If you feel tired or weak and no good night’s sleep can cure it, that bone-deep weariness might be from a vitamin B-12 nutrient deficiency.New breakthrough studies suggest that vitamin B-12 is not only important for supporting a healthy brain, but can be critical for supporting and maintaining healthy muscles, especially as people age. In a 2026 study published in the journal of Nutrition, researchers at Cornell University and the University of Alabama at Birmingham found that low vitamin B-12 levels could stop mitochondria from functioning properly for skeletal muscle cells.Mitochondria are cellular structures that turn what you eat and the oxygen you breathe into energy that can get you out of a chair and lift heavy packages. And in this study, researchers created a mild B-12 deficiency measured by the functional biomarkers in mice; this deficiency led to notably impaired skeletal muscular function and energy production.The mice “had physically smaller muscles,” Martha Field, an author of the study and an associate professor in the division of nutritional sciences at Cornell University, told HuffPost. “We see impacts on muscle mitochondrial function with what I think is a mild deficiency.” “It needs a human trial, it’s preclinical, but it sets the stage for an impact of B-12 status on muscle function,” Field said. Losing muscular strength and mass, or sarcopenia, is a common frustration with aging ― but perhaps vitamin B-12 can help slow this down. Field’s collaborator, Anna Thalacker-Mercer of University of Alabama at Birmingham, did a similar study on aging mice that received a vitamin B-12 supplementation for 12 weeks, and found that B-12 supplementation improved mitochondrial biology.“We think that the B-12 supplementation was helping to maintain muscle mass in the older mice,” Thalacker-Mercer told HuffPost.In this way, B-12 supplementation can improve mitochondrial health and help combat weakened muscular function.“It’s not the holy grail for muscle aging or sarcopenia, but it could really help at least some people to attenuate the loss,” Thalacker-Mercer suggested. A boy and the sea via Getty ImagesVitamin B-12 is a misunderstood nutrient. It has been linked to anemia, but new studies are suggesting it is critical for maintaining muscle mass and strength as we age.Increasing Vitamin B-12 Could Support Aging Muscles Vitamin B-12 is a nutrient your body cannot make on its own. It is found primarily in animals, so vegans and vegetarians are prone to being deficient in B-12. But just eating more eggs and steaks won’t automatically cure a B-12 deficiency. Exact numbers vary, but one estimate found that 1 in 8 adults over 50 reported low B-12 status. “Even if our diet stays the same throughout our entire life, our ability to absorb these things through the intestines and the [gastrointestinal tract], it changes” as we age, Thalacker-Mercer said. A sedentary lifestyle and being less physically active already impact how muscles age, but perhaps B-12 deficiency is another important driver people should not ignore, these studies suggest. Field said B-12 deficiencies are not mimicking the signs of aging, but could be “exacerbating” impaired muscle function. For an older adult, a vitamin B-12 deficiency might appear as no longer being able to walk through the entire grocery store without feeling fatigue, Thalacker-Mercer said as an example. If your energy levels keep dropping, “that could be due to mitochondrial dysfunction and B-12,” Thalacker-Mercer said. Field and Thalacker-Mercer’s studies were done on mice, not humans, but there have been other observational studies done on humans that show a link between improving B-12 levels and strengthening muscle mass and function. One small 2024 study done on vitamin B-12 deficient elderly people found that their muscle strength improved after three-month regimen of oral B-12 supplements. If you suspect you are low on B-12, it’s best to seek professional medical help, so your physician can assess if you are deficient in this nutrient and determine the best supplemental dosage. You don’t want to take too much B-12 without medical guidance –– one 2020 study in JAMA Network Open suggests that very high concentrations of vitamin B-12 were linked to early deaths. “I always tell people the best thing you can do is know your status,” Field said. “Talk to your doctor, have your [B-12] status measured, and then make an informed decision whether a supplement would be appropriate.”“At the very least, have your doctor check your B-12 levels, because a lot of doctors don’t, and it’s an easy test to run,” Thalacker-Mercer said.