Metabolism is the series of chemical reactions within the body required to sustain life. It is typically divided into two broad categories. Catabolism is the breakdown of organic matter resulting in the release of energy. Anabolism uses energy to synthesize complex organic materials such as tissues and enzymes.
The oxidation of biological food substrates (carbohydrates, proteins, fats) produces carbon dioxide, water, and energy. The energy is used to form adenosine triphosphate (ATP), the energy currency of the body.
The unit of energy is calorie, defined as the amount of energy required to raise 1 g of water (1 mL) by 1°C. A kilocalorie or dietary calorie is equal to 1000 calories, and is used in nutritional context. The kilocalorie is commonly referred to as a calorie in lay terminology. According to the International System of Units, the unit of energy is joule. One calorie is equal to 4.2 joules.
Basal metabolic rate (BMR) is the amount of energy expended at rest. This energy is sufficient for vital organ function. Basic metabolic rate can be calculated using the Harris–Benedict equation:
Since 1919, various alternatives for calculating BMR have been formulated. Today, metabolic rate can be calculated using methods of direct or indirect calorimetry. Metabolic rate increases with increased lean body mass, muscle exertion, food digestion, thermogenesis, temperature extremes, growth, reproduction, lactation, increased thyroid activity, and increased stress. Both sepsis and burns can drastically increase BMR.
Respiratory quotient (RQ) is the ratio of volume of carbon dioxide eliminated to oxygen consumed in a steady state. A pure carbohydrate diet will result in an RQ = 1. Fats have an RQ = 0.7, and proteins have an RQ = 0.82. When eating a balanced diet, the RQ = approximately 0.8. In critically ill patients, such as in sepsis and burns, the body will shift to fat and protein breakdown, and an RQ of 0.6–0.7. These patients will require fat and protein supplementation in addition to carbohydrates in their diets or total parental nutrition.
Ingested carbohydrates are divided into two groups, simple and complex. Simple carbohydrates are composed of single (monosaccharide) or double (disaccharide) sugar units. Monosaccharides include glucose, fructose, and galactose. Disaccharides are lactose, maltose, and sucrose. Complex carbohydrates have structures consisting of three or more sugars. Digestive enzymes break down complex carbohydrates, while simple carbohydrates can absorb freely into the bloodstream.
Once in the bloodstream, glucose uptake into cells occurs via two methods, facilitated diffusion and secondary active transport. Once intracellular, glucose is phosphorylated into glucose-6-phosphate via two enzymes, hexokinase in muscle and fat or glucokinase in liver. The energy cost is one ATP and magnesium is also required. Glucose-6-phosphate cannot easily cross cell ...