Vegan and vegetarian diets: which blood tests to have once a year

A well-balanced vegetarian or vegan diet is safe at every stage of life, as confirmed by position statements from leading dietetic associations. The key word is “balanced”. Several nutrients are present in plant-based diets in small amounts, in less readily absorbed forms or not at all, and deficiencies develop slowly, often without symptoms for a long time. Regular blood tests can identify a problem at a stage when adjusting your diet or supplements is enough. Below is a list of tests worth having after switching to a plant-based diet, and then repeating once a year.

Vitamin B12: the number one test

Vitamin B12 is found almost exclusively in animal-derived foods. Plants do not contain it, while spirulina, tempeh and fermented vegetables provide inactive forms that do not meet your needs. The liver stores enough for 2–5 years, so deficiency takes time to become apparent: first as fatigue and numbness in the limbs, and later as megaloblastic anaemia and nerve damage, some of which is irreversible. Everyone following a vegan diet should take B12 supplements, as should vegetarians who eat little dairy or few eggs.

The serum B12 level alone can be misleading: a result at the lower end of the reference range may already indicate a deficiency at cellular level. More accurate markers are holotranscobalamin (active B12), methylmalonic acid (MMA) and homocysteine, which rises with deficiencies of B12, folate and B6. In practice: B12 plus homocysteine once a year, with holoTC or MMA testing if results are borderline.

Iron and ferritin

A plant-based diet contains plenty of iron (pulses, grains, seeds and green vegetables), but in the non-haem form, which is absorbed several times less efficiently than iron from meat. Phytates and polyphenols (tea, coffee) further limit absorption, while vitamin C increases it. Vegetarians have lower iron stores on average, although they do not necessarily have anaemia more often. Ferritin is the best measure of iron stores; a full blood count with MCV and RDW shows whether deficiency is already affecting blood cell production. Women who menstruate, athletes and people who have lost a substantial amount of weight are at the greatest risk.

Vitamin D

This is not a problem specific to plant-based diets (the main source is production in the skin through sunlight exposure), but plant-based food sources provide very little, and many fortified products contain D2, which raises levels less effectively than D3. Measuring 25-OH-D in autumn helps determine the supplement dose; plant-derived forms of D3 (from lichens) are available.

Omega-3 fatty acids and lipid profile

A plant-based diet usually improves the lipid profile: it lowers total cholesterol and LDL. It is worth documenting this, as it is one of the most firmly established benefits. On the other hand, no fish means no ready-made EPA and DHA; the body produces them from alpha-linolenic acid (linseed, walnuts, chia), but with low efficiency. Testing the omega-3 index (Omega-3 Index) is available at larger laboratories, although it is not a routine test; many people choose to take algae-derived DHA supplements without testing.

Zinc, iodine, selenium and calcium

Iodine in a plant-based diet comes mainly from iodised salt; people who limit salt or use Himalayan salt or non-iodised sea salt may develop a deficiency that puts a strain on the thyroid. Iodine is measured in urine, while the effects of deficiency can be seen in TSH and FT4 results. Zinc testing in serum has limited sensitivity, but a clearly low result is significant in cases of hair loss, recurrent infections and skin changes. Selenium levels depend on its content in the soil and are low in Poland regardless of diet. Calcium in serum is tightly regulated and rarely falls outside the reference range; bone densitometry and vitamin D provide more information about bone health, while a dietary history helps assess calcium intake (fortified plant-based drinks, tofu, kale, poppy seeds and sesame seeds).

Protein, albumin and creatinine

With an adequate calorie intake, a plant-based diet provides enough protein, but deficiency is a real possibility in people who eat few pulses, have a low body weight or are older. Serum total protein and albumin only change when deficiency is substantial; an earlier sign is a loss of muscle mass and strength. Creatinine may be lower in vegans (because of a lower intake of creatine from meat), which should be taken into account when assessing kidney function.

TSH and full blood count as general checks

TSH rules out a thyroid problem that may be mistakenly attributed to diet (fatigue, hair loss, feeling cold). A full blood count with differential provides a basic overview: B12 and folate deficiencies cause large blood cells (high MCV), iron deficiency causes small ones (low MCV), and their coexistence may produce an MCV “within range” alongside a high RDW.

When to test

• 3–6 months after switching to a plant-based diet: full blood count, ferritin, B12, homocysteine, vitamin D, lipid profile and TSH.
• Every year: the same set of tests; if you take supplements, checking B12 and ferritin assesses whether the doses are sufficient.
• Additional testing during pregnancy, in children and teenagers following a plant-based diet, in athletes and after the age of 60 (reduced B12 absorption).

Attend for testing in the morning, on an empty stomach, after 3 days without alcohol. It is best to stop B12 and iron supplements for 24–48 hours before blood collection so that the result does not reflect the most recent dose, unless your doctor advises otherwise. Interpreting results and deciding on treatment are the responsibility of a doctor or clinical dietitian.

Frequently asked questions

Does a vegetarian who eats eggs and dairy need B12 supplements?

It depends on the amounts: two portions of dairy and one egg a day usually meet your needs, but B12 and homocysteine results determine this, not an assumption.

Ferritin within range, but still feeling tired: what next?

Check B12, vitamin D and TSH, and have a full blood count; fatigue has many causes, and ferritin is only one factor to consider.

Do you need a referral for these tests?

No. All the tests listed are available privately, with online purchase and sample collection at a collection point without booking a doctor’s appointment.

Sources

• Bakaloudi DR et al., Clinical Nutrition, Intake and adequacy of the vegan diet. A systematic review of the evidence, 2021. sciencedirect.com
• Narodowe Centrum Edukacji Żywieniowej (National Centre for Nutrition Education), NIZP-PZH (Narodowy Instytut Zdrowia Publicznego – Państwowy Zakład Higieny; National Institute of Public Health – National Institute of Hygiene), Dieta wegetariańska i wegańska: gdzie szukać składników niedoborowych (Vegetarian and vegan diets: where to find nutrients that may be lacking), 2020, updated 2023. ncez.pzh.gov.pl
• Academy of Nutrition and Dietetics, J Acad Nutr Diet, Position of the Academy of Nutrition and Dietetics: Vegetarian Diets, 2016. jandonline.org