Vegan nutrient deficiencies and the tests that catch them

Vegans most often run low on vitamin B12, then iodine, the long-chain omega-3s EPA and DHA, vitamin D, iron, zinc and calcium. A standard blood panel catches most of these late, so a reassuring result can arrive years after the intake gap began. With those gaps covered, a well-planned vegan diet can meet nutrient needs at every age, with professional support in pregnancy and for children ¹.

Which nutrients vegans run low on

The Academy of Nutrition and Dietetics, the largest US body of dietitians, reviewed the evidence in 2016 and concluded that vegan diets work across the life cycle when they are planned ¹. A systematic review of 141 studies, published at the end of 2021, then mapped where plant-based eaters fall short: intake and status of vitamin B12, vitamin D, iron, zinc, iodine and calcium were generally lower than in meat eaters, EPA and DHA intake was lower, and vegans had the lowest B12, calcium and iodine intakes of all ⁵. Each of these vegan deficiencies has a known fix and a specific test, and for most of the list the useful test is not the one on a routine panel.

Appropriately planned vegetarian, including vegan, diets are healthful, nutritionally adequate, and may provide health benefits for the prevention and treatment of certain diseases. These diets are appropriate for all stages of the life cycle, including pregnancy, lactation, infancy, childhood, adolescence, older adulthood, and for athletes.Academy of Nutrition and Dietetics, position paper on vegetarian diets, 2016 ¹

Vitamin B12 has no reliable plant source

Vitamin B12 is made by bacteria, and animal foods carry it because animals store it. Seaweed, spirulina, chlorella and tempeh are sometimes sold as vegan sources, but much of their B12 is a look-alike form (an inactive analogue) that the body cannot use ¹⁶. In the EPIC-Oxford study of 689 British men, 52% of the vegans had serum B12 below 118 pmol/L (about 160 pg/mL, the unit most US labs use), against 7% of vegetarians and one meat eater out of 226 ².

The Vegan Society gives three equivalent routes: fortified foods two or three times a day for at least 3 µg in total, a daily supplement of at least 10 µg, or a weekly one of at least 2,000 µg ¹⁶. It recommends cyanocobalamin, the most stable and best-studied form; methylcobalamin also works, but there is no good evidence that it is better for people who absorb B12 normally.

The liver holds several years of B12, so a person who went vegan last year can have a normal serum B12 while stores fall. Methylmalonic acid (MMA), a by-product that builds up when cells lack B12, and homocysteine, an amino acid that needs B12 and folate to be cleared, usually rise before serum B12 drops below its cutoff. MMA also rises when the kidneys filter less well, so a high MMA is read alongside creatinine or eGFR. Active B12 (holotranscobalamin, the share of B12 bound to the protein that delivers it to cells) falls early too. Symptoms, causes and treatment are on the B12 deficiency page, and the high homocysteine page reads that result.

Iodine comes mainly from dairy and fish

Iodine feeds the thyroid, and in the UK and much of Europe most of it comes from milk, dairy and fish. A 2017 study of UK milk-alternative drinks found most were not fortified with iodine, and the unfortified ones held about 2% of the iodine in cow's milk ³. In the US, iodized salt helps, but the salt in packaged food and sea salt is usually not iodized. The US recommended intake is 150 µg a day for adults, 220 µg in pregnancy and 290 µg when breastfeeding, the stages when iodine shapes a baby's brain development ¹⁴.

Seaweed is an unreliable answer. Nori carries modest amounts, while kelp can carry so much that it disturbs the thyroid in the other direction, so seaweed snacks or kelp tablets should never stand in for a measured source. If you have a thyroid condition or take thyroid medication, check with your prescriber before adding an iodine supplement.

For testing, urine iodine reflects the last day or two of intake and is the status test used in population surveys; one spot sample is noisy for an individual. TSH and free T4 move only after the thyroid can no longer compensate. The thyroid panel guide explains those numbers, and a raised TSH is covered on the hypothyroidism page. If you have no clear iodine source, ask your clinician whether a urine iodine test or a supplement suits you, especially before pregnancy.

How flax omega-3 differs from fish omega-3

Flax, chia, hemp and walnuts provide ALA (alpha-linolenic acid), a short-chain omega-3. The body converts only a small fraction of ALA to EPA (eicosapentaenoic acid) and DHA (docosahexaenoic acid), the long-chain forms found in fish and in brain and heart tissue, and DHA conversion is the lowest of all ¹². The omega-3 index measures EPA plus DHA as a percentage of the fat in red cell membranes.

In a 2015 study of 165 long-term vegans, the average omega-3 index was 3.7%. In a subgroup that started below 4%, four months of an algae-derived supplement giving 254 mg of EPA plus DHA a day raised the index from 3.1% to 4.8% ⁷. Fish get their DHA from algae, so algae oil supplies the same molecules. The European Food Safety Authority sets 250 mg of EPA plus DHA a day as adequate for adults ¹⁸, and many algae oils supply that in one or two capsules; the fish oil vs algae oil comparison sets the two side by side, and the omega-3 page covers what the trials found.

