Why Am I Tired After Going Vegan?

Written by the Nuvirox Research Team

Short answer: fatigue after going vegan is usually traceable to B12 or iron status, not to plant-based eating itself. The research is fairly consistent that a well-planned, supplemented vegan diet performs comparably to an omnivorous one on most nutritional markers — the fatigue risk shows up specifically when supplementation is skipped.

Why would going vegan cause fatigue at all?

Vitamin B12 exists reliably only in animal-source foods (or fortified products and supplements). It's essential for red blood cell formation and nerve function, and low B12 directly causes fatigue through impaired oxygen transport. A cross-sectional study comparing German omnivores, vegetarians, and vegans found vegans had the lowest intake of B12 from foods, compared to omnivores and lacto-ovo-vegetarians, which is exactly what you'd expect given the food sources involved.

Does this mean all vegans end up B12 deficient?

No — and this is the important honest nuance. The same study found that analysis of B12 status revealed adequacy in omnivores and vegans, and a poorer B12 status amongst lacto-ovo-vegetarians, because a large majority of the vegan participants were actively supplementing. The number of participants supplementing vitamin B12 was highest in vegans (92%) in that sample, taking a median dose of 250 micrograms daily. In other words, vegans who supplement tend to do fine; the risk clusters in people who switch to a vegan diet without addressing B12 directly.

A broader scoping review of 70 studies on vegan B12 status reached a similar conclusion, noting that insufficient intake of these nutrients among vegans can result in health issues such as fatigue, anemia, depression, and cognitive impairment, while also emphasizing that regular monitoring and appropriate supplementation address the gap effectively.

What about iron?

Plant-based (non-heme) iron absorbs less efficiently than the heme iron found in meat, and menstruating women following vegetarian or vegan diets face a compounded risk since monthly blood loss increases iron needs. Low iron produces fatigue through a similar mechanism to B12 deficiency — reduced oxygen-carrying capacity in the blood. Pairing iron-rich plant foods with vitamin C, and being deliberate about iron-dense choices like legumes, tofu, and fortified grains, meaningfully improves absorption. Anyone specifically noticing fatigue that tracks with their menstrual cycle, on any diet, may find our piece on why energy dips before and during your period a useful complementary read, since cyclical iron loss compounds with dietary iron availability.

What actually helps if you're tired after going vegan?

A daily B12 supplement (fortified foods alone are often inconsistent), attention to iron-rich plant foods paired with vitamin C, and a basic bloodwork check (B12, ferritin, complete blood count) 2–3 months after transitioning are the most evidence-supported steps. These aren't guesses — they map directly onto the nutrients research repeatedly flags as the ones most likely to run low on an unsupplemented plant-based diet. General nutrient gaps like this are a recurring theme across fatigue research broadly, and our overview of supplements for fatigue over 40 covers several other commonly overlooked deficiencies worth ruling out alongside B12 and iron.

It's also worth checking whether your fatigue is tracking with total calorie intake rather than nutrient quality alone — plant-based diets are often less calorie-dense per volume, and an inadvertent calorie deficit during the transition can produce a very similar tired, low-energy feeling that resolves once intake catches up, independent of any specific nutrient gap.

What a vegan diet, on its own, won't fix

If fatigue persists despite supplementation and a few months on the diet, or if you notice numbness, tingling, or memory issues alongside the tiredness, that combination warrants bloodwork and a conversation with a doctor rather than further dietary guessing — B12 deficiency that goes untreated long enough can produce neurological symptoms that don't fully reverse.

It's also worth acknowledging a limitation in how this research generalizes: much of the strongest B12 supplementation data comes from health-conscious, engaged study volunteers who are more likely than the general population to supplement consistently and correctly. Real-world adherence to a daily B12 regimen is almost certainly less uniform, which is part of why periodic bloodwork, rather than assuming supplementation alone has "solved" the issue, remains the more reliable check.

Frequently asked questions

How soon after going vegan would B12-related fatigue show up?
B12 stores in the liver can take months to years to deplete, so fatigue from B12 deficiency specifically often appears later rather than immediately — early fatigue is more likely tied to overall dietary adjustment or reduced calorie intake, which typically resolves faster as your body adapts to new meal patterns and food sources.

Can I get enough B12 from fortified foods alone?
It's possible but inconsistent, since fortification levels vary by product and serving size; most of the research on adequate vegan B12 status involves direct supplementation rather than fortified foods alone.

Is fatigue after going vegan more common in women?
Iron-related fatigue risk is disproportionately higher in menstruating women on any lower-iron-availability diet, vegan or not, simply due to higher iron needs from monthly blood loss.

Should I get bloodwork before or after going vegan?
A baseline check before transitioning, and a follow-up a few months in, gives you the clearest picture of whether your specific supplementation approach is actually working for your body.

Are there other nutrients besides B12 and iron worth watching on a vegan diet?
Vitamin D, zinc, and omega-3 fatty acids (specifically the long-chain forms EPA and DHA) are also commonly flagged in vegan nutrition research as worth monitoring, though the fatigue link is most directly established for B12 and iron specifically.

Is a vegan diet inherently lower in energy-supporting nutrients than an omnivorous one?
Not inherently — the German cross-sectional study found comparable energy intake and macronutrient distribution across omnivore, vegetarian, and vegan groups, with the meaningful differences concentrated in specific micronutrients rather than overall energy adequacy, reinforcing that this is a supplementation and planning issue, not a structural flaw in plant-based eating patterns generally.

From Nuvirox

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The bottom line: fatigue after going vegan is well-documented in the research, but it's tied to specific, correctable nutrient gaps — primarily B12 and iron — rather than being an unavoidable consequence of plant-based eating. Supplementation and periodic bloodwork are the evidence-backed fix.

References

1. A cross-sectional study of nutritional status in healthy, young, physically-active German omnivores, vegetarians and vegans reveals adequate vitamin B12 status in supplemented vegans. Ann Med. 2023. PMCID: PMC10586079.
2. Exploring Vitamin B12 Supplementation in the Vegan Population: A Scoping Review of the Evidence. Nutrients. 2024;16(10):1442. PMCID: PMC11124153.

*These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease. This article is for informational purposes only and is not a substitute for professional medical advice.