Water-Soluble Vitamins: Structure, Function, and Importance in Human Nutrition

Water-Soluble Vitamins

Introduction to Vitamins

Vitamins are essential organic micronutrients required in small amounts for various physiological functions. They do not provide energy but are crucial as cofactors in metabolic processes. Vitamins are classified based on their solubility: water-soluble (B vitamins and vitamin C) and fat-soluble (A, D, E, K).

• Organic – contain carbon

• Micronutrients – needed in small amounts

• Essential – must be obtained from the diet

• Not energy-yielding

• Classified as water-soluble or fat-soluble

• Act as cofactors (helpers) in cell functions

Classification of Vitamins

• Water-Soluble Vitamins: Vitamin C, B Vitamins (Thiamin, Riboflavin, Niacin, Pantothenic acid, Pyridoxine, Biotin, Folate, Cobalamin)

• 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 lymph, stored in tissues (e.g., liver, adipose), and may be toxic in excess.

Vitamins in Our Food

Vitamins are present in almost all foods. The amount of a vitamin in food depends on:

• The amount naturally present

• What is added (fortification/enrichment)

• How the food is processed, prepared, and stored

Finding Vitamins in Foods

Whole Grains	Vegetables & Fruit	Protein Foods	Healthy Fats
Thiamin Riboflavin Niacin Pantothenic acid Vitamin B6 Folate	Riboflavin Niacin Vitamin B6 Folate Vitamin C Vitamin A Vitamin E Vitamin K	Thiamin Riboflavin Niacin Pantothenic acid Biotin Folate Vitamin B12 Vitamin A Vitamin D Vitamin K	Vitamin E

Fortified and Enriched Foods

• Fortification: Adding nutrients to foods not originally present (e.g., calcium to orange juice).

• Enrichment: Adding nutrients back to foods that lost them during processing (e.g., B vitamins to white rice).

Health Canada regulates both mandatory and voluntary fortification to prevent deficiencies but excessive intake may cause toxicity. Examples of mandatory fortification include table salt with iodine, milk with vitamin D, and grains with thiamin, riboflavin, niacin, iron, and folic acid.

Supplementation

About 40% of Canadian adults use supplements. Groups who may benefit include:

• Those on calorie-restricted diets

• Vegans

• Infants and children

• Young females and pregnant women

• Individuals with dark skin or who cover their bodies outdoors (vitamin D)

• Individuals taking medications

• Cigarette smokers and alcohol users

Absorption and Bioavailability

Vitamins in the Digestive Tract

Vitamins are released from food during digestion and absorbed primarily in the small intestine. Fat-soluble vitamins require dietary fat and bile for absorption, while water-soluble vitamins may require specific transport molecules.

Bioavailability of Vitamins

• Bioavailability is the extent to which the body can absorb and use a nutrient.

• Approximately 40–90% of vitamins are absorbed.

• Fat-soluble vitamins require fat for absorption; water-soluble vitamins may require transporters.

• Some vitamins are absorbed as inactive provitamins or precursors and must be converted to active forms by the body.

Factors affecting bioavailability include:

• Efficiency of digestion and GI transit time

• Previous nutrient intake and nutritional status

• Other foods consumed at the same time

• Method of food preparation

• Source of the nutrient (synthetic, fortified, or natural)

Provitamins/Precursors

• Provitamins are inactive forms of vitamins found in food, converted to active forms in the body.

• Example: Provitamin A (β-carotene) has about 1/6 the biological activity of retinol (vitamin A); the body uses an enzyme to convert β-carotene to retinol.

Water-Soluble Vitamins: Functions, Sources, and Deficiency

Thiamin (Vitamin B1)

• Function: Coenzyme for glucose breakdown, metabolism of sugars and amino acids, synthesis of ribose and deoxyribose, nerve function (neurotransmitter synthesis).

• Sources: Bran layer of whole grain, enriched grains, pork, legumes, seeds.

• RDA: Male: 1.2 mg/day; Female: 1.1 mg/day

• Deficiency: Beriberi (weakness, nerve degeneration, heart changes), Wernicke-Korsakoff syndrome (in alcoholics: mental confusion, psychosis, memory disturbances, coma).

• Excess: No reported effects; no UL set.

Riboflavin (Vitamin B2)

• Function: Forms two active coenzymes for ATP production, antioxidant properties, conversion of other vitamins to active forms, important in the citric acid cycle.

• Sources: Dairy, red meat, poultry, fish, whole/enriched grains, asparagus, broccoli, mushrooms, leafy greens.

• RDA: Male: 1.3 mg/day; Female: 1.1 mg/day

• Deficiency: Poor wound healing, cracking of lips/corners of mouth, sensitivity to light, eye irritation, skin flaking around nose, eyebrows, earlobes.

• Excess: Turns urine bright yellow; no other reported effects.

Niacin (Vitamin B3)

• Function: Coenzyme in glucose metabolism, synthesis of fatty acids and cholesterol, can be synthesized from tryptophan.

• Sources: Meats, fish, peanuts, whole/enriched grains, legumes, wheat bran.

• RDA: Male: 16 mg NE/day; Female: 14 mg NE/day

• Deficiency: Pellagra (fatigue, decreased appetite, indigestion, 4Ds: dermatitis, diarrhea, dementia, death).