Vitamin D runs low for most people in winter

Few vegan foods contain vitamin D beyond fortified plant milks and UV-exposed mushrooms, and the 2021 review found vegans' vitamin D intake and status lower than meat eaters' ⁵. In the UK, sunlight from October to March is too weak to make vitamin D in the skin, so UK advice, based on the Scientific Advisory Committee on Nutrition (SACN) report, is that everyone over four consider a supplement in those months ¹⁷.

Most D3 is made from sheep's wool lanolin, but vegan D3 from lichen exists, and D3 raised blood levels more than D2 in a 2012 meta-analysis by Tripkovic and colleagues ⁹; the D3 vs D2 comparison has the detail. The standard blood test is 25-OH vitamin D (25-hydroxyvitamin D, the stored form), reported in ng/mL in the US and nmol/L elsewhere. The UK treats under 25 nmol/L (10 ng/mL) as deficient ¹⁷; US guidance treats under 30 nmol/L (12 ng/mL) as deficient and 50 nmol/L (20 ng/mL) or above as adequate ¹⁹, and some labs use 75 nmol/L (30 ng/mL) as a target. Low results are read on the vitamin D deficiency page.

CalculatorVitamin D converterConvert a 25-OH vitamin D result between ng/mL and nmol/L before comparing it with a cutoff.Work it out

Iron stores run lower, anemia less often

Plant foods carry only non-heme iron, which the gut absorbs less well than the heme iron in meat. For that reason the US recommended iron intake for vegetarians is 1.8 times the figure for meat eaters ⁶. Vegans' iron status tends to be lower ⁵, but low ferritin (the protein that stores iron) does not always mean anemia, because the gut absorbs a larger share of iron as stores fall. Women who menstruate carry the highest risk.

Ferritin is the first test, read with transferrin saturation (how full the iron-carrying protein is) and hemoglobin; the iron studies guide walks through all of them, and the ferritin vs iron comparison explains why serum iron alone misleads. Vitamin C in the same meal raises non-heme absorption, while tea, coffee and calcium supplements taken with meals cut it; the iron-rich foods guide has the food lists. A low result is read on the iron deficiency page.

Zinc and calcium have no reliable blood test

Phytates, the phosphorus stores in whole grains, beans and seeds, bind zinc in the gut, and vegetarians may need up to 50% more zinc than meat eaters ¹³. Soaking, sprouting and fermenting (as in sourdough and tempeh) lower phytate content. Serum zinc is a blunt test: it drops after meals, later in the day and during any inflammation, so a single low value needs a fasting morning repeat. Zinc supplements taken for months can lower copper. The zinc deficiency page covers signs such as slow wound healing and brittle nails.

Calcium has no useful blood test for intake. The body holds serum calcium within a narrow band by pulling calcium from bone, so a normal serum calcium says nothing about diet. In EPIC-Oxford, vegans had a 43% higher risk of total fractures and 2.3 times the risk of hip fracture compared with meat eaters, partly explained by lower body weight and lower calcium and protein intakes ⁴. An earlier EPIC-Oxford analysis found no extra fracture risk in vegans eating at least 525 mg of calcium a day ⁸. Fortified plant milks and calcium-set tofu close the gap; bone tests are explained in the bone markers guide and on the osteoporosis page.

Supplements with evidence for the vegan gaps

Strongest evidenceVitamin B12Corrects low B12 status in vegans; oral forms work for most people with normal absorptionIn our shop

Good evidence for raising the indexOmega-3 (algae oil)Algae EPA and DHA raised the omega-3 index in vegans within four monthsIn our shop

Strongest evidenceVitamin DRaises 25-OH vitamin D reliably; D3 raises it more than D2In our shop

Strong evidence, if deficientIronRestores ferritin when a test confirms deficiency; no benefit when stores are normalIn our shop

Mixed evidenceZincRaises zinc status; long use can lower copperIn our shop

Mixed evidenceCalciumBone outcomes depend on total intake from food and supplements combinedIn our shop

Selenium, vitamin A, choline, protein and creatine

Why a normal result can still mislead

For a vegan, the body holds the routine marker for several nutrients steady while stores drain. The table pairs each marker with what fools it and the test that fills the gap.

Vegans eat a lot of folate, and in EPIC-Oxford their folate levels were the highest of any group ². Folate keeps red blood cells normal-sized even when B12 is short, so the anemia that would flag B12 deficiency may never show while nerve damage progresses. The folate deficiency page explains how the two vitamins work together, and a raised MCV is read on the high MCV page.

Blood tests for vegans: which to ask for, and when

A core panel many dietitians and doctors use for vegans is serum B12 with active B12 or MMA, ferritin with a full blood count, 25-OH vitamin D and folate. Useful add-ons are urine iodine, the omega-3 index, zinc and TSH. One marker usually reads better on a vegan diet: LDL cholesterol tends to run lower ¹, and the lipid panel guide explains it. For a wider view, see which blood tests to consider. Your doctor can decide which of these fit your history and how often to repeat them.

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