Why Veganism Doesn’t Work for Everyone

A well-planned vegan diet can be healthy for many people, but genetics, biology, and life circumstances create real obstacles that make it impractical or even risky for others. The reasons go well beyond willpower or planning skills. Some people carry gene variants that cripple their ability to convert plant nutrients into the forms the body actually uses. Others face absorption barriers baked into plant chemistry itself. And for many, the socioeconomic realities of food access make the whole enterprise unrealistic. The picture is more layered than either side of the vegan debate tends to acknowledge.

Your Genes Decide How Well You Use Plant Nutrients

One of the least discussed reasons veganism fails for certain people is genetic variation in nutrient metabolism. Two conversions matter enormously on a plant-only diet: turning beta-carotene into vitamin A, and turning short-chain omega-3 fats into the long-chain forms your brain and body need. Both of these conversions depend on enzymes, and how well those enzymes work varies dramatically from person to person based on inherited gene variants.

Vitamin A from animal foods arrives as preformed retinol, ready to use. Plants provide only beta-carotene, a precursor your body must convert. The enzyme responsible for that conversion is called BCMO1, and two common variants in its gene can slash its efficiency. In a study of female volunteers, carriers of both variant forms showed a roughly 69% reduction in their ability to convert beta-carotene into usable vitamin A. The enzyme’s catalytic activity dropped by about 57% in lab tests of the double-mutant form. These are not rare mutations: the variant allele frequencies sit at around 42% and 24% in the population studied.¹ Some research suggests that up to 45% of healthy individuals are “poor converters” of beta-carotene, meaning they could eat enormous amounts of carrots and sweet potatoes and still not meet their vitamin A needs without supplementation or animal sources.

A similar story plays out with omega-3 fatty acids. Plant foods like flaxseed and walnuts supply alpha-linolenic acid (ALA), a short-chain omega-3. But your body needs the long-chain forms, EPA and DHA, for brain function, inflammation control, and cardiovascular health. The FADS1 and FADS2 genes encode the enzymes that handle this conversion, and they are among the most genetically variable regions in the human genome.² A scoping review found that individuals carrying certain minor alleles in FADS genes consistently had lower circulating levels of EPA and DHA, with at least 15 specific gene variants identified that reduce the efficiency of this conversion pathway.³ The frequency of these variants differs across ancestral populations, meaning the suitability of a plant-only diet is partly shaped by evolutionary history.

Research on FADS haplotypes has revealed that modern humans carry two common versions of this gene cluster that differ dramatically in how efficiently they produce long-chain fatty acids from plant precursors. The more efficient version appears to have evolved after the lineage leading to modern humans split from Neanderthals, and it shows signs of positive selection, likely because it helped populations survive on diets with limited access to preformed EPA and DHA from fish or meat.⁴ If you inherited the less efficient version, relying entirely on plant-based ALA for your omega-3 needs leaves you at a disadvantage that no amount of chia seeds can fully overcome.

A recent nutrigenetic review pulled these threads together, noting that conversion efficiency of plant-derived precursors into biologically active forms varies according to polymorphisms in FADS, BCMO1, and several other genes affecting choline metabolism, mineral absorption, and vitamin B12 transport.⁵ The upshot: for a meaningful fraction of the population, plant nutrients are not biologically equivalent to animal nutrients, no matter how carefully a diet is designed.

The Vitamin B12 Problem Is Harder to Manage Than It Sounds

Vitamin B12 is the nutrient most commonly flagged as a risk for vegans, and with good reason. It occurs naturally almost exclusively in animal foods. Vegans depend on fortified products or supplements, and the margin for error is thinner than people assume. A case study monitoring a healthy vegan who stopped B12 supplements found that active B12 markers began dropping within four weeks, and by 16 weeks the situation had deteriorated enough that supplementation had to be restarted. The amino acid homocysteine, a marker that rises when B12 is inadequate, climbed to elevated levels within 20 weeks.⁶ That timeline is much shorter than the years of stored B12 that popular vegan guidance sometimes implies you have in reserve.

The broader population data confirms this is not a one-off problem. A global review of studies on vegetarians and vegans found reduced average B12 levels and elevated homocysteine concentrations across the board, with vegans showing the most pronounced deficiencies. Elevated homocysteine is linked to higher cardiovascular risk, which is ironic for a diet often promoted as heart-protective.⁷ A German study of long-term vegans found that roughly half had B12 concentrations below the threshold for adequacy, and their homocysteine levels were mildly elevated as a group. The duration of following a vegan diet was one of the strongest predictors of how depleted someone’s B12 had become.⁸

None of this means B12 deficiency on a vegan diet is inevitable. Consistent supplementation works. But “consistent” is the operative word. Life gets in the way: travel, cost, forgetting, supplement fatigue, or distrust of pills. The genetic angle resurfaces here too, since variants in genes involved in B12 transport can make some individuals more susceptible to functional deficiency even when their intake looks adequate on paper.

