Water-Soluble Vitamins: Structure, Function, and Dietary Sources

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Water-Soluble Vitamins

Introduction to Vitamins

Vitamins are organic micronutrients essential for normal physiological functions. They are required in small amounts, do not provide energy, and are classified based on their solubility as either water-soluble or fat-soluble. Water-soluble vitamins include the B-complex group and vitamin C, while fat-soluble vitamins include A, D, E, and K.

Organic: Contain carbon atoms.
Micronutrients: Needed in small quantities.
Essential: Must be obtained from the diet.
Not energy-yielding: Do not provide Calories.
Cofactors: Many act as cofactors in enzymatic reactions.

Classification of vitamins by solubility and food sources

Classification of Vitamins

Water-Soluble Vitamins: Thiamin (B1), Riboflavin (B2), Niacin (B3), Pantothenic acid, Pyridoxine (B6), Biotin, Folate (B9), Cobalamin (B12), Vitamin C.
Fat-Soluble Vitamins: Vitamins A, D, E, K.

Water-soluble vitamins dissolve in water, are easily absorbed and excreted, are not stored extensively in tissues, and seldom reach toxic levels. Fat-soluble vitamins dissolve in lipids, require bile for absorption, are transported in the lymph, stored in tissues, and may be toxic in excess.

Water-soluble vitamin dissolving in water

Vitamins in Foods

Vitamins are present in almost all foods. The amount of a vitamin in food depends on its natural content, fortification or enrichment, and how the food is processed, prepared, and stored.

Table of vitamin content in food groups

Fortification and Enrichment

Fortification: Addition of nutrients to foods that do not naturally contain them (e.g., adding calcium to orange juice).
Enrichment: Addition of nutrients lost during processing (e.g., adding B vitamins to white rice).

Health Canada regulates both mandatory and voluntary fortification to prevent deficiencies but excessive intake may cause toxicity.

Grains, a common vehicle for vitamin enrichment

Vitamin Supplementation

Supplementation may benefit individuals with restricted diets, vegans, infants, children, pregnant women, those with dark skin or limited sun exposure (vitamin D), medication users, and smokers or alcohol users.

Digestion and Absorption of Vitamins

Absorption Pathways

Water-soluble vitamins are absorbed directly into the blood, while fat-soluble vitamins require bile and are absorbed via the lymphatic system. Bioavailability refers to the extent to which the body can absorb and use a nutrient, influenced by digestion efficiency, nutrient status, food preparation, and nutrient source.

Absorption of water- and fat-soluble vitamins in the digestive tract

Provitamins and Precursors

Some vitamins are available as inactive precursors (provitamins) in foods and must be converted to active forms in the body. For example, β-carotene is a provitamin A converted to retinol.

B Vitamins: Structure, Function, and Sources

Role in Energy Metabolism

B vitamins act as coenzymes in energy metabolism, facilitating the breakdown of carbohydrates, fats, and proteins to produce ATP.

B vitamins in energy metabolism pathways

Thiamin (Vitamin B1)

Function: Coenzyme in glucose metabolism, nerve function, and synthesis of neurotransmitters.
RDA: Male: 1.2 mg/day; Female: 1.1 mg/day.
Sources: Whole grains, enriched grains, pork, legumes, seeds.
Deficiency: Beriberi (weakness, nerve degeneration, heart changes), Wernicke-Korsakoff syndrome in alcoholics.
Excess: No reported effects; no UL set.

Beriberi, a thiamin deficiency disease Food sources of thiamin

Riboflavin (Vitamin B2)

Function: Coenzyme in ATP production, antioxidant, activates other vitamins, important in the citric acid cycle.
RDA: Male: 1.3 mg/day; Female: 1.1 mg/day.
Sources: Dairy, red meat, poultry, fish, whole/enriched grains, vegetables.
Deficiency: Poor healing, cracked lips, sensitivity to light, skin and eye issues.
Excess: Turns urine bright yellow; no other reported effects.

Food sources of riboflavin

Niacin (Vitamin B3)

Function: Coenzyme in glucose metabolism, synthesis of fatty acids and cholesterol, can be synthesized from tryptophan.
RDA: Male: 16 mg NE/day; Female: 14 mg NE/day.
Sources: Meats, fish, peanuts, grains, legumes, wheat bran.
Deficiency: Pellagra (4Ds: dermatitis, diarrhea, dementia, death).
Excess: Supplements can cause flushing, rash, GI symptoms, liver issues.

Food sources of niacin

Biotin

Function: Coenzyme in the citric acid cycle, glucose synthesis.
AI: 30 μg/day.
Sources: Liver, egg yolks, yogurt, nuts (avoid raw egg whites due to avidin).

Pantothenic Acid

Function: Part of coenzyme A, involved in fatty acid and cholesterol synthesis.
AI: 5 mg/day (adults), 6 mg/day (pregnancy), 7 mg/day (lactation).
Sources: Meat, eggs, whole grains, legumes.

