Chapter 8. Introduction to Micronutrients
Introduction to Vitamins
Vitamins are organic (carbon-containing) compounds. The body can synthesize some vitamins, but others must be obtained from the diet and there are 13 essential to health. Vitamins are structurally different from macronutrients as they are individual units that do not require any digestion to function. We also need to consume vitamins and any micronutrients in smaller amounts compared to macronutrients (e.g., µg and mg vs. g)
Vitamins are traditionally assigned to two groups: fat-soluble (hydrophobic) and water-soluble (hydrophilic). This classification determines where they act in the body. Water-soluble vitamins act in the cytosol of cells or extracellular fluids such as blood; fat-soluble vitamins are largely responsible for protecting cell membranes from free radical damage.
One major difference between fat-soluble and water-soluble vitamins is how they are absorbed in the body. Vitamins are absorbed primarily in the small intestine and their bioavailability is dependent on one’s diet. Fat-soluble vitamins are absorbed along with dietary fat. Therefore, if a meal is very low in fat, the absorption of fat-soluble vitamins will be impaired. Once fat-soluble vitamins have been absorbed in the small intestine, they are packaged and incorporated into chylomicrons along with other fatty acids and transported in the lymphatic system to the liver. Water-soluble vitamins, on the other hand, are absorbed in the small intestine but are transported to the liver through blood vessels. (Figure 8.2 “Absorption of fat-soluble and water-soluble vitamins”).
Fat-soluble vitamins include vitamins A, D, E, and K. These vitamins are found in the fatty parts of food and as previously mentioned, they are absorbed with dietary fat. Fat-soluble vitamins are mainly stored in our adipose tissue, which means that we don’t need to consume them daily, but if we consume high amounts, they can be toxic and even result in organ damage or death.
Water-soluble vitamins include vitamins C and B. They are found in a variety of foods including fruits, vegetables, dairy, meats, and whole grains. We do not typically store water-soluble vitamins (except B12), so they must be consumed regularly. Typically, if you consume excess water-soluble vitamins, they are excreted in the urine, which means that toxicity is rare. However, toxicity can occur with high doses of certain vitamins, which is why these have a UL.
Summary of Water-Soluble Vitamins
| Vitamin | Sources | Recommended Intake for adults | Major Functions | Deficiency diseases and symptoms | Groups at risk of deficiency | Toxicity | UL |
| Vitamin C (ascorbic acid) | Orange juice, grapefruit juice, strawberries, tomato, sweet red pepper | 75-90 mg/day | Antioxidant, collagen synthesis, hormone and neurotransmitter synthesis | Scurvy, bleeding gums, joint pain, poor wound healing, | Smokers, alcoholics, elderly | Kidney stones, GI distress, diarrhea | 2000 mg/day |
| Thiamin (B1) | Pork, enriched and whole grains, fish, legumes | 1.1-1.2 mg/day | Coenzyme: assists in glucose metabolism, RNA, DNA, and ATP synthesis | Beriberi: fatigue, confusion, movement impairment, swelling, heart failure | Alcoholics, older adults, eating disorders | None reported | ND |
| Riboflavin (B2) | Beef liver, enriched breakfast cereals, yogurt, steak, mushrooms, almonds, eggs | 1.1-1.3 mg/day | Coenzyme: assists in glucose, fat and carbohydrate metabolism, electron carrier, other B vitamins are dependent on | Ariboflavinosis: dry scaly skin, mouth inflammation and sores, sore throat, itchy eyes, light sensitivity | None | None reported | ND |
