24/10/2026
B12 is the vitamin people reach for when they worry about memory. In 1,359 older hospital patients, the lowest thinking scores belonged to people low in both B12 and vitamin D, and vitamin D carried most of the weight.
Avram and colleagues (Nutrients 2026) measured blood vitamin D and B12 once in patients aged 65 and older in Cluj-Napoca, Romania, and scored thinking on a standard 30-point test.
The middle score was 23 out of 30 with neither deficiency, 22 with low B12 only, 20 with low vitamin D only and 18 with both. After accounting for age, weight, kidney function and blood protein, people low in both still scored about 2.3 points lower than people low in neither, and people low in vitamin D alone about 1.5 points lower. Low B12 on its own showed no link to thinking, mood or frailty.
The two deficiencies added up rather than amplifying each other. The authors tested that directly. The double-deficient group was also the only one with more depression symptoms, but that result faded when low B12 was defined at a higher cutoff, so it is preliminary. Only 96 people, about 1 in 14, were low in both.
The biology fits a cumulative picture. Vitamin D receptors are spread through the brain and muscle. B12 is needed to keep nerve insulation healthy and to clear homocysteine, which tracks with dementia risk in observational studies.
A single blood draw cannot show which came first. Older adults who are sick or frail get outside less and eat less, and both lower vitamin D. These were hospital patients, not healthy people at home, and the basic B12 blood test used can miss true deficiency.
Trials temper the vitamin D story. In VITAL, 3,424 adults aged 60 and older took 2,000 IU of vitamin D3 a day or a placebo, and thinking declined at the same rate over almost three years. DO-HEALTH gave the same dose to 2,157 adults aged 70 and older and found no effect on the same 30-point test.
If you are 75 or older, the Endocrine Society's 2024 guideline suggests a daily vitamin D supplement, for a possible lower risk of death rather than for the brain, and prefers small daily doses over large occasional ones. A single 500,000 IU dose once a year raised falls and fractures in 2,256 women aged 70 and older. If you are over 50, stomach acid often drops, and acid is what frees B12 from food, so the NIH advises getting most of it from fortified foods or supplements. If you take metformin, the label says to measure B12 every 2 to 3 years, and long-term acid reducers have been linked to deficiency.
B12 alone was not the memory vitamin in this study. Low vitamin D tracked with most of the gap, and low B12 on top of it added a little more.
Avram et al., Nutrients 2026;18(18):3078
Kang et al. (VITAL), Sci Rep 2021;11:23253
Bischoff-Ferrari et al. (DO-HEALTH), JAMA 2020;324(18):1855-1868
Demay et al. (Endocrine Society guideline), J Clin Endocrinol Metab 2024;109(8):1907-1947
Sanders et al., JAMA 2010;303(18):1815-1822