Physiological Aspects of Obesity: Energy Expenditure, Adipose Tissue, and Genetic Factors

Energy Balance and Regulation of Body Weight

Complexity of Body Weight Regulation

The regulation of body weight is a multifactorial process involving physiological, behavioral, and environmental factors. Energy balance is determined by the relationship between energy intake and energy expenditure.

• Energy intake: Influenced by the nervous system, endocrine system, gut microbiota, emotional stress, and medications.

• Energy expenditure: Includes resting energy expenditure, thermic effect of nutrients, controlled ambient temperatures, sleep patterns, and physical activity.

• Levels of influence: Individual, social/family environment, organizations/institutions, communities, and public policies.

Components of Energy Expenditure

Overview of Energy Expenditure

Total energy expenditure is the sum of several components, each contributing differently to overall energy balance.

• Resting energy expenditure: Includes basal metabolic rate (BMR) and obligatory thermogenesis; energy needed to maintain vital functions.

• Thermic effect of food (TEF): Energy required to metabolize meals, also known as post-prandial thermogenesis.

• Energy spent in physical activity: Includes both exercise and non-exercise activity thermogenesis (NEAT and EAT); most modifiable component.

Typical contributions:

• BMR: ~70%

• Physical activity: ~20%

• TEF: ~10%

Resting Energy Expenditure: Contributing Factors

Resting energy expenditure is affected by several physiological and demographic factors.

• Age: Younger individuals have higher BMR.

• Sex: Males typically have higher BMR than females.

• Body size and composition: Greater lean muscle mass increases BMR.

• Hormones: Thyroid hormones play a significant role.

Basal Metabolic Rate (BMR)

BMR is the energy expended at rest to maintain vital bodily functions. Several factors can increase or decrease BMR.

Factors that Increase BMR	Factors that Decrease BMR
Higher lean body mass	Lower lean body mass
Greater height (more surface area)	Lower height
Younger age	Older age
Elevated levels of thyroid hormone	Depressed levels of thyroid hormone
Stress	Starvation or fasting
Male gender	Female gender
Pregnancy and lactation
Certain drugs (stimulants, caffeine, tobacco)

Formula for BMR (Harris-Benedict Equation):

$\text{BMR}_{\text{men}} = 88.362 + (13.397 \times \text{weight in kg}) + (4.799 \times \text{height in cm}) - (5.677 \times \text{age in years})$

$\text{BMR}_{\text{women}} = 447.593 + (9.247 \times \text{weight in kg}) + (3.098 \times \text{height in cm}) - (4.330 \times \text{age in years})$

Physical Activity

Physical activity is the second largest contributor to total energy expenditure and is highly variable among individuals.

• Non-exercise activity thermogenesis (NEAT): Includes walking, cleaning, stair climbing.

• Exercise activity thermogenesis (EAT): Includes jogging, yoga, sports.

• Modifiability: Physical activity is the most modifiable component of energy expenditure.

• Contribution: 15–30% depending on lifestyle behaviors.

Thermic Effect of Food (TEF)

TEF is the energy required for digestion, absorption, transport, metabolism, and storage of nutrients after a meal.

• Post-prandial thermogenesis: Refers to the increase in metabolic rate after eating.

• Influencing factors: Amount and type of food consumed.

• Macronutrient effect: High protein foods have the highest TEF, while high fat foods have the lowest.

Example: Consuming a high-protein meal increases TEF more than a high-fat meal.

Anatomy and Function of Adipose Tissue

Types of Tissues

Tissues are groups of cells similar in structure and function. Major types include:

• Epithelial tissue: Covering and lining surfaces.

• Muscle tissue: Movement.

• Nerve tissue: Control and communication.

• Connective tissue: Support and binding.

• Adipose tissue: Storage, protection, and endocrine functions.

Types of Adipose Tissue

Adipose tissue is specialized for fat storage and exists in several forms:

• White adipose tissue (WAT): Most abundant, primarily stores triglycerides, and secretes bioactive substances (adipokines).

