Do you recommend taking B12 supplement on a vegan diet? Would you only recommend it if symptoms occur (tingling in your fingers etc)? Could low B12 levels be causing inability to lose weight? If you recommend supplementing B12 what type of supplement would you recommend?
Deficiency in B12 is a concern for people eating a plant-based diet. This review paper nicely summarized early deficiency status for vegetarians and vegans of different age brackets:
However, because the markers used to measure deficiency are indicative of early deficiency, there may not yet be any symptoms present. This is an important point to consider - when are we calling something a deficiency because of low indicators that alert us to possible eventual symptoms and when are we calling something a deficiency because actual symptoms are exhibited?
Supplements have specific effects on the body, and it would be more accurate to consider them as medications, not as food. It is best to have your nutrient levels checked by your physician on a regular basis through routine bloodwork, and then to work with your physician to monitor the effects of any supplementation he/she feels is necessary. For general advice about whether or not take B12 supplements, Dr. McDougall's newsletter on B12 offers an excellent history on the subject and general guidelines to follow. His answer is yes, it is wise to consume a B12 supplement, and methylcobalamin or hydroxycobalamin may work better than cyanocobalamin.
In fact, B12 deficiency is a problem that is not limited to plant-based eaters - it exists broadly throughout the population, particularly in the elderly, as discussed in this paper: Vitamin B12 and older adults
B12 is not typically a reason for an inability to lose weight, but it is involved in metabolism and it is always possible that there are subtle effects we are not yet aware of. However based on what we know now, it is much more likely that consuming even small amounts of animal foods or processed foods that contain added sweeteners and/or oil are the culprits. Lack of exercise also plays a role, as can stress.