Principles of Dietetics · 12 min read
Vitamins, Minerals and Water
Micronutrient questions test four things: the biochemical role that explains the deficiency signs, the at-risk groups, the tolerable upper intake level (UL) and its critical effect, and interactions with other nutrients and drugs. Values below are adult (19–50 y) RDAs or AIs unless stated; requirements change with life stage.
Water-Soluble Vitamins
Water-soluble vitamins are generally not stored in large amounts (B12 is the exception, with liver stores lasting years), are lost in cooking water, and have low toxicity from food. Most B vitamins act as coenzymes in energy metabolism, so deficiencies often appear together in alcohol use disorder, malabsorption and poor diets.
| Vitamin | Coenzyme / Function | Deficiency | UL / Toxicity | Key Sources | Adult RDA / AI |
|---|---|---|---|---|---|
| Thiamin (B1) | TPP: pyruvate dehydrogenase, α-ketoglutarate dehydrogenase, transketolase (carbohydrate metabolism) | Beriberi (wet: high-output heart failure; dry: neuropathy); Wernicke encephalopathy (confusion, ataxia, ophthalmoplegia) → Korsakoff; risk in alcohol use, refeeding, bariatric surgery, hyperemesis | No UL | Pork, whole and enriched grains, legumes | 1.2 mg M / 1.1 mg F |
| Riboflavin (B2) | FAD, FMN (redox reactions) | Ariboflavinosis: cheilosis, angular stomatitis, glossitis, seborrheic dermatitis | No UL | Milk, eggs, meat, enriched grains; destroyed by light | 1.3 mg M / 1.1 mg F |
| Niacin (B3) | NAD, NADP; 60 mg tryptophan = 1 mg niacin equivalent (NE) | Pellagra: dermatitis, diarrhea, dementia, death; risk with corn-based diets (bound niacin), carcinoid syndrome, Hartnup disease, isoniazid | 35 mg (supplements/fortificants): flushing; hepatotoxicity at pharmacologic doses | Meat, poultry, fish, peanuts, enriched grains | 16 mg NE M / 14 mg NE F |
| Pantothenic acid (B5) | Coenzyme A (fatty acid synthesis and oxidation) | Rare; ‘burning feet’ syndrome | No UL | Widespread | AI 5 mg |
| Vitamin B6 | PLP: transamination, neurotransmitter and heme synthesis, glycogenolysis, tryptophan → niacin | Microcytic (sideroblastic) anemia, cheilosis, glossitis, depression, confusion, seizures; risk with isoniazid, alcohol | 100 mg: sensory neuropathy | Poultry, fish, potatoes, bananas, fortified cereals | 1.3 mg (≥51 y: 1.7 M / 1.5 F) |
| Biotin (B7) | Carboxylases (gluconeogenesis, fatty acid synthesis) | Dermatitis, alopecia, neurologic signs; raw egg white avidin binds biotin | No UL; high-dose supplements interfere with some lab immunoassays (e.g., troponin, thyroid) | Egg yolk, liver, nuts, legumes; gut bacteria | AI 30 mcg |
| Folate (B9) | THF: one-carbon transfer, DNA synthesis, homocysteine → methionine | Megaloblastic anemia, glossitis; neural tube defects; elevated homocysteine; risk with methotrexate, phenytoin, alcohol | 1,000 mcg folic acid (synthetic): may mask B12 deficiency | Leafy greens, legumes, liver, enriched grains | 400 mcg DFE (pregnancy 600) |
| Vitamin B12 (cobalamin) | Methionine synthase (with folate), methylmalonyl-CoA mutase; myelin | Megaloblastic anemia plus neurologic damage (paresthesia, subacute combined degeneration) that may be irreversible; ↑ methylmalonic acid and homocysteine | No UL | Animal foods only and fortified foods/supplements | 2.4 mcg; adults ≥51 should get most from fortified foods or supplements |
| Vitamin C | Collagen hydroxylation, carnitine and neurotransmitter synthesis, antioxidant, ↑ nonheme iron absorption | Scurvy: bleeding gums, perifollicular hemorrhages, corkscrew hairs, poor wound healing | 2,000 mg: GI distress, osmotic diarrhea; ↑ oxalate kidney stone risk | Citrus, berries, peppers, broccoli, tomatoes, potatoes | 90 mg M / 75 mg F; smokers +35 mg |
| Choline | Acetylcholine, phosphatidylcholine, methyl donor (betaine) | Fatty liver, muscle damage | 3,500 mg: fishy body odor, hypotension | Eggs, liver, meat, soybeans | AI 550 mg M / 425 mg F (pregnancy 450, lactation 550) |
Distinguishing Folate From B12 Deficiency
- Both cause macrocytic, megaloblastic anemia with hypersegmented neutrophils and elevated homocysteine.
