Vitamin B6

Pyridoxine and its relatives, used in over a hundred enzyme reactions, particularly those handling amino acids, and in making haemoglobin and several neurotransmitters. Unusual among water-soluble vitamins in that too much causes real harm.

Involved in over a hundred enzyme reactions, most of them in protein metabolism. Needed to make haemoglobin and several neurotransmitters, including serotonin and GABA.

• Chickpeas
• Tuna and salmon
• Poultry
• Bananas
• Potatoes
• Fortified cereals

1.3 mg · rising to 1.7 mg (men) and 1.5 mg (women) over 50 for most adults. The upper limit, past which harm becomes likely, is 100 mg.

Reference intakes are population figures. Pregnancy, illness, age and medication all move an individual’s needs, which is one of several reasons this page is information rather than advice.

Most at risk: Kidney disease, autoimmune conditions, heavy alcohol use.

Alcohol dependence, kidney disease and dialysis, autoimmune conditions, and medicines including isoniazid, penicillamine and some anticonvulsants.

Anaemia, cracked lips, a swollen tongue, low mood, confusion, and a weakened immune response.

Every sign here has other, often likelier, explanations. A list of symptoms is a reason to look into something, never a diagnosis.

Plasma pyridoxal 5-phosphate is the usual measure, and falls with inflammation as several others do.

Poultry, fish, potatoes with skin, chickpeas, bananas and fortified cereals. Widely distributed and easy to obtain.

This is the one to watch. Sustained high doses cause peripheral nerve damage — numbness and unsteadiness — which may not fully reverse. It is a common ingredient in high-strength B complexes and energy products, so doses stack without anyone noticing.

One of the few water-soluble vitamins with real toxicity. Sustained high doses cause peripheral neuropathy — numbness and loss of coordination — which may not fully reverse. Harm has been reported below the official limit.

Interactions: Reduces the effectiveness of levodopa when taken without carbidopa. Some medicines deplete it, including isoniazid.