B can be supplemented by pill or injection and appear to be equally effective in those with low levels due to absorption problems.

When large doses are given by mouth its absorption does not rely on the presence of intrinsic factor or an intact ileum. Generally 1 to 2 mg daily is required as a large dose. Even pernicious anemia can be treated entirely by the oral route. These supplements carry such large doses of the vitamin that 1% to 5% of high oral doses of free crystalline B is absorbed along the entire intestine by passive diffusion.

Very high doses of B over many years has been linked to an increase in lung cancer risk in male smokers.

The National Institutes of Health has found that "Large amounts of folic acid can mask the damaging effects of vitamin B deficiency by correcting the megaloblastic anemia caused by vitamin B deficiency without correcting the neurological damage that also occurs", there are also indications that "high serum folate levels might not only mask vitamin B deficiency, but could also exacerbate the anemia and worsen the cognitive symptoms associated with vitamin B deficiency". Due to the fact that in the United States legislation has required enriched flour to contain folic acid to reduce cases of fetal neural-tube defects, consumers may be ingesting more than they realize. To counter the masking effect of B deficiency the NIH recommends "folic acid intake from fortified food and supplements should not exceed 1,000 μg daily in healthy adults." Most importantly, B deficiency needs to be treated with B repletion. Limiting folic acid will not counter the irrevocable neurological damage that is caused by untreated B deficiency.

Folate is found in leafy green vegetables. Multi-vitamins also tend to include Folate as well as many other B vitamins. B vitamins, such as Folate, are water-soluble and excess is excreted in the urine.

When cooking, use of steaming, a food steamer, or a microwave oven can help keep more folate content in the cooked foods, thus helping to prevent folate deficiency.

Folate deficiency during human pregnancy has been associated with an increased risk of infant neural tube defects. Such deficiency during the first four weeks of gestation can result in structural and developmental problems. NIH guidelines recommend oral B vitamin supplements to decrease these risks near the time of conception and during the first month of pregnancy.

Medications can interfere with folate utilization, including:

- anticonvulsant medications (such as phenytoin, primidone, carbamazepine or valproate )

- metformin (sometimes prescribed to control blood sugar in type 2 diabetes)

- methotrexate, an anti-cancer drug also used to control inflammation associated with Crohn's disease, ulcerative colitis and rheumatoid arthritis.

- sulfasalazine (used to control inflammation associated with Crohn's disease, ulcerative colitis and rheumatoid arthritis)

- triamterene (a diuretic)

- birth control pills

When methotrexate is prescribed, folic acid supplements are sometimes given with the methotrexate. The therapeutic effects of methotrexate are due to its inhibition of dihydrofolate reductase and thereby reduce the rate "de novo" purine and pyrimidine synthesis and cell division. Methotrexate inhibits cell division and is particularly toxic to fast dividing cells, such as rapidly dividing cancer cells and the progenitor cells of the immune system. Folate supplementation is beneficial in patients being treated with long-term, low-dose methotrexate for inflammatory conditions, such as rheumatoid arthritis (RA) or psoriasis, to avoid macrocytic anemia caused by folate deficiency. Folate is often also supplemented before some high dose chemotherapy treatments in an effort to protect healthy tissue. However, it may be counterproductive to take a folic acid supplement with methotrexate in cancer treatment.

Adverse effects have been documented from vitamin B supplements, but never from food sources. Damage to the dorsal root ganglia is documented in human cases of overdose of pyridoxine. Although it is a water-soluble vitamin and is excreted in the urine, doses of pyridoxine in excess of the dietary upper limit (UL) over long periods cause painful and ultimately irreversible neurological problems. The primary symptoms are pain and numbness of the extremities. In severe cases, motor neuropathy may occur with "slowing of motor conduction velocities, prolonged F wave latencies, and prolonged sensory latencies in both lower extremities", causing difficulty in walking. Sensory neuropathy typically develops at doses of pyridoxine in excess of 1,000 mg per day, but adverse effects can occur with much less, so doses over 200 mg are not considered safe. Symptoms among women taking lower doses have been reported.

