The Dietary Guidelines for Americans are unusually direct about this: “Vegan diets show broader shortfalls in vitamins A, D, E, B6, and B12; riboflavin; niacin; choline; calcium; iron; magnesium; phosphorus; potassium; zinc; and protein.” That is a long list, and it is not an argument against the diet — the Academy of Nutrition and Dietetics holds that well-planned vegetarian and vegan patterns “can be nutritionally adequate and can offer long-term health benefits.” It is an argument for knowing which items on that list actually need action, and what the action is.
The short version
- B12 is not optional. Plant foods do not naturally contain it. Fortified foods or a supplement, permanently.
- Iron requirements are 1.8× higher on a vegetarian diet — that figure is the NIH’s, not an estimate.
- Spirulina and algae do not supply usable B12. They contain an inactive analogue.
- Spinach is not a calcium source — about 5% is absorbed, against 27% from milk.
- ALA converts to EPA and DHA at under 15%, which is why algal oil exists.
- Soaking, sprouting and fermenting cut phytate by up to 86% when combined.
Vitamin B12 — the one that is not negotiable
The NIH states it plainly: “Plant foods do not naturally contain vitamin B12.” The adult RDA is 2.4 mcg a day, rising to 2.6 in pregnancy and 2.8 while breastfeeding.
What works, per the NIH: “fortified breakfast cereals and fortified nutritional yeasts are readily available sources of vitamin B12 that have high bioavailability.” Fortified nutritional yeast supplies 8.3 to 24 mcg per quarter cup. The Academy adds that supplements “may be the easiest and most reliable way to ensure adequate B12 status among all vegetarians, particularly for vegans.”
Two things sold as B12 that are not. Spirulina and other cyanobacteria contain pseudovitamin B12, which is “biologically inactive in humans” — the researchers who measured it concluded these products “are not suitable for use as sources of Vitamin B12 for vegetarians.” Tempeh is often listed as a source; the NIH puts it at 0.1 mcg per half cup, about 4% of the daily requirement, and where higher values appear they trace to bacterial contamination during production rather than anything reproducible.
One dosing note that explains why supplements look absurdly high: absorption falls sharply with dose. It is roughly 50% below 1–2 mcg, but “only about 2% at doses of 500 mcg and 1.3% at doses of 1,000 mcg.” A 1,000 mcg tablet is not 400 days of B12; it is brute force against poor absorption.
The group where this matters most: exclusively breastfed infants of mothers consuming no animal products “might have very limited reserves” and can become deficient. That is a conversation for a clinician, not a blog.
Iron — the 1.8× rule
This is the clearest number in the whole topic. The NIH: “The requirement for iron is 1.8 times higher for people who follow vegetarian diets than those for people who include animal products in their diet.”
The reason is absorption: 14–18% from mixed diets containing meat, seafood and vitamin C, against 5–12% from vegetarian diets. So a woman aged 19–50 whose RDA is 18 mg is effectively looking at something closer to 32 mg from plant sources.
| Plant source | Iron | %DV |
|---|---|---|
| Fortified breakfast cereal, 1 serving | 18 mg | 100% |
| White beans, canned, 1 cup | 8 mg | 44% |
| Lentils, boiled, ½ cup | 3 mg | 17% |
| Firm tofu, ½ cup | 3 mg | 17% |
| Spinach, boiled, ½ cup | 3 mg | 17% |
| Chickpeas, boiled, ½ cup | 2 mg | 11% |
| Cashews, 1 oz | 2 mg | 11% |
Vitamin C in the same meal enhances absorption; phytate in grains and beans and certain polyphenols reduce it. The Academy notes that premenopausal vegetarian women have a high prevalence of low ferritin and “may benefit from using an iron supplement” — which is a testing question, not a self-prescribing one.
Zinc — and the number everyone gets wrong
Zinc absorption is blocked by the same compound as iron: “Phytates … bind some minerals such as zinc in the intestine and form an insoluble complex that inhibits zinc absorption.” The RDA is 11 mg for men and 8 mg for women.
You will often read that vegetarians need 50% more zinc. That figure is not in the NIH fact sheet. What exists is a broader statement from the Institute of Medicine about low-bioavailability diets: such individuals should consume “intakes that are at least equal to the RDA, and perhaps up to as much as twice the RDA.” A range, not a multiplier.
The NIH’s practical advice is specific and cheap: “soaking beans, grains, and seeds in water for several hours before cooking them. Canned and pre-cooked items are another option” — and “organic acids in fermented foods might increase zinc absorption.” Roasted pumpkin seeds are the densest ordinary source at 2.2 mg per ounce.
What soaking, sprouting and fermenting actually do
These are usually mentioned vaguely. They have been measured. In a controlled study on maize:
| Treatment | Phytate reduction |
|---|---|
| Soaking alone | 12.6% |
| Germination (sprouting) alone | 31.9% |
| Spontaneous fermentation, 24 hours | 51.8% |
| Fermentation with a yogurt culture | 68.7% |
| All three combined | 85.6% |
The honest caveat: this brought the phytate-to-zinc ratio below the threshold considered adequate for absorption, while the phytate-to-iron ratio improved but stayed above it. Preparation helps zinc materially. It does not solve iron.
