The Water-Soluble Vitamins: B Vitamins and Vitamin C

Overview of Vitamins

• Vitamins are essential micronutrients that regulate body processes involved in supporting growth and maintaining life.
• Structure: Vitamins are individual units and are not linked together into chains (unlike energy nutrients like carbohydrates or proteins).
• Function: They do not yield energy themselves but assist enzymes that participate in the release of energy from carbohydrate, fat, and protein.
• Food Contents: Required in small amounts, measured in milligrams ($mg$) or micrograms ($\mu g$ or $mcg$).
• Standards: Intake recommendations are defined by the Estimated Average Requirement (EAR) and the Recommended Dietary Allowance (RDA).

Solubility, Absorption, and Transport

• Vitamins are classified based on their solubility in either water or fat, which determines how they are absorbed, transported, and stored.
• Water-Soluble Vitamins: Include the $B$-complex vitamins and Vitamin $C$.     * Absorption: Absorbed directly into the blood.     * Transport: Travel freely throughout the body.     * Storage: Circulate freely in water-filled compartments of the body.     * Excretion: The kidneys identify excess levels and remove them through urine.     * Toxicity: Possible to reach toxic levels when consumed as supplements, though less likely than fat-soluble vitamins.     * Dosage: Required frequently (every $1$ to $3$ days) because the body does not store them significantly.
• Fat-Soluble Vitamins: Include Vitamins $A$, $D$, $E$, and $K$.     * Absorption: First enter the lymph, then the blood.     * Transport: Many require specific transport proteins.     * Storage: Stored in the liver and fatty tissues.     * Excretion: Less readily excreted; they tend to remain in storage sites.     * Toxicity: Likely to reach toxic levels if consumed in excess, especially via supplements.     * Dosage: Required in periodic doses (weeks or months) because the body can draw on its stores.

Bioavailability and Stability

• Bioavailability: Refers to the amount of a nutrient absorbed and used by the body. Calculation depends on:     * Efficiency of digestion and the time of transit through the gastrointestinal (GI) tract.     * Previous nutrient intake and overall nutrition status.     * Method of food preparation (raw, cooked, or processed).     * Source of the nutrient (synthetic, fortified, or naturally occurring).     * Other foods consumed at the same time (nutrient interactions).
• Precursors: Some vitamins are available in foods in inactive forms known as precursors or provitamins. For example, Beta-carotene (a red-orange pigment) is a precursor to Vitamin $A$.
• Organic Nature: Vitamins are organic and can be destroyed during processing or storage:     * Thiamin: Destroyed by prolonged heating.     * Riboflavin: Destroyed by ultraviolet ($UV$) rays from sunlight or fluorescent light.     * Vitamin $C$: Destroyed by exposure to oxygen.
• Upper Intake Levels (UL): The highest amount of a nutrient likely to pose no harm to most healthy people. High-dose supplements of Niacin, Vitamin $B_6$, Folate, Choline, and Vitamin $C$ have established ULs.

The B Vitamins and Coenzymes

• Several B vitamins—thiamin, riboflavin, niacin, biotin, and pantothenic acid—form essential parts of coenzymes.
• Coenzyme Function: These are small organic molecules that work with enzymes to facilitate chemical reactions.     * The vitamin portion allows the reaction to occur.     * The remaining portion of the coenzyme binds to the enzyme.     * Without its specific coenzyme, an enzyme cannot function.
• Metabolic Role: B vitamins are involved in the pathways that release energy from macronutrients.

Thiamin (Vitamin $B_1$)

• Chemical Form: Part of the coenzyme TPP (thiamin pyrophosphate).
• Recommendations: The RDA is based primarily on thiamin's role in enzyme activity.
• Deficiency Disease: Beriberi.     * Dry Beriberi: Involves damage to the nervous system and is characterized by muscle weakness in the arms and legs.     * Wet Beriberi: Involves damage to the cardiovascular system. It is characterized by dilated blood vessels, which causes the heart to overwork and leads the kidneys to retain salt and water, resulting in edema.
• Food Sources: The pork family is exceptionally rich in thiamin. Whole-grain or enriched grains are also good sources.