Existing authorizations and valuations vary considerably worldwide. As noted, the U.S. Institute of Medicine set an adult UL at 100 mg/day. The European Community Scientific Committee on Food defined intakes of 50 mg of vitamin B per day as harmful and established a UL of 25 mg/day. The nutrient reference values in Australia and New Zealand recommend an upper limit of 50 mg/day in adults. "The same figure was set for pregnancy and lactation as there is no evidence of teratogenicity at this level. The UL was set based on metabolic body size and growth considerations for all other ages and life stages except infancy. It was not possible to set a UL for infants, so intake is recommended in the form of food, milk or formula." The ULs were set using results of studies involving long-term oral administration of pyridoxine at doses of less than 1 g/day. "A no-observed-adverse-effect level (NOAEL) of 200 mg/day was identified from the studies of Bernstein & Lobitz (1988) and Del Tredici "et al" (1985). These studies involved subjects who had generally been on the supplements for five to six months or less. The study of Dalton and Dalton (1987), however, suggested the symptoms might take substantially longer than this to appear. In this latter retrospective survey, subjects who reported symptoms had been on supplements for 2.9 years, on average. Those reporting no symptoms had taken supplements for 1.9 years."

A deficiency of vitamin B alone is relatively uncommon and often occurs in association with other vitamins of the B complex. The elderly and alcoholics have an increased risk of vitamin B deficiency, as well as other micronutrient deficiencies. Evidence exists for decreased levels of vitamin B in women with type 1 diabetes and in patients with systemic inflammation, liver disease, rheumatoid arthritis, and those infected with HIV. Use of oral contraceptives and treatment with certain anticonvulsants, isoniazid, cycloserine, penicillamine, and hydrocortisone negatively impact vitamin B status. Hemodialysis reduces vitamin B plasma levels.

Treatment involves a diet which includes an adequate amount of riboflavin containing foods. Multi-vitamin and mineral dietary supplements often contain 100% of the Daily Value (1.3 mg) for riboflavin, and can be used by persons concerned about an inadequate diet. Over-the-counter dietary supplements are available in the United States with doses as high as 100 mg, but there is no evidence that these high doses have any additional benefit for healthy people.

As a chemical compound, riboflavin is a yellow-orange solid substance with poor solubility in water compared to other B vitamins. Visually, it imparts color to vitamin supplements (and bright yellow color to the urine of persons taking a lot of it).

There is some evidence that zinc may have an effect on cancer and further study is recommended.

Five interventional strategies can be used:

- Adding zinc to soil, called agronomic biofortification, which both increases crop yields and provides more dietary zinc.

- Adding zinc to food, called fortification.

- Adding zinc rich foods to diet. The foods with the highest concentration of zinc are proteins, especially animal meats, the highest being oysters. Per ounce, beef, pork, and lamb contain more zinc than fish. The dark meat of a chicken has more zinc than the light meat. Other good sources of zinc are nuts, whole grains, legumes, and yeast. Although whole grains and cereals are high in zinc, they also contain chelating phytates which bind zinc and reduce its bioavailability.

- Oral repletion via tablets (e.g. zinc gluconate) or liquid (e.g. zinc acetate). Oral zinc supplementation in healthy infants more than six months old has been shown to reduce the duration of any subsequent diarrheal episodes by about 11 hours.

- Oral repletion via multivitamin/mineral supplements containing zinc gluconate, sulfate, or acetate. It is not clear whether one form is better than another. Zinc is also found in some cold lozenges, nasal sprays, and nasal gels.

Iodine deficiency is treated by ingestion of iodine salts, such as found in food supplements. Mild cases may be treated by using iodized salt in daily food consumption, or drinking more milk, or eating egg yolks, and saltwater fish. For a salt and/or animal product restricted diet, sea vegetables (kelp, hijiki, dulse, nori (found in sushi)) may be incorporated regularly into a diet as a good source of iodine.

The recommended daily intake of iodine for adult women is 150–300 µg for maintenance of normal thyroid function; for men it is somewhat less at 150 µg.

However, too high iodine intake, for example due to overdosage of iodine supplements, can have toxic side effects. It can lead to hyperthyroidism and consequently high blood levels of thyroid hormones (hyperthyroxinemia). In case of extremely high single-dose iodine intake, typically a short-term suppression of thyroid function (Wolff–Chaikoff effect) occurs. Persons with pre-existing thyroid disease, elderly persons, fetuses and neonates, and patients with other risk factors are at a higher risk of experiencing iodine-induced thyroid abnormalities. In particular, in persons with goiter due to iodine deficiency or with altered thyroid function, a form of hyperthyroidism called Jod-Basedow phenomenon can be triggered even at small or single iodine dosages, for example as a side effect of administration of iodine-containing contrast agents. In some cases, excessive iodine contributes to a risk of autoimmune thyroid diseases (Hashimoto's thyroiditis and Graves' disease).

In areas where there is little iodine in the diet, typically remote inland areas and semi-arid equatorial climates where no marine foods are eaten, iodine deficiency gives rise to hypothyroidism, symptoms of which are extreme fatigue, goiter, mental slowing, depression, weight gain, and low basal body temperatures.

