Key Takeaways
- Plant based diets often leave weak spots in B12, iodine, protein and DHA.
- Animal foods supply dense nutrition in forms people usually absorb well.
- Low animal food intake can strain bone thyroid brain and muscle function.
- Plant protein often falls short on digestibility and key amino acids.
- Careful planning lowers risk but does not erase every trade off.
Nutrient Gaps
Vitamin B12
Vitamin B12 is one of the hardest nutrients to cover on a fully plant based diet because dependable natural sources come from animal foods. Reviews comparing vegans with meat eaters keep finding the same problem. Vegans tend to have lower intake and poorer blood status, with the risk rising further when fortified products or supplements are inconsistent (1, 2). Low B12 does not always show up fast. Some people notice fatigue, poor concentration, tingling or low mood long before a problem is confirmed. A diet can look tidy and still leave this gap wide open, especially when someone assumes vegetables and legumes cover more than they really do.
Iodine & Thyroid Support
Iodine is another common blind spot. Dairy, eggs and seafood are major sources in many diets, so intake often drops when those foods disappear. A recent systematic review found that vegan and vegetarian diets were linked with a higher risk of low iodine status, which raises concern for thyroid function over time (3).
People often assume sea salt or seaweed solves the issue, yet day to day intake can swing from too little to far too much. Thyroid tissue works best with steady supply. A diet that relies heavily on plant foods while offering little iodine can leave people dragging through the day with cold hands, dry skin and low energy.
Long Chain Fats
Plant based diets can also come up short in long chain omega 3 fats. Plants give ALA, but the body still has to convert it into EPA and DHA, and that conversion is limited. In one study of British men, vegetarians and vegans had lower plasma levels of these long chain fats than meat eaters (4).
Animal foods make this easier. Oily fish, eggs from well fed hens and cod liver oil provide preformed fats the body can use directly. Cod liver oil also brings retinol and vitamin D, which gives it a wider nutritional range than a standard fish oil capsule.
Protein Quality
Digestibility
Protein discussions often get stuck on total grams, yet digestibility is part of the real story. A meal can look decent in an app while still giving a weaker result in the body. Research using DIAAS scoring found lower protein quality in vegetarian athletes than in non vegetarian athletes, which fits the broader concern that plant proteins are often less digestible (5).
Animal foods usually need less juggling. Meat, eggs, fish and dairy deliver dense protein with fewer barriers to absorption. Plant sources can still contribute, but doing well on them often takes more volume, more planning and more tolerance for foods that do not suit everyone.
Amino Acid Balance
Amino acid balance is the other half of the issue. Many plant proteins are lower in one or more indispensable amino acids, especially when the diet leans hard on grains or legumes without much else. Reviews of vegetarian diets note that protein intake and amino acid quality can be lower than in omnivorous diets, even when total intake seems passable on paper (2, 6). This gets more serious in older adults, active people and anyone eating too little overall. Muscle tissue responds best to a clear protein signal. When each meal is bulky but light on usable amino acids, recovery and body composition can drift in the wrong direction.
Bone & Brain Effects
Bone Strength
Bone health is one of the least discussed weak spots in plant based eating. Bone is not just a calcium story. It depends on adequate protein, fat soluble vitamins and a good mineral supply over many years. A systematic review and meta analysis found lower bone mineral density in people following plant based diets compared with omnivores (7).
Prospective data adds another layer. In EPIC Oxford, vegans had a higher risk of total fracture and hip fracture than meat eaters, with body weight and nutrient intake likely part of the picture (8). When animal foods stay very low, the diet can become thin in several bone related nutrients at the same time.
Brain & Nerve Function
Brain and nerve tissue depend on a steady supply of B12, iron, iodine, cholesterol and long chain fats. A plant based menu can look clean while still making those nutrients harder to obtain together. Lower blood levels of vitamin D and long chain omega 3 fats have also been reported in vegetarian and vegan groups, which adds more friction for people already feeling flat or foggy (4, 9).
