Two micrograms is an almost unimaginably small amount. Its weight is less than a small piece of a grain of table salt. However, adults only need this amount of vitamin B12 each day, depending on the guidelines used, to support red blood cells, nerves, and DNA production.
2026 marks 100 years since George Minott and William Murphy reported that a liver-rich diet could treat pernicious anemia, a deadly disease at the time. Their research transformed medicine, eventually leading scientists to identify vitamin B12 as a substance in the liver that treats disease.
However, the path to breakthrough began with an unexpected clue from animal experiments. American physician and pathologist George Whipple showed that liver can help dogs reverse anemia caused by blood loss. Blood loss anemia occurs when the body loses red blood cells through bleeding. Pernicious anemia is different. The problem is not the bleeding, but the malabsorption of vitamin B12. Still, Whipple’s experiments pointed researchers to the liver as a source of potent hematopoietic factors.
Patients who were on the verge of death from pernicious anemia often improved dramatically within a few weeks after eating a diet rich in liver. Successful liver treatments eventually led scientists to isolate the deep red compound now known as vitamin B12 or cobalamin.
often mistaken
Despite decades of research, vitamin B12 deficiency remains common, especially among older adults, vegans, vegetarians, and people with diseases that affect absorption. Some people don’t get enough vitamin B12, as B12 is primarily found naturally in animal foods such as meat, fish, eggs, and dairy products. Some people have a hard time absorbing it properly.
This becomes more common with age. Some older people produce less stomach acid, which is needed to release vitamin B12 from food. Some people develop autoimmune gastritis. In this case, the immune system damages the stomach cells responsible for producing acid and intrinsic factor (a protein needed for the absorption of vitamin B12). Weight loss surgery and some medications used for diabetes and acid reflux disease can also reduce absorption.
Deficiency symptoms can develop slowly and are often mistaken for normal aging. You may feel tired, weak, and short of breath. Some people develop numbness or tingling in their limbs, loss of balance, memory problems, or what many people describe as “brain fog.” These symptoms are not specific to vitamin B12 deficiency, so persistent fatigue, tingling, and balance issues should be checked instead of assuming it’s a simple vitamin problem.
People at higher risk, such as vegans, vegetarians, older adults, and people taking medications that affect stomach acid or diabetes treatment, may need testing and supplement advice from a health care professional.
Doctors have traditionally associated fatigue caused by vitamin B12 deficiency with anemia. Without enough vitamin B12, your bone marrow cannot produce healthy red blood cells. Instead, abnormally large and immature cells are released, unable to efficiently transport oxygen throughout the body.
But anemia may not be the only reason people with low vitamin B12 feel tired.
low energy
In humans, vitamin B12 is directly required only by two enzymes, which are proteins that help in the chemical reactions that occur in the body. One, it helps the body make the DNA it needs when cells divide. The other helps mitochondria process certain fat and protein components. Mitochondria are small structures within cells that help turn food into usable energy.
This role of mitochondria is of increasing interest from researchers studying aging, muscle function, and vitamin B12 status. A 2026 study investigated what happens when cells don’t have enough vitamin B12. Researchers found that low B12 can interfere with DNA in mitochondria, leading to decreased energy production in laboratory models of skeletal muscle (muscle cells studied outside the human body).
A related study in older female mice found that B12 supplementation improved several indicators of muscle mitochondrial health, including mitochondrial number and structure. Taken together, this study shows one reason why some people with low vitamin B12 report fatigue before overt anemia is detected.
These findings do not mean that vitamin B12 supplements reverse aging or act as an energy booster for people whose vitamin B12 levels are already normal.
Scientists have long suspected a link between B12 and mitochondrial function because one of the two B12-dependent enzymes functions within mitochondria. Previous research has also suggested that lower vitamin B12 levels may be associated with decreased muscle function in older adults, but much of this research is observational and cannot prove causation.
So, if you’re feeling persistently tired, is it worth getting a vitamin B12 injection at a wellness clinic or medspa? For most people, it’s not. Vitamin B12 injections are an established treatment for diagnosed vitamin B12 deficiency, particularly when there is malabsorption, and the NHS uses hydroxocobalamin injections for vitamin B12 deficiency anemia.
However, there is little evidence that taking vitamin B12 improves energy, weight loss, or performance in people who already have normal vitamin B12 levels. A more useful first step is to determine the cause of your fatigue.
The story of vitamin B12 is unusual because even though the amount of vitamin B12 required by the body is very small, the effects of deficiency can be severe. Long before scientists understood its chemical properties, doctors knew that something in the liver could restore strength, appetite, and vitality in critically ill patients.
A century later, researchers are still discovering that this tiny cobalt-containing molecule can do more than just prevent anemia. It may also help explain how cells maintain energy and function as the body ages.