- Only B12 deficiency raises methylmalonic acid (MMA) and causes neurologic damage.
- Large folic acid doses can correct the anemia of B12 deficiency while neurologic damage progresses; always assess B12 before treating macrocytosis with folate.
- B12 risk groups: older adults (atrophic gastritis), pernicious anemia (loss of intrinsic factor), gastrectomy/bariatric surgery, ileal disease or resection, vegans, long-term metformin or acid-suppressing drugs.
Fat-Soluble Vitamins
Vitamins A, D, E and K need bile and micelle formation for absorption, travel in chylomicrons, and are stored in liver and adipose tissue. Deficiency accompanies fat malabsorption (cystic fibrosis, cholestatic liver disease, pancreatic insufficiency, short bowel, orlistat, bariatric surgery); toxicity is mainly from supplements, especially vitamins A and D.
| Vitamin | Function | Deficiency | UL / Toxicity | Sources | Adult RDA / AI |
|---|---|---|---|---|---|
| A (retinol; provitamin A carotenoids) | Vision (rhodopsin), epithelial differentiation, immunity, reproduction, growth | Night blindness (first sign), xerophthalmia, Bitot spots, keratomalacia, follicular hyperkeratosis, ↑ infection | 3,000 mcg preformed retinol: liver toxicity, teratogenic (birth defects), bone loss; carotenoids cause harmless carotenodermia | Liver, fortified milk, egg yolk; carotenoids in orange and dark green produce | 900 mcg RAE M / 700 mcg RAE F |
| D (D₂, D₃ → 25(OH)D → calcitriol) | Calcium and phosphorus absorption, bone mineralization, PTH regulation, immune and muscle function | Rickets (children), osteomalacia (adults), secondary hyperparathyroidism; risk: dark skin, limited sun, obesity, older age, breastfed infants, fat malabsorption, CKD | 4,000 IU (100 mcg): hypercalcemia, soft-tissue calcification, kidney stones | Fatty fish, fortified milk and plant milks, egg yolk, UV-exposed mushrooms; skin synthesis from UVB | 600 IU (15 mcg); >70 y 800 IU (20 mcg) |
| E (α-tocopherol) | Lipid-soluble antioxidant protecting PUFA in membranes | Hemolytic anemia (premature infants), peripheral neuropathy, ataxia (fat malabsorption) | 1,000 mg supplemental: bleeding risk, antagonizes vitamin K | Vegetable oils, nuts, seeds, wheat germ | 15 mg |
| K (K₁ phylloquinone; K₂ menaquinones) | γ-Carboxylation of clotting factors II, VII, IX, X, proteins C and S, osteocalcin | Bleeding, ↑ prothrombin time/INR; newborn vitamin K deficiency bleeding (prevented by IM vitamin K at birth) | No UL; consistent intake matters with warfarin | Leafy greens, broccoli, soybean and canola oils; K₂ from fermented foods and gut bacteria | AI 120 mcg M / 90 mcg F |
Units and Conversions
- Vitamin A: 1 mcg RAE = 1 mcg retinol = 12 mcg dietary β-carotene = 24 mcg other dietary provitamin A carotenoids = 2 mcg supplemental β-carotene.