Iodine deficiency is the leading cause of preventable mental retardation, a result which occurs primarily when babies or small children are rendered hypothyroidic by a lack of the element. The addition of iodine to table salt has largely eliminated this problem in the wealthier nations, but as of March 2006, iodine deficiency remained a serious public health problem in the developing world.

Iodine deficiency is also a problem in certain areas of Europe. In Germany it has been estimated to cause a billion dollars in health care costs per year. A modelling analysis suggests universal iodine supplementation for pregnant women in England may save £199 (2013 UK pounds) to the health service per pregnant woman and save £4476 per pregnant woman in societal costs.

Raw eggs should be avoided in those with biotin deficiency, because egg whites contain high levels of the anti-nutrient avidin. The name avidin literally means that this protein has an "avidity" (Latin: "to eagerly long for") for biotin. Avidin binds irreversibly to biotin and this compound is then excreted in the urine.

A vitamin deficiency can cause a disease or syndrome known as an avitaminosis or hypovitaminosis. This usually refers to a long-term deficiency of a vitamin. When caused by inadequate nutrition it can be classed as a "primary deficiency", and when due to an underlying disorder such as malabsorption it can be classed as a "secondary deficiency". An underlying disorder may be metabolic as in a defect converting tryptophan to niacin. It can also be the result of lifestyle choices including smoking and alcohol consumption.

Examples are vitamin A deficiency, folate deficiency, scurvy, vitamin D deficiency, vitamin E deficiency, and vitamin K deficiency. In the medical literature, any of these may also be called by names on the pattern of "hypovitaminosis" or "avitaminosis" + "[letter of vitamin]", for example, hypovitaminosis A, hypovitaminosis C, hypovitaminosis D.

Conversely hypervitaminosis is the syndrome of symptoms caused by over-retention of fat-soluble vitamins in the body.

- Vitamin A deficiency can cause keratomalacia.

- Thiamine (vitamin B1) deficiency causes beriberi and Wernicke–Korsakoff syndrome.

- Riboflavin (vitamin B2) deficiency causes ariboflavinosis.

- Niacin (vitamin B3) deficiency causes pellagra.

- Pantothenic acid (vitamin B5) deficiency causes chronic paresthesia.

- Vitamin B6

- Biotin (vitamin B7) deficiency negatively affects fertility and hair/skin growth. Deficiency can be caused by poor diet or genetic factors (such as mutations in the BTD gene, see multiple carboxylase deficiency).

- Folate (vitamin B9) deficiency is associated with numerous health problems. Fortification of certain foods with folate has drastically reduced the incidence of neural tube defects in countries where such fortification takes place. Deficiency can result from poor diet or genetic factors (such as mutations in the MTHFR gene that lead to compromised folate metabolism).

- Vitamin B12 (cobalamin) deficiency can lead to pernicious anemia, megaloblastic anemia, subacute combined degeneration of spinal cord, and methylmalonic acidemia among other conditions.

- Vitamin C (ascorbic acid) short-term deficiency can lead to weakness, weight loss and general aches and pains. Longer-term depletion may affect the connective tissue. Persistent vitamin C deficiency leads to scurvy.

- Vitamin D (cholecalciferol) deficiency is a known cause of rickets, and has been linked to numerous health problems.

- Vitamin E deficiency causes nerve problems due to poor conduction of electrical impulses along nerves due to changes in nerve membrane structure and function.

- Vitamin K (phylloquinone or menaquinone) deficiency causes impaired coagulation and has also been implicated in osteoporosis

Mild iron deficiency can be prevented or corrected by eating iron-rich foods and by cooking in an iron skillet. Because iron is a requirement for most plants and animals, a wide range of foods provide iron. Good sources of dietary iron have heme-iron, as this is most easily absorbed and is not inhibited by medication or other dietary components. Three examples are red meat, poultry, and insects. Non-heme sources do contain iron, though it has reduced bioavailability. Examples are lentils, beans, leafy vegetables, pistachios, tofu, fortified bread, and fortified breakfast cereals.

Iron from different foods is absorbed and processed differently by the body; for instance, iron in meat (heme-iron source) is more easily absorbed than iron in grains and vegetables ("non-heme" iron sources). Minerals and chemicals in one type of food may also inhibit absorption of iron from another type of food eaten at the same time. For example, oxalates and phytic acid form insoluble complexes which bind iron in the gut before it can be absorbed.

