The B vitamins get bundled together on a label and then discussed as if they were one thing. They are not, and the differences are the useful part: some are almost impossible to overdo, one is hard to get on a plant-based diet, and one is detectable with an ordinary blood test.
As on every page here, the intakes below are the ones authorities publish. They are not our recommendation to you.
Can I take too much of these?
For the water-soluble ones, mostly not from food. Your body flushes the excess, which is why the safety picture for this group looks so different from vitamin A's.
Thiamin toxicity from food is very unlikely; there is no established toxic level and excess is flushed through urine.
Multiple health authorities agree
Clinical review pending
More than one independent health authority states this.
The human body excretes excess thiamine in the urine. No toxicity from high thiamine intake from food or supplements is evident... no established upper limit for thiamine intake that causes toxicity is reported in the literature.
No evidence shows biotin toxicity in humans even at high intakes; excess is excreted because it is water-soluble.
Stated in a health authority's guidance
Clinical review pending
A health authority states this in its published guidance.
Biotin is a safe and nontoxic vitamin that the body excretes when in excess... Since biotin is a water-soluble vitamin, it is excreted in the urine when in excess.
More than a hundred separate jobs, including brain development during pregnancy. Deficiency has a recognisable pattern rather than a vague one.
The body needs vitamin B6 for more than 100 enzyme reactions in metabolism, brain development during pregnancy and infancy, and immune function.
Stated in a health authority's guidance
Clinical review pending
A health authority states this in its published guidance.
"The body needs vitamin B6 for more than 100 enzyme reactions involved in metabolism. Vitamin B6 is also involved in brain development during pregnancy and infancy as well as immune function."
Vitamin B6 deficiency is associated with microcytic anemia, EEG abnormalities, dermatitis with cheilosis and glossitis, depression/confusion, and weakened immunity.
Stated in a health authority's guidance
Clinical review pending
A health authority states this in its published guidance.
"Vitamin B6 deficiency is associated with microcytic anemia, electroencephalographic abnormalities, dermatitis with cheilosis (scaling on the lips and cracks at the corners of the mouth) and glossitis (swollen tongue), depression and confusion, and weakened immune function"
It shows up in hair, skin and nails, which is also why biotin is sold so heavily for exactly those things.
Biotin deficiency symptoms include thinning hair, scaly rashes around eyes/nose/mouth, and brittle nails; if confirmed, eat more biotin-rich foods or consider a supplement.
Stated in a health authority's guidance
Clinical review pending
A health authority states this in its published guidance.
[Signs of] biotin deficiency typically appear gradually and can include thinning hair with progression to loss of all hair on the body; scaly, red rash around body openings (eyes, nose, mouth, and perineum); ... brittle nails
Why is B12 the one to watch on a plant-based diet?
Because it is barely present outside animal foods. That is not an argument against eating plant-based. It is an argument for knowing this one fact, and for the blood test below, which is ordinary and cheap.
Vitamin B12 helps nerve function and produces healthy red blood cells and DNA.
Stated in a health authority's guidance
Clinical review pending
A health authority states this in its published guidance.
Vitamin B12 is required for the development, myelination, and function of the central nervous system; healthy red blood cell formation; and DNA synthesis [1,4,5].
B12 is naturally present mainly in animal products, with some fortified plant foods (liver, clams, tuna, fortified plant milks, etc.).
Stated in a health authority's guidance
Clinical review pending
A health authority states this in its published guidance.
Vitamin B12 is found in foods of animal origin. It is not naturally present in plant foods, but some plant foods are fortified with vitamin B12, including many breakfast cereals and nutritional yeasts.
A basic blood test can detect B12 deficiency; deficiency may be treated with supplements or B12 injections.
Stated in a health authority's guidance
Clinical review pending
A health authority states this in its published guidance.
These conditions can often be diagnosed based on your symptoms and the results of a blood test... Vitamin B12 supplements are usually given by injection at first. Then, depending on whether your B12 deficiency is related to your diet, you'll either require B12 tablets between meals or regular injections.
What is homocysteine, and why does it keep coming up?
It is a compound the B vitamins help clear. It comes up in fertility research because of what has been observed in women whose levels run high, and the finding below is an association rather than a demonstrated cause.
Women with high homocysteine are more likely to have early miscarriages and preeclampsia.
Supported by a systematic review
Clinical review pending
A systematic review of the research supports this.
'there is a connection between raised homocysteine levels and pregnancy issues such as early spontaneous abortions'; 'There is enough evidence to support the link between HHcy and PE.'