- Vitamin D: 1 mcg = 40 IU (so 15 mcg = 600 IU). Serum 25-hydroxyvitamin D is the best status marker; calcitriol is not.
- Folate: 1 mcg DFE = 1 mcg food folate = 0.6 mcg folic acid taken with food = 0.5 mcg folic acid on an empty stomach. Worked example: 200 mcg food folate + 200 mcg folic acid in fortified cereal = 200 + (200 × 1.7) = 540 mcg DFE.
- Niacin: 1 mg NE = 1 mg niacin = 60 mg tryptophan.
Major Minerals
| Mineral | Functions | Deficiency / Imbalance | UL / Toxicity | Sources | Adult RDA / AI |
|---|---|---|---|---|---|
| Calcium | Bone and teeth (99%), muscle contraction, nerve transmission, clotting | Osteopenia, osteoporosis; hypocalcemia → tetany (usually from PTH, vitamin D or Mg problems rather than diet) | 2,500 mg (19–50); 2,000 mg (≥51): hypercalcemia, kidney stones, constipation, ↓ iron and zinc absorption | Dairy, fortified plant milks and juice, calcium-set tofu, canned fish with bones, low-oxalate greens (kale, bok choy) | 1,000 mg; 1,200 mg women ≥51 and men ≥71 |
| Phosphorus | Bone, ATP, DNA/RNA, phospholipids, acid–base buffer | Hypophosphatemia in refeeding syndrome, alcohol use, antacid overuse: weakness, respiratory failure, hemolysis | 4,000 mg (3,000 mg >70) | Protein foods, dairy, whole grains; phosphate additives (highly absorbed) | 700 mg |
| Magnesium | Cofactor for >300 enzymes, ATP, nerve and muscle function, PTH secretion | Tremor, tetany, arrhythmia; refractory hypokalemia and hypocalcemia; risk: diuretics, PPIs, alcohol, diarrhea, refeeding | 350 mg from supplements/drugs only: diarrhea | Nuts, seeds, legumes, whole grains, leafy greens | 400–420 mg M / 310–320 mg F |
| Sodium | Extracellular fluid volume, membrane potential, nutrient transport | Hyponatremia usually reflects water excess, not dietary deficit | No UL; CDRR: reduce intake if above 2,300 mg | Processed and restaurant foods (most intake), salt (1 tsp ≈ 2,300 mg) | AI 1,500 mg (all adults) |
| Potassium | Intracellular cation, nerve and muscle (cardiac) excitability, blunts BP effect of sodium | Hypokalemia: weakness, arrhythmias (diuretics, vomiting, refeeding) | No UL for food; risk of hyperkalemia with CKD, ACE inhibitors, ARBs, K-sparing diuretics | Fruits, vegetables, legumes, potatoes, dairy | AI 3,400 mg M / 2,600 mg F |
| Chloride | Fluid balance, gastric HCl | Hypochloremic metabolic alkalosis from vomiting | 3,600 mg | Salt | AI 2,300 mg |
| Sulfur | Part of methionine, cysteine, thiamin, biotin | Not seen with adequate protein | — | Protein foods | No DRI |
Trace Minerals
| Mineral | Functions | Deficiency | UL / Toxicity | Sources | Adult RDA / AI |
|---|---|---|---|---|---|
| Iron | Hemoglobin, myoglobin, cytochromes | Microcytic hypochromic anemia; fatigue, pica (pagophagia), koilonychia, impaired cognition; ferritin falls first | 45 mg: GI distress; overload in hemochromatosis; leading poisoning cause in young children | Heme: meat, poultry, fish (15–35% absorbed). Nonheme: legumes, fortified grains, greens (2–20%) | 8 mg M; 18 mg F 19–50; 27 mg pregnancy; vegetarians ×1.8 |
| Zinc | Cofactor for >100 enzymes; immunity, wound healing, growth, taste, DNA synthesis | Growth failure, hypogeusia, alopecia, dermatitis, poor wound healing, delayed sexual maturation; acrodermatitis enteropathica | 40 mg: copper deficiency, ↓ HDL, immune suppression | Oysters, red meat, poultry, legumes, whole grains (phytate reduces absorption) | 11 mg M / 8 mg F |
