Scientists warn that current vitamin B12 guidelines may be putting your brain at risk, while a vitamin D supplement may have a “previously unknown” downside
Recent scientific research has raised important questions about whether current nutritional guidelines are truly sufficient to protect long-term human health. In particular, new studies examining vitamin B12 and vitamin D suggest that accepted standards for supplementation and deficiency may overlook subtle but significant effects on the brain, nervous system, and immune function. These findings have prompted researchers to reconsider whether “normal” nutrient levels are always optimal and whether some commonly used supplements may be less beneficial than previously believed.
One major area of concern involves vitamin B12, an essential nutrient responsible for supporting DNA production, red blood cell formation, and healthy nerve tissue. Researchers at University of California, San Francisco recently found evidence suggesting that current vitamin B12 guidelines may not adequately protect the aging brain. Their study revealed that older adults with lower levels of active vitamin B12, even when those levels remained within the accepted normal range, showed measurable signs of neurological decline.
The research, published in Annals of Neurology, focused on healthy older individuals who did not have dementia or mild cognitive impairment. Despite appearing clinically healthy, participants with lower active B12 levels demonstrated slower thinking, delayed visual processing, and greater damage to the brain’s white matter. White matter consists of the nerve fibres that allow communication between different regions of the brain, making it essential for efficient cognitive functioning.
According to the researchers, these findings suggest that the current threshold used to define vitamin B12 deficiency may fail to identify early functional changes occurring within the nervous system. In other words, individuals may technically meet existing medical guidelines while still experiencing subtle neurological strain that could contribute to cognitive decline over time. The study’s senior author, Ari J. Green, MD, of the UCSF Departments of Neurology and Ophthalmology and the Weill Institute for Neurosciences, argued that current standards may focus too heavily on preventing severe deficiency while overlooking the nutrient levels required for optimal brain health.
At the same time, scientists are also reassessing assumptions surrounding vitamin D supplementation. A separate study conducted by researchers from the University of Surrey, John Innes Centre, and Quadram Institute Bioscience found that vitamin D2 supplements may unintentionally reduce the body’s levels of vitamin D3, the more biologically effective form of vitamin D.
Vitamin D is widely recognised for its role in maintaining bone health and supporting immune function, and millions of people take supplements to meet recommended daily intake levels, particularly during winter months when sunlight exposure is limited. However, the new findings suggest that vitamin D2 and vitamin D3 may not function equally within the body. Researchers discovered that vitamin D3 not only improved vitamin D status more effectively, but also appeared to play a unique role in strengthening immune responses against viruses and bacteria.
The study raises the possibility that some vitamin D supplements may inadvertently interfere with the body’s ability to maintain adequate levels of the more beneficial form of the vitamin. As a result, scientists are beginning to question whether vitamin D3 should become the preferred choice for supplementation. More broadly, the findings challenge the assumption that all forms of a nutrient provide identical biological benefits.
Taken together, these studies reflect a broader shift in nutritional science. Traditionally, dietary guidelines have focused on preventing obvious deficiencies and maintaining minimum acceptable nutrient levels. However, emerging research suggests that optimal health may require more nuanced standards that consider subtle effects on cognition, neurological integrity, immune function, and long-term disease risk.
The findings also demonstrate the complexity of human nutrition. Nutrient effectiveness may depend not only on quantity, but also on the specific biological form of a vitamin, age-related changes in metabolism, and the body’s ability to absorb and utilise nutrients effectively. As populations age and chronic neurological conditions become increasingly common, understanding these subtler nutritional influences may become essential for preventive healthcare.
Ultimately, the research highlights the importance of continually reassessing medical and nutritional guidelines in light of new scientific evidence. While existing recommendations remain valuable for preventing severe deficiency diseases, they may not fully account for the levels required to support optimal brain and immune health throughout life. Future research may therefore lead to more personalised and precise nutritional strategies aimed at preserving cognitive function, strengthening immunity, and improving overall wellbeing in aging populations.
Source
Vitamin B12 Levels Association with Functional and Structural Biomarkers of Central Nervous System Injury in Older Adults. Annals of Neurology, 2025; 97 (6): 1190 DOI: 10.1002/ana.27200
Effect of Vitamin D2 Supplementation on 25-Hydroxyvitamin D3 Status: A Systematic Review and Meta-Analysis of Randomized Controlled Trials Nutrition Reviews, 2025; DOI: 10.1093/nutrit/nuaf166