I just tweeted a few studies about how low vitamin B12 levels affect memory and contribute to depression. There are more where those came from.

I don’t think we take B12 deficiency seriously, myself included.

Vitamin B-12 Concentration, Memory Performance, And Hippocampal Structure In Patients With Mild Cognitive Impairment, American Journal of Clinical Nutrition, April 2016

Conclusions: Low Vitamin B12 concentrations within the normal range are associated with poorer memory performance, which is an effect that is partially mediated by the reduced microstructural integrity of the hippocampus.

Vitamin B(12) Deficiency And Depression In Physically Disabled Older Women: Epidemiologic Evidence From The Women’s Health And Aging Study, American Journal of Psychology, May 2000

Conclusions: In community-dwelling older women, metabolically significant vitamin B(12)deficiency is associated with a twofold risk of severe depression.

Vitamin B12 Supplementation In Treating Major Depressive Disorder: A Randomized Controlled Trial, The Open Neurology Journal, November 2013
This study had two groups. Both had depression, both had low vitamin B12 levels, both took antidepressants. One group (73 patients) added B12, the other (39 patients) didn’t.

100% of the B12 treatment group showed at least a 20% reduction in [depression] score. (vs. 69% in controls)

Those at risk of vitamin B12 deficiency:

• Older adults
• People with gastrointestinal disorders, such as Crohn’s disease or celiac disease
• Those who have had gastrointestinal surgeries, such as bariatric surgery or bowel resection surgery
• People on vegetarian diets
• People who take metformin (for diabetes)
• People who take proton pump inhibitors (for heartburn: e.g. Prilosec, Nexium)