Because iron from plant sources is less easily absorbed than the heme-bound iron of animal sources, vegetarians and vegans should have a somewhat higher total daily iron intake than those who eat meat, fish or poultry. Legumes and dark-green leafy vegetables like broccoli, kale and oriental greens are especially good sources of iron for vegetarians and vegans. However, spinach and Swiss chard contain oxalates which bind iron, making it almost entirely unavailable for absorption. Iron from non-heme sources is more readily absorbed if consumed with foods that contain either heme-bound iron or vitamin C. This is due to a hypothesised "meat factor" which enhances iron absorption.

Following are two tables showing the richest foods in heme and non-heme iron.

In both tables, food serving sizes may differ from the usual 100g quantity for relevancy reasons. Arbitrarily, the guideline is set at 18 mg, which is the USDA Recommended Dietary Allowance for women aged between 19 and 50.

Iron deficiency can have serious health consequences that diet may not be able to quickly correct; hence, an iron supplement is often necessary if the iron deficiency has become symptomatic.

In plants a micronutrient deficiency (or trace mineral deficiency) is a physiological plant disorder which occurs when a micronutrient is deficient in the soil in which a plant grows. Micronutrients are distinguished from macronutrients (nitrogen, phosphorus, sulfur, potassium, calcium and magnesium) by the relatively low quantities needed by the plant.

A number of elements are known to be needed in these small amounts for proper plant growth and development. Nutrient deficiencies in these areas can adversely affect plant growth and development. Some of the best known trace mineral deficiencies include: zinc deficiency, boron deficiency, iron deficiency, and manganese deficiency.

The European Food Safety Authority concluded that chromium is not an essential nutrient, making this the only mineral for which the United States and the European Union disagree. The proposed mechanism for cellular uptake of Cr via transferrin has been called into question. There is no proof that chromium supplementation has physiological effects on body mass or composition, and its use as a supplement may be unsafe. A 2014 systematic review concluded that chromium supplementation had no effect on glycemic control, fasting plasma glucose levels, or body weight in people with or without diabetes.

Chromium may be needed as an ingredient in total parenteral nutrition (TPN), since deficiency may occur after months of intravenous feeding with chromium-free TPN. For this reason, chromium is added to normal TPN solutions for people with diabetes, and in nutritional products for preterm infants.

Iron is needed for bacterial growth making its bioavailability an important factor in controlling infection. Blood plasma as a result carries iron tightly bound to transferrin, which is taken up by cells by endocytosing transferrin, thus preventing its access to bacteria. Between 15 and 20 percent of the protein content in human milk consists of lactoferrin that binds iron. As a comparison, in cow's milk, this is only 2 percent. As a result, breast fed babies have fewer infections. Lactoferrin is also concentrated in tears, saliva and at wounds to bind iron to limit bacterial growth. Egg white contains 12% conalbumin to withhold it from bacteria that get through the egg shell (for this reason, prior to antibiotics, egg white was used to treat infections).

To reduce bacterial growth, plasma concentrations of iron are lowered in a variety of systemic inflammatory states due to increased production of hepcidin which is mainly released by the liver in response to increased production of pro-inflammatory cytokines such as Interleukin-6. This functional iron deficiency will resolve once the source of inflammation is rectified; however, if not resolved, it can progress to Anaemia of Chronic Inflammation. The underlying inflammation can be caused by fever, inflammatory bowel disease, infections, Chronic Heart Failure (CHF), carcinomas, or following surgery.

Reflecting this link between iron bioavailability and bacterial growth, the taking of oral iron supplements in excess of 200 mg/day causes a relative overabundance of iron that can alter the types of bacteria that are present within the gut. There have been concerns regarding parenteral iron being administered whilst bacteremia is present, although this has not been borne out in clinical practice. A moderate iron deficiency, in contrast, can provide protection against acute infection, especially against organisms that reside within hepatocytes and macrophages, such as malaria and tuberculosis. This is mainly beneficial in regions with a high prevalence of these diseases and where standard treatment is unavailable.

If untreated, pellagra can kill within four or five years. Treatment is with nicotinamide, which has the same vitamin function as niacin and a similar chemical structure, but has lower toxicity. The frequency and amount of nicotinamide administered depends on the degree to which the condition has progressed.

Treatment is possible but unless continued daily, problems may arise. Currently, this is done through supplementation of 5–10 mg of oral biotin a day. If symptoms have begun to show, standard treatments can take care of them, such as hearing aids for poor hearing.

Micronutrient deficiency or dietary deficiency is a lack of one or more of the micronutrients required for plant or animal health. In humans and other animals they include both vitamin deficiencies and mineral deficiencies, whereas in plants the term refers to deficiencies of essential trace minerals.

