Nutrition
A Carbohydrate (CHO) metabolism
1. CHO usually makes up 40% to 45% of the total food intake but may reach 60%.
2. Amylases and disaccharidases hydrolyze complex starches and sugars to monosaccharides before they are absorbed in the small intestine.
a. One half of the ingested CHO is digested to glucose, and the rest are digested mainly to fructose and galactose.
b. Most of the glucose circulates as blood sugar and is taken up by the body cells and metabolized for energy.
c. Fructose, galactose, and some glucose are converted to glycogen, some in the liver and the rest in muscle.
3. Glucose is also manufactured from amino acids and other products of intermediary metabolism by a process called gluconeogenesis, which protects the glycogen reserves.
a. In the fasting state, liver glycogen is stimulated by catecholamine and glucagon to undergo glycogenolysis and form blood glucose.
b. Glucose then undergoes glycolysis to pyruvate.
(1) Pyruvate may undergo three basic metabolic processes:
(a) It can move into the mitochondria and be converted to acetyl coenzyme A (acetyl-CoA) for oxidation in the citric acid cycle.
(b) Diversion of pyruvate for oxidation to CO2 and H2O in the citric acid cycle and the respiratory chain is known as aerobic glycolysis.
(2) When O2 is not present for aerobic glycolysis, pyruvate is reduced to lactate.
(3) Anaerobic glycolysis is a rapid method for energy production.
(a) It does not need large quantities of O2.
(b) However, only limited energy production occurs via this mechanism.
(c) Pyruvate can also be carboxylated to oxaloacetate in the mitochondria by pyruvate carboxylase.
(d) Oxaloacetate can then form glucose in a process that is the reverse of glycolysis, known as gluconeogenesis.
1. Lipid is the main energy substrate in the body.
a. Lipid provides approximately 90% of the total body caloric reserve.
b. Most body fat exists as fatty acids that are esterified with glycerol to form tri-glycerides.
2. Dietary lipid is hydrolyzed in the intestinal tract, absorbed, and then resynthesized to triglycerides.
a. These are transported from the jejunal wall as lipid-protein complexes called chylomicrons.
b. Chylomicrons enter the blood via the intestinal lymphatics (lacteal) and thoracic duct.
c. Short- and medium-chain free fatty acids enter directly into the portal blood without conversion into chylomicrons.
3. Lipoproteins may be modified in the liver before going to adipose tissue or may go there directly.
a. In adipose tissue, lipoproteins are hydrolyzed, releasing fatty acids.
b. The fatty acids are reesterified and stored as triglycerides.
4. Triglycerides may also be formed from CHOs by lipogenesis.
a. When CHO intake is decreased, triglycerides are mobilized to glycerol and fatty acids.
b. The glycerol is partly converted to glucose by gluconeogenesis.
5. The free fatty acids are oxidized to produce acetyl-CoA, which is converted to energy via the Krebs cycle.
1. Protein normally comprises approximately 20% of the lean body mass or approximately 15% of the total body weight.
2. An intake of approximately 0.8 to 1 g/kg of body weight is needed each day in a normal individual.
3. If CHOs and lipids meet energy demands, protein is used entirely for:
4. Specific enzymes in the intestinal tract hydrolyze ingested protein to peptides and ultimately to amino acids.
5. The body cannot synthesize essential amino acids:
6. Amino acids not used anabolically for protein synthesis undergo catabolism (e.g., they may be transaminated or deaminated). After deamination the residue may be converted by way of either:
7. The nitrogen released is excreted in the urine as urea.
8. Branched chain amino acids (e.g., valine, leucine, and isoleucine) are used by muscle for metabolism.
d. Branched chain amino acids given intravenously can be used as a source of energy for muscles and can preserve lean body mass.
9. Alanine is the major gluconeogenic amino acid.
D Summary of energy metabolism
1. Glucose is the principal energy source of anaerobic metabolism:
2. The Krebs cycle is the main source of aerobic metabolism.
a. It requires some glucose to prime it.
b. Total energy needs cannot be supplied by fatty acid metabolism.
4. Triglycerides can be manufactured from excess CHO, lipid, and protein intake.
II Determination of Caloric and Protein Needs
1. Energy is commonly measured in units of calories or joules.
B Energy used by the body can be measured by direct or indirect calorimetry.
1. Direct calorimetry is based on the postulate that all energy expended in the body eventually becomes heat.
a. In a resting person, the amount of energy expended over a given time, or metabolic rate, can be determined by measuring the heat liberated.
2. Indirect calorimetry is based on the postulate that >95% of the energy expended by a person is derived from the reaction of oxygen with food (oxidation).
1. The BMR is defined as the rate of energy expenditure under a given set of basal conditions.
2. The BMR gives an estimate of energy expended when no extra demands are placed on a patient and would include such functions as:
D Resting energy expenditure (REE)
1. If energy expenditure is measured after eating, energy expenditure is increased.
2. The REE is defined as the BMR plus the component of SDA of food.
3. The Harris-Benedict equation will give an estimation of REE:
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(1)
(2)where A = age in years, H = height in centimeters, and W = weight in kilograms.
1. The final factor that must be included to determine the total energy expenditure (TEE) is exercise or physical work performance and degree of stress.
2. TEE = (REE × Activity factor) × Stress factor (Table 24-1)
TABLE 24-1
Factors Indicating Increases in Basal Metabolic Rate Under Certain Physiologic Conditions
| Condition | Factor |
| Confined to bed | 1.2 |
| Normal activity | 1.3 |
| Minor operation | 1.2 |
| Skeletal trauma | 1.35 |
| Major sepsis | 1.6 |
| Severe thermal burn | 2.1 |
3. The BMR has been found to increase under certain conditions (see Table 24-1).
1. The normal man engaging in an average amount of exercise requires 0.8 to 1.0 g of protein/kg body weight/day.
2. In the moderately stressed individual (e.g., infection and major surgery) protein requirement is 1.5 to 2.0 g/kg body weight/day.
3. In the severely stressed person (e.g., burns and major trauma) protein requirement is 2.0 to 4.0 g/kg body weight/day.
4. A calorie-to-nitrogen ratio of 150:1 is adequate for most catabolic patients. Higher ratios are used for those in severe stress.
5. Daily protein requirements are:
(3)III Vitamin Requirements and Deficiency States
A General functions of vitamins
1. Vitamins generally participate in metabolism in the interconversion and degradation of protein and amino acids.
2. They also participate in the extraction of energy from CHO and fat sources.
a. Function in biochemical systems
(1) Cellular hydrogen transport
(2) Component of coenzymes nicotinamide adenine dinucleotide (NAD) and nicotinamide adenine dinucleotide phosphate (NADP)
a. Function in biochemical systems
(1) Transamination in amino acid decarboxylation
(2) A primary coenzyme in metabolism of protein, CHO, fat, and nonoxidative degradation of amino acids
