According to a 2012 report, 2% of the US population follows a vegan diet, which is a strict plant-based diet that excludes all animal-derived foods.1 Increasingly, people are choosing to follow this diet for ethical, environmental, religious, and health concerns.2 The increase in patients following a vegan diet can lead to requests for provider advice about the safety of this dietary pattern. Several studies have demonstrated health benefits of plant-based diets, including improvements in ischemic heart disease mortality, as well as decreases in hypertension, hyperlipidemia, obesity, type 2 diabetes mellitus, and cancer incidence.3-13 Despite these benefits, could vegans be at risk for deficiency of essential nutrients? The present article concisely summarizes the recent literature examining potential negative consequences of a vegan diet and assists clinicians in their conversations with patients.8,14
In December 2014, we performed an independent literature review of the PubMed, EMBASE, and Medline databases from 2004 to the present. The following search terms were used: vegan, risk, safety, vitamin B12, protein, amino acids, iron, calcium, omega-3 fatty acids, eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA). We included all prospective cohort, cross-sectional, and review articles on the proposed risks associated with vegan diets. Non-English studies and studies regarding the health benefits of a vegan diet and effects in pregnancy and breastfeeding were excluded. We identified 167 titles and abstracts and reviewed relevant full texts and pertinent references. The Table identifies nutrients of concern and sources of those nutrients for vegans, and the Figure provides suggested actions for primary care physicians.
|Vitamin B12||Cereal, fortified soy and rice milk, meat analogs, nutritional yeast, supplements|
|Calcium||Blackstrap molasses, collard greens, fortified plant milks and juices|
|Vitamin D||Fortified plant milk; ultraviolet B–exposed mushrooms|
|Omega-3 fatty acids (EPA, DHA)||Algae (EPA and DHA), chia, flaxseed, walnuts|
|Iron||Spinach, collard greens, lentils, soybeans, tofu|
|Essential amino acids||Soy, quinoa, buckwheat, hempseed, chia, legumes|
Abbreviations: DHA, docosahexaenoic acid; EPA, eicosapentaenoic acid.
Plant foods contain a negligible amount of vitamin B12; thus, vegans have difficulty meeting the recommended daily allowance (2.4 μg) and maintaining adequate serum concentrations of B12. In a cross-sectional study of 689 men, 121 of 232 vegans (52%) were B12 deficient (<118 pmol/L), whereas only 1 of 226 omnivores (0%) was B12 deficient.15 A prospective study of 20 omnivores who adopted a vegan diet over 5 years showed that B12 levels were only maintained in the patients who consumed B12-fortified products.16
Vitamin B12 deficiency can cause cognitive disorders, neuropathies, weakness, and macrocytic anemia, which can result in fatigue and low energy.17 Deficiency also affects digestion and absorption of nutrients and metabolism of serotonin and melatonin.17 Vegetarians have a lower risk of death from ischemic heart disease5,18; however, low B12 levels have been associated with worsening of some cardiovascular markers. A cross-sectional study of 154 vegans revealed an elevated homocystine level in 66% of participants, of whom 93% also had low B12 levels (<250 pmol/L).19 A 2014 review6 identified elevated homocysteine level, impaired brachial flow-mediated endothelial dilation, and increased carotid intimal medial thickness in patients with low B12 levels. These effects were improved with B12 supplementation.6
Considering the frequency of B12 deficiency in vegans, providers should advise vegans to take supplemental B12 or eat B12-fortified foods. To ensure adequate intake, periodic monitoring of B12 levels should be considered.
Iron deficiency is one of the most common nutritional deficiencies worldwide.20 Although the overall iron intake in vegans is surprisingly greater than that in omnivores, levels of ferritin, which stores iron, are often lower in vegans.14,21,22 Absorption and storage is affected by the bioavailability of nonheme iron (plant sources) being less than that of heme iron (animal sources).14,20 The vegan diet contains nutrients that both enhance and inhibit iron absorption, such as organic acids and phytates, respectively, leading to variable iron levels.4,20 Iron deficiency is more prevalent in young vegan women compared with older vegan women20 and men and women following omnivore diets.21
Periodic assessments of anemia and iron status should be considered for all vegans but especially for vegan children and women of childbearing age. Health care professionals should encourage patients with documented low ferritin levels to take a bioavailable supplement with absorption-enhancing foods.