| Copper | Ceruloplasmin (iron oxidation for transport), cytochrome c oxidase, collagen cross-linking, SOD | Anemia, neutropenia, myeloneuropathy; seen after bariatric surgery or excess zinc | 10,000 mcg: liver damage; Wilson disease = copper accumulation | Organ meats, shellfish, nuts, seeds, cocoa | 900 mcg |
| Iodine | Thyroid hormones T₃, T₄ | Goiter, hypothyroidism; in pregnancy → cretinism (impaired neurodevelopment) | 1,100 mcg: thyroid dysfunction | Iodized salt, seafood, seaweed, dairy, eggs | 150 mcg; pregnancy 220; lactation 290 |
| Selenium | Glutathione peroxidase, thyroid deiodinases | Keshan disease (cardiomyopathy), Kashin-Beck disease | 400 mcg: selenosis (hair and nail loss, garlic breath) | Brazil nuts, seafood, meats, grains (soil-dependent) | 55 mcg |
| Fluoride | Strengthens tooth enamel (fluorapatite) | Dental caries | 10 mg: fluorosis (enamel mottling, skeletal) | Fluoridated water, tea, marine fish | AI 4 mg M / 3 mg F |
| Chromium | Potentiates insulin action | Impaired glucose tolerance (long-term PN) | No UL | Meat, whole grains, broccoli | AI 35 mcg M / 25 mcg F |
| Manganese | Enzyme cofactor (bone, metabolism, SOD) | Rare | 11 mg: neurotoxicity; accumulates in cholestatic liver disease | Whole grains, nuts, legumes, tea | AI 2.3 mg M / 1.8 mg F |
| Molybdenum | Xanthine oxidase, sulfite oxidase | Rare (long-term PN) | 2,000 mcg | Legumes, grains, nuts | 45 mcg |
Water and Electrolytes
Water is about 50–60% of adult body weight (higher in infants, lower with age and adiposity); two-thirds is intracellular. The AI for total water (beverages plus food) is 3.7 L/day for men and 2.7 L/day for women; roughly 80% comes from beverages and 20% from food. Metabolic water from oxidation adds about 250–350 mL/day. Clinical estimates for adults are 30–35 mL/kg or 1 mL/kcal; children use Holliday-Segar.
| Route | Approximate Adult Losses (mL/day) | Increases With |
|---|---|---|
| Urine | ≈1,000–2,000 (obligatory minimum ≈500) | High protein or sodium intake (solute load), diuretics, hyperglycemia |
| Skin (insensible and sweat) | ≈450–1,900 | Fever, heat, exercise, burns |
| Lungs | ≈250–350 | Fever, tachypnea, dry air, mechanical ventilation without humidification |
| Feces | ≈100–200 | Diarrhea, fistulas, ostomy output |
| Electrolyte | Typical Serum Range | Low Values: Common Causes | High Values: Common Causes |
|---|---|---|---|
| Sodium | 135–145 mEq/L | Excess free water (SIADH, hypotonic fluids, heart failure, cirrhosis) | Water deficit: dehydration, inadequate free water with tube feeding, diabetes insipidus |
| Potassium | 3.5–5.0 mEq/L | Diuretics, vomiting, diarrhea, refeeding, insulin | CKD, K-sparing drugs, acidosis, tissue breakdown, hemolyzed sample |
| Chloride | 98–106 mEq/L | Vomiting, gastric suction | Dehydration, excess saline, diarrhea (non-gap acidosis) |
| Magnesium | 1.7–2.2 mg/dL | Alcohol, diarrhea, PPIs, diuretics, refeeding | CKD with Mg-containing antacids or laxatives |
| Phosphorus | 2.5–4.5 mg/dL | Refeeding, alcohol use, binders | CKD |
| Calcium (total) | 8.5–10.5 mg/dL | Low albumin (correct: add 0.8 mg/dL per 1 g/dL albumin below 4), vitamin D or Mg deficiency | Hyperparathyroidism, malignancy, vitamin D toxicity |
Nutrient Interactions
Nutrients compete for transporters, act as cofactors for one another, or are bound by food components. Interactions explain many exam scenarios, from iron supplements given with tea to copper deficiency caused by zinc supplements.
