Adults need only about two micrograms of vitamin B12 each day, but the nutrient’s role in health is substantial. It supports red blood cell formation, nerve function and DNA production. In 2026, medicine marks a century since George Minot and William Murphy showed that liver-rich diets could treat pernicious anemia.
That discovery followed George Whipple’s animal studies, which found that liver helped dogs recover from blood-loss anemia. Pernicious anemia has a different cause—poor B12 absorption—but the work directed researchers toward liver as a source of a blood-forming factor. Scientists later isolated the compound now called cobalamin, or vitamin B12.
Deficiency remains common, especially among older adults, vegans, vegetarians and people with absorption problems. B12 is found naturally mainly in animal foods, including meat, fish, eggs and dairy. Ageing can reduce stomach acid, needed to release B12 from food, and autoimmune gastritis, weight-loss surgery or some diabetes and reflux medicines can also limit absorption.
Symptoms can develop gradually and may be confused with normal ageing. They include fatigue, weakness, breathlessness, numbness or tingling, balance difficulties, memory problems and “brain fog.” Because these signs have many possible causes, health professionals advise assessment rather than assuming a vitamin deficiency. Higher-risk groups may need testing or guidance on supplementation.
Doctors have long linked B12 deficiency fatigue to anemia, since insufficient B12 disrupts production of healthy red blood cells. Newer research is examining another pathway: mitochondria, the cell structures that convert food into usable energy. Muscle studies and mouse research suggest low B12 may impair mitochondrial DNA and energy processes, though evidence is still developing.
Researchers caution that these findings do not show B12 supplements reverse ageing or boost energy in people with normal levels. Injections are established treatment for diagnosed deficiency, particularly impaired absorption. For tiredness, the recommended first step is identifying the cause.