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Series 4: Vegan or flexitarian - which diet is healthier?

In the quest for sustainable healthy diets, it is common to hear recommendations to reduce the amount of animal-sourced foods consumed in high-income countries. But what level of reduction is optimal for human health? For the best nutritional and health outcomes, should we be eating low-meat “flexitarian” diets, or entirely plant-based diets? In this Letterbox exchange, nutrition researcher Flaminia Ortenzi and physician Dr Tushar Mehta share their perspectives on the relative health and nutritional impacts of purely vegan diets compared to those that contain low levels of animal-sourced foods. Flaminia focuses on the nutrient density of different foods, while Tushar discusses data on the health outcomes of different diets.


Flaminia Ortenzi

Flaminia Ortenzi

Flaminia Ortenzi is a multi-disciplinary researcher in the fields of nutrition, sustainable food systems, and global/public health. She currently holds a Research Associate position at the Global Alliance for Improved Nutrition (GAIN) in Geneva.

She has been working on various projects in a research and knowledge dissemination capacity in the areas of healthy diets, food safety and security, undernutrition and micronutrient deficiencies, sustainability of food systems, obesity, and non-communicable diseases prevention and control. She is passionate about finding innovative, holistic, evidence-based solutions to the triple burden of malnutrition and other global health challenges, toward the achievement of the Sustainable Development Goals (SDGs). 

Flaminia holds a MSc in Global Health from the University of Geneva (Switzerland, and a BSc in Food Science and Human Nutrition from the Bio-medical Campus University of Rome, Italy.

A list of her publications is available here.

Dr Tushar Mehta

Tushar Mehta

Dr. Tushar Mehta completed medical school and residency at the University of Toronto.  Currently, he practices Emergency Medicine and participates in international health projects.  Formerly an annual volunteer in rural India, he is now doing work in Haiti ( He has also works with small NGOs and is a medical advisor for Project CANOE in Toronto.

Along with a small team, Tushar co-founded Plant Based Data, an online database which collects and organises academic literature regarding the impact of plant and animal agriculture on health, environment, food security, and pandemics.

Tushar is interested in an evidence and data based approach to ecological issues, embedded in humanitarianism. 

Tushar receives no funding for his work, and Plant Based Data receives no direct or indirect funding from food industry sources nor has any other financial conflicts of interest.

Flaminia Ortenzi

Dear Tushar,

As you know, developing guidelines for diets that simultaneously improve human health and sustainability of global food systems has been a priority research focus for nutrition and environmental scientists in recent years. It is generally recommended to substantially limit consumption of animal-source foods (ASFs), especially in relation to current high intake levels of ASFs in high-income countries, in favour of plant-source foods (PSFs). The issue I want to explore in this letter series is whether it is ‘healthier’ to follow completely vegan diets – as recommended by some actors, particularly in wealthier countries – or to maintain low-to-moderate amounts of ASFs in diets.

Given the low intakes of minimally processed PSFs, such as fruits and vegetables, legumes, nuts and seeds, in many countries worldwide, increased consumption certainly holds potential to improve population health. However, issues of nutrient adequacy, affordability, and acceptability have not yet been fully integrated into the above recommendations. Indeed, minimally processed ASFs rich in highly bioavailable micronutrients, make a significant contribution to diet quality globally, especially among population groups vulnerable to malnutrition such as women and children. Thus, major reductions in the consumption of ASFs could lead to increased micronutrient deficiencies, and may also be socio-culturally inappropriate in many contexts.

Within this context, my co-author Ty Beal and I recently conducted a study entitled Priority Micronutrient Density in Foods, to identify the top intrinsic food sources of six ‘priority’ micronutrients commonly lacking in diets globally: iron, zinc, folate, vitamin A, calcium, and vitamin B12. We built an aggregated global food composition database comprising 41 foods and food groups, and calculated recommended nutrient intakes for five population groups with different requirements (adults ≥25 years; children 2–4 years; adolescents; women of reproductive age – WRA; and pregnant women). We then developed an approach to rate foods according to their density in each and all priority micronutrients for the selected population groups, while accounting for estimated differences in iron and zinc bioavailability across foods.

We found that the top sources of these six priority micronutrients are organ meats (liver, spleen, kidney, and heart), small fish, dark green leafy vegetables, bivalves, crustaceans, goat meat, beef, eggs, milk, canned fish with bones, lamb, and mutton. Our results show that adding just small amounts of particularly nutrient dense ASFs (e.g., organs, small fish, and bivalves) to largely plant-based diets would go a long way toward ensuring adequacy of commonly lacking micronutrients. These findings are not only applicable to low- and middle-income countries (LMICs), which suffer from the highest burden of micronutrient deficiencies, but also to high-income countries (HICs), especially among population groups with increased nutritional needs such as pregnant women and WRA, who may often be deficient in micronutrients such as iron, zinc, and folate.

