You Can't Get All Your Nutrients from Plants Alone: Here's the Science

The mainstream story is simple: eat plants, feel good. The scientific story is messier. Some nutrients plants provide poorly or not at all. And when you remove animal foods entirely, you're betting that supplementation can fully replace them. Sometimes it can. Sometimes it cannot.

Vitamin B12: the non-negotiable animal nutrient

Let's start with the hardest truth. Vitamin B12 is found almost exclusively in animal foods. There is no meaningful plant source.¹ Plants don't make B12. Animals store it from bacterial sources or synthesise it themselves. If you eat no animals, you get no B12 unless you supplement.

B12 is essential for brain development, nervous system function, energy production, and red blood cell formation. Deficiency develops silently and can take years to manifest. By the time symptoms appear (fatigue, neuropathy, cognitive decline, anaemia), damage has often accumulated substantially.

Supplementation works, but it requires consistency and correct dosing. Many plant-based people discover years into their diet that they're deficient because supplementation is easy to skip or get wrong. The challenge isn't that B12 supplementation is ineffective. It's that it requires vigilance that many people don't maintain. The safest plant-based approach is to accept that B12 supplementation isn't optional. It's necessary.

Iron: not all iron is equal

Plants contain iron. But iron from plants behaves differently from iron from meat.

Heme iron (from animal sources) is highly absorbable. Your body absorbs a substantially higher fraction of heme iron than non-heme iron from plants.², depending on what else you're eating. Vitamin C, certain spices, and acidity improve absorption. Phytates, tannins, and oxalates (abundant in plants) block it severely.

You can optimise plant-based iron absorption. Pair iron-rich plants with vitamin C. Cook in cast iron. Avoid tea and coffee with meals. But the starting point is worse. You're fighting harder to absorb less. And the variance is huge. One person might absorb 20 per cent, another 3 per cent, depending on their individual physiology.

Additionally, plant-based iron doesn't automatically regulate absorption well. Your body tightly controls how much heme iron it absorbs based on current stores. Non-heme iron? Not the same. People on plant-based diets often show either deficiency or excessive iron, depending on the combination of foods and supplements they're taking. The regulation system doesn't work as well.

Retinol: why beta-carotene conversion fails

Your body needs vitamin A for vision, immune function, and skin health. Plants provide beta-carotene, a precursor that theoretically converts to vitamin A.

Theoretically is the operative word. Children convert as little as 3 to 6 per cent of the beta-carotene they eat into usable vitamin A. Conversion is variable across individuals.³

More troubling, the conversion rate depends on genetics, gut health, thyroid function, and whether you've eaten fat with the plant. A substantial proportion of the population carry BCMO1 genetic variants that reduce beta-carotene to retinol conversion efficiency.³ If you're unlucky genetically, you're trying to get vitamin A from a plant source that converts at 2 to 3 per cent efficiency. That's nearly useless.

Animal sources provide preformed vitamin A (retinol), which requires no conversion. Liver is extraordinarily rich in it. Eggs contain it. Dairy contains it. If you're plant-based, you're entirely reliant on conversion efficiency from plants, hoping your genetics are good, and planning for the possibility that you're not getting enough. The gamble is real.

Creatine: the nutrient your brain desperately needs

Creatine isn't just for muscles. Your brain uses creatine. Your mitochondria use it. It's a primary energy shuttle in cells that work hard (brain, muscle, immune cells).

Creatine is found almost exclusively in animal meat.⁴ Your body can synthesise some from arginine and glycine, but the amount is modest and varies with genetics. If you eat no meat, you're relying on endogenous synthesis, which often doesn't meet demand.

This becomes particularly relevant if you're training hard, if you're older, or if you're pregnant or breastfeeding. All of these states increase creatine demand. Plant-based people in these categories often experience energy crashes, slower cognitive function, or weaker recovery. Creatine supplementation helps, but it's another supplement required on top of B12.

Carnosine and anserine: muscle and brain antioxidants

Carnosine and anserine are dipeptides found almost exclusively in animal muscle. They're powerful antioxidants, particularly in fast-twitch muscle fibres and in the brain.