Animal foods are simply more nutrient dense here. They package protein, fats and micronutrients in combinations people have used for a very long time. When someone feels sharper and more stable after bringing back eggs, dairy, seafood or meat, the explanation is often nutritional rather than mysterious.
What Makes It Hard
Fortified Foods & Substitutes
Many plant based diets lean on fortified foods to patch recurring gaps. Breakfast cereal, plant milks and meat substitutes can provide added vitamins and minerals, yet this approach often replaces real food with industrial products. It also leaves nutrition dependent on label design instead of the natural food matrix. A person can hit a target on paper while still eating a diet full of fillers, gums and seed oils. That is one reason some people look well nourished by numbers alone while feeling underfed in real life.
Meal Rhythm
Meal rhythm also changes the outcome. Many people eating mostly plant foods end up snacking all day because the meals do not hold them for long. A better approach is one to three solid meals centered on protein and fat, with no grazing between them.
Animal foods make satiety much easier to reach. Eggs at breakfast, beef or lamb at lunch or fish at dinner give a steadier result than a constant stream of crackers, fruit, smoothies and fortified bars.
Support
Anyone staying vegan or vegetarian needs to be more deliberate than the average public health leaflet suggests. B12 and iodine need active attention, and protein quality deserves more respect than it usually gets. Cod liver oil can be a useful choice for people who want support for omega 3 fats, retinol and vitamin D from one traditional source rather than a pile of isolated products.
For any health concerns or questions about a medical condition, get guidance from a physician or another appropriately trained clinician. Before changing your diet, supplements or health routine, talk with a licensed healthcare professional.
FAQs
What are the main downsides of a plant based diet?
The main downsides are low B12, low iodine, weaker protein quality and lower intake of long chain omega 3 fats. Bone and thyroid support can also become harder.
Can a plant based diet cause vitamin B12 deficiency?
Yes. Fully plant based diets carry a clear risk of low B12 because dependable natural sources are found in animal foods.
Is plant protein as good as animal protein?
Plant protein can help, but it is often less digestible and less complete in key amino acids. Animal protein usually gives more per serving.
Can a vegan diet affect bone health?
Yes. Bone health can suffer when protein, vitamin D and other key nutrients stay low over time.
How can someone reduce the downsides of a plant based diet?
They can reduce risk by paying close attention to B12, iodine and protein quality, using targeted supplements and including animal foods when possible.
Research
Niklewicz, A., Hannibal, L., Hvas, A.M. and Sanders, T.A.B. (2024) A systematic review and meta analysis of functional vitamin B12 status among adult vegans. Nutrition Bulletin, 49(4), pp. 463 to 479.
Neufingerl, N. and Eilander, A. (2021) Nutrient intake and status in adults consuming plant based diets compared to meat eaters. A systematic review. Nutrients, 14(1), 29.
Eveleigh, E.R., Coneyworth, L. and Welham, S.J.M. (2023) Systematic review and meta analysis of iodine nutrition in modern vegan and vegetarian diets. British Journal of Nutrition, 130(9), pp. 1580 to 1594.
Rosell, M.S., Lloyd Wright, Z., Appleby, P.N., Sanders, T.A.B., Allen, N.E. and Key, T.J. (2005) Long chain n 3 polyunsaturated fatty acids in plasma in British meat eating vegetarian and vegan men. American Journal of Clinical Nutrition, 82(2), pp. 327 to 334.
Ciuris, C., Lynch, H.M., Wharton, C. and Johnston, C.S. (2019) A comparison of dietary protein digestibility, based on DIAAS scoring, in vegetarian and non vegetarian athletes. Nutrients, 11(12), 3016.
Mariotti, F. and Gardner, C.D. (2019) Dietary protein and amino acids in vegetarian diets. A review. Nutrients, 11(11), 2661.