Contrarily to what one might think, micronutrient deficiencies are not only a problem in LMICs. Nutrient density matters both in LMICs and in HICs such as the US, where inadequate micronutrient intakes are common (Devarshi et al., 2021). For example, recent evidence shows that 44% of US women don’t consume enough calcium, 37% have an inadequate intake of vitamin A, 23% of folate, 18% of zinc, and 6% of iron (Passarelli S. et al., 2022). Other recent studies, such as Sun H. and Weaver C.M., 2021, found much larger percentages of US women not meeting daily dietary iron intake requirements (almost 20% in 2018), and estimated that 9% of women in the country suffer from anaemia. Although estimates may vary due to methodological differences, and the exact numbers may be disputed, the burden of micronutrient deficiencies in HICs is apparent and well-documented.

Although carefully constructed plant-based diets could provide adequate amounts of all six priority micronutrients for the general population (except for vitamin B12 which would need to be consumed through fortified foods or supplements), I am concerned about the feasibility of such diets at the population level, even in HICs, where there is greater access to nutritious and safe foods as compared to LMICs.

Indeed, there is large variation in the availability, accessibility, and affordability of nutrient-dense PSFs across countries. For instance, the supply of pulses in the US and that of vegetables in the Netherlands are way below the respective global averages. In Finland, the cost of legumes, nuts, and seeds is more than twice that of starchy staples (e.g., wheat, rice or potatoes), and in Germany the cost of vegetables is 3.5 times that of starchy staples(compared on the basis of the recommended amount per day of vegetables or starchy staples). In addition, building a healthy, nutrient-adequate vegan diet requires a certain degree of nutritional knowledge, which the general population may be lacking, exposing them to an increased risk of micronutrient deficiencies. Issues of sociocultural acceptability of plant-based diets may also arise in some countries/contexts.

Finally, even when carefully designed, completely vegan diets may be unable to meet the increased nutritional requirements of certain population groups, such as WRA, pregnant women, young children, and adolescents, that will likely need fortification or supplementation for other micronutrients in addition to vitamin B12, such as iron and zinc. However, evidence on the efficiency and effectiveness of supplements is inconclusive, with some studies reporting limited-to-no benefits, and others mentioning significant side effects. For example, a Cochrane review identified an increased prevalence of gastrointestinal side effects in women taking iron supplements, including diarrhea, constipation, and abdominal pain. A recent study on nutrient intakes of US women found that the addition of dietary supplements to their usual diets did not markedly decrease the percentage of women at risk for inadequate intakes of iron and zinc.

In conclusion, in view of the evidence presented above, I believe that consumption of small-to-moderate quantities of highly nutrient-dense animal-source foods is not only of crucial importance in LMICs, but is also advisable in HICs, particularly among population groups at increased risk of micronutrient deficiencies.

I look forward to hearing your views on this topic.

Best wishes,

Flaminia Ortenzi

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Dr Tushar Mehta

Dear Flaminia

Thank you for your letter.

You conclude that it is advisable from a health perspective to consume small-to-moderate amounts of nutrient-dense animal source foods (ASFs), on the grounds that completely vegan diets present an increased risk of micronutrient deficiencies, even in high-income countries (HICs) and even with supplementation and fortification. As decided prior to our exchange of letters, our discussion will focus on the context of HICs.

I believe that the evidence demonstrates that, in HICs, completely vegan diet patterns have roughly equal long term health impacts to plant-based diets that include some animal products. I have three issues to discuss in this letter: first, the links between ASF consumption and critical diseases; second, the role of supplementation and fortification; third, some methodological queries about your paper.

By way of context, I will respond strictly as a physician with no direct or indirect funding from any agriculture or food industry, prioritise references free from such funding, and evaluate studies for methodology and the details of comparison groups within studies. I prioritise evidence-based medicine regarding clinical outcomes (morbidity and mortality), which give a more direct indication of health impacts compared to biomarkers or expert opinion.

As a Canadian emergency and family physician, I regularly treat people with severe diseases causing suffering and death linked to animal based and highly processed foods (many of which are also animal based). Diseases increased by ASFs correlates with the global burden of disease in HICs.

