Nutritional biochemistry forms the foundation for understanding how the body utilizes food components to support life processes. For Nutrition and Dietetics professionals in Kenya, mastering the biochemical nature of macro and micronutrients is essential to assess dietary quality and to design appropriate nutrition interventions. This chapter explores the molecular types fundamental to nutrition, linking their biochemical roles to practical health outcomes. Understanding these molecules enables practitioners to advise on nutrient metabolism, energy provision, and the synthesis of vital substances.

1.1 Types of Macro Molecules

In the Kenyan nutrition context, macro molecules are the primary nutrients required in large amounts to sustain bodily functions, support growth, and provide energy. These include carbohydrates, proteins, lipids, and nucleic acids, each playing unique roles in metabolism and health. Biochemical knowledge of these molecules helps dietitians tailor nutritional plans that meet the needs of diverse populations such as children in county hospitals or adults managing chronic diseases in urban clinics.

1.1.1 Role of Biochemistry in Energy Provision and Substance Synthesis Molecules

Biochemistry elucidates how macro molecules are metabolized to produce energy and how they contribute to the synthesis of essential body substances. This understanding is critical for dietitians to recommend diets that optimize metabolic health and prevent nutrient deficiencies common in Kenyan communities.

Energy Provision through Metabolism

• Cellular Respiration: Macro molecules undergo enzymatic breakdown in cells to release energy stored in chemical bonds, primarily in the form of ATP. For example, glucose from carbohydrates is metabolized via glycolysis and the Krebs cycle in liver cells, fueling bodily functions.
• Energy Yield Variance: Different macro molecules yield varied energy amounts; carbohydrates and proteins provide approximately 4 kcal/g, whereas lipids yield about 9 kcal/g due to their dense hydrocarbon chains.
• Metabolic Flexibility: The body adapts to utilize available macro molecules for energy, such as increased fat oxidation during prolonged fasting, a consideration in managing malnutrition in Kenyan rural areas.
• Anabolism and Catabolism: Biochemistry explains the balance between building complex molecules (anabolism) like muscle proteins and breaking down molecules (catabolism) for energy, guiding nutritional rehabilitation strategies.
• Enzyme Function: Specific enzymes catalyze biochemical reactions to convert macro molecules into usable forms, with deficiencies impacting nutrient utilization, as seen in some inherited metabolic disorders diagnosed at Kenyan referral hospitals.

Synthesis of Body Substances

• Structural Components: Proteins and lipids serve as building blocks for cell membranes, enzymes, and hormones, essential for tissue repair and immune function.
• Nucleic Acid Formation: Nucleotides derived from dietary nucleic acids are vital for DNA and RNA synthesis, supporting cell replication and genetic expression.
• Coenzyme Synthesis: Vitamins and minerals interact with macro molecules to form coenzymes necessary for metabolic pathways, such as NAD+ in carbohydrate metabolism.
• Biochemical Signaling: Lipids act as precursors for signaling molecules like prostaglandins, influencing inflammation and vascular function, relevant in managing non-communicable diseases in Kenya.
• Detoxification and Excretion: Biochemical pathways transform harmful substances into excretable forms, a process dependent on adequate protein intake for enzyme production.

1.1.2 Carbohydrates

Carbohydrates are the most abundant source of dietary energy, particularly important in Kenya where staple foods like maize, sorghum, and cassava dominate diets. Their biochemical properties influence digestion, absorption, and energy metabolism.

Classification and Structure

• Monosaccharides: Simple sugars such as glucose and fructose are the basic units absorbed directly into the bloodstream, providing immediate energy.
• Disaccharides: Composed of two monosaccharides, including sucrose and lactose, disaccharides require enzymatic hydrolysis before absorption.
• Polysaccharides: Complex carbohydrates like starch and dietary fiber consist of long chains of glucose units; starch is digestible while fiber aids gastrointestinal health.
• Glycemic Index: Carbohydrates vary in their impact on blood glucose levels, with high-GI foods leading to rapid glucose spikes, important in managing diabetes in Kenyan patients.
• Storage Forms: Plants store carbohydrates as starch, while animals store glycogen in liver and muscle tissues, serving as a readily mobilizable energy reserve.

