I am Dr. David Marconi, a physician at Internal Medicine, Lipid, and Wellness. This blog is inspired by real patients, their concerns, their health journeys, and the insights I gain while caring for them – offering a window into the clinical reasoning and practical wisdom that shape my approach to medicine.
Vitamin B12 plays a critical role in human physiology, serving as an essential cofactor in DNA synthesis, neurologic function, and cellular metabolism. It is fundamental to red blood cell production and the maintenance of healthy nerve tissue, particularly through its involvement in myelin synthesis. Deficiency can manifest in a wide spectrum of clinical presentations, ranging from fatigue and anemia to neuropathy and cognitive changes. Given its broad physiologic impact, maintaining adequate B12 levels is not simply about correcting a lab value, but about preserving systemic function at both the cellular and neurologic levels.
What you, my patients, have taught me, however, is that B12 replacement is far more nuanced than I was originally trained to believe. While traditional teaching often treats supplementation as uniform, clinical experience reveals meaningful variability in patient response to different cobalamin forms, including cyanocobalamin, methylcobalamin, hydroxocobalamin, and adenosylcobalamin. Genetic testing aimed at identifying methylation patterns can provide some guidance, often narrowing the field to one or two more likely candidates. However, even with this information, it rarely eliminates the need for individualized trial and observation. In practice, I have found that monitoring B12 levels alongside functional markers such as homocysteine and methylmalonic acid—combined with careful attention to symptom response—offers the most clinically useful insight. By rotating through different formulations, patterns of improvement emerge that help tailor therapy to the individual.
I am grateful to the patients who have partnered with me in this process, those willing to explore, adjust, and reflect alongside me as we work toward better health. One patient said it best: “We practice medicine together.” That sentiment captures something essential about this work. The lessons learned are not mine alone; they are shared, built over time through trust, curiosity, and collaboration. It is through these partnerships that I continue to grow as a physician, carrying each insight forward to help the next person who sits across from me.