The honest downsides of eating vegan, and why you should still get tested

This post is general information, not medical advice. It is not a substitute for a consultation with a doctor or dietitian who knows your history and can see your blood results. If anything here rings true for you, get tested and talk to a professional rather than acting on a journal entry.

Most of this site is an argument that plant-based food is good, varied, and worth cooking. I still believe that. But a site that only ever tells you the upside is not being honest with you, and the people this site is written for, those with real dietary stakes, deserve the whole picture. So this is the other half. A vegan diet has genuine gaps, some of them serious, and the difference between a healthy vegan and an unwell one is almost entirely whether those gaps get planned for.

None of this is an argument against eating plant-based. It is an argument against doing it carelessly, and against treating the diet as an identity that overrides what your own body and your own doctor are telling you.

The one that is not optional: vitamin B12

If you take one thing from this post, take this. Vitamin B12 is not reliably present in any plant food. It is made by bacteria, and in a mixed diet it reaches people through animal products. Remove those, and unless you actively replace it, your intake falls far below what your body needs, not by a little, but to a fraction of the requirement.

This is not a fringe worry or an anti-vegan talking point. It is the settled consensus of the nutrition literature, and every serious dietary body treats B12 supplementation for vegans as non-negotiable. The reason it is dangerous is that a deficiency builds quietly. It can take a long time to show, and by the time it produces symptoms, fatigue, brain fog, nerve problems, some of the damage can be hard to reverse. You do not feel it draining, which is exactly why people ignore it until a blood test or a symptom forces the issue.

The fix is simple and cheap. A B12 supplement, or genuinely consistent fortified foods, and periodic blood tests to confirm your levels are where they should be. There is no plant-based way around this one. If you are vegan and not supplementing B12, that is the first thing to change, today, before anything else in this post.

Iron, and why plant iron works harder for less

Plant foods do contain iron, so this is not about absence. It is about absorption. The iron in plants, called non-heme iron, is less readily absorbed by the body than the form found in meat. So a vegan can eat what looks like plenty of iron on paper and still run low, and iron-deficiency anaemia shows up more often in poorly planned plant-based diets as a result.

The good news is that this one is very manageable with knowledge rather than pills. Vitamin C dramatically improves absorption of plant iron, so pairing iron-rich foods, lentils, beans, tofu, dark leafy greens, with something high in vitamin C in the same meal, a squeeze of lemon, some pepper, a tomato, makes a real difference. It is worth knowing too that tea and coffee hinder iron absorption, so the cup right alongside the meal is working against you.

Omega-3, the conversion problem

Omega-3 fats matter for the heart and the brain, and the most useful forms, EPA and DHA, are found mainly in oily fish. Plants offer a different form, ALA, found in flax, chia and walnuts, which the body can convert into the useful forms. The catch is that this conversion is inefficient, so relying on it alone tends to leave vegans lower in EPA and DHA than others.

The clean solution here is an algae-based omega-3 supplement. It is worth knowing that this is the one case where the vegan source is arguably the better source, since the fish get their omega-3 from algae in the first place, so an algae supplement simply skips the middle fish.

Vitamin D, iodine, zinc, and calcium: the quiet shortfalls

These do not carry the same urgency as B12, but they are the nutrients that most often run low on a plant-based diet, and they are worth knowing by name so you can check them.

Vitamin D is a shortfall for many people regardless of diet, especially in winter and in less sunny climates, and it is worth testing and supplementing if you are low. Iodine can fall short without iodised salt or sea vegetables. Zinc is present in plants but, like iron, absorbed less efficiently. Calcium intake often lands below recommendations in vegans, which matters for long-term bone health, so fortified plant milks and calcium-rich foods earn their place. None of these are reasons not to eat plant-based. They are simply the boxes a well-planned vegan diet has to tick on purpose rather than by accident.

A word on collagen, carefully

You may have read that vegan diets lack collagen. That framing is not quite right, and the accurate version is more useful. No diet contains collagen unless you are eating animal connective tissue. Your body builds its own, from amino acids like glycine, proline and lysine, plus cofactors including vitamin C, zinc and copper.

Where a plant-based diet can fall short is in supplying those building blocks in the right amounts. Lysine in particular is sometimes the limiting amino acid in vegan diets, and vitamin C is essential for the body to assemble collagen at all. So the honest concern is not that veganism strips collagen out of you, but that a poorly planned one can under-supply the raw materials your body needs to make it. The answer, once again, is planning: enough protein, lysine-rich legumes and soy, and plenty of vitamin C. If you have specific concerns about joints, skin or connective tissue, that is a conversation for a doctor and a blood test, not for a diet forum.

The real lesson: listen to your body and your doctor, not to ideals

Here is the thread running through all of it. Every single one of these gaps is manageable. None of them is an argument for abandoning plant-based eating, which remains linked to lower rates of heart disease, type 2 diabetes and several other conditions. A well-planned vegan diet is, by the consensus of the same nutrition science, perfectly adequate for almost everyone.

But that phrase, well-planned, is doing enormous work, and this is where ideology becomes dangerous. It is easy, inside any strongly held way of eating, to treat the diet as an identity to defend rather than a set of choices to examine. When someone feels persistently tired, or a blood test comes back low, the ideological response is to explain it away and carry on. The honest response is to take the result seriously.

So the real message of this post is simple. Get your blood tested regularly, especially B12, iron and vitamin D. If a result comes back low, act on it, supplement, adjust, or see a specialist, rather than defending the diet against the evidence. If a doctor who has seen your bloodwork tells you something your beliefs do not want to hear, listen to the doctor. The science is not against plant-based eating. But the science is against pretending a diet has no downsides, and no set of ideals is worth more than your actual health.

Eat plants. Cook them well. And check, honestly and regularly, that your body is getting everything it needs. That is not a betrayal of the diet. It is what makes it sustainable.