Metabolism, Energy Pathways, and Body Composition: Study Guide for Nutrition Students

Metabolism & Energy Pathways

Overview of Metabolic Pathways

Metabolism refers to the sum of all chemical reactions that occur within living cells to maintain life. These reactions are organized into metabolic pathways, which are sequences of chemical reactions catalyzed by enzymes.

• Glycolysis: The series of reactions converting glucose to pyruvate is called glycolysis. This is the first step in cellular respiration.

• Catabolism: The breakdown of molecules (such as glucose) to release energy is termed catabolism.

• Aerobic vs. Anaerobic Metabolism:

  • Aerobic reactions require oxygen and yield more ATP, producing carbon dioxide and water as byproducts.

  • Anaerobic reactions do not require oxygen and produce less ATP, with lactic acid as a byproduct.

• ATP Production: Aerobic metabolism produces approximately 36 ATP per glucose molecule, while anaerobic metabolism produces only 2 ATP per glucose molecule.

• Reversibility: Many metabolic pathways are reversible, allowing cells to convert molecules back to their original forms as needed.

Example: During intense exercise, muscles rely on anaerobic metabolism, leading to lactic acid buildup.

Energy Yield from Nutrients

When energy is needed, amino acids may enter the energy pathway at all stages except for fatty acid synthesis. Fatty acids can be used in metabolism only at the stage of acetyl-CoA formation.

• Glucose: The primary energy source for most cells.

• Fatty Acids: Used for energy after conversion to acetyl-CoA.

• Amino Acids: Can be used for energy, especially during fasting or starvation.

Example: During prolonged fasting, the body uses amino acids from muscle protein for energy.

Body Mass Index (BMI) & Body Composition

BMI Calculation and Classification

Body Mass Index (BMI) is a widely used measure to assess body weight relative to height. It is calculated using the following formula:

BMI Formula:

$\text{BMI} = \frac{\text{weight (lb)} \times 703}{\text{height (inches)}^2}$

BMI is classified as follows:

• Underweight: <18.5

• Healthy Weight: 18.5–24.9

• Overweight: 25.0–29.9

• Obese: >29.9

Example: A person weighing 150 lbs and 65 inches tall has a BMI of:

$\text{BMI} = \frac{150 \times 703}{65^2} = 25.0$

Waist-to-Hip Ratio

The waist-to-hip ratio is used to assess fat distribution and risk for chronic diseases. It is calculated as:

$\text{Waist-to-hip ratio} = \frac{\text{Waist measurement}}{\text{Hip measurement}}$

Example: Waist = 37", Hip = 40"; Ratio = 0.93

Basal Metabolic Rate (BMR)

BMR is the rate at which the body uses energy while at rest to maintain vital functions. It is influenced by age, sex, weight, and height.

Harris-Benedict Equation for BMR:

• Men: $\text{BMR} = 66 + (13.7 \times \text{weight in kg}) + (5 \times \text{height in cm}) - (6.8 \times \text{age in years}) \times \text{AF}$

• Women: $\text{BMR} = 655 + (9.6 \times \text{weight in kg}) + (1.8 \times \text{height in cm}) - (4.7 \times \text{age in years}) \times \text{AF}$

Conversion Factors:

Unit	Conversion
1 inch	2.54 cm
1 kg	2.2 lbs

Example: A 25-year-old male, 175 lbs, 70 inches tall:

Convert weight: $175 \div 2.2 = 79.5$ kg

Convert height: $70 \times 2.54 = 177.8$ cm

Plug into formula for BMR.

Body Composition Assessment

Body composition refers to the proportion of fat and non-fat mass in the body. Methods include:

• BMI: Simple, but does not distinguish between fat and muscle.

• Waist-to-hip ratio: Indicates fat distribution.

• Skinfold measurements: Estimate body fat percentage.

• Bioelectrical impedance: Measures body composition using electrical currents.

Example: Athletes may have a high BMI due to muscle mass, not excess fat.

Energy Balance & Weight Management

Energy In vs. Energy Out

Energy balance is achieved when energy intake equals energy expenditure. If energy intake exceeds expenditure, weight is gained; if expenditure exceeds intake, weight is lost.

• Energy In: Calories consumed from food and beverages.

• Energy Out: Calories burned through basal metabolism, physical activity, and thermic effect of food.

Basal Metabolic Rate (BMR): BMR is lowest during sleep and highest during periods of growth or physical activity.

Factors Affecting Metabolism

• Age: Metabolism slows with age.

• Sex: Males generally have higher BMR than females.

• Body Composition: More muscle increases BMR.

• Hormones: Thyroid hormones increase metabolic rate.

Appetite Regulation & Weight Control

Hormones Influencing Appetite

• Ghrelin: Produced by the stomach, stimulates appetite.

• Leptin: Produced by fat cells, suppresses appetite.

Example: After eating, leptin levels rise, reducing hunger.

SMART Goals for Weight Management

SMART is an acronym for setting effective goals:

• S: Specific

• M: Measurable

• A: Achievable

• R: Relevant

• T: Time-bound

Summary Table: BMI Classifications

BMI Category	BMI Range
Underweight	<18.5
Healthy Weight	18.5–24.9
Overweight	25.0–29.9
Obese	>29.9

Additional info:

• Some content inferred from context and standard nutrition curriculum, such as definitions and examples.

• Equations and tables formatted for clarity and academic completeness.