Multiple studies have documented lower intake of calcium and vitamin D in vegans,20,23 yet the implications of a vegan diet in bone health, particularly bone mineral density (BMD), have been varied. Several investigations have demonstrated lower BMD in vegans compared with nonvegans,23-26 whereas 1 study showed no difference.27
Data on fracture incidence are also mixed. In 1 study, fracture incidence was 30% higher in vegans than in nonvegans, though participants with a minimum daily calcium intake of 525 mg had no increase in fractures.28 A prospective longitudinal study of Asian women found no statistically significant difference in fracture incidence between vegans and nonvegans.29
Gastrointestinal calcium absorption increases when dietary calcium intake is low, and lower animal protein intake decreases renal calcium excretion. Thus, the skeletal effects of a vegan diet might be attenuated. The higher intake of vitamin K and phytoestrogens and increased alkalinity of vegan diets are also thought to improve BMD.
Many factors affect bone health. Osteoporosis, fracture risk, and vitamin D deficiency are not unique to the vegan diet; nevertheless, health care professionals should advise vegans to consume at least 525 mg of calcium and 15 μg of vitamin D per day. Periodic monitoring of BMD could be considered, but recent evidence is not conclusive.
Two essential fatty acids must be acquired in the diet because the human body cannot make them: linoleic acid (LA) and α-linolenic acid (ALA). Linoleic acid is the main component of most vegetable-based oils; consequently, the vegan diet need only include vegetable fat to avoid deficiency. α-Linolenic acid is found in many vegan foods: soy, soybean oil, canola (rapeseed) oil, walnuts, and flaxseeds. Two additional fatty acids, EPA and DHA, are deficient in the vegan diet. Vegans do not ingest EPA or DHA unless they consume large amounts of seaweed, so they are dependent on synthesis of these fatty acids from ALA. The health effects of lower levels of EPA and DHA remain a concern. Both play major roles in metabolism, including EPA’s effect on platelet aggregation (via thromboxane) and inflammation (via leukotrienes) and DHA’s effect on brain and visual functioning.
A review of the literature reveals that these worries have not been substantiated. Studies of children brought up with a vegan diet have not shown problems in visual or cognitive development.30 A study of vegan men showed no effect on IQ (intelligence quotient) compared with nonvegan men, and a study of Seventh Day Adventist vegetarians showed no increase in dementia.31,32
There is no convincing evidence that vegans have health consequences from lower levels of omega-3 fatty acids. Concerned vegans can increase their consumption of foods containing ALA, and health care professionals can inform patients that plant-based DHA supplements (such as those derived from algae) are available.
Concerns about protein deficiency in vegans abound, yet little is known about the likelihood of vegans ingesting less than the minimum recommended intake. The accuracy of reporting protein intake is complicated, as many foods are sources of protein. Modern diets, including the vegan diet, tend to include higher protein intake than the Institute of Medicine’s recommended daily intake for adult women (46 g) and men (56 g).33 In a study of British vegans, the total reported protein intake was lower compared with the national average for men (65 vs 87 g/d) and women (47 vs 66 g/d)34 but still met the Institute of Medicine’s recommendations. A prospective trial of omnivores placed on a vegan diet for 18 weeks showed a decline of only 1.1% in total protein calories.35
Despite these reports, protein intake remains a concern. Essential amino acids must be ingested because the body is incapable of manufacturing them independently. Plant sources are commonly believed to be incomplete sources of protein and to lack at least 1 essential amino acid. A specific food source may have low levels of a specific amino acid, but none is completely absent.36 No evidence has shown that vegans become deficient in specific amino acids.
To ensure appropriate protein levels, providers should advise vegans that they will achieve adequate essential amino acid and protein intake by varying the plant sources that comprise their diet.37
A vegan diet has many health benefits but is not without risk of dietary deficiencies. Health care professionals should ask patients about their dietary practices and assess for possible nutrient deficiencies through physical examination, periodic serum monitoring (including complete blood cell count, vitamin B12, iron, calcium, and vitamin D), and perhaps BMD monitoring.
