Water-Soluble Vitamins: Structure, Function, and Clinical Relevance

Water-Soluble Vitamins

Overview of Water-Soluble Vitamins

Water-soluble vitamins are essential micronutrients that dissolve in water and are not stored in significant amounts in the body. They must be consumed regularly in the diet. There are nine water-soluble vitamins, including vitamin C and eight B-complex vitamins.

• Vitamin C

• B-complex vitamins:

  • Thiamin (B1)

  • Riboflavin (B2)

  • Niacin (B3)

  • Pantothenic acid (B5)

  • Biotin (B7)

  • Vitamin B6

  • Folate (B9)

  • Vitamin B12

These vitamins are easily destroyed by heat, light, and oxidation.

Comparison: Water-Soluble vs. Fat-Soluble Vitamins

	Water-Soluble Vitamins	Fat-Soluble Vitamins
Absorbed in the	Small Intestine	Small Intestine
Hydrophobic or Hydrophilic	Hydrophilic	Hydrophobic
Absorbed into the	Blood	Lymph
Stored in the body	Not Generally	Yes
Can build up and become toxic	Not Generally	Yes
Need to consume daily	Yes	No

Digesting and Absorbing Water-Soluble Vitamins

Water-soluble vitamins are primarily absorbed in the small intestine. They are transported via the blood and, because they are not stored in large amounts, excess is excreted in urine.

Primary Functions of Water-Soluble Vitamins

Coenzyme Activity

• B vitamins act as coenzymes in metabolic processes, catalyzing reactions that build or break apart compounds.

• They are essential for transforming carbohydrates, proteins, and fats into ATP (energy).

Examples of B vitamins with coenzyme roles:

• Thiamin

• Riboflavin

• Niacin

• Pantothenic acid

• Biotin

• Vitamin B6

Other Critical Roles

• Antioxidant: Vitamin C

• Nerve function: Thiamin

• Protein synthesis: Niacin

• Red blood cell formation: Folate and vitamin B12

• Several B vitamins promote heart health.

Thiamin (Vitamin B1)

Properties and Absorption

• First B vitamin discovered.

• Sensitive to pH; destroyed in basic solutions, protected in acidic solutions.

• Absorbed in the jejunum of the small intestine, transported in blood, and excreted in urine.

Metabolic Functions

• Acts as a coenzyme in carbohydrate metabolism to produce ATP.

• Supports appetite and nerve function.

• Involved in the transmission of nerve impulses.

Daily Needs and Food Sources

• RDA: Males 1.2 mg/day, Females 1.1 mg/day.

• Major sources: Enriched/fortified foods, wheat germ, whole grains, pork, ham, bacon, liver, legumes, nuts.

Toxicity and Deficiency

• No known toxicity.

• Deficiency:

  • Beriberi: Loss of appetite, weight loss, confusion, muscle weakness, peripheral neuropathy.

  • Wernicke-Korsakoff syndrome: Caused by chronic alcohol abuse; includes mental confusion, muscle and nerve problems, heart problems.

Riboflavin (Vitamin B2)

Properties and Absorption

• Exists as flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD).

• Stable during cooking except in ultraviolet light.

• Absorbed by active transport in the small intestine after being released from proteins by stomach acid.

Metabolic Functions

• Supports normal vision and skin health.

• Breaks down fatty acids for energy.

• Enhances the function of other B vitamins.

Daily Needs and Food Sources

• RDA: Males 1.3 mg/day, Females 1.1 mg/day.

• Major sources: Milk, yogurt, cottage cheese, meat, leafy green vegetables, enriched/fortified cereals and grains.

Toxicity and Deficiency

• No known toxicity; excess excreted in urine.

• Deficiency (Ariboflavinosis):

  • Inflammation/swelling of tissues lining the throat, mouth, tongue, lips.

  • Sore throat, stomatitis, glossitis, cheilosis.

Niacin (Vitamin B3)

Properties and Absorption

• Two active forms: nicotinic acid and nicotinamide.

• Converted to NAD+ and NADP+ (coenzymes).

• Bioavailability: Less in plant foods, more in dairy and meat.

• Absorbed in the small intestine, circulates to the liver.

Metabolic Functions

• Coenzyme in energy metabolism (carbohydrate metabolism, glycogen synthesis).

• Keeps skin and digestive system healthy.

Daily Needs and Food Sources

• RDA: Males 16 mg/day, Females 14 mg/day, Upper level 35 mg/day.

• Major sources: Meats, fish, poultry, milk, eggs, enriched/fortified grains and cereals, nuts, all protein-containing foods.

• Stable in foods; not destroyed by heat or UV light, but can leach into water.

Toxicity and Deficiency

• No toxicity from normal intake; excess supplements/fortified foods can cause flushing, nausea, vomiting, heartburn, liver toxicity, raised blood glucose.

• Deficiency (Pellagra): The "four Ds"—Dermatitis, Diarrhea, Dementia, Death.

Pantothenic Acid (Vitamin B5)

Properties and Absorption

• Part of coenzyme A, used to make acetyl CoA.

• Absorbed in the small intestine.

Metabolic Functions

• Fat metabolism: Synthesizes fatty acids and converts them to energy.

