Vitamins are compounds that catalyze or participate in the synthesis of body molecules or metabolic reactions.   Some vitamins are required by all animals.   They are organic molecules normally located in the body (Vitamin D is the exception) and obtained naturally from sources outside the body or produced in it in inadequate amounts that are essential for normal life and growth.   Vitamin molecules come in the dextro- (right-handed) and levo- (left-handed) isomers.   The body can only use the levo- form.   In addition to natural vitamins, synthesized vitamins are manufactured in large quantities and are absolutely the same as natural vitamins.   They are extremely cheap (see unit costs) because they obviate the need to extract vitamins from plant and animal sources.   Other organic molecules are essential to human life and growth, e.g., choline and inositol, but are not considered vitamins because they are produced in sufficient amounts without deficiencies.   Other so-called "vitamins" popular with fadists, e.g., laetrile and pangamic acid, are not recognized by scientists as having any known benefit to humans.   Vitamins contain no energy.   They are some of the materials usually, but not necessarily, included in what most people call food.   Fat-soluble vitamins (A, D, E, and K) are absorbed through the intestines into the body with the aid of fats and liver bile.   Therefore, they can be taken irregularly, say, every few days, and not daily.     These vitamins can accumulate to toxic levels, so excessive intake can be dangerous.   Water-soluble vitamins (8 B and C) are not absorbed by the body to any significant degree.   They leave the body soon in urine and sweat, so they must be taken daily.   These vitamins are fragile and are destroyed or washed away by cooking, storage or processing.   The vitamin content of food varies considerably with conditions under which the food was grown and processed. ^{Brody 157}   The 13 vitamins listed below are required by humans:

Vitamin A comes in two forms: preformed vitamin A or retinol is found in foods of animal origin.   provitamin A or carotene is found in foods of both animal and plant origin.   Retinol is absorbed quickly, but carotene is converted in the body into a retinol-like vitamin A before it can be absorbed. Sources of retinol: liver, fish-liver oil.   Sources of carotene: yellow and dark green fruits and vegetables, cheese, butter, fortified margarine.   Vitamin A aids healthy hair, skin, and mucuous membranes; ability to see in dim light, promotes bone growth, tooth development, reproduction; assists in treating eye disorders, resists acne, respiratory infections, promotes bone growth.   A deficiency results in night blindness, rough skin and mucuous membranes, infection of mucuous membrances, drying of eyes, slow bone and tooth enamel growth. ^{Brody 159, Netzer 695}   Since rice is deficient in carotene, eating rice as a staple results in vitamin A deficiency.   Researchers have developed a vitamin A rice ("golden rice") that is expected to reduce this deficiency in rice eaters.  

Vitamin B₁ helps release energy from carbohydrates, maintains muscles, nerves, and heart.   Sources: pork, especially ham, liver, oysters, whole-grain and enriched cereals, pasta, bread, wheat germ, oatmeal, peas, lima beans.   A deficiency results in beriberi disease: mental confusion, muscle weakness, heart swelling, leg cramps. ^{Brody 161, Netzer 696}

Vitamin B₂ helps release energy from carbohydrates, fats and proteins, helps maintain mucouis membranes, good vision, skin, hair and nails.   Sources: Liver and other organ meats, poultry, milk, eggs, brewer's yeast, whole grains, enriched bread, cereal and pasta, almonds, dried beans and peas, dark green vegetables, mushrooms.   A deficiency results in skin disorders, especially around the nose and lips, cracks at corners of mouth, sensitivity of eyes to light. ^{Brody 161, Netzer 697}

Vitamin B₃ assists B₁ (thiamin) and B₂ (riboflavin) produce cell energy, promotes a healthy nervous and digestive system, maintains healthy skin and hair, aids blood circulation, assists in the breakdown of carbohydrates, fats, and proteins.   Sources: liver, poultry, meat, tuna, eggs, whole-grain and enriched cereals, pasta, bread, nuts, dried peas and beans.   A deficiency results in pellagra: skin disorders, especially on body parts exposed to sun, diarrhea, mental confusion, irritability, mouth swelling, smooth tongue. ^{Brody 161, Netzer 697}

