Does Your Vitamin B12 Level Put You at Risk of Dementia?

Two groups who are at risk of vitamin B12 deficiency are adults over 60 and those who eat a vegan or near-vegan diet. Vitamin B12 is produced by bacteria and scarce in plant foods; animal foods contain B12 that is produced by the bacteria in the gastrointestinal tracts of the animals and then absorbed into the tissues. Of course, eating a diet almost exclusively of unrefined whole plant foods is a healthful way to eat because it dramatically reduces the risk of chronic illnesses, such as diabetes and cancers.

  But these healthful diets do require a supplemental source of vitamin B12.

The risk of B12 deficiency increases with age, because as we age, our capability to absorb this important vitamin declines. In addition, common medications, such as proton pump inhibitors and metformin are thought to interfere with B12 absorption. About 20 percent of adults over age 60 are either insufficient or deficient vitamin B12. Even young people may be deficient, since research suggests we absorb only a small proportion of the B12 we ingest at each sitting. So supplementation is probably appropriate for most age groups, and absolutely essential for most vegans to achieve sufficient B12 status.

Vitamin B12 is required for important biological functions like red blood cell production, nervous system function, and DNA synthesis. A severe deficiency causes anemia, but other symptoms include pins and needles, tremors, depression, confusion, fatigue, digestive issues, and nerve damage.

 One important role of B12 is to convert the amino acid homocysteine to methionine; this chemical reaction is part of a system of intersecting cycles of reactions that facilitates DNA synthesis and methylation, neurotransmitter synthesis, and other cellular processes. Low levels of B12 (or B6 or folate) can cause high homocysteine levels, a known risk factor for cardiovascular disease.

Elevated homocysteine is also thought to have a negative effect on bone health, increasing the risk of fracture.

Serum B12 levels are associated with cognitive impairment in some but not all studies, this may be in part because serum vitamin B12 is not necessarily a good indicator of the amount of B12 in the body’s tissues.  Studies using different markers of B12 status – homocysteine, methylmalonic acid (MMA), and holotranscobalamin – have found links between lower or declining vitamin B12 status and either cognitive impairment or Alzheimer’s disease.

High homocysteine has been linked to a higher risk of Alzheimer’s disease in many studies. In a seven-year longitudinal study, researchers looked at the relationship between homocysteine, vitamin B12, and Alzheimer’s disease diagnosis in 271 healthy people ages 65-79. Elevated homocysteine was associated with increased risk, and increased vitamin B12 was associated with a decreased risk of Alzheimer’s disease.

Since we already know that vitamin B12 is an important supplement to take, preventing Alzheimer’s disease may be an important added benefit to maintaining adequate levels of this vitamin.

I recommend taking B12 daily, in greater amounts than the RDA, because when taken once a day as a supplement the percentage absorbed is limited.

I suggest most people on a plant-rich diet with limited animal product consumption take 100-250 mcg daily. It is also advisable to test your B12 levels with both a B12 and a MMA, every five years after the age of 60, which will provide a more accurate picture of your B12 status than B12 alone. If your B12 blood level is above 500 pg/ml, you don't need an MMA to check if you have enough. If it's below 200, you're definitely deficient, but if you're in the 200-500 range, then an MMA test would determine whether you have a deficiency.


Allen LH: How common is vitamin B-12 deficiency? Am J Clin Nutr 2009;89:693S-696S.

Hooshmand B, Solomon A, Kareholt I, et al: Homocysteine and holotranscobalamin and the risk of Alzheimer disease: a longitudinal study. Neurology 2010;75:1408-1414.

O'Leary F, Samman S: Vitamin B12 in health and disease. Nutrients 2010;2:299-316.

Smith AD, Refsum H: Vitamin B-12 and cognition in the elderly. Am J Clin Nutr 2009;89:707S-711S.

Van Dam F, Van Gool WA: Hyperhomocysteinemia and Alzheimer's disease: A systematic review. Arch Gerontol Geriatr 2009;48:425-430.


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