1. NewportF.In U.S., 5% consider themselves vegetarians. Gallup website.2012; http://www.gallup.com/poll/156215/consider-themselves-vegetarians.aspx. Posted July 26, 2012. Accessed September 27, 2014. Search in Google Scholar
2. CrawfordE.Vegan is going mainstream, trend data suggests. Food Navigator-USA website. http://www.foodnavigator-usa.com/Markets/Vegan-is-going-mainstream-trend-data-suggests. Posted March 17, 2015. Accessed December 18, 2015. Search in Google Scholar
3. CraigWJ,MangelsAR;American Dietetic Association. Position of the American Dietetic Association: vegetarian diets.J Am Diet Assoc.2009;109(7):1266-1282. Search in Google Scholar
4. ClarysP,DeliensT,HuybrechtsI, et al.Comparison of nutritional quality of the vegan, vegetarian, semi-vegetarian, pesco-vegetarian and omnivorous diet.Nutrients.2014;6(3):1318-1332. doi:10.3390/nu6031318. Search in Google Scholar
5. HuangT,YangB,ZhengJ,LiG,WahlqvistML,LiD.Cardiovascular disease mortality and cancer incidence in vegetarians: a meta-analysis and systematic review.Ann Nutr Metab.2012;60(4):233-240. doi:10.1159/000337301. Search in Google Scholar
6. WooKS,KwokTCY,CelermajerDS.Vegan diet, subnormal vitamin B-12 status and cardiovascular health.Nutrients.2014;6(8):3259-3273. doi:10.3390/nu6083259. Search in Google Scholar
7. LeLT,SabatéJ.Beyond meatless, the health effects of vegan diets: findings from the Adventist cohorts.Nutrients.2014;6(6):2131-2147. doi:10.3390/nu6062131. Search in Google Scholar
8. KeyTJ,ApplebyPN,RosellMS.Health effects of vegetarian and vegan diets.Proc Nutr Soc.2006;65(1):35-41. Search in Google Scholar
9. BarnardND,CohenJ,JenkinsDJ, et al.A low-fat vegan diet and a conventional diabetes diet in the treatment of type 2 diabetes: a randomized, controlled, 74-wk clinical trial.Am J Clin Nutr.2009;89(5):1588S-1596S. doi:10.3945/ajcn.2009.26736H. Search in Google Scholar
10. EsselstynCBJr,GendyG,DoyleJ,GolubicM,RoizenMF.A way to reverse CAD?J Fam Pract.2014;63(7):356-364b. Search in Google Scholar
11. OrnishD,BrownSE,ScherwitzLW, et al.Can lifestyle changes reverse coronary heart disease? The Lifestyle Heart Trial.Lancet.1990;336(8708):129-133. Search in Google Scholar
12. TonstadS,ButlerT,YanR,FraserGE.Type of vegetarian diet, body weight, and prevalence of type 2 diabetes.Diabetes Care.2009;32(5):791-796. doi:10.2337/dc08-1886. Search in Google Scholar
13. YokoyamaY,BarnardND,LevinSM,WatanabeM.Vegetarian diets and glycemic control in diabetes: a systematic review and meta-analysis.Cardiovasc Diagn Ther.2014;4(5):373-382. doi:10.3978/j.issn.2223-3652.2014.10.04. Search in Google Scholar
14. CraigWJ.Nutrition concerns and health effects of vegetarian diets.Nutr Clin Pract.2010;25(6):613-620. doi:10.1177/0884533610385707. Search in Google Scholar
15. GilsingAM,CroweFL,Lloyd-WrightZ, et al.Serum concentrations of vitamin B12 and folate in British male omnivores, vegetarians and vegans: results from a cross-sectional analysis of the EPIC-Oxford cohort study.Eur J Clin Nutr.2010;64(9):933-939. doi:10.1038/ejcn.2010.142. Search in Google Scholar
16. MadryE,LisowskaA,GrebowiecP,WalkowiakJ.The impact of vegan diet on B-12 status in healthy omnivores: five-year prospective study.Acta Sci Pol Technol Aliment.2012;11(2):209-212. Search in Google Scholar
17. Sridevy,Baby P.Overcoming vitamin B12 deficiency in vegan diet.Nitte Univ J Health Sci.2014;4(1):104-105. Search in Google Scholar
18. KeyTJ,FraserGE,ThorogoodM, et al.Mortality in vegetarians and non-vegetarians: a collaborative analysis of 8300 deaths among 76,000 men and women in five prospective studies.Public Health Nutr.1998;1(1):33-41. Search in Google Scholar