In a 2021 UK Biobank study which studied the correlation between 25 common conditions and meat intake, the risk of the following diseases were increased per 100 g of additional meat intake (both processed and unprocessed) per day: ischemic heart disease, cerebrovascular disease, ischemic stroke, hemorrhoids, pneumonia, diverticular disease, colon polyps, gallbladder disease, osteoarthritis, female genital prolapse, uterine fibroids, diabetes, and cellulitis. For a general population, disease rates listed in the supplementary data should be considered without adjusting for obesity, since consumption of animal products itself is a cause of increased population obesity.

Clinical results of a single study, such as the one above, should not be accepted in isolation. My project Plant Based Data contains a library of relevant studies. Numerous studies show that predominantly plant based diets reduce, or certain animal products increase, the risk of cardiovascular disease, diabetes, obesity and dementia as well as breast, prostate, colorectal and pancreatic cancers. Plant diet protection from general cancer risk is more important than specific cancers.

“All-cause mortality” is perhaps the strongest clinical measure of medical or nutritional parameters. A 2020 JAMA paper demonstrated that a moderate replacement of 3% of energy from animal protein for plant protein resulted in a 10% decrease in all cause mortality (note that the study was observational rather than interventional). A 2016 paper in JAMA demonstrated similar results in those with at least one unhealthy lifestyle factor, which notably includes overweight and obesity.

Purely vegan diets have very occasionally been compared to other plant based diets, which include vegetarian, pescatarian, flexitarian, and other subtypes. Data is extremely limited due to 1) the low number of vegans, 2) the heterogeneity of study populations including vegan subgroups. We must recognise that vegans are small in number, and were more so when data collection for these studies started 10-20 years ago. Vegan dietary best practices are also new and evolving. That said, existing data shows that vegan and vegetarian diets both offer similar levels of health benefits when compared to omnivorous diets. Two major cohort studies that contain a small vegan subgroup, EPIC Oxford and the Adventist Health Study-2, show similar benefits for both vegans and plant based subgroups that contain animal products. Vegans have solid evidence of advantages over vegetarians regarding diabetes, and possible extra protection against prostate cancer.

Conversely, a study shows increased fractures in vegan women, which seems most strongly correlated with low body mass index, but may also relate to low calcium, vitamin D, or B12 intake, and lifestyle issues such as child bearing or hormone use. This requires education and mitigation, as well as further study. Other than this, I am not aware of any non-industry funded clinical data showing either increased all-cause mortality or increased incidence of other diseases in vegans compared to those following omnivorous diets or plant diets containing some animal products.

Despite solid evidence of morbidity and mortality benefits for predominantly plant based diets, including evidence related to vegan diets, the most exciting evidence shows the quality of plant diets determining the strength of benefits. Both plant based and vegan diets are highly varied and ultimately optimising their quality will maximise benefits. This potential should continue to be explored.

When looking at nutrient intake and biomarkers rather than health outcomes, adult vegans perform well overall, though have deficiency risk for B12, calcium, zinc, and selenium. Vegan children show good health, though some potential deficiencies, where diet optimisation and supplements/fortification can help.

Vegans require B12 fortified foods and/or supplements, noting that B12 deficiency is multifactorial. General plant diets have an increased risk of B12 deficiency, but so do elderly people consuming omnivorous diets. Iron deficiency anemia is also multifactorial, and prevalent amongst omnivores. Diet is not an exclusive factor, though must be attended. I have never diagnosed a case of clinical disease due to Zinc deficiency while practicing for over 20 years in Canada, but let us presume that vegans should take more precaution, and look forward to more clinical research.

I have some queries about your paper’s methodology. Other than dark green leafy vegetables, your paper omitted common plant sources of beta carotene (a form of Vitamin A), such as sweet potato, pumpkin, squash and carrots. Significant plant based sources of folate are also not listed, nor are sources of zinc, and sources of iron. Your paper’s scoring system may therefore not adequately assess the ability of plant diets to provide these nutrients. Also, it specifically does not measure clinical evidence of death or disease from diets containing or lacking combinations of the listed sources, in the context of HICs.

Note also that Hypervitaminosis A could be caused by moderate but long term consumption of the dense sources highlighted in your references. Birth defects, liver toxicity, bone degradation and other issues could result. A precaution should be included. Furthermore, recognition of the risk from animal product consumption and the major diseases listed above is also needed.

