B12 deficiency linked with brain shrinkage in later life

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Vitamin B12 is a nutrient found naturally in animal products such as meat, fish and eggs. The nutrient is perhaps best known for its relationship with a condition known as ‘pernicious anaemia’ – a form of anaemia caused by B12 deficiency which itself is caused by impaired absorption of B12 from the gut. However, for some time now there has been specific interest in the role the B12 plays in brain function. In particular, B12 deficiency is believed to play a potential role in the declining brain function (cognitive impairment) often seen in ageing.

One of the proposed mechanisms here concerns ‘homocysteine’ – an amino acid that is linked with adverse effects on health including cardiovascular issues such as heart disease and stroke. Low levels of B12 can cause raised levels of homocysteine (as can low levels of other nutrients including folic acid and vitamin B6). Measuring homocysteine levels is sometimes used as a proxy for B12 status.

In a study published this week in the journal Neurology, vitamin B12 levels and brain structure and function was assessed in a group of individuals aged 65 or more. B12 status was assessed with five tests including homocysteine levels and serum (blood) B12 levels. Four and a half years later, the individuals were assessed via tests for cognitive function as well as MRIs of the brain.

In general, tests which indicated low B12 status (e.g. raised homocysteine levels) were associated with lower cognitive function tests scores and smaller brain volume. This was true for all of the 5 markers for B12 except serum B12 levels.

What this evidence suggests is that B12 may have an important role to play in the ageing brain, and that assessing levels via the standard blood test is not particularly useful.

This situation is reminiscent of tests for iron levels in the body. Serum iron is a generally useless test of actual iron levels in the body, while other tests (notably ‘ferritin’) are much more useful in practice.

The absorption of B12 is a quite complex process, and ageing may well lead to an impairment of B12 absorption and increase the risk of deficiency. There is a risk, of course, that using the most common test for B12 levels (serum B12) runs the risk of missing genuine B12 deficiency. The end result might be someone languishing in a B12-deficient state, which may have profound implications for their health and wellbeing.

For more information about B12 deficiency and how to assess this you might like to listen to this podcast featuring Sally Pacholok, author of the book ‘Could it be B12?’. You can learn more about the book here.


1. Tangney CC, et al. Vitamin B12, cognition, and brain MRI measures – A cross-sectional examination. Neurology 2011;77(13):1276-1282

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