Phytate and the Mineral Absorption Gap

Iron and zinc are present in plenty of plant foods, but a compound called phytate complicates the picture. Phytate is found in grains, legumes, nuts, and seeds, and it binds tightly to iron and zinc in the gut, preventing absorption.⁹ This is not a minor effect. Phytate is described in the nutrition literature as one of the most potent inhibitors of iron and zinc absorption, and its impact is especially pronounced in diets that rely heavily on whole grains and legumes as staple foods.¹⁰

Vegans eat more phytate-rich foods than omnivores do, and there is no reliable way to fully neutralize phytate through cooking or food preparation alone. Soaking, sprouting, and fermenting grains and legumes can reduce phytate levels, but they do not eliminate it. The result is that a vegan whose diet looks adequate for iron and zinc on paper may still develop functional deficiencies because so much of those minerals passes through unabsorbed. This helps explain why iron-deficiency anemia remains a concern for vegans despite legumes and leafy greens being rich in non-heme iron. Some researchers have explored whether plant ferritin, a form of iron stored in legume seeds, might bypass phytate inhibition. Early evidence suggests ferritin iron may be absorbed through a different cellular pathway, which is promising, but the research is still in relatively early stages.¹¹

Oxalates create a parallel problem for calcium. Many vegetables that are high in calcium, like spinach and Swiss chard, are also high in oxalates, which bind calcium and reduce its availability. People eating larger quantities of vegetables to compensate for removing dairy face higher oxalate intake as a side effect.¹² Low-oxalate greens like kale and bok choy are better calcium sources, but you need to know that distinction exists and have reliable access to those foods.

Protein Quality Is Not Just About Quantity

The standard rebuttal to protein concerns on a vegan diet is that combining different plant sources provides all essential amino acids. This is true in principle, but it glosses over a real issue with digestibility. The Digestible Indispensable Amino Acid Score (DIAAS) measures how much of a protein’s amino acids your body can actually absorb and use, not just how much is present in the food. By this measure, most plant proteins score substantially lower than animal proteins. A comprehensive review found that pork, casein, egg, and potato proteins scored above 100 (excellent), while soy and whey were classified as high quality with scores at or above 75. Most other plant proteins, including pea, rice, oat, hemp, fava bean, and corn, fell below the threshold for even a “quality” claim.¹³

For a young, healthy adult with a large appetite and time to plan meals, this gap can be bridged by eating more total protein and combining sources strategically. For older adults trying to preserve muscle mass, athletes with high protein turnover, or people recovering from illness, the lower digestibility of plant proteins means you need to eat considerably more food volume to achieve the same net amino acid delivery. That is not always practical, particularly for people with small appetites or digestive sensitivities.

Thyroid Risks and Iodine Gaps

Iodine is a nutrient that rarely gets headline attention in vegan nutrition discussions, but it deserves more. It is essential for thyroid hormone production, and deficiency is one of the most common causes of hypothyroidism worldwide.¹⁴ The richest dietary sources of iodine are fish, dairy, and eggs, all of which are eliminated on a vegan diet. Seaweed provides iodine but in wildly variable and sometimes dangerously high amounts, making it unreliable as a sole source. Iodized salt helps, but many health-conscious vegans use sea salt or specialty salts that contain little or no iodine.

Compounding the problem, several plant foods commonly eaten on vegan diets contain goitrogenic compounds that interfere with thyroid function. Cruciferous vegetables like broccoli, kale, and Brussels sprouts, along with soy products, contain substances that can impair iodine uptake by the thyroid when consumed in large quantities, particularly if iodine intake is already marginal. For someone eating large amounts of soy milk, tofu, tempeh, and cruciferous vegetables while skipping iodized salt, the combined effect can push the thyroid into underperformance. This tends to manifest slowly as fatigue, weight gain, and cold sensitivity, symptoms that are easy to attribute to other causes.

Why People Actually Quit

Survey data from Faunalytics, which tracked former vegetarians and vegans, reveals that the reasons for abandoning plant-based diets are more practical than ideological. Among over 900 former vegetarians and vegans who gave a primary reason for quitting, the top answer was dissatisfaction with the food itself, cited by about a third. Health concerns came second at roughly a quarter. Social difficulties and inconvenience each accounted for about 13%, while cost and loss of motivation were less common but still present.¹⁵ The health category likely captures a mix of genuine nutrient deficiency symptoms and vaguer feelings of malaise that may or may not have had a biological basis.

