Body Weight, Energy Balance & Weight Management

Core equation: $\text{Energy In} = \text{Energy Out}$
Positive balance \text{In} > \text{Out} → fat storage & weight gain
Negative balance \text{In} < \text{Out} → fat mobilization & weight loss
Approx. $3500\;\text{cal} \approx 1\;\text{lb fat}$ (average, not absolute)
Small daily surpluses/deficits compound: $100\;\text{cal} \times 365 = 10.4\;\text{lb yr}^{-1}$

Basal Metabolic Rate (BMR): ≈ $\tfrac{2}{3}$ of total; affected by age, sex, body size, muscle mass, hormones
Physical Activity: most variable; includes exercise + non-exercise movement
Thermic Effect of Food (TEF): ~10 % of intake; highest for protein but overall small (~20 cal day)
Adaptive Thermogenesis: temporary ↑ expenditure under stress/illness; usually ignored in estimates
Estimated Energy Requirement (EER) ≈ $\text{BMR}+\text{Activity}+\text{TEF}$ (TEF often omitted)

Internal cues (physiological): stomach stretch, blood glucose, insulin, digestive hormones, $\text{CCK}$ , leptin, ghrelin
Hunger = need for fuel (unpleasant); Satiety/Satiation = “stop” signals; Appetite = desire/pleasure (can override signals)
External cues: sight/smell, social settings, emotions, conditioning → drive eating independent of hunger

Leptin (from fat): signals fullness, ↑ expenditure; obesity often shows leptin resistance
Ghrelin (stomach): triggers hunger; levels ↑ with weight loss & sleep deprivation
LPL (lipoprotein lipase): promotes fat storage; ↑ with dieting, estrogen, weight loss
HSL (hormone-sensitive lipase): releases fat; ↑ with exercise, ↓ with fasting/dieting

Body Mass Index (BMI): $\frac{\text{kg}}{\text{m}^2}$ ; 18.5–24.9 “healthy”, >30 obese (Class I 30–34.9, II 35–39.9, III ≥40)
Limitations: high muscle or sarcopenic elderly may misclassify
% Body Fat tools: skinfolds (least accurate), bio-impedance, DEXA, hydro-weighing (most accurate)
Fat distribution: Visceral/“apple” (waist > $40''$ men, $35''$ women) ↑ CVD, diabetes, metabolic syndrome vs. “pear”
Brown & brite adipose: uncouple oxidation → heat; exercise promotes “browning” of white fat

Multifactorial: genetics (predisposition), hormones, environment (cheap, high-calorie food + low activity), epigenetics
Positive energy balance epidemic: ↑ portion sizes, sugary foods, sedentary lifestyles

Red-flag traits: gimmick, rapid loss claims, severe restriction, ignore energy balance
Rapid, very-low-calorie or keto (< 10 g CHO) diets → water & muscle loss, ↓BMR (8–22 %), hormonal shifts (↑ghrelin, ↓leptin), rebound bingeing
Weight-loss industry profits from repeat failure; supplements lack proven efficacy

GLP-1 agonists (semaglutide: Ozempic/Wegovy, etc.): 12–18 % loss, $\downarrow$ appetite; high cost, GI effects, regain after discontinuation, long-term data limited
Bariatric surgery: significant durable loss for some; requires lifelong dietary vigilance, risk of malnutrition

Moderate deficit: ↓ $300–500\;\text{cal day}^{-1}$ + ↑ activity $200–400\;\text{cal day}^{-1}$ ⇒ 0.5–2 lb week
Eat nutrient-dense foods; include protein + fiber each meal; reasonable portions; avoid extreme restriction
Move more: target ≈ $2000\;\text{cal wk}^{-1}$ exercise; combine aerobic + strength; increase daily NEAT (non-exercise activity)
Address emotional/behavioral factors: mindful eating, coping skills, professional support if needed
Adopt supportive mindsets: food as nourishment, plan for obstacles, rely on strategies not willpower alone

Weight change always requires negative or positive energy balance
Quick-fix diets undermine long-term success by lowering BMR, altering hormones, and provoking rebound eating
Sustainable loss combines modest calorie control, increased movement, and psychological support
Health risk assessment should consider BMI, waist circumference, activity level, and metabolic markers, not weight alone