Pregnancy , hyperthyroidism, and advanced stages of cancer may increase the body's requirement for B12, sometimes leading to a deficiency state. Low stomach acid , known as hypochlorhydria, interferes with the absorption of B12 from food but not from supplements. Aging is associated with a decrease in the normal secretion of stomach acid. As a result, some older people with normal levels of intrinsic factor and with no clear cause for malabsorption will become vitamin B12 deficient unless they take at least a few micrograms per day of vitamin B12 from supplements. Nerve damage caused by vitamin B12 deficiency can be irreversible.
Most symptoms can occur before the deficiency is severe enough to cause anemia.
Healthcare professionals have a series of laboratory tests that can determine B12 deficiency at earlier stages that are not accompanied by anemia. Pernicious anemia is a serious medical condition. When fatigue, often the first symptom of pernicious anemia, is present, a qualified healthcare practitioner should be consulted. Symptoms of pernicious anemia can be caused by other conditions, none of which would respond to vitamin B12 supplementation. Thus, only vegans would be helped significantly by consuming foods containing vitamin B12.