• Brown adipose tissue (BAT): Least abundant, located in specific regions (e.g., interscapular, supraclavicular), contributes to adaptive thermogenesis and body temperature regulation.

• Beige adipose tissue: Intermediate characteristics between white and brown adipose tissue.

Adipocyte: The fat cell, containing nucleus, mitochondria, Golgi apparatus, and endoplasmic reticulum, with a characteristic fat reservoir storing triglycerides.

Distribution of Adipose Tissue

Adipose tissue is distributed in two main compartments:

• Subcutaneous adipose tissue: Located under the skin, stores >80% of total body fat.

• Visceral adipose tissue: Surrounds internal organs (e.g., intra-abdominal, perirenal, pericardial), associated with higher risk of metabolic abnormalities.

Visceral fat accumulation correlates most strongly with obesity-related health risks.

Adipose Tissue Remodeling and Health Risks

Adipose Tissue Remodeling

Remodeling refers to changes in adipose tissue mass and cellular composition, impacting health risks.

• Healthy adipose tissue: Normal number and size of adipocytes, few resident immune cells, adequate blood supply.

• Remodeling processes:

  • Hyperplasia: Increase in cell number; considered protective.

  • Hypertrophy: Increase in cell size; associated with greater health risks.

• Hypertrophy consequences: Promotes inflammation, insulin resistance, and vascular changes due to altered adipokine production.

Adipokine Production and Functional Changes

Adipocytes produce and release proteins/hormones (adipokines) that regulate physiological functions.

• Healthy fat: Produces anti-inflammatory adipokines (e.g., adiponectin, omentin-1).

• Dysfunctional fat (obesity): Increased production of pro-inflammatory adipokines (e.g., resistin, leptin), decreased anti-inflammatory adipokines.

• Effects: Influence food intake, metabolism, blood pressure, immunity, and vascular function.

Cardiometabolic Health Risks

Remodeling of adipose tissue, especially hypertrophy and visceral fat accumulation, increases risk for:

• Insulin resistance

• Type 2 diabetes

• Cardiovascular disease

• Stroke

• Local cell dysfunction and death (lipotoxicity)

Genetic Factors in Obesity

Role of Genetics

Genetics play a significant role in determining body weight and susceptibility to obesity.

• Heritability: Up to 70% of variation in body weight may be due to genetic factors.

• Twin studies: Monozygotic twins show higher concordance in body mass than dizygotic twins, even when raised apart.

Genes and Obesity

Genes are units of heredity composed of DNA, encoding instructions for protein synthesis. Humans have over 20,000 genes, with two copies of each gene inherited from each parent.

• Alleles: Variations in the same gene contribute to individual differences.

• Monogenic obesity: Rare form resulting from mutation in a single gene, often characterized by early-onset obesity and hyperphagia (constant hunger).

• Example: Congenital leptin deficiency leads to severe, early-onset obesity due to lack of satiety and increased food-seeking behavior.

Application: Identification of obesity-related genes provides insight into mechanisms of weight regulation and potential targets for treatment and prevention.

Summary Table: Components of Energy Expenditure

Component	Description	Typical Contribution (%)	Modifiability
Basal Metabolic Rate (BMR)	Energy needed to maintain vital functions at rest	~70%	Low
Physical Activity	Energy spent in exercise and non-exercise activities	~20%	High
Thermic Effect of Food (TEF)	Energy required to digest, absorb, and metabolize food	~10%	Low

Key Takeaways

• Total energy expenditure is determined by BMR, physical activity, and TEF.

• BMR is the largest contributor and is influenced by age, sex, body composition, and hormones.

• White adipose tissue is most abundant and distributed into subcutaneous and visceral compartments.

• Adipose tissue remodeling, especially hypertrophy, underlies the severity of obesity-related health risks.

• Genetic factors explain much of the variation in body weight; monogenic forms of obesity are rare but informative.