Hello Vitamin B12 is essential vitamin that is found in foods like meat,eggs, milk, etc and is absorbed in the small intestine with the help of a protein called intrinsic factor( released from stomach).
If there is gastric atrophy , it leads to low or no production of intrinsic factor and ultimatly the absorption of vitamin B12 is impaired which leads to low levels of B12 in the blood.
Low levels of B12 in body leads to many problems . most notable is megaloblastic anemia (low hemoglobin with bigger red cells) and also neurological problems.
There are several possible causes of low B12 like
Strict vegetarian diet Stomach problems.
Problem with absorption in small intestine.
Problem with pancreas
Problem with transport in the blood
Problem with B12 metabolism at cell levels
. Heavy prolonged alcoholism.
You need a complete work up in order to know the underlying cause behind your low B12.
Its low level in blood is associated with anemia( low hemoglobin) Neurological problems like paresthesias or numbness, weakness, gait problem etc. In advance stages it may also cause memory problems.
You should take B12 shots as advised by your physician.
Consume diet comprising of egs, milk, meat etc.
you can also take oral B12 in the form of multivitamins.
Have complete work up by endocrinologist.
In early stages specially treated with B12 shots, it should not effect your daily life.
But as long as you have anemia( low hemoglobin) because of B12 deficiency, you should not be involved in any sternous physical exercise.
Best wishes for your health.