Nutrition, Metabolism, and Energy Balance

Body Weight and Energy Balance

Body weight is regulated by the balance between energy intake and energy output, maintaining a homeostatic set point. Heredity accounts for 30-50% of individual variation, while eating and exercise habits contribute to the rest.

• Stable weight: Achieved when caloric intake equals caloric expenditure.

• Homeostatic set point: The body's natural tendency to maintain a certain weight.

• Heredity: Influences metabolic rate and fat storage.

Calories and Nutrients

Calories are units of energy. One calorie is the amount of heat required to raise the temperature of 1 gram of water by 1°C. Nutrients are classified as macronutrients (carbohydrates, fats, proteins) and micronutrients (minerals, vitamins).

• Fats: 9 kcal/g

• Carbohydrates and proteins: 4 kcal/g

• Macronutrients: Needed in large amounts for energy and structure.

• Micronutrients: Needed in small amounts for metabolic functions.

Essential Nutrients and RDA

Essential nutrients cannot be synthesized by the body and must be obtained from the diet. The Recommended Daily Allowance (RDA) is a safe estimate of daily intake for standard needs.

• Essential nutrients: Minerals, vitamins, 8 essential amino acids, 1-3 essential fatty acids.

• RDA: Guidelines for daily nutrient intake.

Carbohydrates

Structure and Function

Carbohydrates, also known as saccharides, are organic compounds with the formula (CH2O). They are the primary source of energy for the body, stored as blood glucose and glycogen.

• Blood glucose: C6H12O6, regulated by insulin and glucagon.

• Glycogen: Branched glucose polymer for energy storage.

Dietary Sources and RDA

Most dietary carbohydrates come from plants. The RDA for carbohydrates is higher than for any other nutrient, and intake should favor complex carbohydrates over simple sugars.

• Sources: Grains, legumes, fruits, root vegetables.

• Simple sugars: Sucrose (table sugar), fructose (fruits, corn syrup), maltose (cereal), lactose (milk).

• Complex carbohydrates: Provide additional nutrients and fiber.

Carbohydrate Metabolism

Dietary carbohydrates are metabolized rapidly, providing fuel for cellular processes. The main metabolic pathway is cellular respiration, which transfers energy from glucose to ATP.

• Equation:

• ATP production: Energy from glucose is transferred to ATP.

Glycogenesis, Glycogenolysis, and Gluconeogenesis

Excess glucose is stored as glycogen (glycogenesis), which can be broken down to glucose (glycogenolysis) or synthesized from noncarbohydrate sources (gluconeogenesis) when needed.

• Glycogenesis: Stimulated by insulin and epinephrine.

• Glycogenolysis: Stimulated by glucagon during fasting.

• Gluconeogenesis: Synthesis of glucose from fats and amino acids.

Dietary Fiber

Dietary fiber is fibrous material that resists digestion. It is important for digestive health, softening stool, increasing bulk, stimulating peristalsis, and lowering blood cholesterol.

• RDA: 30 g/day

• Sources: Oats, beans, peas, carrots, brown rice, fruits.

Lipids

Types and Functions

Lipids include fats, phospholipids, and steroids. They are hydrophobic, energy-dense, and serve diverse functions such as structural components, chemical precursors, and energy storage.

• Fats: Main energy reserve, compact storage.

• Phospholipids: Structural component of cell membranes.

• Cholesterol: Precursor for steroids, bile salts, and vitamin D.

• Fat-soluble vitamins: A, D, E, K absorbed with dietary fat.

Fat Requirements and Sources

Most fatty acids are synthesized by the body, but essential fatty acids must be consumed. Saturated fats are of animal origin, while unsaturated fats are found in plant sources.

• Saturated fats: Meat, egg yolks, dairy.

• Unsaturated fats: Nuts, seeds, vegetable oils.

• Cholesterol: Egg yolks, cream, shellfish, organ meats.

Lipid Metabolism: Lipolysis and Lipogenesis

Triglycerides are stored in adipocytes. Lipolysis breaks down fat for fuel, while lipogenesis synthesizes fat from other sources such as amino acids and sugars.

• Lipolysis: Fat breakdown for energy.

• Lipogenesis: Fat synthesis from non-lipid sources.

Ketosis

Ketosis occurs during fasting or carbohydrate restriction, when the liver produces ketone bodies from fatty acids. This process provides an alternative energy source for the brain and muscles.

• Ketone bodies: β-hydroxybutyric acid, acetoacetic acid, acetone.

• Clinical relevance: Common in diabetics and prolonged fasting.

Serum Lipoproteins

Lipids are transported in the blood as lipoproteins, which are categorized by density. Chylomicrons, VLDL, LDL, and HDL serve different roles in lipid transport and metabolism.

