Introduction to Human Nutrition and Energy Balance

Fundamentals of Human Nutrition

  • Definition: Human nutrition involves the study of how the body utilizes food to maintain health, often summarized by the phrase "You are what you eat."

  • Impact on Health: Deficiencies in single nutrients (e.g., zinc or B12B_{12}) can cause specific diseases.

  • Diet vs. Nutrients:

    • Diet: A pattern of eating (what, how much, and when).

    • Nutrients: Chemical substances for energy, structure, and regulation. These are classified as Essential (must be consumed via diet) or Non-essential (synthesized by the body).

Disease Development and Prevention

  • Chronic Diseases: Six out of the ten leading causes of death are chronic diseases significantly influenced by diet.

  • Prevention: Physical activity and diet are the leading lifestyle factors in preventing chronic illness.

Dietary Reference Intakes (DRI)

  • EAR (Estimated Average Requirement): Meets the needs of 50%50\% of healthy individuals in a specific group.

  • RDA (Recommended Dietary Allowance): Meets the needs of 97%−98%97\% - 98\% of individuals. Formula: RDA=EAR+2×SDEARRDA = EAR + 2 \times SD_{EAR}.

  • AI (Adequate Intake): Set when scientific evidence for an RDA is insufficient.

  • UL (Upper Limit): The highest average daily intake level that poses no risk of adverse health effects; exceeding the UL leads to toxicity.

  • Monitoring Intake: Assessment methods include food records, 24-hour dietary recall, and food frequency questionnaires.

Body Composition Assessment

  • Measurement Levels: Composition is studied at atomic, molecular, cellular, tissue, and whole-body levels.

  • Methods:

    • Hydrostatic Weighing: The gold standard for accuracy but impractical for routine use.

    • Skin Fold Caliper: Measures subcutaneous fat; accurate and repeatable but less effective for extreme obesity.

    • Bioelectric Impedance Analysis (BIA): Easy and inexpensive, but accuracy depends on hydration levels.

    • Body Mass Index (BMI): Calculated as BMI=Weight (kg)Height (m)2BMI = \frac{\text{Weight (kg)}}{\text{Height (m)}^2}. It does not distinguish between muscle and fat.

    • Waist-to-Hip Ratio: A superior predictor of cardiometabolic risk compared to BMI.

Energy Balance and Metabolism

  • Energy Intake: Derived from Carbs (4 kcal/g4\,kcal/g), Proteins (4 kcal/g4\,kcal/g), and Lipids (9 kcal/g9\,kcal/g).

  • Energy Expenditure Components:

    • Basal Metabolism: 60%−75%60\% - 75\%

    • Physical Activity: 15%−30%15\% - 30\%

    • Thermogenesis: 10%10\%

  • Basal Metabolic Rate (BMR/RMR):

    • Increases: Higher lean body mass, growth, pregnancy, fever, stress, and caffeine.

    • Decreases: Aging, female gender, and fasting/starvation.

  • Regulation: Hunger is stimulated by Ghrelin. Satiety signals include Insulin, Leptin, CCK, and GLP-1.

Weight Management and Obesity

  • Obesity Classification by BMI:

    • Underweight: <18.5

    • Normal: 18.5−24.918.5 - 24.9

    • Overweight: 25.0−29.925.0 - 29.9

    • Obesity class I: 30.0−34.930.0 - 34.9

    • Obesity class II: 35.0−39.935.0 - 39.9

    • Extreme Obesity III: ≥40\ge 40

  • Global Health: Kuwait ranks among the most obese countries (45.36%45.36\% of adults).

  • Medical Treatments: Includes pharmacotherapy such as Xynical (lipase inhibitor) and semaglutide (GLP-1 receptor agonist, e.g., Ozempic, Mounjaro).

  • Eating Disorders: Major conditions include Anorexia Nervosa (starvation), Bulimia Nervosa (binge and purge), and Binge Eating Disorder.