Nutrition and Metabolism: ANP College Study Guide

Nutrients and Their Roles

Types of Nutrients

Nutrients are substances required by the body for energy, growth, and maintenance. They are classified into macronutrients and micronutrients:

• Macronutrients: Carbohydrates, Lipids, Proteins

• Micronutrients: Vitamins, Minerals

Essential nutrients are those that the body cannot synthesize and must be obtained from the diet.

Food Energy

Food energy is measured in kilocalories (kcal). One "Calorie" (with a capital C) on food labels equals 1 kcal, which is the amount of heat required to raise 1 kg of water by 1°C.

Macronutrients

Carbohydrates

Carbohydrates are the primary energy source for cells, especially in the form of glucose, which is used to generate ATP. Excess glucose is stored as glycogen in the liver and muscles. Other monosaccharides like fructose and galactose are converted to glucose in the liver.

• Recommended intake: 45–65% of total calories

• Complex carbohydrates are preferred over simple carbs

• Cellulose is an insoluble fiber that humans cannot digest

Lipids

Lipids include triglycerides, cholesterol, and essential fatty acids. They serve as energy storage, insulation, and are components of cell membranes.

• Triglycerides: Unsaturated (healthy, plant-based), Saturated (animal-based), Trans fats (modified plant oils, unhealthy)

• Cholesterol: Most is synthesized by the liver

• Essential fatty acids: Linoleic (omega-6) and linolenic (omega-3)

Physiological uses include protection, insulation, fuel storage, membrane structure, and hormone synthesis.

Proteins

Proteins are vital for structure, enzymes, and hormones. They are composed of 20 amino acids, 8 of which are essential for adults and 2 additional for infants.

• Daily requirement: ~0.8g per kg body weight

• Too much: Obesity, bone density loss, kidney stones

• Too little: Anemia, tissue wasting, edema, growth deficits

Protein Function and Amino Acid Utilization

The body uses amino acids for protein synthesis or as fuel, depending on availability and hormonal controls. The all-or-none rule states that if any essential amino acid is missing, protein synthesis cannot occur and amino acids are used for energy.

Micronutrients

Vitamins

Vitamins are organic compounds, mostly functioning as coenzymes. Most are essential except for vitamin D, vitamin K, some B vitamins, and vitamin A.

• Water-soluble: B complex and C (excess excreted in urine)

• Fat-soluble: A, D, E, K (absorbed with lipids, stored except K)

• Antioxidants: A, C, E, and selenium neutralize free radicals

Water-Soluble Vitamins Table

Fat-Soluble Vitamins Table

Minerals

Minerals are inorganic elements required for various physiological functions. Seven are needed in moderate amounts, while others are required in trace amounts.

• Major minerals: Ca, P, K, S, Na, Cl, Mg

• Trace minerals: I, Fe, Zn, F, Cu, Se, etc.

Major Minerals Table

Trace Minerals Table

Metabolism

Overview of Metabolism

Metabolism is the sum of all biochemical reactions in the body. It includes:

• Anabolism: Building larger molecules from smaller ones (requires energy)

• Catabolism: Breaking down larger molecules into smaller ones (releases energy)

Stages of Nutrient Processing

1. Digestion, absorption, and transport to cells

2. Synthesis or breakdown in cytoplasm

3. Oxidative breakdown in mitochondria (generates ATP)

Cellular Respiration

Cellular respiration is a catabolic process that converts glucose to ATP through glycolysis, the citric acid cycle, and oxidative phosphorylation.

• Overall equation: $\mathrm{C_{6}H_{12}O_{6} + 6O_{2} \rightarrow 6H_{2}O + 6CO_{2} + 32\ ATP + heat}$

Oxidation-Reduction (Redox) Reactions

Cellular respiration involves redox reactions:

• Oxidation: Loss of electrons (usually as hydrogen atoms)

• Reduction: Gain of electrons (by carrier molecules like NAD+ and FAD)

ATP Synthesis

• Substrate-level phosphorylation: Direct transfer of phosphate to ADP during glycolysis and citric acid cycle

• Oxidative phosphorylation: Uses energy from electron carriers to build a proton gradient, which powers ATP synthase

Carbohydrate Metabolism

Glycolysis

Glycolysis occurs in the cytosol and does not require oxygen. It consists of three phases:

1. Sugar activation: Uses 2 ATP to prime glucose

2. Sugar cleavage: Splits into two 3-carbon fragments (G3P and DHP)

3. Sugar oxidation & ATP formation: Produces 2 NADH and 4 ATP (net gain: 2 ATP)

Fate of Pyruvic Acid

• Aerobic: Pyruvic acid enters mitochondria for further oxidation

• Anaerobic: Pyruvic acid is reduced to lactic acid (lactic acid fermentation)

Citric Acid Cycle

In mitochondria, pyruvic acid is converted to acetyl CoA, which enters the citric acid cycle. This cycle completes the oxidation of glucose, producing CO2, NADH, FADH2, and ATP.

Electron Transport Chain (ETC) and Chemiosmosis

The ETC is a series of protein complexes in the inner mitochondrial membrane. Electron carriers (NADH, FADH2) transfer electrons, creating a proton gradient used by ATP synthase to generate ATP.

Glucose Storage and Synthesis

Glycogenesis

Excess glucose is polymerized to form glycogen for storage in liver and muscles.

Glycogenolysis

When glucose is low, glycogen is hydrolyzed to release glucose.

Gluconeogenesis

In the absence of glucose or glycogen, the liver can synthesize glucose from amino acids and lipids.

Lipid Metabolism

Triglycerides and ATP Production

Triglycerides are broken down into glycerol and fatty acids. Glycerol enters glycolysis, while fatty acids undergo beta oxidation to form acetyl CoA, which enters the citric acid cycle.

Lipogenesis and Lipolysis

• Lipogenesis: Formation of triglycerides when ATP and glucose are high

• Lipolysis: Breakdown of triglycerides for energy

Protein Metabolism

Protein Catabolism

Excess dietary protein is deaminated in the liver, converting amino acids to pyruvic acid or citric acid cycle intermediates for energy production.

Protein Synthesis

Protein synthesis occurs on ribosomes and requires all essential amino acids. Anabolic hormones regulate this process.

Metabolic States

Absorptive State

Occurs after eating; anabolism exceeds catabolism. Excess nutrients are stored as fat, glycogen, or used for protein synthesis. Insulin is the key hormone.

Postabsorptive State

Occurs during fasting; catabolism exceeds anabolism. The main goal is to maintain blood glucose levels. Glucagon and sympathetic nervous system hormones stimulate glycogenolysis, gluconeogenesis, and lipolysis.

Metabolic Role of the Liver

The liver processes nutrients, regulates plasma cholesterol, stores vitamins and minerals, and metabolizes drugs and hormones.

Cholesterol and Lipoproteins

Cholesterol Functions

Cholesterol is not an energy source but is essential for bile salts, steroid hormones, and vitamin D. It is transported in plasma by lipoproteins.

Types of Lipoproteins

• VLDLs: Carry triglycerides from liver to adipose tissue

• LDLs: Carry cholesterol to tissues

• HDLs: Pick up excess cholesterol for excretion

• Chylomicrons: Transport dietary lipids from lymph

Recommended Levels

• Total cholesterol: < 200 mg/dl

• HDL: > 60 mg/dl

• LDL: < 160 mg/dl

Metabolic Rate

Metabolic rate is the total heat produced by chemical reactions and mechanical work. It is influenced by age, gender, body temperature, stress, and thyroid hormone.