Vitamin B6 (Pyridoxine)

Function: Amino acid/protein metabolism, neurotransmitter and hemoglobin synthesis, conversion of tryptophan to niacin.
RDA: 1.3 mg/day (19–50 years).
Sources: Chicken, fish, pork, organ meats, whole grains, legumes, seeds, bananas, vegetables.
Deficiency: Poor growth, skin lesions, anemia, neurological symptoms.
Excess: Supplements can cause nerve impairment.

Functions of vitamin B6 Food sources of vitamin B6

Folate (Folic Acid, B9)

Function: DNA synthesis, amino acid metabolism, neural tube formation, prevents high homocysteine.
RDA: 400 μg/day.
Sources: Enriched grains, leafy greens, legumes, nuts, oranges, liver, yeast.
Deficiency: Neural tube defects, macrocytic anemia, poor growth, nerve issues, increased heart disease risk.
Excess: May mask B12 deficiency, possible cancer risk.

Food sources of folate

Vitamin B12 (Cobalamin)

Function: ATP production from fatty acids, homocysteine to methionine conversion, myelin maintenance, protein metabolism.
RDA: 2.4 μg/day (adults).
Sources: Animal products, fortified foods.
Deficiency: Pernicious anemia, nerve damage, memory loss, paralysis, death.
Excess: No reported effects up to 100 μg/day.

Myelin sheath, maintained by vitamin B12

Vitamin C (Ascorbic Acid)

Functions

Antioxidant: Neutralizes free radicals, regenerates vitamin E, protects DNA and other molecules from oxidative damage.
Collagen Synthesis: Essential for strong connective tissue.
Other Roles: Synthesis of neurotransmitters, hormones, bile acids, carnitine, enhances iron absorption.

Antioxidant action of vitamin C Vitamin C in collagen synthesis

Dietary Sources and Deficiency

RDA: Male: 90 mg/day; Female: 75 mg/day.
Sources: Citrus fruits, strawberries, kiwis, cantaloupe, cabbage-family and dark-green vegetables, peppers, tomatoes, potatoes.
Deficiency: Scurvy (gum/tooth problems, joint pain, bleeding, poor wound healing, fatigue, depression).
Excess: Diarrhea, nausea, cramps, possible kidney stones.

Food sources of vitamin C

Choline

Functions and Sources

Function: Synthesis of acetylcholine (neurotransmitter), cell membrane structure, lipid transport, homocysteine metabolism.
AI: Male: 550 mg/day; Female: 425 mg/day.
Sources: Egg yolks, liver, meat, fish, wheat germ, nuts.
Deficiency: May impair fetal brain development, cause fatty liver and muscle damage in adults.
Excess: Fishy odor, sweating, low blood pressure, liver damage (at high supplemental intakes).

Antioxidants and Free Radicals

Mechanism and Health Implications

Antioxidants are substances that neutralize free radicals—unstable molecules with unpaired electrons that can damage DNA, proteins, and cell membranes. Oxidative stress from free radicals is linked to aging, heart disease, cancer, and neurodegenerative diseases. Vitamin C is a key antioxidant in blood and tissues.

Antioxidant action of vitamin C

Summary Table: Water-Soluble Vitamins

Vitamin	Main Functions	Major Sources	Deficiency Symptoms	Excess/Toxicity
Thiamin (B1)	Energy metabolism, nerve function	Whole grains, pork, legumes	Beriberi, Wernicke-Korsakoff	None reported
Riboflavin (B2)	ATP production, antioxidant, vitamin activation	Dairy, meat, grains, vegetables	Cracked lips, skin/eye issues	Bright yellow urine
Niacin (B3)	Energy metabolism, DNA repair	Meat, fish, peanuts, grains	Pellagra (4Ds)	Flushing, liver issues (supplements)
Vitamin B6	Amino acid metabolism, neurotransmitters	Meat, grains, vegetables	Anemia, nerve issues	Nerve impairment (supplements)
Folate (B9)	DNA synthesis, neural tube formation	Leafy greens, grains, legumes	Neural tube defects, anemia	Masks B12 deficiency
Vitamin B12	Myelin maintenance, energy, protein metabolism	Animal products, fortified foods	Pernicious anemia, nerve damage	None reported
Vitamin C	Antioxidant, collagen synthesis	Citrus, fruits, vegetables	Scurvy	GI upset, kidney stones (supplements)
Biotin	Energy metabolism	Liver, eggs, nuts	Rare	None reported
Pantothenic Acid	Coenzyme A, fatty acid synthesis	Meat, eggs, grains	Rare	None reported
Choline	Neurotransmitter synthesis, cell membranes	Eggs, meat, fish, nuts	Fatty liver, muscle damage	Fishy odor, liver damage (high intake)