Blood Tests for Vegetarians and Vegans: The Deficiency Screen
Plant-based diets have significant health benefits — but several nutrients are either absent from plants or poorly absorbed without animal products. These are the five blood tests every vegetarian and vegan should get annually.
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Whole-food plant-based diets are among the most evidence-backed dietary patterns for reducing cardiovascular risk, type 2 diabetes, and certain cancers. However, several micronutrients are either found only in animal products or are present in plant foods in forms that are poorly bioavailable. Without targeted supplementation and monitoring, deficiencies develop silently over months to years — and can cause irreversible neurological damage (B12) or anaemia (iron, B12, folate) long before symptoms appear.
Essential Tests for Plant-Based Eaters
| Nutrient | Test | Target Level | Plant-Based Risk Factor |
|---|---|---|---|
| Vitamin B12 | Serum B12 + MMA (methylmalonic acid) | B12 above 400 pg/mL; MMA normal | B12 is found almost exclusively in animal products — absent from plants. Supplements or fortified foods are essential for all vegans. |
| Iron (ferritin) | Ferritin + serum iron + transferrin saturation | Ferritin above 50 ng/mL | Plant iron (non-haem iron) has 2–3x lower bioavailability than haem iron. Phytates in beans and grains further reduce absorption. |
| Vitamin D | 25-OH vitamin D | 40–60 ng/mL | Vitamin D3 (the more effective form) comes primarily from oily fish and eggs. D2 (mushrooms, fortified foods) is less potent. |
| Omega-3 fatty acids | Omega-3 index (EPA+DHA % of red blood cell phospholipids) | Above 8% | ALA from plants converts to EPA/DHA at under 10% efficiency. Without algae-based omega-3 supplements, EPA and DHA are typically very low in vegans. |
| Zinc | Serum zinc | 80–110 µg/dL | Phytate in plant foods blocks zinc absorption; vegans have ~50% lower zinc absorption than omnivores |
| Calcium | Serum calcium (limited) — calcium intake via diet tracking is more useful | Serum 8.5–10.5 mg/dL | Dairy-free vegans must rely on fortified plant milks, tofu (calcium-set), leafy greens — easy to under-consume |
| Iodine | Spot urine iodine:creatinine ratio | Above 100 µg/g creatinine | Iodine comes primarily from dairy, seafood, and iodised salt. Vegans relying on sea salt or kosher salt may be deficient. |
Why Serum B12 Is Not Enough
Serum B12 can remain in the "normal" range for years after deficiency develops — because inactive B12 analogues from plant foods, nutritional yeast, and supplements cross-react with the assay. Methylmalonic acid (MMA) and homocysteine are functional markers that rise when B12 is genuinely deficient at the cellular level. If your serum B12 is in the 200–400 pg/mL grey zone, ordering MMA (and homocysteine) will tell you whether the cells actually have enough B12. Neurological damage from B12 deficiency — peripheral neuropathy, subacute combined degeneration of the cord — can be irreversible. Do not wait for symptoms.
Optimal Supplementation Targets for Vegans
Iron Absorption: Making the Most of Plant Sources
Vitamin C consumed with iron-rich plant foods (lentils, beans, tofu, fortified cereals, spinach) dramatically increases non-haem iron absorption — by up to 3–6 fold. Conversely, tea, coffee, and calcium supplements consumed with iron-rich meals inhibit absorption. If your ferritin is low, optimising these food combinations should be the first step before supplementation. If ferritin remains below 30 ng/mL despite dietary optimisation, iron supplementation — ferrous sulfate or bisglycinate (better tolerated) — is indicated.
Medical Disclaimer
This article is for educational purposes only and does not constitute medical advice. Reference ranges, supplement dosages, and nutritional information mentioned are general educational guidance from published research—not personalised recommendations. Do not use this content to self-diagnose or self-treat any condition. Always consult a qualified healthcare provider before making any changes to your health regimen, medications, or supplements.
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