The social friction is worth lingering on because it compounds over time. Eating differently from everyone around you creates constant friction at family meals, work events, restaurant outings, and travel. A broader review of barriers to plant-based diet adoption found that cultural and social norms were a top-three barrier, reported in nearly 60% of the studies analyzed. Taste and palatability concerns topped the list at around 69%, and economic constraints were close behind at about 62%.¹⁶ Adoption rates among high-income groups were roughly triple those in low-income groups in the same analysis, underscoring that veganism is far easier to sustain when you have money, a well-stocked grocery store nearby, and the time to cook.

Pregnancy and Other Vulnerable Windows

Certain life stages raise the stakes of any nutrient shortfall. Pregnancy is the most obvious. Demand for iron, B12, iodine, choline, calcium, and DHA all spike during pregnancy, and deficiencies in any of them can affect fetal development. Research from the University of Michigan noted that pregnant women eating plant-based diets may face higher risk of B12 and iron deficiencies, though no significant increase in severe adverse outcomes was identified in the studies reviewed. The absence of severe outcomes does not mean subclinical deficiencies are harmless; it means the evidence for outright catastrophic effects is not strong, while the evidence for lower nutrient status is consistent.

Infants and young children present another concern. Their protein and micronutrient needs per unit of body weight are higher than adults’, and their capacity to eat large food volumes is limited. A young child eating a vegan diet relies entirely on parents who understand fortification, supplementation, and food combining at a level that most nutrition professionals would consider advanced. The margin for error is slim and the consequences of getting it wrong, especially with B12 and iron during brain development, are serious. Pediatric nutrition guidelines in several countries recommend that vegan children be monitored closely and supplemented with B12, vitamin D, iodine, and sometimes iron and zinc as a matter of routine.

The Microbiome Wildcard

Switching to a vegan diet reshapes the gut microbiome, and the direction of that reshaping is not uniformly beneficial. A systematic review of interventional studies on plant-based diets found contradictory results at the bacterial family level. Some bacterial groups associated with gut health, like Ruminococcaceae, consistently increased on plant-based diets across multiple studies. But results for other groups were mixed or inconsistent, with some studies showing increases and others showing decreases in the same bacterial families depending on the specific population and study design.¹⁷

People with existing digestive conditions face particular challenges. The high fiber content of a vegan diet, often touted as a benefit, can worsen symptoms in people with irritable bowel syndrome, small intestinal bacterial overgrowth, or certain inflammatory bowel conditions. FODMAPs, a category of fermentable carbohydrates found in many legumes, onions, garlic, and wheat, are staples of vegan cooking and common triggers for digestive distress. Someone with IBS who also has moral convictions about veganism can find themselves caught between their values and their gut.

When “Plant-Based” Means Ultra-Processed

The rapid growth of plant-based meat and dairy alternatives has made veganism more accessible, but it has also introduced a new wrinkle. Many of these products are highly processed and formulated with long ingredient lists that include isolated proteins, stabilizers, and flavorings. A recent review comparing ultra-processed plant foods to their unprocessed animal counterparts found some genuine advantages: plant-based alternatives tend to be lower in saturated fat and cholesterol, and they provide fiber that animal products lack. Substituting dairy milk with soy milk, for example, was associated with reductions in total and LDL cholesterol.¹⁸

The complication is that many people switching to veganism lean heavily on these convenience products rather than whole foods. A diet built around plant-based burgers, nuggets, cheese substitutes, and protein bars may technically qualify as vegan, but it can be high in sodium, low in certain micronutrients, and nutritionally unimpressive in ways that have little to do with the absence of animal products. The gap between what veganism can look like in theory (diverse whole grains, legumes, vegetables, nuts, seeds, fermented foods) and what it often looks like in practice (processed substitutes plus pasta) is part of why the diet fails for people who try it without deep engagement with cooking and nutrition.

The Orthorexia Connection

An underexplored angle is the psychological relationship some people develop with restrictive eating. Research has identified a positive association between plant-based diets and orthorexia nervosa, a pattern of obsessive focus on eating “correctly” that can paradoxically lead to nutritional deficiency and psychological distress.¹⁹ The causal direction is murky: it is unclear whether veganism attracts people prone to obsessive dietary restriction, whether the restrictions of veganism push some people toward orthorexic patterns, or both. Either way, the association means that for some individuals, adopting a vegan diet can interact with existing psychological vulnerabilities in ways that make the diet unsustainable or harmful. Clinicians working with eating disorders increasingly recognize plant-based diets as a potential complicating factor in treatment, since eliminating an entire food category can reinforce or mask restrictive behaviors.