Vitamin B12 and Peripheral Neuropathy in Vegans and Vegetarians

Vitamin B12 (cobalamin) occurs naturally almost exclusively in animal-derived foods — meat, fish, eggs, and dairy. This means people following a vegetarian, and especially vegan, diet are structurally exposed to lower dietary intake of this vitamin, regardless of how well-balanced the rest of their diet is in other respects. We cover this issue in more depth in our entry on vitamin B12, where we also discuss its absorption mechanism and role in the body.

B12 deficiency usually develops slowly, because the body stores reserves in the liver sufficient for several years of function even with zero dietary intake — which, paradoxically, makes the problem harder to catch early, since symptoms appear only once those reserves run out, sometimes years after switching to a plant-based diet. Among the most serious, potentially irreversible consequences of chronic B12 deficiency is damage to the myelin sheath of nerve fibers, leading to peripheral neuropathy.

Even though the link between B12 deficiency and neuropathy has long been known, relatively few studies have measured this problem using concrete, objective functional tests in people on plant-based diets — most existing knowledge was based on descriptions of advanced, clinically overt cases, rather than a systematic look at subclinical changes at an early stage.

In other words: nearly a quarter of healthy, young adults on a long-term plant-based diet already had marginal or deficient B12 levels — and those with worse B12 status performed worse on objective hand dexterity tests, despite being young and reporting good overall physical fitness. This matters because it shows that subclinical, detectable functional changes can appear before a B12 deficiency becomes severe enough to produce clear, obvious clinical symptoms.

The 26% rate of participants with marginal or deficient B12 in this specific study is consistent with broader epidemiological data cited by the authors, indicating that B12 deficiency prevalence among adult vegetarians in the US may exceed 30%. That's significantly higher than in the general population and shows the problem isn't marginal or limited to extreme, poorly balanced cases of plant-based eating — it affects a substantial share of people who might consider their diet fully healthy and well thought out.

Vitamin B12 is essential for proper synthesis and repair of the myelin sheath — the fatty covering around nerve fibers that enables fast, efficient nerve impulse conduction. Chronic deficiency leads to myelin degeneration, most often first in the long peripheral nerve fibers, which explains why symptoms usually appear first in the hands and feet — the body parts farthest from the central nervous system, served by the longest nerve fibers.

The manual dexterity and balance tests used in the study discussed here are precisely sensitive, objective indicators of these long nerve fibers' function — hence their choice as tools for measuring subclinical neuropathy before more obvious symptoms, such as numbness, tingling, or gait disturbances, appear.

This distinction has practical significance: relying solely on subjective wellbeing ("nothing's bothering me, so I probably have enough B12") isn't a reliable way to rule out deficiency in someone on a long-term plant-based diet without supplementation. The only way to actually check is a specific blood test.

Despite its small sample, this study has real practical value: it shows that the consequences of B12 deficiency on a plant-based diet can be measurable, objective, and earlier than we might expect based on wellbeing alone. For anyone who's eaten fully plant-based for years and feels fine, it's a signal not to treat the absence of symptoms as proof of adequate B12 intake.