"Most likely everybody would like to slow down brain atrophy. Who wouldn't like more brain power? But is that even possible? Take a look at the results from this research.
168 elderly people (≥70 y) with mild cognitive impairment were included and randomly assigned to either a placebo group or to a group taking a high-dose of B vitamins.
This is what they  took daily, 0.8 mg of folic acid, 20 mg of vitamin B-6, and 0.5 mg of vitamin B-12 (Jernerén F, et.al., 2015).
The participants underwent cranial magnetic resonance imaging scans when they started and 2 years later. The omega 3 fatty acid levels of EPA and DHA was also measured.
This what the researchers found.
The B vitamin treatment slowed the average atrophy rate by 40.0% compared with the placebo group. This happened however only in the participants who had high levels of omega 3 fatty acids at the start of the study.
Another study investigating cognitive decline in 266 participants 70 years or older found similar interesting results (Oulhaj A, et.al., 2016).
When omega-3 fatty acid concentrations are low, treatment with B vitamins had no effect on cognitive decline, but when omega-3 levels were in the upper normal range, B vitamins slowed cognitive decline.
Eating a lot of fish is not a good way anymore to increase your omega 3 fatty acid levels because all fish is now contaminated. A better choice is to use a high quality fish oil with high amounts of EPA and DHA, since that would not expose you to the same levels of contaminants.
It is also a good idea to take a B-complex formula that has the metabolite of folic acid, 5-methyltetrahydrofolate, because many people does not metabolize folic acid effectively.
References
Jernerén F, Elshorbagy AK, Oulhaj A, Smith SM, Refsum H, Smith AD. Brain atrophy in cognitively impaired elderly: the importance of long-chain ω-3 fatty acids and B vitamin status in a randomized controlled trial. Am J Clin Nutr. 2015 Jul;102(1):215-21.
Oulhaj A, Jernerén F, Refsum H, Smith AD, de Jager CA.Omega-3 Fatty Acid Status Enhances the Prevention of Cognitive Decline by B Vitamins in Mild Cognitive Impairment. J Alzheimers Dis. 2016;50(2):547-57.