The study uses 100mg Vitamin B6 (~60x RDI) and 1000mcg Vitamin B12 (~417x RDI) for a month. High doses have been previously associated with toxicity symptoms:
For vitamin B12, the best source is liver, e.g. chicken liver (16.6 mcg / 100 g raw chicken liver) (when eating liver, one should take care to not eat too much, as liver, especially other kinds of liver than chicken liver, e.g. turkey liver, can provide too much vitamin A, but the quantity of chicken liver that provides enough B12 is much less than that which provides too much vitamin A).
For vitamin B6, garlic has a higher content (1.2 mg / 100 g) than all the sources listed at the link given above, with the exception of some kinds of nuts, e.g. pistachios.
Liver is a nutrient powerhouse! Chicken, beef, lamb liver all good choices in moderation. However, few are unsafe for consumption even in small quantities - https://www.nature.com/articles/164530a0
For vitamin B6, garlic has a higher content (1.2 mg / 100 g) than all the sources listed at the link given above, with the exception of some kinds of nuts, e.g. pistachios.