A plant-based diet offers well-documented benefits for heart health and longevity, but it comes with real nutritional gaps and practical challenges that can affect your health if you don’t plan carefully. The most significant negatives include a high risk of vitamin B12 deficiency (affecting roughly half of all vegans), lower absorption of key minerals like iron and zinc, difficulty getting enough high-quality protein and omega-3 fatty acids, and increased fracture risk tied to lower bone density.
Vitamin B12 Deficiency Is Common and Serious
B12 is the single biggest nutritional vulnerability on a plant-based diet. This vitamin exists almost exclusively in animal foods, and without supplementation, deficiency is nearly inevitable. Studies estimate that about 52% of vegans have deficient blood levels of B12, compared to just 1% of omnivores. Across multiple countries, 50 to 70% of vegetarians and vegans show subnormal B12 status.
What makes B12 tricky is that deficiency develops slowly and symptoms can be vague at first: fatigue, trouble concentrating, insomnia. As levels drop further, the consequences become more serious. B12 is essential for nerve function and red blood cell production, so prolonged deficiency can cause anemia, depression, anxiety, cognitive impairment, and in severe cases, dementia-like symptoms. In infants born to deficient mothers, case reports describe developmental regression, including loss of the ability to hold up their heads or grasp objects. Supplementation is non-negotiable for anyone eating fully plant-based.
Iron Absorption Drops Significantly
Plants contain iron, but your body handles it differently than the iron found in meat. Animal foods provide heme iron, which is absorbed at a rate of about 25%. Plant foods provide non-heme iron, absorbed at 17% or less. When you look at overall diet patterns, people eating animal products absorb 14 to 18% of dietary iron, while plant-based eaters absorb just 5 to 12%.
The gap widens because many plant foods contain compounds that actively block mineral absorption. Phytates, found in grains, legumes, nuts, and seeds, bind tightly to iron, zinc, and calcium, forming complexes your body can’t break down. Oxalates in spinach, beets, and sweet potatoes do the same thing with calcium and iron. Polyphenols in tea, coffee, and red wine further reduce iron uptake. Vitamin C helps counteract these effects, so pairing iron-rich plant foods with citrus or peppers makes a meaningful difference, but it requires consistent attention to food combinations that most omnivores never need to think about.
Protein Quality Is Lower
The issue with plant protein isn’t just quantity. It’s quality. Protein quality is measured by how well your body can digest and use the amino acids in a given food. The current gold standard for this measurement, called DIAAS, consistently scores plant proteins below animal proteins. Dairy proteins like whey and milk protein concentrate score significantly higher than soy protein isolate, pea protein concentrate, soy flour, and wheat protein.
Most plant proteins are low in one or more essential amino acids. Grains tend to lack lysine. Legumes are low in methionine. You can compensate by eating a variety of protein sources throughout the day, but it takes more total food volume to match the amino acid profile you’d get from a single serving of meat, eggs, or dairy. This matters most for older adults trying to maintain muscle mass, athletes with high protein demands, and growing children.
Omega-3 Fatty Acids Are Hard to Get
Plant foods like flaxseed, chia seeds, and walnuts provide a type of omega-3 called ALA. But the omega-3s your brain and heart primarily need are EPA and DHA, found directly in fatty fish and seafood. Your body can convert ALA into EPA and DHA, but the conversion rate is remarkably low. In healthy young men, about 8% of ALA converts to EPA and somewhere between 0 and 4% converts to DHA. Women do somewhat better, converting roughly 21% to EPA and 9% to DHA, likely due to estrogen’s role in the process.
This means that even if you eat plenty of ALA-rich seeds and oils, you may still fall short on the long-chain omega-3s that support brain health, reduce inflammation, and protect against heart disease. Algae-based DHA supplements are the most reliable plant-based workaround.
Bone Fracture Risk Increases
One of the more striking findings in the research comes from the EPIC-Oxford study, a large prospective study tracking tens of thousands of people over many years. Compared to meat eaters, vegans had a 43% higher risk of total fractures after adjusting for body mass index and lifestyle factors. The risk was even more pronounced for specific sites: vegans had more than double the risk of hip fractures (a 131% increase) and roughly double the risk of leg fractures.
Several factors likely contribute. Vegans tend to have lower calcium intake, partly because plant sources of calcium are often bound up with oxalates that limit absorption. Lower protein intake can also affect bone density over time. And vitamin D, which is critical for calcium absorption, presents its own challenge on a plant-based diet. The plant-derived form of vitamin D (D2) is substantially less effective at raising blood levels than the animal-derived form (D3). Research has found that D3 is roughly two to three times more potent than D2 at restoring vitamin D levels, even when D2 is given at double the dose. Lichen-based D3 supplements exist but aren’t as widely available.
Iodine Deficiency Affects Thyroid Function
Iodine is easy to overlook, but it’s essential for your thyroid, the gland that regulates metabolism, energy, and body temperature. The richest dietary sources are fish, dairy, and eggs. Without these, vegans are at particular risk for iodine deficiency, which can lead to hypothyroidism, goiter, and thyroid nodules.
Research consistently shows that iodine nutrition is inadequate among vegetarians and especially poor among vegans. A study of women of childbearing age found that vegan women had lower iodine levels than omnivores, with both groups falling below the threshold for adequate iodine status. This is especially concerning for pregnant or breastfeeding women, since iodine deficiency during pregnancy can impair fetal brain development. Ironically, some vegans overcorrect by consuming large amounts of seaweed, which can deliver iodine in unpredictable and sometimes excessive quantities, creating a different set of thyroid problems.
Ultra-Processed Plant Foods Carry Their Own Risks
The growing market of plant-based burgers, sausages, nuggets, and cheese alternatives makes it easier than ever to eat plant-based without eating many actual plants. A UK Biobank analysis published in The Lancet Regional Health found that while whole, unprocessed plant foods were associated with lower cardiovascular risk, ultra-processed plant foods were linked to a 5% increased risk of cardiovascular disease and 12% higher cardiovascular mortality.
The reasons go beyond just high sodium and unhealthy fat content. Certain additives common in these products, along with contaminants formed during industrial processing, have been linked to inflammation, insulin resistance, and changes in gut bacteria. Processing also strips away protective compounds like polyphenols that make whole plant foods beneficial in the first place. A plant-based diet built around these convenience products can end up being nutritionally worse than a balanced omnivorous diet.
Digestive Discomfort During the Transition
Shifting to a plant-based diet usually means a sharp increase in fiber intake, and your gut needs time to adjust. Bacteria in your colon break down and ferment fiber, producing carbon dioxide, hydrogen, and methane in the process. Legumes are a particularly strong trigger because they’re rich in oligosaccharides, a highly fermentable type of fiber, along with resistant starch, pectin, and inulin. Fiber can also slow the movement of gas through your digestive tract, making bloating worse.
For most people, these symptoms ease as the gut microbiome adapts, but the transition period can last several weeks and may be uncomfortable enough to derail the diet entirely. Increasing fiber gradually, soaking beans before cooking, and staying well hydrated all help, but some people with irritable bowel syndrome or other digestive conditions find that the high-fiber, legume-heavy nature of plant-based eating remains a persistent problem.