Carbohydrates, Lipids, and Proteins: Structure, Metabolism, and Health Implications

Carbohydrates

Origin and Dietary Sources

Carbohydrates are organic compounds primarily produced by plants through the process of photosynthesis. They are a major energy source in the human diet, found in grains, fruits, vegetables, and dairy products. Humans can also synthesize limited amounts of carbohydrates via gluconeogenesis, a metabolic pathway that generates glucose from non-carbohydrate substrates.

• Photosynthesis: Plants convert carbon dioxide and water into glucose and oxygen using sunlight.

• Dietary Sources: Grains (wheat, rice), fruits (apples, bananas), vegetables (potatoes, carrots), dairy (milk, yogurt).

• Human Production: Gluconeogenesis occurs mainly in the liver.

Structure and Classification

Carbohydrates have the general formula $(CH_2O)_n$. They are classified based on the number of sugar units:

• Monosaccharides: Single sugar units such as glucose, fructose, and galactose.

• Disaccharides: Two monosaccharides linked together (e.g., sucrose = glucose + fructose; lactose = glucose + galactose; maltose = glucose + glucose).

• Polysaccharides: Long chains of monosaccharides (e.g., starch, glycogen, cellulose).

Types of Carbohydrates

• Simple Carbohydrates: Sugars (monosaccharides and disaccharides) that are quickly absorbed and provide rapid energy.

• Complex Carbohydrates: Starches and polysaccharides that are digested more slowly, providing sustained energy.

• Dietary Fibre: Non-digestible carbohydrates, classified as soluble (e.g., pectin) and insoluble (e.g., cellulose).

Metabolism

Carbohydrate metabolism involves several key steps:

• Digestion: Begins in the mouth with salivary amylase, continues in the small intestine.

• Glycolysis: Breakdown of glucose to pyruvate, producing ATP.

• TCA Cycle (Krebs Cycle): Further oxidation of pyruvate for energy production.

• Glycogen Storage: Excess glucose is stored as glycogen in liver and muscle.

• Gluconeogenesis: Synthesis of glucose from non-carbohydrate sources.

Key Equation:

$\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} + \text{energy (ATP)}$

Disease Links

• Type 2 Diabetes: Impaired glucose regulation due to insulin resistance.

• Obesity: Excessive intake of refined carbohydrates can contribute to weight gain.

• Dental Caries: Sugars promote bacterial growth and acid production, leading to tooth decay.

• Non-Alcoholic Fatty Liver Disease (NAFLD): High intake of sugars, especially fructose, is a risk factor.

• Protective Role of Fibre: Dietary fibre reduces risk of cardiovascular disease, diabetes, and aids digestive health.

Lipids

Origin and Dietary Sources

Lipids are a diverse group of hydrophobic molecules obtained from both animal and plant sources. The body can also synthesize lipids via de novo lipogenesis, and certain fatty acids are essential and must be obtained from the diet.

• Dietary Fats/Oils: Found in meats, dairy, nuts, seeds, and vegetable oils.

• De Novo Lipogenesis: Synthesis of fatty acids from carbohydrates in the liver.

• Essential Fatty Acids: Omega-3 (e.g., alpha-linolenic acid) and omega-6 (e.g., linoleic acid) cannot be synthesized by humans.

Structure and Classification

• Fatty Acids: Hydrocarbon chains with a carboxyl group; can be saturated (no double bonds), unsaturated (one or more double bonds), or trans (hydrogen atoms on opposite sides of the double bond).

• Triglycerides: Three fatty acids esterified to a glycerol backbone; main storage form of fat.

• Phospholipids: Glycerol backbone, two fatty acids, and a phosphate group; major component of cell membranes.

• Sterols: Complex ring structures; cholesterol is the most well-known sterol.

Types of Lipids

• Saturated Fats: No double bonds; solid at room temperature; found in animal fats and some tropical oils.

• Unsaturated Fats: One or more double bonds; liquid at room temperature.

