depletion may also delay optimal muscle functioning, which is important for supporting increased respira-tory demands and early mobilization of the patient.
The resting metabolic rate can be measured (but often inaccurately) using indirect calorimetry (known as a metabolic cart) or by estimating energy expenditure using standard nomograms (such as the Harris–Benedict equation), yielding an approxima-tion of daily energy requirements. Alternatively, a simple and practical approach assumes that patients require 25–30 kcal/kg daily. The weight is usually taken as the ideal body weight or adjusted body weight. Even though nutritional requirements can increase greatly above basal levels with certain condi-tions (eg, burns), the more often relevant reason fordetermining the daily requirements is to ensure that patients are not unnecessarily overfed. In this regard, obese patients require adjusting the body weight based on the degree of obesity to prevent overfeeding.