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 Intro to Nutrition, you'll be 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. Expect exam questions that 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 a free-response question about 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 form rhodopsin, which enables sight in low-light conditions.
• Supports epithelial tissue integrity and mucous membrane function, creating physical barriers against pathogens.
• Regulates gene expression for cell differentiation through the retinoic acid form, 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 by promoting their absorption in the intestine.
• Synthesized endogenously when UVB radiation converts 7-dehydrocholesterol in skin to cholecalciferol ($D_3$). The liver and kidneys then convert it to its active form, calcitriol.
• Deficiency causes rickets in children and osteomalacia in adults due to impaired bone mineralization from insufficient calcium absorption.

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. It 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.
• Two key forms: $K_1$ (phylloquinone) comes from green leafy vegetables, while $K_2$ (menaquinone) is produced by gut bacteria and also found in fermented foods.

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Water-Soluble Vitamins: Coenzymes in Energy Metabolism

The B-vitamins function primarily as coenzymes, which are 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 form: thiamin pyrophosphate (TPP). It's required by pyruvate dehydrogenase, the enzyme that 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 chronic alcoholics, who often have poor thiamin intake and impaired absorption.

Riboflavin (B2)

• Forms two coenzymes: FAD and FMN. These are essential electron carriers in oxidation-reduction reactions throughout metabolism.
• FAD plays a direct role in the electron transport chain and the citric acid cycle (e.g., succinate dehydrogenase uses FAD to accept electrons).
• Deficiency presents as ariboflavinosis, characterized by cracked lips (cheilosis), inflamed tongue (glossitis), 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, while NADP supports anabolic (building) pathways like fatty acid synthesis.
• Deficiency causes pellagra, remembered by 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 (PLP) for over 100 enzyme reactions, primarily in amino acid metabolism including transamination (shuffling amino groups between molecules) and decarboxylation.
• Required for neurotransmitter synthesis. PLP converts precursors into serotonin, dopamine, and GABA.
• Supports hemoglobin production by enabling heme synthesis. Deficiency causes microcytic anemia (small, pale red blood cells), which is distinct from the megaloblastic anemia caused by folate or B12 deficiency.

Folate (B9)

• Essential for one-carbon transfer reactions, which are critical for DNA and RNA synthesis during cell division.
• Prevents neural tube defects (like spina bifida) when adequate levels are present during the first 28 days of pregnancy, often before a person knows they're pregnant. That's why supplementation is recommended before conception.
• Deficiency causes megaloblastic anemia. Red blood cells grow large but can't divide properly because DNA synthesis stalls.

Vitamin B12 (Cobalamin)

• Required for methionine synthase, which regenerates folate to its active form. This links B12 and folate metabolism tightly together.
• Maintains myelin sheath integrity around nerve fibers. Deficiency causes neurological damage that can become irreversible if not caught early.
• Found only in animal-derived foods (meat, dairy, eggs). Vegans require fortified foods or supplements. Older adults may also need supplements due to decreased stomach acid, which is needed to release B12 from food proteins.

Biotin (B7)

• Coenzyme for carboxylase enzymes, which add carbon dioxide ($CO_2$) to substrates in gluconeogenesis and fatty acid synthesis.
• Supports pyruvate carboxylase, which converts pyruvate to oxaloacetate, replenishing citric acid cycle intermediates (a process called anaplerosis).
• Deficiency is rare but can occur with excessive raw egg white consumption. Raw egg whites contain avidin, a protein that tightly binds biotin and prevents its absorption. Cooking denatures avidin.

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Antioxidant Vitamins: Cellular Defense Against Oxidative Stress

Reactive oxygen species (ROS) are unstable molecules that damage DNA, proteins, and lipids. Antioxidants neutralize ROS by donating electrons without becoming dangerously reactive themselves. 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 plasma).
• Required cofactor for collagen synthesis. It hydroxylates proline and lysine residues to stabilize collagen's triple-helix structure. Without it, collagen is weak, leading to scurvy (bleeding gums, poor wound healing, bruising).
• Enhances non-heme iron absorption by reducing $Fe^{3+}$ to $Fe^{2+}$ in the intestine. This is especially important for people on plant-based diets, since plant iron (non-heme) is less readily absorbed than animal iron (heme).

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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 the 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. Why are chronic alcoholics at high risk for thiamin deficiency? What metabolic pathway is disrupted, and what are the clinical consequences?