I also posted this on MC's thread, but wanted to keep the B12 info together. This is from
www.felineconstipation.org/prevention.html (bolding is mine):
Nerve sheaths, the outer protective covering of nerves, need good body levels of Vitamin B12 to ensure nerve integrity, so that the nerves inside the sheaths are not vulnerable to exposure and do not 'short out' as it were. The usual form of supplemental B12, added to most commercial cat foods and supplements, is cyanocobalamin which requires conversion in the body to an active form. Cyanocobalamin is actually an artifact of the purification process when synthesizing Vitamin B12. The methylcobalamin form of Vitamin B12 does not require the same conversion and is now more readily available for purchase for cats who require supplemental Vitamin B12.
The entire family of B vitamins called B Complex are important to help maintain gut health and to prevent constipation. Vitamins are not nutrients, they are co-enzymes, that is they assist enzymes to do their jobs. Enzymes are not only involved in digestion, enzymes play a role in all metabolic functions and each enzyme is specific to its task. Without adequate B vitamins such as folic acid and Vitamin B12, enzymatic function suffers. First to suffer are those areas of the body with faster cellular turnover such as the gut wall and the blood components. If gut barrier function is reduced, the body itself is more vulnerable to trouble and the enteric nervous system is at risk of damage.
I should add that the paragraph following what I posted warns against oversupplementing. “The B vitamins are water soluble and so more 'forgiving' as any extra can more easily be dumped into urine rather than accumulating in the body. Still unnecessary supplementation, above demonstrated or diagnosed need, is to be avoided.” I’m personally not concerned about over-supplementing B12, but I’m not an expert and wanted to include this so the information is as completed and balance as possible.