When a 40-year-old sports fan was referred to Professor Matthew Kiernan suffering from frequent, uncontrollable spasms, the Sydney neurologist suspected an autoimmune disease affecting the man's nerves.
“This is extreme,” said Kiernan, who is now CEO and director of Neuroscience Research Australia (NeuRA). “This guy came to us thinking he might have motor neurone disease, so you can imagine his situation.”
The man even said that he is interested in bodybuilding and takes vitamin supplements, some of which contain vitamin B6 (sometimes called pyridoxine).
Through research, Kiernan discovered that high doses of vitamins can irritate or damage nerves outside the brain and spinal cord. Tests showed that the level of vitamin B6 in the man's blood was "extremely high."
The man's case, which Kiernan wrote about in the Australian Medical Journal , is a clear example of the dangers of dietary supplements sold free in bulk.
Dietary supplements containing vitamin B6 or pyridoxine have been linked to more than 1,000 adverse events reported to the Therapeutic Goods Administration since 2010. Reports of adverse events are submitted by individuals and do not necessarily mean that the event has been confirmed.
The most common symptoms include nausea, hypersensitivity and peripheral neuropathy, in which nerve damage causes tingling, burning and numbness throughout the body.
"If you consume two or three times more of something than you need, your body's mechanisms won't be able to work fast enough to get rid of it," Kiernan says. “The longer the nerve, the more vulnerable it is to injury. Typically, people's legs are longer than their arms, so the legs lose sensation, and it gradually spreads to the feet, then to the fingers and toes."
From 2020, the TGA requires all supplements containing a daily dose of more than 50 milligrams of B6 to carry a warning statement. The threshold was lowered to 10 milligrams in 2022 after the agency raised concerns about a lack of public awareness of the risks of taking large amounts of vitamin B6.