• Chylomicrons: Formed in the small intestine, transport dietary fats.

• VLDL: Produced by the liver, transport lipids to adipose tissue.

• LDL: Deliver cholesterol to cells for membrane repair and steroid synthesis.

• HDL: Remove cholesterol from arteries, deliver it to the liver for elimination.

Desirable Lipoprotein Levels and Cholesterol

High HDL and low LDL levels are desirable for cardiovascular health. Total cholesterol should be maintained below 200 mg/dL. Lifestyle modifications such as exercise and dietary restrictions help manage cholesterol levels.

• Recommendations: Exercise regularly, avoid smoking, saturated fats, coffee, and stress.

Proteins

Structure and Functions

Proteins make up 12-15% of body mass, primarily in skeletal muscles. They serve structural, functional, and regulatory roles in the body.

• Structural proteins: Collagen, keratin.

• Globular proteins: Antibodies, myoglobin, enzymes.

• Plasma proteins: Blood osmolarity and viscosity.

Protein Requirements and Dietary Sources

The RDA for protein is 44-60 g/day. Nutritional value depends on the proportions of amino acids. Complete proteins supply all essential amino acids and are found in animal sources, while plant sources must be combined for completeness.

• Complete proteins: Meat, eggs, dairy.

• Complementary plant proteins: Beans and rice.

Protein Metabolism

Amino acids are used to synthesize new proteins. When used as fuel, they must be deaminated, producing ammonia, which is converted to urea by the liver and excreted in urine.

• Deamination: Removal of NH2 group.

• Urea cycle: Detoxifies ammonia.

Minerals

Functions and Dietary Sources

Minerals are essential for bone and teeth formation, enzyme function, oxygen transport, and electrolyte balance. Dietary sources include vegetables, legumes, milk, eggs, fish, and animal tissues.

• Calcium and phosphorus: Bones and teeth.

• Iron: Hemoglobin and myoglobin.

• Chlorine: Component of stomach acid (HCl).

• Electrolytes: Govern nerve and muscle cell function.

Vitamins

Classification and Functions

Vitamins are organic compounds required for metabolic processes. Some are synthesized from precursors, while others must be obtained from the diet. They are classified as water-soluble (C, B) and fat-soluble (A, D, E, K).

• Water-soluble vitamins: Absorbed with water in the small intestine.

• Fat-soluble vitamins: Absorbed with dietary lipids.

Appetite Regulation

Hormonal Control

Appetite is regulated by hormones such as leptin, ghrelin, peptide YY (PYY), and insulin. Leptin decreases appetite, ghrelin stimulates hunger, PYY induces satiety, and insulin has a weaker effect similar to leptin.

• Leptin: Secreted by adipocytes, signals fat stores.

• Ghrelin: Produced by stomach, stimulates hunger.

• PYY: Secreted by ileum, induces satiety.

• Insulin: Pancreatic hormone, regulates glucose and appetite.

Obesity and Body Mass Index (BMI)

Definition and Factors

Obesity is defined as being 20% above the recommended norm for age, sex, and weight. BMI is used to assess weight status. Heredity plays a significant role, and childhood obesity can worsen the effect.

• BMI formula:

• BMI ranges: 20-25 optimal, >25 overweight, >30 obese.

Absorptive and Postabsorptive States

Absorptive State

The absorptive state lasts about 4 hours during and after a meal, when nutrients are absorbed and used for energy. Insulin regulates this state by increasing cellular uptake of glucose and stimulating glycolysis, glycogenesis, lipogenesis, and protein synthesis.

• Carbohydrates: Glucose used for ATP synthesis; excess stored as glycogen or fat.

• Fats: Taken up by fat cells.

• Amino acids: Used for protein synthesis.

Postabsorptive State

The postabsorptive state occurs between meals, when stored fuels are used. Glucagon regulates this state by stimulating glycogenolysis and gluconeogenesis to maintain blood glucose levels.

• Carbohydrates: Glucose from glycogen reserves or synthesized from other compounds.

• Fat: Glucose from adipocytes.

• Protein: Used as fuel when glycogen and fat reserves are depleted.

Metabolic Rate

Definition and Factors

Metabolic rate is the amount of energy used by the body in a given period (kcal/hr or kcal/day). Basal metabolic rate (BMR) is measured under relaxed, awake, fasting conditions at comfortable temperature.

• BMR: Adult male ~2000 kcal/day; slightly less for females.

• Factors affecting MR: Size, gender, age, pregnancy, anxiety, fever, eating, thyroid hormones, depression, exercise, muscle/fat mass, activity level, body temperature.