Vitamin Functions

Why This Matters

Vitamins aren't just items on a nutrition label—they're the molecular machinery that keeps your body running. In Introduction to Nutrition, you're being tested on how these micronutrients function at the cellular level, why deficiencies cause specific symptoms, and how vitamins interact with each other and with macronutrients. The exam will expect you to connect fat-soluble vs. water-soluble properties, coenzyme roles in metabolism, and antioxidant mechanisms to real physiological outcomes.

Don't just memorize that "Vitamin C helps immunity." Know why—it's an electron donor that neutralizes free radicals and supports collagen synthesis for barrier tissues. When you understand the mechanism, you can answer any question they throw at you, whether it's multiple choice or an FRQ asking you to explain a deficiency disease. Master the categories below, and you'll see vitamins as a system rather than a list.

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Fat-Soluble Vitamins: Stored in Tissue, Absorbed with Fat

Fat-soluble vitamins (A, D, E, K) dissolve in lipids and require dietary fat for absorption. Because they're stored in adipose tissue and the liver, toxicity is possible with excessive intake, but deficiency develops slowly.

Vitamin A

• Retinal is the active form for vision—it combines with opsin proteins in rod cells to enable sight in low-light conditions
• Supports epithelial tissue integrity and mucous membrane function, creating physical barriers against pathogens
• Regulates gene expression for cell differentiation, making it critical during growth and immune cell development

Vitamin D

• Functions as a hormone rather than a traditional vitamin—it regulates calcium and phosphorus homeostasis through intestinal absorption
• Synthesized endogenously when UVB radiation converts 7-dehydrocholesterol in skin to cholecalciferol ($D_3$)
• Deficiency causes rickets in children and osteomalacia in adults due to impaired bone mineralization

Vitamin E

• Primary fat-soluble antioxidant—alpha-tocopherol donates electrons to neutralize free radicals in cell membranes
• Protects polyunsaturated fatty acids (PUFAs) from lipid peroxidation, preserving membrane fluidity and function
• Works synergistically with Vitamin C, which regenerates oxidized Vitamin E back to its active form

Vitamin K

• Essential cofactor for carboxylation reactions—activates clotting factors II, VII, IX, and X in the coagulation cascade
• Supports bone metabolism by activating osteocalcin, a protein that binds calcium to bone matrix
• Produced by gut bacteria ($K_2$), while dietary sources provide $K_1$ from green leafy vegetables

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Water-Soluble Vitamins: Coenzymes in Energy Metabolism

The B-vitamins function primarily as coenzymes—molecules that bind to enzymes and enable metabolic reactions. Because they're water-soluble, excess is excreted in urine, making toxicity rare but requiring consistent daily intake.

Thiamin (B1)

• Coenzyme for pyruvate dehydrogenase—converts pyruvate to acetyl-CoA, linking glycolysis to the citric acid cycle
• Critical for carbohydrate metabolism, meaning high-carb diets increase thiamin requirements
• Deficiency causes beriberi (peripheral neuropathy, cardiac dysfunction) and Wernicke-Korsakoff syndrome in alcoholics

Riboflavin (B2)

• Forms FAD and FMN coenzymes—essential electron carriers in oxidation-reduction reactions throughout metabolism
• Supports the electron transport chain where FAD accepts electrons during ATP production
• Deficiency presents as ariboflavinosis—characterized by cracked lips (cheilosis), sore throat, and light sensitivity

Niacin (B3)

• Forms NAD and NADP coenzymes—involved in over 400 enzymatic reactions including glycolysis and the citric acid cycle
• NAD carries electrons to the electron transport chain for ATP synthesis; NADP supports anabolic pathways
• Deficiency causes pellagra—the "4 Ds": dermatitis, diarrhea, dementia, and death if untreated

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B-Vitamins in Cell Synthesis and Blood Formation

Beyond energy metabolism, several B-vitamins are essential for DNA synthesis, amino acid metabolism, and red blood cell production. Deficiencies in this group often manifest as anemia or neurological symptoms.

Vitamin B6 (Pyridoxine)

• Coenzyme for over 100 enzyme reactions—primarily in amino acid metabolism including transamination and decarboxylation
• Required for neurotransmitter synthesis—converts precursors into serotonin, dopamine, and GABA
• Supports hemoglobin production by enabling heme synthesis; deficiency causes microcytic anemia

Folate (B9)

• Essential for one-carbon transfer reactions—critical for DNA and RNA synthesis during cell division
• Prevents neural tube defects when adequate levels are present during the first weeks of pregnancy
• Deficiency causes megaloblastic anemia—red blood cells grow large but cannot divide properly without DNA synthesis

Vitamin B12 (Cobalamin)

• Required for methionine synthase—regenerates folate to its active form, linking B12 and folate metabolism
• Maintains myelin sheath integrity around nerve fibers; deficiency causes irreversible neurological damage
• Found only in animal products—vegans require fortified foods or supplements to prevent deficiency

Biotin (B7)

• Coenzyme for carboxylase enzymes—adds carbon dioxide to substrates in gluconeogenesis and fatty acid synthesis
• Supports pyruvate carboxylase which converts pyruvate to oxaloacetate, replenishing citric acid cycle intermediates
• Deficiency is rare but can occur with excessive raw egg white consumption (avidin binds biotin)

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Antioxidant Vitamins: Cellular Defense Against Oxidative Stress

Antioxidants neutralize reactive oxygen species (ROS) that damage DNA, proteins, and lipids. The body's antioxidant network includes both fat-soluble (Vitamin E) and water-soluble (Vitamin C) vitamins working in different cellular compartments.

Vitamin C (Ascorbic Acid)

• Water-soluble antioxidant that donates electrons to neutralize free radicals in aqueous environments (cytoplasm, blood)
• Required cofactor for collagen synthesis—hydroxylates proline and lysine residues to stabilize collagen structure
• Enhances non-heme iron absorption by reducing $Fe^{3+}$ to $Fe^{2+}$ in the intestine, important for plant-based diets

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Quick Reference Table

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Self-Check Questions

1. Which three B-vitamins form coenzymes directly involved in the electron transport chain and citric acid cycle, and what are those coenzymes called?

2. Compare and contrast the roles of Vitamin D and Vitamin K in bone health—how do their mechanisms differ?

3. A patient presents with megaloblastic anemia and peripheral neuropathy. Which vitamin deficiency is most likely, and why can't folate deficiency alone explain all symptoms?

4. Vitamin C and Vitamin E both function as antioxidants. Explain how their solubility determines where in the cell each one works and how they interact with each other.

5. If an FRQ asks you to explain why alcoholics are at high risk for thiamin deficiency, what metabolic pathway would you discuss, and what are the clinical consequences?