Digestion and Absorption

• Enzymatic Breakdown: Amylase enzymes in saliva and pancreatic secretions cleave starch into maltose and glucose, facilitating absorption in the small intestine.
• Transport Mechanisms: Glucose absorption involves active transport via sodium-glucose linked transporters (SGLTs) and facilitated diffusion through GLUT transporters.
• Role of Fiber: Non-digestible polysaccharides influence gut microbiota, improve bowel motility, and reduce cholesterol absorption, contributing to cardiovascular health.
• Metabolic Fate: Absorbed glucose is either used immediately for energy, stored as glycogen, or converted to fat under excess caloric intake.
• Clinical Relevance: Understanding carbohydrate metabolism supports nutritional therapy for conditions such as kwashiorkor and diabetes mellitus common in Kenyan healthcare settings.

Functions in Nutrition and Health

• Primary Energy Source: Carbohydrates supply approximately 50-60% of daily caloric needs in typical Kenyan diets, crucial for brain and muscle function.
• Protein Sparing Effect: Adequate carbohydrate intake prevents protein catabolism for energy, preserving muscle mass and supporting growth.
• Fat Metabolism: Carbohydrates are necessary for complete fat oxidation; deficiency leads to ketone body formation, relevant in malnourished children.
• Dietary Fiber Benefits: Fiber intake reduces the risk of colon cancer, regulates blood glucose, and supports weight management.
• Influence on Satiety: Complex carbohydrates promote longer-lasting fullness, aiding in appetite control and obesity prevention.

1.1.3 Proteins

Proteins are fundamental for body structure, function, and regulation. In Kenya, protein-energy malnutrition remains a public health challenge, making protein biochemistry crucial for dietitians.

Structure and Composition

• Amino Acids: Proteins are polymers of 20 amino acids linked by peptide bonds; essential amino acids cannot be synthesized and must be obtained from diet.
• Primary to Quaternary Structure: Protein functionality depends on folding levels; for example, enzymes have specific tertiary structures critical for activity.
• Protein Quality: Animal proteins generally have higher biological value due to complete amino acid profiles, whereas plant proteins may lack one or more essential amino acids.
• Denaturation: Heat, pH changes, or chemicals can alter protein structure, affecting digestibility and nutritional value.
• Sources: Common Kenyan protein sources include beans, milk, fish, and meat, each varying in amino acid content and digestibility.

Digestion and Absorption

• Gastric Digestion: Pepsin initiates protein breakdown in the stomach, cleaving peptide bonds to smaller polypeptides.
• Pancreatic Enzymes: Trypsin and chymotrypsin further degrade peptides into di- and tri-peptides in the small intestine.
• Absorption Mechanisms: Amino acids and small peptides are absorbed via active transport into enterocytes and then released into circulation.
• Nitrogen Balance: Protein intake must meet nitrogen requirements to maintain tissue synthesis and repair, critical in recovery from illness.
• Clinical Application: Protein metabolism understanding guides nutritional support for surgical patients and those with chronic infections in Kenyan hospitals.

Biological Functions

• Tissue Repair and Growth: Proteins provide amino acids for synthesis of muscle, skin, enzymes, and hormones.
• Enzymatic Activity: All enzymes are proteins catalyzing biochemical reactions essential for metabolism.
• Immune Function: Antibodies are protein molecules vital for immune defense mechanisms.
• Transport and Storage: Hemoglobin and myoglobin transport oxygen; ferritin stores iron, both protein-based.
• Fluid Balance: Plasma proteins maintain oncotic pressure, preventing edema, a condition monitored in malnourished children.

1.1.4 Lipids

Lipids serve as dense energy sources and structural components of cells, with important roles in health and disease. Kenyan diets often include lipids from oils, nuts, and animal fats, necessitating comprehension of their biochemical roles for effective dietary advice.

