Here’s what neurologists and a dietitian want you know about Vitamin B12, plus its potential impact on your current and future brain health. The researchers drilled down on the data to focus on a biologically active component of vitamin B12, which indicates the amount of B12 the body can actually use. “Low B12 levels can lead to mental fog.” Symptoms of low B12 levels can also mimic signs of dementia. But, again, if you suspect that low levels of B12 are an issue, talk to your healthcare provider.