Under normal conditions, the human body converts nutrients from food into energy or energy storage forms necessary for maintaining normal metabolism and physiological functions. In states of hunger, trauma, or stress-related illness, the body undergoes a series of metabolic changes to sustain the functions of tissues and organs and support survival during these conditions.
Substance Metabolism Under Normal Conditions
During normal physiological activities, the human body requires a continuous intake of various nutrients, which are transformed and utilized to maintain metabolism. Essential nutritional substrates include carbohydrates, fats, proteins, water, electrolytes, trace elements, and vitamins. After entering the body, these nutrients participate in a variety of metabolic processes. Through anabolic pathways, they contribute to growth, development, repair, and regeneration of bodily structures while providing the energy necessary for life processes.
Carbohydrates
Carbohydrates represent a primary component of human dietary intake, serving as a major energy source and a critical component of cellular structures. Under normal conditions, carbohydrates supply approximately 55–65% of the energy required for adults to maintain normal physiological functions. Certain tissues and cells, such as brain neurons, adrenal cells, and blood cells, rely exclusively on glucose oxidation for energy. Additionally, some glucose metabolites are essential substrates for other metabolic pathways and serve as important components of human tissue structures. Dietary carbohydrates primarily exist as starch, which must be digested and broken down into monosaccharides in the digestive tract before being absorbed by intestinal epithelial cells.
The metabolism of carbohydrates in the body is primarily reflected in glucose metabolism. In normal states, the entry and exit of glucose from the bloodstream remain relatively balanced, maintaining blood glucose levels at 4.5–5.5 mmol/L. Sources of blood glucose include the digestion and absorption of dietary sugars, glycogenolysis in the liver, and hepatic gluconeogenesis. Utilization pathways include uptake and usage by peripheral tissues and the liver, glycogen synthesis, conversion into non-carbohydrate substances, or incorporation into other carbohydrate-containing compounds. The regulation of blood glucose levels reflects coordinated metabolism of carbohydrates, fats, and amino acids, as well as cooperation among various organs and tissues such as the liver, muscles, and adipose tissue.
Proteins
Proteins are essential components of living organisms and play critical roles in life processes. Their primary physiological functions include forming the structural basis for various tissues and cells, supporting growth, renewal, and repair, participating in numerous physiological activities, and serving as a source of oxidative energy. Dietary protein is the primary source of proteins for the human body. Under the action of proteases and peptidases, dietary proteins are hydrolyzed into oligopeptides and amino acids, which are then absorbed.
Under normal physiological conditions, proteins in the body undergo continuous dynamic renewal through the processes of protein synthesis and degradation. The coordinated balance between these processes is essential for maintaining the structure and function of tissues and cells, regulating growth, and controlling the biological activity of various enzymes in the body.
Fats
The main physiological roles of fats include providing energy, forming bodily structures, supplying essential fatty acids, and transporting fat-soluble vitamins. Dietary fats are the major source of lipids for the human body. Since lipids are insoluble in water, their digestion requires the action of bile salts, pancreatic lipase, phospholipase A2, and cholesterol esterase, which metabolize fats into monoglycerides, fatty acids, cholesterol, and lysophospholipids. These substances are then emulsified into smaller micelles and further digested by enzymes.
Triglycerides composed of short- and medium-chain fatty acids are emulsified by bile salts and absorbed, after which lipases in intestinal mucosal cells hydrolyze them into fatty acids and glycerol, which enter the bloodstream through the portal vein. Triglycerides composed of long-chain fatty acids combine with phospholipids, cholesterol, and apolipoproteins to form chylomicrons, which enter the bloodstream via the lymphatic system. Triglycerides serve as the body's primary form of energy storage.
Energy Metabolism and Requirements
Energy metabolism refers to the processes of energy release, transfer, and utilization that occur during the metabolism of carbohydrates, proteins, and fats within the body. Understanding and accurately measuring energy expenditure under different clinical conditions is fundamental for providing rational and effective nutritional support and determining the appropriate amount and proportion of required nutrients.
Components, Measurement, and Calculation of Energy Expenditure
The body’s daily energy expenditure consists of several components, including basal energy expenditure (or resting energy expenditure), the thermic effect of food, adaptive thermogenesis, and the energy expenditure from physical activity. Among these, basal energy expenditure (BEE) accounts for the largest proportion (60–70%) of total daily energy expenditure. BEE represents the energy required to maintain normal physiological functions and internal homeostasis. Since the conditions required for measuring basal metabolic rate are highly stringent, resting energy expenditure (REE) is typically measured in clinical practice as an alternative.