Carnosine buffers lactic acid during intense exercise. It protects neurons from oxidative damage. It supports muscle recovery. It's found in meat, fish, and poultry. Plants don't provide it.

Your body can synthesise carnosine from beta-alanine and histidine, but synthesis is modest. For athletes and for people doing intense cognitive work, dietary carnosine from animal sources is significantly different from relying on synthesis alone. The gap is real and measurable.

This matters more than people realise. Plant-based athletes often recover more slowly, experience more muscle soreness, and see smaller strength gains despite similar training, partly because they lack dietary carnosine. It's not the only factor, but it's a consistent one.

Vitamin K2: mostly found in animal foods

Vitamin K2 exists in the fermented plant foods. Natto (fermented soybeans) is extraordinarily rich in K2. But most plant sources provide it poorly or in the wrong form.

K2 is a family of molecules. MK-4 is found primarily in animal products; MK-7 is found in fermented foods such as natto.⁵ MK-4, the form your body uses most readily, is made by animals from dietary K1 or synthesised de novo. You don't get much MK-4 from plants.

K2 is essential for bone mineralisation, cardiovascular health, and preventing arterial calcification. Deficiency is associated with increased fracture risk and calcified arteries. Plant-based people relying solely on natto and other fermented sources often show suboptimal K2 status without realising it.

B12, heme iron, retinol, creatine, carnosine, and K2 are difficult or impossible to get from plants in sufficient quantities. This is not ideology. It is biochemistry.

A nuanced take

This is not an argument against plant-based eating. This is an argument for honesty about the trade-offs.

If you choose to eat plants, you can do so very healthily. But you're making a trade-off. You're removing your most bioavailable sources of certain critical nutrients. You then need to either accept supplementation (B12 is non-negotiable, but K2, creatine, and others might be wise), or you need to be very deliberate about including the small number of animal foods that fill these gaps.

Some plant-based people include eggs. Some include fish. Some include dairy. Those choices dramatically improve nutrient status without requiring supplementation. Others are fully plant-based and supplement comprehensively. Both approaches can work if done thoughtfully.

What doesn't work is assuming that a plant-based diet without supplementation provides equal nutrition to an omnivorous one. The science doesn't support this. The evidence suggests that careful, supplemented plant-based eating can be adequate, sometimes. But it's harder, it's less forgiving, and it requires knowledge that most people don't have or maintain long-term.

If you're plant-based or considering it, know what you're trading. Know which nutrients are difficult to obtain. Know whether you're willing to supplement, and be consistent if you do. The plant-based people who thrive are those who acknowledge these limitations and address them systematically, not those who pretend they don't exist.

Testing and monitoring on a plant-based diet

If you're committed to plant-based eating, supplement strategically, and monitor your status. B12 is non-negotiable. Get your level tested annually. Iron, zinc, and vitamin D status should also be checked every 12 to 18 months. If you're showing deficiency signs, address them immediately, not after a year of progressive decline.

Consider working with a nutritionist who understands both plant-based nutrition and bioavailability. They can help you optimise supplementation and identify foods and preparation methods that improve absorption.

The plant-based people who thrive long-term are those who treat their diet with the same rigour they'd apply to medication. They test. They supplement. They adjust. They don't pretend that nutrient density is equal across all food sources. They work with reality.

References

1. 1. National Institutes of Health, Office of Dietary Supplements. Vitamin B12: Fact Sheet for Health Professionals.
2. 2. National Institutes of Health, Office of Dietary Supplements. Iron: Fact Sheet for Health Professionals.
3. 3. Lietz G et al. Single nucleotide polymorphisms upstream from the β-carotene 15,15\'-monooxygenase gene influence provitamin A conversion efficiency in female volunteers. J Nutr. 2012;142(1):161S-5S. PMID: 22113863.
4. 4. Brosnan ME, Brosnan JT. The role of dietary creatine. Amino Acids. 2016;48(8):1785-91. PMID: 27108136.
5. 5. National Institutes of Health, Office of Dietary Supplements. Vitamin K: Fact Sheet for Health Professionals.