Persistent fatigue that a good night's sleep cannot fix may stem from a vitamin B-12 deficiency, according to new breakthrough studies. The research indicates that B-12 is not only crucial for brain health but also critical for maintaining healthy muscles, particularly as people age.
Study Links Low B-12 to Muscle Impairment
A 2026 study published in the Journal of Nutrition by researchers at Cornell University and the University of Alabama at Birmingham found that low vitamin B-12 levels can disrupt mitochondrial function in skeletal muscle cells. Mitochondria convert food and oxygen into energy needed for movement and physical activity. In mice with a mild B-12 deficiency, the researchers observed notably impaired skeletal muscle function and energy production.
“The mice had physically smaller muscles,” said Martha Field, an author of the study and associate professor in the division of nutritional sciences at Cornell University. “We see impacts on muscle mitochondrial function with what I think is a mild deficiency.”
B-12 Supplementation Improves Mitochondrial Health
Field’s collaborator, Anna Thalacker-Mercer of the University of Alabama at Birmingham, conducted a similar study on aging mice that received vitamin B-12 supplementation for 12 weeks. The supplementation improved mitochondrial biology and helped maintain muscle mass in older mice.
“We think that the B-12 supplementation was helping to maintain muscle mass in the older mice,” Thalacker-Mercer said. She noted that while B-12 is not a “holy grail” for muscle aging or sarcopenia, it could significantly help some people attenuate muscle loss.
B-12 Absorption Declines with Age
Vitamin B-12 is not produced by the body and is primarily found in animal products, making vegans and vegetarians prone to deficiency. However, even those with adequate dietary intake may have trouble absorbing it as they age. One estimate suggests that 1 in 8 adults over 50 have 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,” Thalacker-Mercer explained.
A sedentary lifestyle and reduced physical activity already impact muscle aging, but B-12 deficiency may be an additional important driver. Field emphasized that B-12 deficiencies do not mimic aging but can “exacerbate” impaired muscle function. For older adults, a deficiency might manifest as fatigue during routine activities, such as walking through a grocery store.
Human Studies Support Findings
While the mouse studies are preclinical, observational human studies also link improved B-12 levels to better muscle mass and function. A small 2024 study on vitamin B-12 deficient elderly individuals found that muscle strength improved after a three-month regimen of oral B-12 supplements.
Consult a Doctor Before Supplementing
Experts advise seeking medical guidance before taking B-12 supplements. A 2020 study in JAMA Network Open suggested 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.” Thalacker-Mercer added, “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.”