The excellent study you referenced regarding nutrient adequacy in USA women of child bearing and menopausal ages stated that current supplement strategies did not remove all deficiencies, but was positive about the potential to improve both nutrition and supplement strategies, as am I. Furthermore, the study was in a general USA population of women, hence mostly omnivores. Though supplements did not fill nutrient gaps, it seems that massive USA meat consumption also fails to reduce gaps, reflecting the multifactorial causes of deficiency.  

Since the scope of your paper does not include clinical evidence on health outcomes, it is not possible draw reliable conclusions from your research on whether good quality vegan diet results in worse health than diets containing small amounts of animal products. Hence, your concern, although well taken, is a hypothesis based on your expert opinion (I am using the term in the sense of the Evidence Hierarchy of evidence-based practice).

The level of difficulty in optimising a vegan diet is also a matter of opinion, and one that may change after a more complete listing of common plant foods containing the nutrients of concern. Nevertheless, I do share this concern, but equally extend it to all plant diets that contain some animal products, and even more concern to the conventional consumption of animal products.

I am very thankful for the questions you bring forward. They are an excellent opportunity for education and action to optimise vegan and plant-based diets, and then to conduct more health research on the potential of more optimised plant diets.

Best wishes,

Tushar Mehta

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Flaminia Ortenzi

Dear Tushar,

Thank you for your thoughtful and informative letter. In this follow-up letter, I will respond to the issues you raised point-by-point and provide some additional references to give us more food for thought. 

I will start from addressing your queries on our paper. First, I invite you to check the Supplementary Material, which provides a list of all individual food items included in our analysis. We created a category for ‘vitamin-A-rich fruits and vegetables’, comprising carrots, pumpkin, squash, mangoes, and papayas, among others. Thus, your claim that we omitted common plant sources of vitamin A is incorrect, as are your claims that we did not include important plant sources of folate, zinc, and iron, for which I also refer you to our Supplementary Material.

Second, our paper in no way discourages the adoption of plant-rich diets, which have well-recognized health benefits (as exhaustively described in your letter), but rather encourages consumption of small amounts of minimally processed nutrient-dense animal-source foods (ASFs) as part of diverse, balanced diets. 

Third, our methodology already accounts for the risks associated with certain foods containing high densities of individual micronutrients (e.g., vitamin A). Indeed, each micronutrient’s contribution to the aggregate score is capped at 100% of recommended intakes. The rationale for this methodological choice is explained in detail in our paper.

Fourth, I completely agree that massive meat consumption in high-income countries (HICs) has not and will never fix micronutrient deficiencies. In fact, we never suggested that consumers in HICs continue this trend; on the contrary, we recommend consuming small quantities of specific nutrient-dense ASFs – i.e., not just meat and not any kind of meat. In particular, we encourage introducing organ meat, dried fish, bivalves, crustaceans, canned fish with bones, fresh fish, eggs, milk, yoghurt, cheese, ruminant meat, and pork within a balanced diet rich in a wide variety of plant-source foods (PSFs), especially highly nutrient-dense ones such as dark green leafy vegetables. 

Finally, our paper has not undergone validation for health outcomes because it was never meant to propose a new global model for a healthy and sustainable diet, but rather to inform policies and programmes focused on addressing micronutrient malnutrition. My concern that vegan diets may result in increased micronutrient inadequacies is not solely founded on our paper, but on a multitude of factors which will be presented below. A growing body of literature reveals that micronutrient gaps are much more common than we believed – including in HICs – and that fortification and supplementation alone are insufficient to address them effectively and should be complemented by approaches focused on improving diet quality. Even the EAT-Lancet Planetary Health Diet, which was designed by globally renowned nutrition and public health experts and recommends an ASF intake of about half the average in HICs, has been shown to be lacking in several essential micronutrients (i.e., vitamin B12, calcium, iron, and zinc), particularly for women of reproductive age. If the current gold standard plant-based diet (which is not even fully vegan) is inadequate, I have little hope that the average vegan consumer follows a micronutrient-adequate diet.

Moving on to the other issues you raised, I completely agree that there is growing evidence supporting the association between consumption of ultra-processed foods (UPFs) and an increased risk of overweight/obesity and non-communicable diseases (NCDs). For instance, a recent randomized controlled trial revealed that diets high in UPFs consistently led to overeating and weight gain compared to minimally processed diets, despite meals within both diets being matched for calories, macronutrients, sugar, sodium, and fiber. However, there are just as many UPFs of plant origin as there are of animal origin. You can think of sugar-sweetened beverages, which, according to the Global Burden of Diseases study, are the greatest contributor to consumers’ total sugar intake globally; or of breakfast cereals, biscuits, snack bars, and potato chips, many of which are completely vegan and have extremely long, ‘questionable’ (to say the least) ingredients lists. Similarly, most popular meat replacements are ultra-processed and often higher in sodium, unhealthy fats, sugar and refined starch than the corresponding meat products.