Calcium — where the plant sources actually are
Absorption varies enormously by food. From spinach about 5% is absorbed; from milk about 27% — and kale and broccoli absorb similarly to milk. Oxalic and phytic acid are why. Collard greens, sweet potatoes, rhubarb and beans are also high in oxalate.
The NIH’s conclusion for people avoiding dairy is not encouragement to eat more greens: they “typically need to eat foods fortified with calcium or take supplements.” Calcium-set firm tofu is one of the better whole-food options at 253 mg per half cup, and fortified plant milks, juices and cereals do the rest. The Academy recommends “at least 2 to 3 daily servings of calcium-rich foods with good bioavailability.”
Iodine — the quiet one
The adult RDA is 150 mcg, and the NIH lists vegans among those who “might not obtain sufficient amounts.” Two facts make this trickier than it looks.
Iodised salt works — it contains about 45 mcg of iodine per gram, roughly an eighth to a quarter of a teaspoon. But specialty salts do not: “Specialty salts, such as sea salt, kosher salt, Himalayan salt, and fleur de sel, are not usually iodized.” A kitchen that switched to sea salt years ago quietly removed its iodine source.
And seaweed is not a controlled substitute. Commercially available seaweeds range from 16 to 2,984 mcg of iodine per gram — a nearly two-hundred-fold spread, which makes dosing impossible.
Omega-3 — why algal oil exists
The adequate intake for ALA is 1.6 g a day for men and 1.1 g for women, which flax, chia and walnuts cover easily. The problem is downstream: “ALA can be converted into EPA and then to DHA, but the conversion … is very limited, with reported rates of less than 15%.”
Algal oil is the direct route: it usually supplies 100–300 mg of DHA, and the NIH notes that “the bioavailability of DHA from algal oil is equivalent to that from cooked salmon.” For pregnancy specifically, the NIH advises women on vegetarian or vegan diets to consult a healthcare provider about whether EPA/DHA supplementation is necessary.
Vitamin D and creatine
Vitamin D. The RDA is 600 IU to age 70 and 800 IU after. Few foods contain it naturally, and the best of them are fatty fish. Fortified plant milks carry about 120 IU per cup, similar to fortified cow’s milk. For supplements, D3 raises and holds blood levels better than D2 — and an animal-free D3 sourced from lichen is available, so the vegan option is no longer limited to D2.
Creatine. Worth knowing because it is the one supplement where being plant-based changes the maths. The body makes about 1 g a day, and dietary creatine comes essentially only from meat and fish. The NIH observes that “vegetarians, for example, with their lower muscle creatine content, might have greater responses to supplementation than meat eaters.” Standard maintenance is 3–5 g a day, with no loading phase required.
The whole thing in one table
| Nutrient | What to do |
|---|---|
| Vitamin B12 | Supplement or fortified foods, permanently. Not optional |
| Iron | Aim 1.8× the RDA; pair with vitamin C; test ferritin rather than guess |
| Zinc | Soak, sprout or ferment; pumpkin seeds and legumes; up to twice the RDA on low-bioavailability diets |
| Calcium | Fortified foods and calcium-set tofu; kale and broccoli, not spinach |
| Iodine | Iodised salt — and check that yours is. Not sea salt, not seaweed |
| Omega-3 | ALA from flax and walnuts; algal oil for EPA/DHA |
| Vitamin D | Fortified plant milk plus a supplement; lichen D3 if avoiding animal sources |
| Creatine | Optional, but plant-based people may respond more. 3–5 g/day |
FAQ
Can I get B12 from nutritional yeast?
Only from fortified nutritional yeast, which the NIH lists as a high-bioavailability source at 8.3–24 mcg per quarter cup. Unfortified yeast contains none, because no plant food does.
Does spirulina count as a B12 source?
No. Cyanobacteria contain pseudovitamin B12, which is biologically inactive in humans, and the researchers who measured it concluded these products are not suitable as a B12 source.
Is spinach good for iron and calcium?
It contains both, but oxalate limits what you absorb — around 5% of its calcium, against 27% from milk. Kale and broccoli absorb far better.
Do I need an iron supplement?
Not automatically. Requirements are 1.8× higher, but supplementation should follow a blood test. Premenopausal vegetarian women are the group the Academy flags as most likely to benefit.
Is sea salt fine for iodine?
No. The NIH states that sea salt, kosher salt, Himalayan salt and fleur de sel are not usually iodised. If you use them, iodine has to come from somewhere else.
Where this fits
Protein targets and how to reach them are in building muscle on a plant-based diet, and the head-to-head evidence in plant vs animal protein for muscle. Which form of a supplement to buy — and where the marketing runs ahead of the evidence — is in supplement forms.
Every figure here is linked to its source in the text: the NIH Office of Dietary Supplements fact sheets on vitamin B12, iron, zinc, calcium, iodine, omega-3 fatty acids and vitamin D; the Dietary Guidelines for Americans 2025–2030; the Academy of Nutrition and Dietetics 2025 position paper; the Institute of Medicine Dietary Reference Intakes; and peer-reviewed research in Nutrients and Frontiers in Nutrition. Checked against those sources on 2 September 2026. Educational information, not medical advice — see our medical disclaimer.
This content is for educational purposes only and is not medical advice. Always consult a qualified healthcare professional for personalised guidance.