Riboflavin (Vitamin $B_2$)

• Coenzyme Forms: Flavin mononucleotide (FMN) and Flavin adenine dinucleotide (FAD).
• Action: These coenzymes accept and then donate two hydrogens during the TCA cycle, delivering them to the electron transport chain.
• Deficiency: Causes inflammation of the membranes of the mouth, skin, eyes, and GI tract.
• Toxicity: No known harm from excess riboflavin; no UL is established.
• Food Sources: Milk and milk products are the most common sources. Other sources include whole grains and leafy green vegetables.

Niacin (Vitamin $B_3$)

• Forms: Nicotinic acid and nicotinamide (niacinamide).
• Coenzyme Forms: Nicotinamide adenine dinucleotide (NAD) and its phosphate form (NADP).
• Recommendations: The body can synthesize niacin from the amino acid tryptophan after protein needs are met.     * Conversion Factor: $60\,mg$ of tryptophan = $1\,mg$ of niacin.     * Intake is expressed as Niacin Equivalents (NE).
• Deficiency: Pellagra, characterized by the "four Ds": Diarrhea, Dermatitis, Dementia, and Death.
• Toxicity: Large doses of nicotinic acid from supplements cause "niacin flush" (dilated capillaries and painful tingling). It may also cause liver damage.
• Food Sources: Meat, poultry, fish, legumes, and enriched or whole grains.

Biotin

• Role: Plays a critical role in the TCA cycle. It serves as a coenzyme that participates in gluconeogenesis, fatty acid synthesis, and the breakdown of amino acids.
• Recommendations: Needed only in very small amounts; an AI (Adequate Intake) is set.
• Deficiency: Rare symptoms include skin rash, hair loss, and neurological impairment.
• Food Sources: Egg yolks; also synthesized by GI-tract bacteria.

Pantothenic Acid

• Chemical Role: Part of the chemical structure of Coenzyme A (CoA).
• Functions: Involved in more than $100$ steps in the synthesis of lipids, neurotransmitters, steroid hormones, and hemoglobin.
• Recommendations: AI has been established.
• Deficiency: Rare symptoms include fatigue, GI distress, neurological disturbances, and "burning feet" syndrome.
• Food Sources: Beef, poultry, whole grains, potatoes, tomatoes, and broccoli.

Vitamin $B_6$

• Forms: Pyridoxal, pyridoxine, and pyridoxamine. All can be converted to the coenzyme PLP (pyridoxal phosphate).
• Recommendations: Based on the amount needed to maintain coenzyme levels.
• Deficiency: Early symptoms include depression and confusion. Advanced symptoms include abnormal brain wave patterns and convulsions.
• Toxicity: Can cause neurological damage if taken at doses of $2\,g$ daily for $2$ months or more.
• Food Sources: Meats, fish, poultry, potatoes, and certain other fruits and vegetables.

Folate

• Also Known As: Folacin or folic acid.
• Coenzyme Form: Tetrahydrofolate (THF).
• Metabolic Functions:     * Transfers $1$-carbon units in metabolism.     * Converts Vitamin $B_{12}$ to its active coenzyme form.     * Synthesizes DNA for rapidly growing cells.     * Regenerates methionine from homocysteine.
• Recommendations and Bioavailability:     * Food folate (polyglutamates) is given full credit.     * Synthetic folate (monoglutamates) in supplements or fortified foods is $1.7$-fold more bioavailable.     * Expressed as Dietary Folate Equivalents (DFE).
• Neural Tube Defects: Taken one month before conception and through the first trimester, folate prevents brain and spinal cord defects (e.g., spina bifida). It may also reduce risks for congenital heart disease, cleft lip/palate, and autism.
• Deficiency: Causes macrocytic or megaloblastic anemia and GI-tract deterioration.
• Toxicity: High intakes can "mask" a Vitamin $B_{12}$ deficiency by clearing up anemia while nerve damage continues.
• Food Sources: Dark green leafy vegetables (foliage), legumes, and fortified grains. Meats and milk are poor sources.

