Vitamin B toxicity
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Vitamin B Toxicity: Overview and Key Concerns
Vitamin B toxicity is generally rare because most B vitamins are water-soluble and excess amounts are excreted in urine. However, certain B vitamins, especially when consumed in high doses through supplements, can cause toxicity and adverse health effects 36.
Vitamin B6 (Pyridoxine) Toxicity and Neuropathy
High-Dose Supplementation and Prevalence
Vitamin B6 toxicity is a well-documented cause of nerve dysfunction, particularly peripheral neuropathy. Many over-the-counter (OTC) multivitamins contain vitamin B6 in amounts exceeding the recommended daily intake (1.7–2.0 mg), with studies showing that 72% of surveyed supplements provided higher doses than recommended. Prolonged daily consumption of these high-dose supplements can lead to toxicity .
Symptoms and Mechanisms
The main symptom of vitamin B6 toxicity is peripheral neuropathy, which involves nerve damage leading to numbness, tingling, and weakness. The mechanism is thought to involve injury to sensory neurons, possibly through inhibition of the enzyme pyridoxal kinase (PDXK), which disrupts neurotransmitter (GABA) signaling in peripheral nerves, leading to neurodegeneration . Neurological side effects are rare and typically occur only with high daily doses or long-term use .
Vitamin B12 (Cobalamin) Toxicity
Case Reports and Symptoms
Vitamin B12 toxicity is uncommon, but there are rare reports of adverse effects following very high oral doses. Symptoms can include acne, palpitations, anxiety, restlessness, facial redness, headache, and insomnia. These symptoms usually resolve after stopping supplementation, and no long-term complications have been reported in such cases .
Potential Risks of High Levels
Elevated vitamin B12 levels have been associated in some studies with immune dysfunction, cancer, and increased mortality, though these findings are still being explored and the clinical significance remains unclear .
Other B Vitamins: General Safety and Rare Toxicity
Most other B vitamins, such as thiamine (B1), riboflavin (B2), niacin (B3), pantothenic acid (B5), biotin (B7), and folate (B9), are considered safe even at relatively high intakes due to their water solubility. Toxicity is rare and usually only occurs with excessive supplementation, not from food sources 36.
Fortification, Supplementation, and Hidden Risks
While food fortification has helped reduce B vitamin deficiencies globally, the use of synthetic forms in supplements and fortified foods can sometimes lead to excessive intake and potential toxicity, especially for vitamin B6 and, less commonly, B12 .
Conclusion
Vitamin B toxicity is rare but can occur, particularly with high-dose supplementation of vitamin B6 and, in rare cases, vitamin B12. The most common and well-documented toxicity is vitamin B6-induced neuropathy, which is preventable by adhering to recommended intake levels. Most other B vitamins are safe at typical dietary and supplemental levels. Awareness of the risks of excessive supplementation is important for both healthcare providers and the general public 123458.
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