Maintenance of normal body mass, composition, structure, and function requires intake of water, energy substrates, and specific nutrients. Ions and compounds that cannot be synthesized from other nutrients are characterized as “essential.” Relatively few essential nutrients are required to form the thousands of compounds that make up the body. Known essential nutrients include 8 to 10 amino acids, 2 fatty acids, 13 vitamins, and approximately 16 minerals.
Energy is normally derived from dietary or endogenous carbohydrates, fats, and protein. Metabolic breakdown of these substrates yields the adenosine 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 (see Figure 33–4). Excess amino acids can 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 essential tissues is spared. As blood glucose concentration begins to fall during fasting, insulin secretion decreases and counterregulatory hormones (eg, 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 triglycerides enters the glycolytic 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 undergoing elective surgery could be fasted for up to a week postoperatively, provided fluid and electrolyte needs are met. Whether early postoperative nutritional support influences outcomes likely relates to the degree of preoperative malnutrition, number of nutrient deficiencies, and severity of the illness, injury, or surgical procedure. The optimal timing and amount of nutrition support following acute illness remain unknown. On the other hand, malnourished patients likely benefit from nutritional repletion prior to and after surgery.
Modern surgical practice has evolved to an expectation of an accelerated (“enhanced”) recovery. Enhanced recovery programs generally include early enteral feeding, even in patients undergoing surgery on the gastrointestinal tract, so prolonged periods of postoperative starvation are no longer common practice. Such protocols often specify a carbohydrate drink the night before surgery and again shortly before operation. Previously well-nourished patients should receive nutritional support after no more than 5 days of postsurgical starvation, and those with ongoing critical illness or severe malnutrition should be given nutritional support immediately. The healing of wounds requires energy, protein, lipids, electrolytes, trace elements, and vitamins. Depletion of any of these substrates may delay wound healing and predispose to complications, such as infection. Nutrient depletion may also delay optimal muscle function, which is important for supporting increased respiratory 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) to approximate the daily energy requirements. Alternatively, a simple and practical approach assumes that patients require 25 to 30 kcal/kg daily. The weight is usually taken as the ideal body weight or adjusted body weight. One determines the daily requirements to ensure that patients are not unnecessarily overfed, recognizing that nutritional requirements can increase greatly above basal levels with certain conditions (eg, burns). Thus, obese patients require an estimation of ideal body weight to prevent overfeeding.