| Interaction | Effect | Practical Application |
|---|---|---|
| Vitamin C + nonheme iron | Reduces Fe³⁺ to Fe²⁺ → ↑ absorption | Pair plant iron sources with a vitamin C food |
| ‘MFP factor’ + nonheme iron | Meat, fish and poultry enhance nonheme iron absorption | Small amounts of meat improve iron from a mixed meal |
| Calcium, tea/coffee polyphenols, phytate, oxalate → iron | ↓ Nonheme iron absorption | Separate iron supplements from calcium, tea and coffee |
| Zinc ↔ copper | High zinc induces intestinal metallothionein, trapping copper | Long-term zinc >40 mg/day can cause copper deficiency anemia |
| Folate ↔ B12 | Folic acid corrects megaloblastic anemia but not neurologic damage of B12 deficiency | Rule out B12 deficiency before high-dose folate |
| Vitamin D → calcium, phosphorus | Calcitriol ↑ intestinal absorption | Pair calcium repletion with vitamin D adequacy |
| Magnesium → PTH, potassium | Low Mg impairs PTH release and causes renal K wasting | Correct Mg first in refractory hypocalcemia or hypokalemia |
| Copper → iron | Ceruloplasmin oxidizes iron for transferrin binding | Copper deficiency mimics iron deficiency anemia |
| Vitamin E (high dose) ↔ vitamin K | Antagonizes vitamin K–dependent clotting | Bleeding risk with anticoagulants |
| Zinc → vitamin A | Zinc needed for retinol-binding protein synthesis | Vitamin A status may not improve until zinc is repleted |
| Riboflavin, B6 → niacin | Both are needed to convert tryptophan to niacin | Multiple B deficiencies coexist |
| Oxalate, phytate → calcium | Bind calcium; spinach calcium is poorly absorbed | Count low-oxalate greens as better calcium sources |
| Drug | Nutrient Effect |
|---|---|
| Warfarin | Keep vitamin K intake consistent; avoid starting or stopping high-dose vitamin E supplements without monitoring |
| Metformin; PPIs and H₂ blockers | ↓ Vitamin B12 absorption; PPIs also ↓ Mg, Ca, iron |
| Isoniazid | Vitamin B6 antagonist (give B6) |
| Methotrexate; phenytoin | Folate antagonism; phenytoin also ↓ vitamin D; separate phenytoin from enteral feeding |
| Loop and thiazide diuretics | ↑ Loss of K and Mg |
| MAO inhibitors | Hypertensive crisis with high-tyramine foods (aged cheese, cured meats, tap beer) |
Exam Focus
- Classic triads: pellagra (3 Ds + death), Wernicke (confusion, ataxia, ophthalmoplegia), scurvy (bleeding gums, perifollicular hemorrhage, poor healing).
- MMA distinguishes B12 from folate deficiency; ferritin is the earliest marker of iron depletion; 25(OH)D reflects vitamin D status.
- ULs to recall: vitamin A 3,000 mcg preformed, vitamin D 4,000 IU, niacin 35 mg, B6 100 mg, folic acid 1,000 mcg, vitamin C 2,000 mg, calcium 2,500/2,000 mg, iron 45 mg, zinc 40 mg.
- Sodium: AI 1,500 mg and CDRR 2,300 mg. Potassium AI 3,400/2,600 mg.
- Correct magnesium before expecting potassium or calcium to normalize; high-dose zinc causes copper deficiency.
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