Plant-based eating has moved from the fringe to the mainstream, and the research has moved with it. The headline finding of the last two decades is reassuring: a well-planned vegetarian or vegan diet meets or exceeds recommendations for most nutrients. Fibre, folate, vitamin C, vitamin E, magnesium and polyunsaturated fats all tend to be higher than in typical meat-eating diets. But "most" is not "all". A short and highly predictable list of nutrients behaves differently once animal foods come off the plate, and those are the ones worth paying attention to. This guide covers what the evidence actually shows about each of them, which markers are worth testing, and what a sensible baseline looks like.
Quick Facts
- A systematic review of 141 studies found plant-based eaters take in more fibre, folate, vitamin C, vitamin E and magnesium — and less EPA, DHA, vitamin B12, iron, zinc and iodine — than meat-eaters.
- Vitamin B12 is the one nutrient a fully plant-based diet cannot reliably supply. A supplement or fortified food is not optional.
- Iron and zinc intakes often look adequate on paper, but absorption from plant sources is lower — so blood status matters more than the number on a food tracker.
- ALA from flax, chia and walnuts converts to EPA and DHA at a low rate; algal oil raises the omega-3 index directly at modest doses.
- Creatine is essentially absent from plant foods, which is why vegetarians tend to show a larger response to supplementation than omnivores.
Why Plant-Based Diets Create Predictable Gaps
The gaps are not random. They fall into two categories, and understanding which category a nutrient belongs to tells you how to fix it.
Category one: absorption, not intake
Some minerals are abundant in plants but poorly absorbed from them. Legumes, whole grains, nuts and seeds are rich in phytate, a storage form of phosphorus that binds minerals in the gut and carries them out unabsorbed. Polyphenols in tea, coffee and cocoa do something similar. The result is that a vegan diet can look excellent on a nutrition tracker while delivering meaningfully less usable mineral.
This is the story for iron, where plant foods supply only non-haem iron — absorbed at roughly 2–20% depending on the meal, against 15–35% for the haem iron in meat. A meta-analysis of vegetarian iron status found consistently lower serum ferritin in vegetarians and vegans than in omnivores, even where reported intakes were similar or higher. The same pattern shows up for zinc: a meta-analysis of 26 studies found lower dietary zinc intake and lower serum zinc concentrations in vegetarians, driven largely by the phytate-to-zinc ratio of the diet rather than by any shortage of zinc-containing foods.
Category two: nutrients plants simply do not make
The second category is more absolute. Vitamin B12 is produced by bacteria and accumulates in animal tissue; no unfortified plant food is a reliable source. Long-chain omega-3s — EPA and DHA — are made by marine algae and concentrated up the food chain into fish; plants supply only the short-chain precursor ALA. Creatine is stored in muscle, so it enters the diet almost exclusively through meat and fish. Iodine depends on where food is grown and whether salt is fortified, and dairy is a major contributor in many countries. For this category, no amount of clever meal planning solves the problem — you either fortify, supplement, or accept lower status.
The Nutrients Worth Watching
Vitamin B12
This is the non-negotiable one. A review of 18 studies using functional markers (methylmalonic acid, holotranscobalamin) rather than serum B12 alone found deficiency in a striking share of unsupplemented vegetarians across every demographic examined — higher in vegans than lacto-ovo vegetarians, and higher still in people raised on the diet from birth. Serum B12 alone can look falsely normal, particularly when folate intake is high, which is exactly the situation on a plant-heavy diet.
Iron
Lower ferritin is common but not universal, and low ferritin is not the same thing as anaemia. Many long-term vegetarians sit in the lower-normal range without symptoms. What matters is trajectory and context: menstruating women, endurance athletes, frequent blood donors and adolescents have higher requirements and less margin.
Zinc
Zinc has no meaningful storage pool, so intake and absorption need to be adequate more or less continuously. Soaking, sprouting, fermenting and leavening all reduce phytate and improve absorption — sourdough bread delivers noticeably more usable zinc than an unleavened equivalent.
Omega-3 (EPA and DHA)
Conversion of ALA to EPA is limited and conversion onward to DHA is lower still, with wide individual variation. The practical consequence shows up in blood: vegans consistently have a lower omega-3 index than fish eaters. Encouragingly, a study of 165 long-term vegans found that a modest algal-derived dose of around 254 mg combined EPA plus DHA per day for four months raised the omega-3 index substantially — the response to supplemental omega-3 was robust precisely because baseline was low.
Iodine
A systematic review of iodine intake across vegans, vegetarians and omnivores in industrialised countries found vegans consistently at the highest risk of inadequate intake. Seaweed is the obvious plant source, but it is a poor default: iodine content varies by orders of magnitude between species and batches, and kelp in particular can deliver excessive doses. Iodised salt and a measured supplement are far more predictable than iodine from seaweed.