Vitamin B₅ aids in the metabolism of carbohydrates, fats, and proteins, aids the formation of hormones, cell building, healing, and nerve-regulating substances.   Sources: all plant and animal foods, especially liver, kidneys, whole-grain cereal and bread, nuts, eggs, dark-green vegetables.   An experimental deficiency results in abdominal cramps, vomiting, fatigue, sleeplessness, tingling hands and feet. ^{Brody 163, Netzer 697}

Vitamin B₆ aids in the absorption and metabolism of proteins and fats, aids in the formation of red blood cells, regulation of body fluids and nervous system, alleviates nausea.   Sources: whole-grain, non-enriched cereals and bread, liver, avocados, spinach, green beans, bananas, fish, poultry, meat, nuts, potatoes, green leafy vegetables, cantaloupe.   A deficiency results in skin disorders, cracks at corners of mouth, smooth tongue, convulsions, dizziness, nausea, anemia, kidney stones. ^{Brody 162, Netzer 698}

Biotin aids in the formation of fatty acids, helps to release carbohydrate energy.   Sources: egg yolk, liver, kidneys, dark-green vegetables, green beans.   A deficiency results in experimental loss of appetite, nausea, vomiting, pallor, depression, fatigue, muscle pain.^{Brody 163}

Folacin acts with B₁₂ to synthesize genetic material, aids in the formation of hemoglobin and all cells, helps maintain a healthy nervous system and mental health.   Sources: liver, kidneys, dark-green leafy vegetables, wheat germ, dried peas and beans, brewer's yeast, broccoli, carrots, asparagus.   A deficiency results in megaloblastic anemia: enlarged red blood cells, smooth tongue, diarrhea. ^{Brody 163, Netzer 698}

Viamin B₁₂ aids in the formation of red blood cells and in the building of genetic material, helps functioning of nervous system, alleviates irritability and increases energy.   Sources: animal foods, such as liver, kidneys, meat, fish, eggs, milk, oysters, nutritional yeast.   A deficiency results in pernicious anemia: anemia, pale skin and mucous membranes, numbness and tingling in fingers and toes, loss of balance and weakness and pain in the arms and legs. ^{Brody 162, Netzer 698}

Vitamin C aids in the formation of collagen and red blood cells, helps maintain capillaries, bones, teeth and gums, helps protect other vitamins from oxidation, helps the absorption of iron, promotes healing.   Sources: citrus fruits, tomatoes, strawberries, melon, green peppers, potatoes, dark-green vegetables, broccoli, Brussels sprouts, cabbage, fresh fruits and vegetables.   A deficiency results in scurvy: bleeding gums, degenerating muscles, wound that won't heal, loose teeth, brown, dry, rough skin, weight and appetite loss, irritability. ^{Brody 164, Netzer 699}

Vitamin D aids in the formation and maintenance of bones and teeth, assists in the absorption and use of calcium and phosphorus, helps in maintaining the nervous system, heart action and blood clotting.   Sources: exposure to sunlight, fortified milk, egg yolk, liver, tuna, salmon, cod liver oil.   A deficiency results in rickets: stunted bone growth, bowed legs, protruding abdomen, osteomalacia (softening of bones). ^{Brody 159, Netzer 699}   Milk was fortified with vitamin D in the 1930s to assure that children obtained enough of it to sustain normal growth.   Vitamin D fortified foods are important to people who are exposed to sufficient sunlight, e.g., institutionalized persons.

Vitamin E aids in the formation of red blood cells, muscles and other tissues, protects vitamin A and essential fatty acids from oxidation, supplies oxygen to the body.   Sources: vegetable oils, margarine, wheat germ, whole-grain cereals and bread, liver, dried beans, green leafy vegetables.   Deficiencies are rare in people. ^{Brody 160, Netzer 700}

Vitamin K aids in the synthesis of substances needs for blood clotting, helps to maintain bone metabolism, prevents internal bleeding, aids liver functioning.   Sources: dark green leafy vegetables, cabbage, cauliflower, peas, potatoes, liver, cereals, kelp, Brussels sprouts, fish liver oils.   A deficiency results in hemorrhage, especially in newborn infants. ^{Brody 160, Netzer 700}   Vitamin K is produced by bacteria in the intestines and is extremely rare, unless the intestines are damaged.