• Excess: Supplements can cause flushing, rash, tingling, nausea, vomiting, diarrhea, high blood sugar, liver abnormalities, blurred vision.

Biotin

• Function: Coenzyme in the citric acid cycle, glucose synthesis.

• Sources: Liver, egg yolks, yogurt, nuts. Avoid raw egg whites (contain avidin, which binds biotin).

• AI: 30 micrograms/day

Pantothenic Acid

• Function: Part of coenzyme A, carrier protein for cholesterol and fatty acid synthesis.

• Sources: Meat, eggs, whole grains, legumes.

• AI: 5 mg/day (6 mg/day pregnancy, 7 mg/day lactation)

Vitamin B6 (Pyridoxine)

• Function: Amino acid and protein metabolism, synthesis of nonessential amino acids, neurotransmitters, hemoglobin, conversion of tryptophan to niacin, glucose and amino acid breakdown.

• Sources: Chicken, fish, pork, organ meats, whole grains, legumes, sunflower seeds, bananas, broccoli, spinach, some fortified cereals.

• RDA: 1.3 mg/day (19–50 years)

• Deficiency: Poor growth, skin lesions, decreased immunity, anemia, neurological symptoms (anxiety, nightmares).

• Excess: Supplements can cause severe nerve impairment.

Folate (Folic Acid, Vitamin B9)

• Function: DNA synthesis, amino acid metabolism, neural tube formation in early pregnancy, prevents homocysteine accumulation (heart disease risk).

• Sources: Enriched grains, leafy greens, asparagus, legumes, nuts, oranges, liver, yeast.

• RDA: 400 micrograms/day

• Deficiency: Neural tube defects (spina bifida, anencephaly), macrocytic/megaloblastic anemia, poor growth, nerve problems, diarrhea, tongue inflammation, increased risk of heart disease and some cancers.

• Excess: May mask B12 deficiency, possible increased breast cancer risk.

Folate Fortification: Has reduced neural tube defects in Canada but may be associated with increased colon cancer rates.

Vitamin B12 (Cobalamin)

• Function: ATP production from fatty acids, conversion of homocysteine to methionine, maintenance of myelin on nerves, amino acid and protein metabolism.

• Sources: Animal products, fortified foods.

• RDA: 2.4 micrograms/day (adults)

• Deficiency: Pernicious anemia, zero energy, increased homocysteine, decreased folate activation, numbness, tingling, gait abnormalities, memory loss, disorientation, paralysis, death.

• Excess: No reported effects up to 100 micrograms/day.

• Note: Vegan diets require supplementation; absorption can be impaired in GI disorders or atrophic gastritis.

Vitamin C (Ascorbic Acid)

• Function: Antioxidant, synthesis and maintenance of collagen, synthesis of neurotransmitters, hormones, bile acids, carnitine, enhances iron absorption, regenerates vitamin E.

• Sources: Citrus fruits, strawberries, kiwis, cantaloupe, cabbage-family and dark green vegetables, peppers, tomatoes, potatoes.

• RDA: Male: 90 mg/day; Female: 75 mg/day

• Deficiency: Scurvy (gum/tooth problems, joint pain, bleeding, poor wound healing, fatigue, depression).

• Excess: Diarrhea, nausea, abdominal cramps, possible increased kidney stone risk.

Choline

• Function: Synthesis of acetylcholine (neurotransmitter), cell membrane structure, lipid transport, homocysteine metabolism.

• Sources: Egg yolks, liver, meat, fish, wheat germ, nuts.

• AI: Male: 550 mg/day; Female: 425 mg/day

• Deficiency: In pregnancy, can interfere with fetal brain development; in adults, causes fatty liver and muscle damage.

• Excess: Fishy body odor, sweating, reduced growth rate, low blood pressure, liver damage (at high supplement levels).

Note: Choline is an essential nutrient but not currently classified as a vitamin.

Antioxidants and Free Radicals

What Are Antioxidants?

Antioxidants are substances that decrease the adverse effects of free radicals—highly unstable molecules with unpaired electrons that can damage DNA and other molecules. Antioxidants neutralize free radicals by donating electrons, preventing cellular damage associated with aging, heart disease, cancer, and neurodegenerative diseases.

Vitamins and Mood/Behaviour

Role of Vitamins in Mood Regulation

• Vitamins are involved in the synthesis of neurotransmitters (e.g., serotonin, dopamine) that regulate mood, sleep, appetite, and cognitive function.

• Deficiencies in certain vitamins (e.g., B vitamins, vitamin D) can contribute to mood disorders, irritability, and cognitive impairment.

Example: Serotonin synthesis requires vitamin B6, folate, and vitamin C. Deficiency can lead to anxiety, depression, poor sleep, and other mood disturbances.

Exam Preparation Tips

• Review vitamin lists daily, use color coding, and practice active recall.

• Study vitamins in reverse order for better retention.

• Utilize multimedia resources (e.g., YouTube) for memory aids.

• Attend study skills workshops for effective learning strategies.

Additional info: The notes also reference the importance of reviewing symptoms of vitamin deficiencies as they may manifest in various body systems (e.g., hands, gums, skin, eyes, mouth, nails, throat), which is useful for clinical application in nutrition assessment.