It has been estimated that vitamin B12 deficiency is present in about 20% of the general population of industrialized countries. 39% of 2,999 subjects in the Framingham Offspring study, were found to be B12 deficient. If this is true of the rest of America, more that 80 million adults may have B12 deficiency - a condition that may be undetected or unrecognized.
Because B12 comes from animal source foods, vegans and vegetarians have been shown to have the lowest dietary intake. It has been reported that between 70% and 80% of vegans have a B12 deficiency and need supplementation.
Over 200,000 gastric bypass surgeries are performed in the US every year. The most popular type of surgery removes part of the stomach that makes Intrinsic Factor which can cause lifelong B12 deficiency. 
Antibodies produced by the body in the progression of autoimmune disease, can destroy Intrinsic Factor or the cells in the stomach that make it. This process is known to cause severe B12 deficiency and is called Pernicious anemia. 
Many diabetics who take a biguanide develop vitamin B12 deficiency. Antacids , PPIs or H2 blockers used for GERD or acid reflux reduce IF production. This also happens when the stomach lining gets thinner as people grow older (>50 years) and this leads to reduction in B12 absorption. [6,7,8,9]
Numbness, tingling, shooting pains in feet and poor balance
Pallor, low energy, light-headedness and weakness
Higher risk for heart attacks, heart failure and stroke
Memory, attention and cognitive decline
Folate, B12 and choline are very important during early pregnancy to promote healthy fetal development. Adequate intake (400mcg per day) of folate by pregnant women may reduce the risk of serious birth malformations like Neural Tube Defects (e.g. spina bifida). These nutrients are especially important to promote normal brain development.
Most current options give massive doses (500 - 5,000mcg) of B12 when the body can only use about 5 mcg/day. The large excess is not only wasted, but has been associated with increased risks of cancer and other serious problems.