Treatment of THB deficiencies consists of THB supplementation (2–20 mg/kg per day) or diet to control blood phenylalanine concentration and replacement therapy with neurotransmitters precursors (L-DOPA and 5-HTP) and supplements of folinic acid in DHPR deficiency.

Tetrahydrobiopterin is available as a tablet for oral administration in the form of "tetrahydrobiopterin dihydrochloride" (BH4*2HCL). BH4*2HCL is FDA approved under the trade name Kuvan. The typical cost of treating a patient with Kuvan is $100,000 per year. BioMarin holds the patent for Kuvan until at least 2024, but Par Pharmaceutical has a right to produce a generic version by 2020. BH4*2HCL is indicated at least in tetrahydrobiopterin deficiency caused by GTPCH deficiency or PTPS deficiency.

Copper deficiency is a very rare disease and is often misdiagnosed several times by physicians before concluding the deficiency of copper through differential diagnosis (copper serum test and bone marrow biopsy are usually conclusive in diagnosing copper deficiency). On average, patients are diagnosed with copper deficiency around 1.1 years after their first symptoms are reported to a physician.

Copper deficiency can be treated with either oral copper supplementation or intravenous copper. If zinc intoxication is present, discontinuation of zinc may be sufficient to restore copper levels back to normal, but this usually is a very slow process. People who suffer from zinc intoxication will usually have to take copper supplements in addition to ceasing zinc consumption. Hematological manifestations are often quickly restored back to normal. The progression of the neurological symptoms will be stopped by appropriate treatment, but often with residual neurological disability.

Pellagra can develop according to several mechanisms, classically as a result of niacin (vitamin B3) deficiency, which results in decreased NAD production leading to most of the pathology (since NAD and its phosphorylated NADP form are cofactors required in many body processes, the pathological impact of pellagra is broad and results in death if not treated).

The first mechanism is simple dietary lack of niacin. Second, it may result from deficiency of tryptophan, an essential amino acid found in meat, poultry, fish, eggs, and peanuts that the body converts into niacin. Third, it may be caused by excess leucine, as it inhibits quinolinate phosphoribosyl transferase (QPRT) and inhibits the formation of Niacin or Nicotinic acid to Nicotinamide mononucleotide (NMN) causing pellegra like symptoms to occur.

Some conditions can prevent the absorption of dietary niacin or tryptophan and lead to pellagra. Inflammation of the jejunum or ileum can prevent nutrient absorption, leading to pellagra, and this can in turn be caused by Crohn's disease. Gastroenterostomy can also cause pellagra. Chronic alcoholism can also cause poor absorption which combines with a diet already low in niacin and tryptophan to produce pellagra. Hartnup disease is a genetic disorder that reduces tryptophan absorption, leading to pellagra.

Alterations in protein metabolism may also produce pellagra-like symptoms. An example is carcinoid syndrome, a disease in which neuroendocrine tumors along the GI tract use tryptophan as the source for serotonin production, which limits the available tryptophan for niacin synthesis. In normal patients, only one percent of dietary tryptophan is converted to serotonin; however, in patients with carcinoid syndrome, this value may increase to 70%. Carcinoid syndrome thus may produce niacin deficiency and clinical manifestations of pellagra. Anti-tuberculosis medication tends to bind to vitamin B and reduce niacin synthesis, since B (aka pyridoxine) is a required cofactor in the tryptophan-to-niacin reaction.

Several therapeutic drugs can provoke pellagra. These include the antibiotics isoniazid, which decreases available B by binding to it and making it inactive, so it cannot be used in niacin synthesis, and chloramphenicol; the anti-cancer agent fluorouracil; and the immunosuppressant mercaptopurine.

Increased consumption of zinc is another cause of copper deficiency. Zinc is often used for the prevention or treatment of common colds and sinusitis (inflammation of sinuses due to an infection), ulcers, sickle cell disease, celiac disease, memory impairment and acne. Zinc is found in many common vitamin supplements and is also found in denture creams. Recently, several cases of copper deficiency myeloneuropathy were found to be caused by prolonged use of denture creams containing high quantities of zinc.

Metallic zinc is the core of all United States currency coins, including copper coated pennies. People who ingest a large number of coins will have elevated zinc levels, leading to zinc-toxicity-induced copper deficiency and the associated neurological symptoms. This was the case for a 57-year-old woman diagnosed with schizophrenia. The woman consumed over 600 coins, and started to show neurological symptoms such as unsteady gait and mild ataxia.