| Niacin (B3) | Meat, poultry, fish, peanuts, enriched grains | 14-16 NE/day | Coenzyme: assists in glucose, fat, and protein metabolism, electron carrier | Pellagra: diarrhea, dermatitis, dementia, death | Alcoholics | Nausea, rash, tingling extremities | 35 mg/day from fortified foods and supplements |
| Pantothenic Acid (B5) | Sunflower seeds, fish, dairy products, widespread in foods | 5 mg/day | Coenzyme: assists in glucose, fat, and protein metabolism, cholesterol and neurotransmitter synthesis | Muscle numbness and pain, fatigue, irritability | Alcoholics | Fatigue, rash | ND |
| B6(Pyridoxine) | Meat, poultry, fish, legumes, nuts | 1.3-1.7 mg/day | Coenzyme; assists in amino-acid synthesis, glycogneolysis, neurotransmitter and hemoglobin synthesis | Muscle weakness, dermatitis, mouth sores, fatigue, confusion | Alcoholics | Nerve damage | 100 mg/day |
| Biotin | Egg yolks, fish, pork, nuts and seeds | 30 mcg/day | Coenzyme; assists in glucose, fat, and protein metabolism, amino-acid synthesis | Muscle weakness, dermatitis, fatigue, hair loss | Those consuming raw egg whites | None reported | ND |
| Folate | Leafy green vegetables, enriched grains, orange juice | 400 mcg/day | Coenzyme; amino acid synthesis, RNA, DNA, and red blood cell synthesis | Diarrhea, mouth sores, confusion, anemia, neural-tube defects | Pregnant women, alcoholics | Masks B12 deficiency | 1000 mcg/day from fortified foods and supplements |
| B12(cobalamin) | Meats, poultry, fish | 2.4 mcg/day | Coenzyme; fat and protein catabolism, folate function, red-blood-cell synthesis | Muscle weakness, sore tongue, anemia, nerve damage, neural tube defects | Vegans, elderly | None reported | ND |
| Choline | Egg yolk, wheat, meat, fish, synthesis in the body | 425-550 mg/day | Synthesis of neurotransmitters and cell membranes, lipid transport | Non-alcoholic fatty liver disease, muscle damage, interfered brain development in fetus | None | Liver damage, excessive sweating, hypotension | 3500 mg/day |
Summary of Fat-Soluble Vitamins
| Vitamin | Sources | Recommended Intake for adults | Major functions | Deficiency diseases and symptoms | Groups at risk of deficiency | Toxicity | UL |
| Vitamin A (retinol, retinal, retinoic acid,carotene, beta-carotene) | Retinol: beef and chicken liver, skim milk, whole milk, cheddar cheese; Carotenoids: pumpkin, carrots, squash, collards, peas | 700-900 mcg/day | Antioxidant, vision, cell differentiation, reproduction, immune function | Xerophthalmia, night blindness, eye infections; poor growth, dry skin, impaired immune function | People living in poverty (especially infants and children), premature infants, pregnant and lactating women, people who consume low-fat or low-protein diets | Hypervitaminosis A: Dry, itchy skin, hair loss, liver damage, joint pain, fractures, birth defects, swelling of the brain | 3000 mcg/day |
| Vitamin D | Swordfish, salmon, tuna, orange juice (fortified), milk (fortified), sardines, egg, synthesis from sunlight | 600-800 IU/day (15-20 mcg/day) | Absorption and regulation of calcium and phosphorus, maintenance of bone | Rickets in children: abnormal growth, misshapen bones, bowed legs, soft bones; osteomalacia in adults | Breastfed infants, older adults people with limited sun exposure, people with dark skin | Calcium deposits in soft tissues, damage to the heart, blood vessels, and kidneys | 4000 IU/day (100 mcg/day) |
| Vitamin E | Sunflower seeds, almonds, hazelnuts,peanuts | 15 mg/day | Antioxidant, protects cell membranes | Broken red blood cells, nerve damage | People with poor fat absorption, premature infants | Inhibition of vitamin K clotting factors | 1000 mcg/day from supplemental sources |
| Vitamin K | Vegetable oils, leafy greens, synthesis by intestinal bacteria | 90-120 mcg/day | Synthesis of blood clotting proteins and proteins needed for bone health and cell growth | Hemorrhage | Newborns, people on long-term antibiotics | Anemia, brain damage | ND |