19. WaldmannA,KoschizkeJW,LeitzmannC,HahnA.German vegan study: diet, life-style factors, and cardiovascular risk profile.Ann Nutr Metab.2005;49(6):366-372. Search in Google Scholar
20. WaldmannA,KoschizkeJW,LeitzmannC,HahnA.Dietary iron intake and iron status of German female vegans: results of the German vegan study.Ann Nutr Metab.2004;48(2):103-108. Search in Google Scholar
21. KajanachumpolS,AtamasirikulK,ViroonudompholD.Iron status among Thai vegans.J Med Technol Assoc Thailand.2012;40(1). Search in Google Scholar
22. CraigWJ.Health effects of vegan diets.Am J Clin Nutr.2009;89(5):1627S-1633S. doi:10.3945/ajcn.2009.26736N. Search in Google Scholar
23. ParsonsTJ,van DusseldorpM,van der VlietM,van de WerkenK,SchaafsmaG,van StaverenWA.Reduced bone mass in Dutch adolescents fed a macrobiotic diet in early life.J Bone Miner Res.1997;12(9):1486-1494. Search in Google Scholar
24. ChiuJF,LanSJ,YangCY, et al.Long-term vegetarian diet and bone mineral density in postmenopausal Taiwanese women.Calcif Tissue Int.1997;60(3):245-249. Search in Google Scholar
25. LauEM,KwokT,WooJ,HoSC.Bone mineral density in Chinese elderly female vegetarians, vegans, lacto-vegetarians and omnivores.Eur J Clin Nutr.1998;52(1):60-64. Search in Google Scholar
26. OutilaTA,KärkkäinenMU,SeppänenRH,Lamberg-AllardtCJ.Dietary intake of vitamin D in premenopausal, healthy vegans was insufficient to maintain concentrations of serum 25-hydroxyvitamin D and intact parathyroid hormone within normal ranges during the winter in Finland.J Am Diet Assoc.2000;100(4):434-441. Search in Google Scholar
27. Ho-PhamLT,NguyenPL,LeTT, et al.Veganism, bone mineral density, and body composition: a study in Buddhist nuns.Osteoporos Int.2009;20(12):2087-2093. doi:10.1007/s00198-009-0916-z. Search in Google Scholar
28. ApplebyP,RoddamA,AllenN,KeyT.Comparative fracture risk in vegetarians and nonvegetarians in EPIC-Oxford.Eur J Clin Nutr.2007;61(12):1400-1406. Search in Google Scholar
29. Ho-PhamLT,VuBQ,LaiTQ,NguyenND,NguyenTV.Vegetarianism, bone loss, fracture and vitamin D: a longitudinal study in Asian vegans and non-vegans.Eur J Clin Nutr.2012;66(1):75-82. doi:10.1038/ejcn.2011.131. Search in Google Scholar
30. SandersTA,ReddyS.Vegetarian diets and children.Am J Clin Nutr.1994;59(5 suppl):1176S-1181S. Search in Google Scholar
31. GiemP,BeesonWL,FraserGE.The incidence of dementia and intake of animal products: preliminary findings from the Adventist Health Study.Neuroepidemiology.1993;12(1):28-36. Search in Google Scholar
32. Lloyd-WrightZ.The Nutritional Status of Vegan Men in the United Kingdom with Particular Reference to Vitamin B12 [doctoral thesis].London, UK; University of London;2005. Search in Google Scholar
33. Institute of Medicine.Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acid, Cholesterol, Protein, and Amino Acids.Washington, DC: The National Academies Press;2005. Search in Google Scholar
34. DraperA,LewisJ,MalhotraN,WheelerE.The energy and nutrient intakes of different types of vegetarian: a case for supplements?Br J Nutr.1993;69(1):3-19. Search in Google Scholar
35. MishraS,XuJ,AgarwalU,GonzalesJ,LevinS,BarnardND.A multicenter randomized controlled trial of a plant-based nutrition program to reduce body weight and cardiovascular risk in the corporate setting: the GEICO study.Eur J Clin Nutr.2013;67(7):718-724. doi:10.1038/ejcn.2013.92. Search in Google Scholar
36. YoungVR,PellettPL.Plant proteins in relation to human protein and amino acid nutrition.Am J Clin Nutr.1994;59(suppl):1203S-1212S. Search in Google Scholar
37. PhillipsF.Vegetarian nutrition.Nutr Bull.2005;30(2):132-167. doi:10.1111/j.1467-3010.2005.00467.x. Search in Google Scholar
© 2016 American Osteopathic Association