Protein quality
Total protein intake on plant-based diets is usually lower than in omnivores but still within recommended ranges. The subtler issue is quality: plant proteins are generally lower in essential amino acids and often limiting in lysine or methionine, and they digest and absorb more slowly. Gram for gram, isolated plant proteins produce a smaller muscle protein synthesis response than dairy or egg protein. This is a solvable problem — a modestly higher total intake, blending sources across the day, and adding lysine-rich legumes to grain-based meals closes most of the gap.
Creatine
Vegetarians start with lower muscle and brain creatine stores, and that appears to matter for more than lifting. In a placebo-controlled crossover trial, five days of creatine monohydrate improved memory performance in vegetarians but not in meat-eaters — a rare case where a supplement's effect depends clearly on how depleted you were to begin with.
What the Evidence Actually Shows
Vitamin B12 must come from a supplement or fortified food on a fully plant-based diet. Evidence level: Well-established. This is one of the least controversial statements in nutrition. Functional-marker studies show depletion across age groups, countries and vegetarian subtypes, and no unfortified plant food has ever been shown to correct it reliably.
Plant-based diets lower iron and zinc status on average without necessarily causing deficiency. Evidence level: Well-established. Meta-analyses find consistently lower ferritin and lower serum zinc in vegetarians and vegans. What they do not show is that most plant-based eaters become clinically deficient — the shift is in the average, and individual risk depends on requirements, meal composition and how long the diet has been followed.
Algal EPA and DHA raise the omega-3 index in people who eat no fish. Evidence level: Well-established. The biochemistry is direct and the trial data agree: supplying preformed EPA and DHA bypasses the conversion bottleneck. What remains less certain is which clinical outcomes improve as a result in this specific population.
Creatine supplementation improves cognitive measures more in vegetarians than in omnivores. Evidence level: Promising. The direction of effect is consistent with the depletion hypothesis and has been replicated for some tasks, but the trials are small, short and use varied cognitive batteries. Treat it as a reasonable expectation, not a guarantee.
Vegan diets are associated with lower bone mineral density and higher fracture risk. Evidence level: Promising. A meta-analysis of 20 studies covering more than 37,000 people found lower bone density at the femoral neck and lumbar spine in vegetarians and vegans, with higher fracture rates in vegans specifically. The finding has been challenged on methodological grounds, and it is confounded by lower body weight, lower protein intake and lower calcium and vitamin D status — all of which are modifiable rather than inherent to the diet.
How to Cover the Gaps in Practice
Test rather than guess
Four markers give you most of the picture: ferritin (with a full blood count), serum 25-hydroxyvitamin D, and either methylmalonic acid or holotranscobalamin for B12 — serum B12 alone is the weakest of the options. An omega-3 index test is optional but useful if you have been avoiding fish for years. Retest annually, or sooner if something changes.
Build meals for absorption
Pair iron-rich plants with a vitamin C source in the same meal — lentils with peppers, beans with tomato, oats with berries. Vitamin C converts non-haem iron to a more absorbable form and can multiply uptake several-fold. Move tea and coffee to between meals rather than with them, since their polyphenols do the opposite. Soak beans and grains before cooking, choose sourdough and sprouted breads, and use fermented soy products like tempeh and miso, all of which cut phytate.
A sensible supplement baseline
For most fully plant-based eaters, a short list covers the predictable gaps: vitamin B12 daily or in a larger weekly dose; algal EPA and DHA a few times a week; iodine at around the daily requirement if you do not use iodised salt; and vitamin D3 through the darker months, as most people need it regardless of diet. Iron and zinc are better addressed by food and testing than by routine supplementation, since both carry real downsides in excess.
Do not neglect protein and training
Aim for a slightly higher total protein intake than an omnivore of the same size, spread across three or four meals, and include a legume or soy source at most of them. If you train, creatine monohydrate is one of the highest-value additions available to a plant-based eater — both for performance and for restoring stores that the diet does not replenish. Resistance training also does more for bone density than any single nutrient.
Safety and Who Should Be Cautious
Pregnancy, breastfeeding, infancy and childhood raise the stakes considerably. B12 and DHA requirements rise, deficiency in a developing nervous system can cause lasting harm, and stores in an infant depend entirely on maternal status. Plant-based diets can be run safely through all of these stages, but they should be planned with a clinician or dietitian rather than improvised.
Two supplements deserve specific caution. Iron should not be taken routinely without a ferritin test — iron overload is genuinely harmful and the body has no efficient way to excrete an excess. Iodine has a narrow window: too little and too much both disrupt thyroid function, and seaweed products are the most common route to an accidental excess. Anyone with existing thyroid disease should discuss iodine with their doctor before supplementing.
