Biological Aspects of Obesity

Disease

• Condition of the body, its parts, organs, or systems or an alteration thereof

• Primarily endocrine system with adipose tissue

• Results from infection, parasites, nutritional, environmental, genetic, or other causes

• Eating practices, obesogenic environments

• Has a characteristic identifiable, marked group of signs or symptoms

• State of being overweight

• Deviates from normal structure or function

Obesity

• A complex disease characterized by a disproportionate body weight for height owing to an excessive accumulation in adipose tissue/adiposity

• Increased total body fat

• Increased central adiposity (android obesity)

• Meets essential criteria for a disease

• Increases the risk of other diseases and health problems

• Health risks are categorized as:

• Biomechanical

• Cardiometabolic

• Driver of metabolic syndrome

• High BP

• High Triglycerides

• Large waistline

• High cholesterol

• Elevated fasting blood sugar

• Three or more of these symptoms at once can result in dangerous/life-threatening obesity

 Assessment of Body Composition

• Diagnosis of obesity requires an assessment of body composition

• Measure of total body adiposity

• Direct measurements of body fat are technically challenging and expensive

• Densitometry (ex: underwater/hydrostatic weighing)

• Imaging-based methods

• Body Mass Index (BMI)

• Relative measure of total body weight to height

• Moderately correlates with direct measurement of body fat

Imperfect measure that doesn’t account for impact of lean body mass

Anatomic Differences in Fat Deposition

• Fat distribution influences health risks

• Android obesity- excess central adiposity

• Associated with greater risk of health complications

• Gynoid obesity- excess adiposity below the waist

• Lower health risks

Adipose Tissue

• Classified into:

• Subcutaneous adipose tissue

• Visceral adipose tissue

• Subcutaneous is larger storage department

• Visceral surrounds internal organs

Physiology

• The branch of biology concerned with the functioning of living organisms, and their constituent tissues and cells

Energy Homeostasis

• Obesity results from an imbalance between caloric intake and energy expenditure

• It is a deviation from energy homeostasis

• Excess energy not needed is stored (and converted to fat)

Food Consumption

• Complex array of physiological, sensory, cognitive, postingestive, and postabsorptive influences

• Hunger → satiety → hunger…

Physiology of Body Weight Regulation

• Involves an endocrine network involving peripheral organs and the CNS

• Hormones help to maintain a balance between energy intake and expenditure

• Homeostatic mechanism functions on a negative feedback system

• Hypothalamus is the main site of sensory integration

• Perception of hunger vs. satiety

• Main peripheral organs:

• Gut/gastrointestinal system- digest and absorb nutrients

• Pancreas- facilitates digestion and regulates metabolism and appetite

• Adipose- releases stored nutrients

Hypothalamus

• Permeable “blood-brain barrier” allows hormones to directly interact with connected systems

• Two sets of (functionally antagonistic) ARC neurons regulate food intake and energy balance

• Orexigenic neurons (appetite-stimulating)

• Anorexigenic neurons (appetite-suppressing)

Peripheral Signals

• Leptin

• Made by adipose tissue

• Produced in levels proportional to whole-body fat stores

• Inhibit food intake and increases energy expenditure

• Insulin

• Secreted by the pancreas upon nutrient ingestion

• Potentiates the satiety action of leptin

• Ghrelin

• Gut hormones have orexigenic and anorexigenic effects

• Ghrelin produced by the stomach stimulates food intake

Microorganisms

• Trillions of microorganisms reside in our guts/GI tract

• Consist of both helpful and harmful microbes (most are symbiotic)

• Microbiota can benefit health

• Digest indigestible foods because they yield short chain fatty acids (SCFA)

• Regulate hormones that can ascend the hypothalamic pathway

Energy Expenditure

• Sum of:

• Resting energy expenditure- resting metabolic rate

• Thermic effect of food- energy needed to metabolize meals

• Energy spent in physical activity- most modifiable

• Contributing factors include: age, sex, body size and composition, hormones

Physical Activity

• Second largest contribution to total energy expenditure (after BMR, around 20%)

• Exercise and non-exercise activity thermogenesis

• Most modifiable component

Thermic Effect of Food (TEF)

• Lowest contribution to energy expenditure

• Energy expended to utilize nutrients following a meal

• Depends on types and amounts of foods consumed