Human Anatomy & Physiology Chapter 27 - Metabolism & Nutrition

Metabolism & Nutrition Overview

  • Focus on Lipid Metabolism, Protein Metabolism, Carbohydrate Metabolism, Cellular Respiration, Metabolic Rate, and Starvation.

Metabolic Rate

  • Definition: Amount of energy used by the body to support metabolic processes.

  • Measurement:

    • Recorded in Calories (actually kilocalories).

    • Defined as the energy needed to raise 1g of water by 1°C. This is an extremely small amount of energy; hence, Calories are often used (i.e., kilocalories).

Measuring Metabolic Rate

  • Methods of Measurement:

    • Can be measured through heat production (calorimetry).

    • More commonly measured via oxygen consumption.

Energy Expenditures Components

  • Energy expenditures consist of

    1. Basal Metabolic Rate (BMR):

    • Represents around 60% of total calories burned.

    • Comprises metabolic activities needed for maintaining homeostasis (e.g., Na+/K+ pumps, cellular activities).

    • Includes resting heart rate, resting breathing rate, and resting nervous system activity.

    1. Active Metabolic Rate (AMR):

    • Involves physical activity of any kind.

    • Proportion of total energy used varies greatly between individuals.

    1. Specific Dynamic Action (SDA):

    • Energy used to digest food.

    • Accounts for 5% to 10% of total calories burned.

    1. Growth:

    • If BMR + AMR + SDA does not equal calorie intake, difference results in growth (positive or negative).

BMR Conditions

  • A person is operating at BMR under the following conditions:

    • Well-rested.

    • Fasted for about 12 hours.

    • No recent physical activity (within the last hour).

    • No physical stress is induced.

    • At a constant and comfortable temperature.

Energy Balance

  • Definition: The balance between energy intake and energy expenditure.

  • Optimal energy homeostasis requires energy intake to match energy expenditure; otherwise, weight is gained or lost.

  • Components of Energy Intake: Consists of food consumption.

  • Components of Energy Expenditure: Encompasses metabolic rate, physical activity, and digestion.

Body Mass Index (BMI)

  • Formula:
    ext{BMI} = \frac{\text{Weight (kg)}}{\text{Height (m)}^2}

  • Categories:

    • Underweight: BMI < 18.5

    • Normal weight: BMI 18.5 - 24.9

    • Overweight: BMI 25 - 29.9

    • Class 1 Obesity: BMI 30 - 34.9 (some risk)

    • Class 2 Obesity: BMI 35 - 39.9 (moderate risk)

    • Class 3 Obesity: BMI ≥ 40 (high risk)

Problems with BMI

  • Not an accurate measure for very tall or short individuals.

  • Does not differentiate between muscle mass and fat mass.

  • Some debate about the definitions of the BMI categories.

Obesity

  • Negative implications include:

    • Decreased life expectancy.

    • Increased rates of atherosclerosis, hypertension, diabetes, joint problems, kidney stone formation, and gallstones.

    • Elevated cancer risk in:

    • Women: breast, uterine, and liver cancers.

    • Men: colon, rectal, and prostate cancers.

Appetite Control

  • Neural Controls:

    • Control centers located in the hypothalamus help regulate hunger and satiety, influenced by blood nutrient levels.

  • Hormonal Control:

    • Insulin and Cholecystokinin (CCK) facilitate satiety.

    • Glucagon, epinephrine, and ghrelin stimulate appetite.

    • Leptin:

    • Hormone produced by adipose tissue that suppresses appetite.

    • In obese individuals, leptin levels are high, but they often become resistant to its effects, resulting in unregulated appetite levels.

  • Psychological Factors:

    • "Comfort eating" may occur, providing stress relief by activating the parasympathetic nervous system, but this behavior is not driven by regular appetite controls.

Starvation

  • Definition: A severe deficiency in caloric intake due to fasting, anorexia, deprivation, or illness.

  • Phases of Starvation:

    1. First Phase:

    • Blood glucose levels are maintained via glycogenolysis for several hours.

    • After glycogen stores are depleted, fats and proteins are catabolized to maintain glucose levels.

    • Fatty acid catabolism occurs for most body cells, while glucose from amino acids is utilized for the central nervous system (CNS).

    1. Second Phase:

    • May last for several days or weeks.

    • The liver produces ketone bodies from fatty acids; the CNS begins to increasingly rely on these ketone bodies.

    • Risk of ketoacidosis may develop due to high ketone levels in the blood.

    1. Third Phase:

    • Initiated once fat reserves are depleted.

    • Proteins become the primary energy source; this leads to depletion of muscle proteins.

    • Essential proteins for cellular function become degraded, resulting in severe changes such as extreme weight loss, apathy, listlessness, and heightened susceptibility to infections.

    • Consequences may include changes in hair color and abdominal edema.