Vitamin B12, including cyanocobalamin, and health across life
What it found
Vitamin B12 deficiency is common, especially in vegetarians, older people and those with absorption problems, and low levels have been linked to problems in pregnancy, heart disease, thinking, bones and eyes.
The long-term effects of mild deficiency are not fully known.
What they found
Studies in people
Neural tube defects
Studies consistently report a 2 to 4 fold increased risk of neural tube defects with low vitamin B12 status. These studies were in a range of population groups, including those exposed to folate fortified foods and non-fortified populations.
Heart disease and homocysteine
Vitamin B12 supplementation at doses from 0.02 to 1 mg per day produced about a 7% reduction in homocysteine, while folate produced a 10 to 30% reduction. Randomised trials that included vitamin B12 with folate did not show the expected reductions in heart disease risk, and the individual impact of vitamin B12 could not be determined.
Cognitive decline
A Cochrane review based on 2 studies found no effect of vitamin B12 supplementation alone on cognitive score in older adults. Raised methylmalonic acid concentrations are associated with cognitive decline and Alzheimer's Disease.
Macular degeneration
A randomised controlled trial with 5205 female health professionals at risk of vascular disease found a 34% reduction in the relative risk of age-related macular degeneration after supplementation with vitamins B12, B6 and folate (daily doses of 1 mg, 50 mg, 2.5 mg respectively).
Studies in people and animals
Bones and fractures
A systematic review found evidence for an association between homocysteine and increased fracture risk, but less conclusive evidence for homocysteine and low bone mineral density, or for vitamin B12 and either fracture risk or low bone mineral density. Intervention trials of B vitamin supplementation have shown mixed results.
Other findings
Forms of vitamin B12
Vitamin B12, also called cobalamin, comes in several forms including cyano-, methyl-, deoxyadenosyl- and hydroxy-cobalamin. The cyano form, used in supplements, is found in trace amounts in food.
Food sources
Vitamin B12 is found only in foods of animal origin. Liver has 26 to 58 micrograms per 100 grams, beef and lamb 1 to 3, chicken trace to 1, eggs 1 to 2.5, and dairy foods 0.3 to 2.4. There are no naturally occurring bioactive forms from plant sources.
Absorption and dose
One study using crystalline vitamin B12 supplements reported that 50% was retained at a 1 microgram dose, 20% at a 5 microgram dose and 5% at a 25 microgram dose. About 1% of large doses (1,000 micrograms) is absorbed even without intrinsic factor.
What the authors conclude
“The effects of sub-clinical deficiency are not fully known and many aspects of vitamin B 12 absorption, bioavailability and metabolism are yet to be determined.”
Also in their conclusions
- They say vitamin B12 is particularly important for women of childbearing age and for older people, but adequate status over the whole lifecycle is needed for optimal health.
- They state that the effects of sub-clinical deficiency are not fully known and many aspects of vitamin B12 absorption, bioavailability and metabolism are yet to be determined.
- They advise that identifying sensitive biomarkers of vitamin B12 status will help clarify the relationships between vitamin B12 and chronic disease and help identify those at risk of clinical and sub-clinical deficiency.
How it was done
The authors wanted to outline how vitamin B12 is processed in the body and to evaluate the causes and consequences of sub-clinical vitamin B12 deficiency.
They reviewed existing research on vitamin B12, covering how it is absorbed, how status is measured, food sources, requirements, deficiency, drug interactions, and links to conditions such as neural tube defects, heart disease, cognitive decline, osteoporosis, macular degeneration and frailty.
What it can’t tell you
- This is a review of other studies, not a new experiment, so it cannot prove cause and effect.
- Many of the findings come from studies that follow people over time or look at groups at one point, which can show links but not prove that vitamin B12 causes or prevents disease.
- The long-term consequences of sub-clinical vitamin B12 deficiency are not fully known.
Who paid
- Authors work at
- University of Sydney, Australia
The paper
- Title
- Vitamin B12 in health and disease
- Type
- ReviewThe authors read earlier studies and describe what they found. It isn’t a new study.
- Evidence
- Studies in people · animals · lab
- Summarised from
- Full text
- Cite
- O'Leary F, Samman S (2010). Vitamin B12 in health and disease. Nutrients. doi:10.3390/nu2030299Free full textPubMed 22254022DOI
Summary written 26 Sep 2026. Check it against the paper before it changes what you eat. How we summarise papers · Report an error