The most commonly used method for determining energy expenditure in clinical settings is indirect calorimetry. This method estimates the rate of substrate oxidation and energy expenditure by measuring gas exchange in the body. The oxidation of proteins, fats, and carbohydrates results in the production of heat and the corresponding consumption of oxygen and generation of carbon dioxide. By measuring the amounts of oxygen consumed (VO2) and carbon dioxide produced (VCO2) over a given period, the energy expenditure or heat production during that time can be calculated.
The Weir equation is used to calculate 24-hour REE based on indirect calorimetry:
REE (kcal/day) = (3.9 × VO2 + 1.1 × VCO2) × 1440
Here, VO2 represents oxygen consumption (L/min) and VCO2 represents carbon dioxide production (L/min), both of which can be measured using noninvasive indirect calorimetry. Additionally, the respiratory quotient (RQ) can be calculated from VO2 and VCO2:
RQ = VCO2 / VO2
The RQ value reflects the oxidative metabolism of different nutrients in the body.
Due to equipment or environmental limitations, it is not always feasible to directly measure a patient's REE in clinical practice. Instead, simplified and practical predictive formulas are frequently used to estimate energy expenditure. The Harris-Benedict equation is a classic formula for estimating BEE:
Male: BEE (kcal/day) = 66 + 13.7 × W + 5.0 × H – 6.8 × A
Female: BEE (kcal/day) = 655 + 9.6 × W + 1.85 × H – 4.7 × A
In these formulas, W represents weight (kg), H represents height (cm), and A represents age (years).
The Harris-Benedict equation provides an estimate of BEE for healthy individuals. However, in clinical settings, actual REE values in patients with various disease states often differ from the estimated BEE. For example, REE in patients undergoing elective surgery increases by approximately 10%, in cases of severe trauma, multiple fractures, or infections it can increase by 20–30%, and in patients with extensive burns, energy expenditure may rise by as much as 100%.
Determination of Energy Requirements
The energy provided must directly correspond to nutritional efficacy and clinical outcomes. Inadequate energy intake can result in protein catabolism, impairing organ structure and function and worsening the patient’s prognosis. Conversely, excessive energy intake can also have adverse effects on clinical outcomes. While indirect calorimetry provides an ideal method for determining energy requirements, it is not yet feasible for real-time measurement in most patients. Predictive equations or clinical experience remain the predominant methods for estimating energy needs in practice.
For non-obese patients, energy intake of 25–30 kcal/(kg·day) is considered sufficient to meet the energy demands of most hospitalized patients. For obese patients with a body mass index (BMI) ≥ 30 kg/m2, the recommended energy intake is typically reduced to 70–80% of the normal target.
Metabolic Changes in States of Starvation and Trauma
Surgical patients are often in a state of starvation, infection, or trauma stress due to illness or surgical interventions. Under such conditions, the body undergoes a variety of metabolic changes to sustain vital tissue and organ functions and ensure survival during the disease state.
Metabolic Changes During Starvation
The lack of external energy substrates and nutrients forms the basis of the starvation response. During starvation, normal metabolic pathways may be partially or completely suppressed, while other pathways are activated or new metabolic pathways arise. The body's survival during starvation depends on utilizing stored fat, glycogen, and intracellular functional proteins.
In the early stages of starvation, glycogen reserves in the liver and muscles are initially utilized for energy until they are depleted, after which the body relies on gluconeogenesis. During this period, energy expenditure decreases, and the breakdown of hepatic and muscle proteins provides precursor substrates for gluconeogenesis, while protein synthesis is reduced.
As starvation continues, fat stores become the primary energy source. Lipolysis is increased, leading to fatty acid oxidation, heightened ketone body production, and enhanced gluconeogenesis. The brain and other tissues progressively rely on ketone bodies for energy, which reduces skeletal muscle protein breakdown, thereby preserving the body's protein stores and extending survival.
Metabolic Changes During Trauma and Stress
In traumatic or stress conditions, such as surgical infection or physical injury, the body experiences a series of metabolic changes characterized by increased REE, hyperglycemia, and enhanced protein catabolism.
During stress, alterations in carbohydrate metabolism manifest as significantly increased endogenous gluconeogenesis, decreased glucose oxidation and utilization in tissues and organs, and peripheral insulin resistance, all contributing to hyperglycemia.
Changes in protein metabolism following trauma include elevated protein breakdown and a resultant negative nitrogen balance. The extent and duration of these changes depend on the degree of stress, pre-trauma nutritional status, patient age, and post-trauma nutrient intake. Hormonal responses also exert significant influence.
Lipids become an essential energy source in stressed patients. During trauma, lipolysis is enhanced, with lipid breakdown products serving as substrates for gluconeogenesis, reducing protein breakdown and preserving body protein stores.