ⓘ High folate intake — very common on plant-based diets — can mask the haematological signs of B12 deficiency while nerve damage continues to progress. This is the main reason functional markers such as methylmalonic acid are preferred over serum B12 alone when assessing status in vegetarians and vegans.
Frequently Asked Questions
Can I get enough B12 from nutritional yeast, algae or fermented foods?
Fortified nutritional yeast counts, because the B12 has been added. Unfortified yeast, spirulina, chlorella, tempeh and other fermented foods do not — several contain B12 analogues that show up on some laboratory assays but are not biologically active in humans, and may even interfere with true B12. Read the label for added cyanocobalamin or methylcobalamin.
Is soy protein good enough if I lift weights?
Yes, with adjustments. Soy is the best-rounded plant protein for essential amino acids, though still slightly behind whey per gram for stimulating muscle protein synthesis. Increasing the dose per meal, spreading protein across the day, and combining soy with other sources compensates for most of the difference.
Do I need to combine proteins at every meal?
No. The older idea that plant proteins must be precisely combined within a single meal has been superseded — the body maintains an amino acid pool across the day. Eating a variety of legumes, grains, nuts and seeds over 24 hours is sufficient for nearly everyone.
Are vegetarians better off than vegans nutritionally?
On these specific markers, usually yes. Dairy and eggs supply B12, iodine, calcium and some highly bioavailable protein, which removes several of the gaps outright. Vegans need a more deliberate plan, which is not the same as a worse diet — planned well, both work.
How long does it take for a deficiency to develop after going plant-based?
It varies enormously by nutrient. Zinc has no real reserve and responds within weeks. Iron stores typically take months to a few years to draw down. B12 stores in the liver can last several years, which is precisely why people often feel fine for a long time and then encounter problems seemingly out of nowhere — the depletion was silent throughout.
Scientific References
- Neufingerl N, Eilander A. Nutrient Intake and Status in Adults Consuming Plant-Based Diets Compared to Meat-Eaters: A Systematic Review. Nutrients. 2022;14(1):29.
- Pawlak R, Parrott SJ, Raj S, Cullum-Dugan D, Lucus D. How prevalent is vitamin B12 deficiency among vegetarians? Nutrition Reviews. 2013;71(2):110–117.
- Haider LM, Schwingshackl L, Hoffmann G, Ekmekcioglu C. The effect of vegetarian diets on iron status in adults: A systematic review and meta-analysis. Critical Reviews in Food Science and Nutrition. 2018;58(8):1359–1374.
- Foster M, Chu A, Petocz P, Samman S. Effect of vegetarian diets on zinc status: a systematic review and meta-analysis of studies in humans. Journal of the Science of Food and Agriculture. 2013;93(10):2362–2371.
- Sarter B, Kelsey KS, Schwartz TA, Harris WS. Blood docosahexaenoic acid and eicosapentaenoic acid in vegans: Associations with age and gender and effects of an algal-derived omega-3 fatty acid supplement. Clinical Nutrition. 2015;34(2):212–218.
- Eveleigh ER, Coneyworth LJ, Avery A, Welham SJM. Vegans, Vegetarians, and Omnivores: How Does Dietary Choice Influence Iodine Intake? A Systematic Review. Nutrients. 2020;12(6):1606.
- Pinckaers PJM, Trommelen J, Snijders T, van Loon LJC. The Anabolic Response to Plant-Based Protein Ingestion. Sports Medicine. 2021;51(Suppl 1):59–74.
- Benton D, Donohoe R. The influence of creatine supplementation on the cognitive functioning of vegetarians and omnivores. British Journal of Nutrition. 2011;105(7):1100–1105.
- Iguacel I, Miguel-Berges ML, Gómez-Bruton A, Moreno LA, Julián C. Veganism, vegetarianism, bone mineral density, and fracture risk: a systematic review and meta-analysis. Nutrition Reviews. 2019;77(1):1–18.
- Melina V, Craig W, Levin S. Position of the Academy of Nutrition and Dietetics: Vegetarian Diets. Journal of the Academy of Nutrition and Dietetics. 2016;116(12):1970–1980.
Disclaimer
This article is provided for educational and informational purposes only and does not constitute medical advice, diagnosis or treatment. It is not intended to replace consultation with a qualified healthcare professional. Dietary supplements are not medicines and should not be used as a substitute for a varied diet and a healthy lifestyle. Individual nutrient requirements vary, and blood test results should always be interpreted by a clinician in the context of your full medical history. If you are pregnant or breastfeeding, feeding a plant-based diet to an infant or child, taking prescription medication, or living with a diagnosed medical condition, speak with your doctor or a registered dietitian before making significant changes to your diet or starting any supplement. Never disregard or delay seeking professional medical advice because of something you have read here.
