This is a common question asked by so many people.
Please refer Nature Cure perspective on deficiency and guidelines for healthy living.
It is important to check our habits even though we are more inclined to check our body.
Assuming health guidelines are being followed, combination of subtle inputs like tender coconut water/green juice followed by sun basking can help synthesize all the necessary nutrients needed by the body.Take Tender coconut water.. In 10 minutes, go for sun basking..Make sure appendix is exposed. The special bacteria living there seem capable of producing small amounts of b12 that we need provided that microbial population is healthy. Peales note that if someone had very low levels of b12 this method may not be sufficient.
Living in a tropical country like India where we have Sun light & heat almost all 365 days, it is unfortunate if we do not make use of it. Get Sun bath every single day and as mentioned above, following all basic NC guidelines, Vitamin D can be produced by the body.
Vitamin B12 is made by bacteria in the gut. Fibre rich foods(read whole plant based foods, unrefined) are food for bacteria and beneficial bacteria thrive on feeding fibre and produce enough Vitamin B12. Fibre is prebiotic.