Li, T., Li, Y. and Wu, S. (2021) Comparison of human bone mineral densities in subjects on plant based and omnivorous diets. A systematic review and meta analysis. Archives of Osteoporosis, 16(1), 95.
Tong, T.Y.N., Appleby, P.N., Armstrong, M.E.G., Fensom, G.K., Knuppel, A., Papier, K., Perez Cornago, A., Travis, R.C. and Key, T.J. (2020) Vegetarian and vegan diets and risks of total and site specific fractures. Results from the prospective EPIC Oxford study. BMC Medicine, 18(1), 353.
Crowe, F.L., Steur, M., Allen, N.E., Appleby, P.N., Travis, R.C. and Key, T.J. (2011) Plasma concentrations of 25 hydroxyvitamin D in meat eaters, fish eaters, vegetarians and vegans. Results from the EPIC Oxford study. Public Health Nutrition, 14(2), pp. 340 to 346.
Haider, L.M., Schwingshackl, L., Hoffmann, G. and Ekmekcioglu, C. (2018) The effect of vegetarian diets on iron status in adults. A systematic review and meta analysis. Critical Reviews in Food Science and Nutrition, 58(8), pp. 1359 to 1374.
Foster, M., Chu, A., Petocz, P. and Samman, S. (2013) Effect of vegetarian diets on zinc status. A systematic review and meta analysis of studies in humans. Journal of the Science of Food and Agriculture, 93(10), pp. 2362 to 2371.
Webster, J., Greenwood, D.C. and Cade, J.E. (2023) Risk of hip fracture in meat eaters, pescatarians and vegetarians. A prospective cohort study of 413,914 UK Biobank participants. BMC Medicine, 21(1), 278.
Ho Pham, L.T., Nguyen, N.D. and Nguyen, T.V. (2009) Effect of vegetarian diets on bone mineral density. A Bayesian meta analysis. American Journal of Clinical Nutrition, 90(4), pp. 943 to 950.
Pawlak, R., Lester, S.E. and Babatunde, T. (2013) How prevalent is vitamin B12 deficiency among vegetarians. Nutrition Reviews, 71(2), pp. 110 to 117.
Thorpe, D.L., Beeson, W.L., Knutsen, R., Rajaram, S., Fraser, G.E. and Knutsen, S.F. (2021) Dietary patterns and hip fracture in the Adventist Health Study 2. Combined vitamin D and calcium supplementation mitigate increased hip fracture risk among vegans. American Journal of Clinical Nutrition, 114(2), pp. 488 to 495.
Appleby, P., Roddam, A., Allen, N. and Key, T. (2007) Comparative fracture risk in vegetarians and nonvegetarians in EPIC Oxford. European Journal of Clinical Nutrition, 61(12), pp. 1400 to 1406.
Weikert, C., Trefflich, I., Menzel, J., Obeid, R., Longree, A., Dierkes, J., Meyer, K., Herter Aeberli, I., Mai, K., Stangl, G.I. and others (2020) Vitamin and mineral status in a vegan diet. Deutsches Ärzteblatt International, 117(35 to 36), pp. 575 to 582.
Menzel, J., Abraham, K., Stangl, G.I., Ueland, P.M., Obeid, R., Schulze, M.B., Herter Aeberli, I. and Weikert, C. (2021) Vegan diet and bone health. Results from the cross sectional RBVD study. Nutrients, 13(2), 685.
Hansen, T.H., Madsen, M.T., Jorgensen, N.R., Cohen, A.S., Hansen, T., Vestergaard, H. and Pedersen, O. (2018) Bone turnover, calcium homeostasis and vitamin D status in Danish vegans. European Journal of Clinical Nutrition, 72(7), pp. 1046 to 1054.
Tucker, K.L. (2014) Vegetarian diets and bone status. American Journal of Clinical Nutrition, 100(Suppl 1), pp. 329S to 335S.
