Fat-soluble vitamins A and D are the most likely to cause symptoms of toxicity if consumed in large quantities due to their ability to accumulate in the body. Iron-containing vitamins are the most toxic, especially in acute pediatric ingestions. Vitamin A is found in both preformed sources and provitamin A carotenoids from plant sources, such as carrots. Vitamin B-1 (thiamine) is found in guts, yeast, eggs, and green leafy vegetables and is a cofactor for pyruvate dehydrogenase in the Krebs cycle.
Vitamin B2 (riboflavin) is found in green vegetables. Vitamin B3 (niacin) is found in green vegetables, yeast, animal proteins, fish, liver, and legumes. Vitamin B6 (pyridoxine) is found in poultry, fish, pork, cereals, and legumes. Vitamin B12 (cyanocobalamin) is found in dairy products, eggs, fish, poultry, and meat.
Vitamin C (ascorbic acid) is found in citrus fruits and vegetables. Vitamin K (phytonadione) is produced by intestinal bacteria (vitamin K-) and found in green leafy vegetables, cow's milk, and soybean oil (vitamin K-1). Folate is found in oranges and green leafy vegetables. Isotretinoin (Accutane), a drug used to treat severe forms of acne, is closely related to vitamin A's chemical structure which means that their pharmacology and toxicology are similar. Birth defects (when taken during pregnancy), intracranial hypertension, depression, and suicidal ideation have been reported with isotretinoin. Vitamin B1 (thiamine) and vitamin B2 (riboflavin) are generally non-toxic but vitamin K3 (menadione) supplements have been banned by the United States Food and Drug Administration (FDA) because of their high toxicity.
Excessive vitamin D intake has been considered in the context of “intoxication” or “hypervitaminosis”; therefore it may be best to view the condition as a relatively acute response. The hallmark of vitamin D poisoning is hypercalcemia which is associated with increased serum levels of 25OHD. Symptoms may appear in less than 4 weeks of continuous overingestion. Vitamin A toxicity can occur from high doses of retinoids which can cause severe headache, blurred vision, nausea, dizziness, muscle pain, dry skin, muscle and joint pain, fatigue, liver problems, bad moods, and coordination problems. In the U. S., the incidence of toxicity is more common than deficiency as it builds up over time due to its fat-soluble nature. Some multivitamin supplements contain high doses of vitamin A so check the label to make sure that most of it supplied is in the form of beta-carotene which appears to be safe. The best available data set for establishing a UL for vitamin D is associated with the onset of hypercalcemia and related toxicity while binders and fillers that manufacturers add to pills make it difficult for these vitamins to be broken down in the body. Vitamin B6 helps protein metabolism, formation of red blood cells and behaves like an antioxidant molecule while decades-old research suggested that taking antioxidant supplements such as vitamin E could help prevent heart disease and cancer but recent findings indicate that people who take these supplements are no better protected against heart disease or cancer than those who don't.In addition people with chronic diarrhea may have problems absorbing sufficient amounts of vitamin K through the intestine so they should consult their doctor to determine if supplementation is necessary. In conclusion it can be said that fat-soluble vitamins A and D, iron-containing vitamins as well as isotretinoin pose the greatest risk of toxicity when consumed in large quantities.
It is important to check labels on multivitamins for high doses of vitamin A as well as consult a doctor if chronic diarrhea persists.
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