Vitamin $B_{12}$ (Cobalamin)

• Activation: Interdependent with folate; $B_{12}$ activates folate by removing a methyl group.
• Other Roles: Maintains nerve fiber sheaths, supports nerve growth, bone cell activity, and metabolism.
• Absorption Process:     * Hydrochloric acid ($HCl$) and pepsin release $B_{12}$ from food in the stomach.     * $B_{12}$ binds with Intrinsic Factor (IF) secreted by the stomach.     * The $B_{12}$-IF complex is recognized by receptors in the small intestine for absorption.     * Enters enterohepatic circulation (reabsorbed from bile).
• Recommendations: RDA for adults is $2.4\,\mu g$.
• Deficiency conditions:     * Atrophic Gastritis: Damage to stomach cells; can be caused by iron deficiency or $Helicobacter\,pylori$ infection.     * Pernicious Anemia: Caused by atrophic gastritis combined with a lack of Intrinsic Factor.
• Food Sources: Exclusively foods of animal origin and fortified foods.

Choline and Nonvitamins

• Choline: Essential nutrient used to make the neurotransmitter acetylcholine and the phospholipid lecithin. The body can make it from methionine. UL is based on its ability to lower blood pressure to life-threatening levels.     * Sources: Milk, liver, eggs, peanuts.
• Nonvitamins:     * Inositol: Part of cell membranes; made by the body.     * Carnitine: Transports fatty acids into mitochondria; made by the body.
• Misleading Names:     * "Vitamin O": Oxygenated saltwater (false claim).     * "Vitamin B15": Pangamic acid (hoax).     * "Vitamin B17": Laetrile (dangerous, falsely marketed as a cancer cure).     * "Vitamin B5": Actual legitimate name for pantothenic acid.

Vitamin C (Ascorbic Acid)

• Role as an Antioxidant: Defends against free radicals (unstable molecules with unpaired electrons). It protects tissues from oxidative stress and may prevent chronic diseases.
• Role as a Cofactor in Collagen Formation: Helps form collagen, the structural protein for connective tissues, bones, teeth, and scar tissue.     * Vitamin $C$ keeps iron in its reduced (active) state so it can act as a cofactor for the enzymes that hydroxylate proline and lysine during collagen synthesis.
• Cold Prevention: High doses ($6$ to $8\,g\,day^{-1}$) may reduce cold duration by deactivating histamine, which causes nasal congestion.
• Recommendations:     * Only $10\,mg$ daily prevents scurvy.     * Cigarette smokers require an additional $35\,mg$ daily because smoke contains oxidants that deplete the body's Vitamin $C$ stores.
• Deficiency: Scurvy. Symptoms include muscle degeneration, weakened bones/teeth, scaly/dry skin, failed wound healing, hemorrhaging, anemia, and depression.
• Toxicity: GI distress, diarrhea, and kidney stones in at-risk individuals. Can cause false positives in medical tests for glucose or ketones. High doses may act as a pro-oxidant in cases of iron overload.
• Food Sources: Citrus fruits, tomatoes, potatoes, broccoli, strawberries. Easily destroyed by heat and oxygen.

Dietary Supplements: Pros and Cons

• Usage: Over half of United States adults take supplements, spending over $40$ billion yearly.
• Arguments For: Correcting overt deficiencies, supporting increased needs (e.g., pregnancy), helping those with very low energy intake (< $1,600\,kcal\,day^{-1}$), and supporting vegans or those with milk allergies.
• Arguments Against:     * Toxicity: Excessive intake often goes unreported.     * Bioavailability/Antagonism: High doses of one nutrient can blocks another (e.g., Iron hinders Zinc, Calcium hinders Magnesium, Vitamin E antagonizes Vitamin K).     * False Security: Belief that supplements can replace a healthy diet or cure serious illnesses.
• Selection: Look for the US Pharmacopeia (USP) seal, which ensures the supplement contains the listed ingredients, lacks contaminants, and will dissolve properly.
• Regulation: The FDA regulates supplements as foods, not drugs. Labels can make nutrient claims but cannot claim to treat, cure, or diagnose specific diseases without a required disclaimer.

Questions & Discussion

• Knowledge Check: Dry beriberi affects the nervous system (muscle weakness), while wet beriberi affects the cardiovascular system (edema and heart strain).
• Riboflavin Check: The coenzymes FMN and FAD facilitate hydrogen transfer to the electron transport chain. Deficiency causes membrane inflammation.
• Folate Discussion: It is vital for all women of childbearing age because neural tube defects occur very early in pregnancy, often before a woman knows she is pregnant. Half of all pregnancies are unplanned.
• Supplement Check: Individuals with an energy intake below $1,600\,kcal\,day^{-1}$ are the most likely group to need multivitamin-mineral supplementation.