  • Monounsaturated Fatty Acids (MUFA): One double bond (e.g., olive oil).

  • Polyunsaturated Fatty Acids (PUFA): Multiple double bonds (e.g., fish oil, sunflower oil).

  • Omega-3 vs Omega-6: Classified by the position of the first double bond from the methyl end.

  • Medium-Chain Triglycerides (MCTs): Fatty acids with 6–12 carbons; rapidly absorbed and metabolized.

Metabolism

• Digestion: Lipases and bile salts emulsify and break down dietary fats in the small intestine.

• Beta-Oxidation: Fatty acids are broken down in mitochondria to generate ATP.

• Ketogenesis: Formation of ketone bodies from fatty acids during prolonged fasting or low carbohydrate intake.

• Lipoprotein Transport: Lipids are transported in the blood as chylomicrons, LDL (low-density lipoprotein), and HDL (high-density lipoprotein).

Key Equation:

$\text{Fatty Acid} + \text{CoA} + \text{ATP} \rightarrow \text{Acyl-CoA} + \text{AMP} + \text{PPi}$

Disease Links

• Cardiovascular Disease: High intake of saturated and trans fats increases risk; unsaturated fats are protective.

• Non-Alcoholic Fatty Liver Disease (NAFLD): Excess fat accumulation in the liver.

• Metabolic Syndrome: Cluster of conditions including high blood pressure, high blood sugar, excess body fat, and abnormal cholesterol levels.

• Gallstones: Formed from cholesterol and bile pigments; associated with high-fat diets.

• Benefits from Omega-3: Anti-inflammatory effects, reduced risk of heart disease.

Proteins

Origin and Dietary Sources

Proteins are essential macromolecules obtained from both animal and plant sources. Complete proteins contain all essential amino acids, while incomplete proteins lack one or more.

• Complete Proteins: Found in meat, eggs, dairy, and soy; provide all essential amino acids.

• Incomplete Proteins: Most plant sources; may lack one or more essential amino acids.

• Endogenous Synthesis: The body can synthesize non-essential amino acids.

Structure and Classification

• Amino Acids: 20 standard amino acids serve as building blocks of proteins.

• Peptide Bonds: Covalent bonds linking amino acids in a protein chain.

• Levels of Protein Structure:

  • Primary: Sequence of amino acids.

  • Secondary: Alpha-helices and beta-sheets formed by hydrogen bonding.

  • Tertiary: Three-dimensional folding driven by side chain interactions.

  • Quaternary: Association of multiple polypeptide chains.

Types of Proteins

• By Completeness: Complete vs incomplete proteins.

• By Function: Structural (collagen), enzymatic (amylase), transport (hemoglobin), hormonal (insulin), immune (antibodies).

• By Shape: Fibrous (collagen, keratin) vs globular (enzymes, antibodies).

Metabolism

• Digestion: Begins in the stomach with pepsin, continues in the small intestine with pancreatic proteases.

• Amino Acid Absorption: Amino acids are absorbed into the bloodstream and transported to cells.

• Transamination: Transfer of amino groups to form non-essential amino acids.

• Urea Cycle: Detoxifies ammonia produced from amino acid breakdown.

• Protein Synthesis: Ribosomes assemble amino acids into proteins according to genetic instructions.

Key Equation (Urea Cycle):

$2\text{NH}_3 + \text{CO}_2 + 3\text{ATP} \rightarrow \text{urea} + 2\text{ADP} + 4\text{Pi} + \text{AMP} + 2\text{H}_2\text{O}$

Disease Links

• Protein-Energy Malnutrition: Includes kwashiorkor (protein deficiency) and marasmus (overall energy deficiency).

• Sarcopenia: Age-related loss of muscle mass and strength.

• Chronic Kidney Disease (CKD): High protein intake can exacerbate kidney damage.

• Allergies: Immune reactions to specific proteins (e.g., gluten in coeliac disease).

• Phenylketonuria (PKU): Genetic disorder affecting metabolism of phenylalanine.