Types and Chemical Structure

• Triglycerides: Composed of glycerol and three fatty acids, they are the main form of stored energy.
• Phospholipids: Contain phosphate groups and fatty acids, essential for cell membrane integrity and function.
• Sterols: Cholesterol is a sterol precursor for steroid hormones and bile acids.
• Fatty Acid Classification: Saturated, monounsaturated, and polyunsaturated fatty acids differ in bond types, affecting physical properties and health implications.
• Essential Fatty Acids: Linoleic and alpha-linolenic acids must be obtained from diet, important for inflammatory regulation and cell signaling.

Digestion and Absorption

• Emulsification: Bile salts from the liver emulsify lipids in the small intestine, increasing surface area for enzyme action.
• Enzymatic Hydrolysis: Pancreatic lipase breaks triglycerides into monoglycerides and free fatty acids.
• Micelle Formation: Lipid digestion products form micelles with bile salts for transport to enterocytes.
• Absorption and Transport: Lipids are re-esterified in enterocytes and packaged into chylomicrons for lymphatic transport.
• Clinical Relevance: Impaired lipid absorption occurs in conditions like cholestasis, requiring specialized nutritional interventions.

Functions in Nutrition and Health

• Energy Storage: Lipids provide 9 kcal/g, serving as a concentrated energy reserve.
• Insulation and Protection: Fat deposits insulate against temperature extremes and cushion organs.
• Cell Membrane Composition: Phospholipids maintain membrane fluidity and permeability.
• Precursor Molecules: Cholesterol-derived hormones regulate metabolism, stress response, and reproductive function.
• Fat-Soluble Vitamins: Lipids facilitate absorption of vitamins A, D, E, and K, essential for vision, bone health, and antioxidant functions.

1.1.5 Nucleic Acids

Though required in smaller amounts, nucleic acids are vital for genetic information storage and transmission. Nutritional biochemistry explains their dietary sources and metabolic importance.

Structure and Components

• Nucleotides: Building blocks consist of a nitrogenous base, pentose sugar, and phosphate group.
• DNA and RNA: DNA stores genetic information, while RNA translates this information into protein synthesis.
• Nitrogenous Bases: Purines (adenine, guanine) and pyrimidines (cytosine, thymine, uracil) form complementary base pairs.
• Polynucleotide Chains: Nucleotides link via phosphodiester bonds forming stable chains.
• Dietary Sources: Foods rich in nucleic acids include organ meats, fish, and legumes, commonly consumed in Kenyan diets.

Digestion and Metabolism

• Enzymatic Breakdown: Nucleases hydrolyze nucleic acids into nucleotides, which are further broken down to nitrogenous bases and sugars.
• Salvage Pathways: Cells recycle bases to synthesize new nucleotides, conserving energy.
• Deamination and Excretion: Excess nitrogen from bases is converted to uric acid for excretion, relevant in gout management.
• Role in Cell Division: Adequate nucleotide supply is essential for DNA replication during growth and repair.
• Clinical Implications: Understanding nucleic acid metabolism aids in managing genetic disorders and cancer nutrition.

Biological Importance

• Genetic Information Storage: DNA encodes instructions for protein synthesis, critical for cell function.
• Protein Synthesis: RNA mediates translation of genetic code into amino acid sequences.
• Cell Signaling: Some nucleotides act as secondary messengers in intracellular signaling pathways.
• Energy Transfer: ATP, a nucleotide derivative, is the universal energy currency in cells.
• Regulation of Metabolism: Nucleotide derivatives regulate enzymatic activities and metabolic flux.

Chapter Summary

This chapter explored the fundamental types of macromolecules essential for life, highlighting the role of biochemistry in providing energy and synthesizing vital substances. It examined carbohydrates as primary energy sources, proteins as structural and functional molecules, lipids for energy storage and cell membrane integrity, and nucleic acids as carriers of genetic information. The discussion then shifted to micronutrients, detailing vitamins and minerals that support various biochemical and physiological functions. The chapter further analyzed the structure and functions of DNA and RNA, emphasizing their roles in genetic information storage and protein synthesis. It described the process of DNA replication, explaining how genetic material is accurately duplicated. Finally, the chapter covered DNA transcription, detailing how genetic instructions are converted into messenger RNA for protein production.