• Carbohydrate metabolism: Converts pyruvate to acetyl CoA.

• Protein metabolism: Converts amino acids to TCA cycle substrates.

• Synthesis of cholesterol, steroid hormones, acetylcholine.

Daily Needs and Food Sources

• AI: 5 mg/day for adults.

• Widespread in foods; highest in whole-grain cereals, nuts, legumes, peanut butter, meat, milk, eggs.

• Destroyed by heat; processed foods have less.

Toxicity and Deficiency

• No known toxicity.

• Deficiency is rare due to widespread availability.

Biotin (Vitamin B7)

Functions

• Coenzyme for enzymes in energy metabolism (fatty acid and glycogen synthesis).

• Cell development and growth; maintains healthy hair and nails.

Daily Needs and Food Sources

• AI: 30 μg/day for adults.

• Widespread in foods.

Toxicity and Deficiency

• Toxicity is rare.

• Deficiency can occur with overconsumption of raw egg whites (avidin binds biotin), leading to hair loss, depression, conjunctivitis, dermatitis, lethargy, hallucinations.

Vitamin B6

Properties and Absorption

• Collective name for several related compounds (pyridoxine, pyridoxal, pyridoxamine).

• Absorbed in the small intestine, transported to the liver.

Functions

• Coenzyme in amino acid and fatty acid metabolism.

• Helps make red blood cells.

• Keeps immune and nervous systems healthy.

Daily Needs and Food Sources

• RDA: Males 1.3–1.7 mg/day, Females 1.3–1.5 mg/day, Upper limit 100 mg/day.

• Sources: Ready-to-eat cereals, meat, fish, poultry, shellfish, fruits, vegetables, nuts, legumes.

Toxicity and Deficiency

• Toxicity: Can cause nerve damage, difficulty walking, tingling in legs and feet (usually from supplements).

• Deficiency: Sore tongue, skin inflammation, depression, confusion, microcytic hypochromic anemia (small, pale red blood cells). Often caused by excessive alcohol consumption.

Folate (Vitamin B9)

Properties and Absorption

• Folate is found in foods; folic acid is the synthetic form in supplements/fortified foods.

• Coenzyme in DNA synthesis, red and white blood cell synthesis, amino acid metabolism.

• Absorbed in the small intestine, transported to the liver.

• Bioavailability: Destroyed by heat/light; raw foods have more; some foods (beans, cabbage) contain inhibitors.

Functions

• DNA and amino acid synthesis.

• Neural tube development (prevents neural tube defects, anencephaly, spina bifida).

Daily Needs and Food Sources

• RDA: 400 μg/day for adults; women of childbearing age need 400 μg/day plus a diet high in natural folate.

• Sources: Dark green leafy vegetables, legumes, seeds, enriched pasta/rice/breads/cereals, liver.

Toxicity and Deficiency

• No risk from foods; excess from supplements/fortified foods can mask B12 deficiency.

• Deficiency: Interferes with red blood cell division, causing macrocytic anemia (large red blood cells), fatigue, weakness, irritability, shortness of breath.

Vitamin B12 (Cobalamin)

Properties and Absorption

• Two forms, both containing cobalt.

• Stored mostly in the liver; excreted in bile and urine.

• Absorption decreases with age (due to reduced stomach acid or intrinsic factor).

Functions

• Coenzyme in cell synthesis and growth.

• Works with folate (activates folate).

• Synthesis of new cells, especially red blood cells.

• Maintains myelin sheath of nerve tissue.

• Catabolism of amino acids and fatty acids for energy metabolism.

Daily Needs and Food Sources

• RDA: 2.4 μg/day for adults.

• Sources: Only found naturally in animal products (meat, fish, poultry, eggs, milk, cheese). Vegans need fortified foods or supplements.

Toxicity and Deficiency

• No known toxicity.

• Deficiency: Causes macrocytic anemia (due to folate deficiency), pernicious anemia, fatigue, shortness of breath, nerve damage (tingling/numbness in arms and legs). Causes include low intake (vegan diets), malabsorption, gastric bypass, lack of intrinsic factor.

Vitamin C (Ascorbic Acid)

Properties and Absorption

• Humans cannot synthesize vitamin C.

• Absorbed in the small intestine, excreted by kidneys, not stored in the body.

Functions

• Collagen synthesis (strengthens blood vessels, forms scar tissue, bone matrix).

• Antioxidant.

• Amino acid metabolism.

• Strengthens resistance to infection (promotes white blood cell synthesis).

• Enhances absorption of nonheme iron from plant sources.

Daily Needs and Food Sources

• RDA: Males 90 mg/day, Females 75 mg/day, Smokers +35 mg/day, Upper limit 2,000 mg/day.

• Sources: Citrus fruits, cabbage-type vegetables, dark green vegetables, other fruits and vegetables.

Toxicity and Deficiency

• Toxicity: >3,000 mg can cause nausea, stomach cramps, diarrhea, kidney stones, aggravate gout, problematic for those with hemochromatosis.

• Deficiency: Scurvy (bleeding gums, coiled hair, wounds won't heal, rough rash). Prevented with 10 mg/day.