The 2021 UK Biobank study you cited found an association between frequent (three or more times/week) meat consumption and higher risks of several diseases; however, the authors concluded that higher Body Mass Index (BMI) ‘accounted for a substantial proportion of these increased risks’ and ‘residual confounding or mediation by adiposity might account for some of the remaining associations’. In other words, overweight/obesity could explain a large part of the associations observed. Not adjusting disease prevalence rates for overweight/obesity on the grounds that ‘consumption of ASFs itself is a cause of increased population obesity’ – as you argued in your letter – is scientifically inaccurate as obesity is a complex, multi-factorial metabolic condition influenced by a wide range of dietary and non-dietary determinants (certainly not only ASF intake).

As for the 2016 and 2020 JAMA papers, the former only found an association between intake of animal- and plant-based protein and cardiovascular and/or all-cause mortality among people with at least one unhealthy lifestyle factor, and both papers accounted for a limited number of potential confounders and acknowledged the possibility that residual confounding could explain some of the observed associations. This points me to a common problem in nutrition science, that is reductionism and the simplistic assumption that we can estimate the quantitative health burden of individual foods. However, the relationship between food and health is extremely complex: foods are consumed as part of the broader diet, which is only one of many individual-level determinants of health, together with other lifestyle and biological factors. In turn, these elements interact with numerous socio-economic, environmental, and commercial determinants of health over the course of a lifetime. Similar concerns have been raised in relation to the Global Burden of Diseases study, which aimed to identify causal relationships between single dietary components and health outcomes, leading to a false sense of confidence in uncertain estimates.

Leaving the discussion on limitations and uncertainties of the existing evidence aside, I recognize there can be health risks associated with ASFs if consumed in certain forms – particularly, though not only, (ultra-)processed – and/or in excess, and if they are not part of balanced, plant-rich diets. Yet, I would also like to draw your and the readers’ attention to the health benefits of ASFs, especially for certain life stages.

ASFs are the only inherent food source of vitamin B12 and offer more bioavailable forms of vitamins A and D, iron, and zinc as compared to PSFs, deficiencies in which can lead to severe, often long-lasting consequences (including birth defects, growth faltering, impaired cognitive development, rickets, anemia, weakened immunity, blindness, and death), particularly if they occur during critical periods of life such as infancy and young childhood, adolescence, pregnancy and lactation, and old age.

In the same UK Biobank study cited above, higher intakes of unprocessed red meat and poultry meat have been associated with lower risk of iron deficiency anemia; this is because, unlike PSFs, ASFs contain heme iron and no iron absorption inhibitors (such as phytic acid), resulting in animal-source iron being 1.5 to 2 times more bioavailable. Similarly, low meat intake has been associated with decreased zinc intake and retention, even when substituting meat with PSFs rich in zinc; this is due to antinutrients in PSFs making zinc 1.7 times less bioavailable as compared to ASFs. ASFs also contain the highest concentrations of vitamin D and choline (especially fatty fish for the former, and organ meats and eggs for the latter), which are commonly lacking worldwide, including in HICs

In addition to micronutrients, ASFs also contain high densities of essential fatty acids and amino acids (EAAs). For instance, fish and seafood are rich in long-chain omega 3 fatty acids (DHA and EPA) that are crucial for fetal development, health protection, and healthy ageing; while most PSFs (except for sea vegetables that are rarely consumed outside of East Asia) contain short-chain omega 3 fatty acids (ALA), of which only 10% is converted into DHA and EPA. As for EAAs, the digestible indispensable amino acid score is much higher for ASFs than for most PSFs (except for certain soy-based products, such as soymilk and tofu). Finally, ASFs provide a wide range of unique bioactive compounds that may play important roles in anti-inflammatory and immune pathways, memory and cognition, and cardiovascular health, to name a few.

You rightly mentioned that micronutrient deficiencies are prevalent in omnivores, which further strengthens my point: if the global population of women of reproductive age (mostly omnivores) is already commonly deficient in iron and zinc, this would only be exacerbated by depriving them of the most dense and bioavailable sources of these nutrients, such as organ meat, ruminant meat and bivalves.

Two recent systematic reviews (Pérez-Roncero, et al.; Achón, et al.) found that consuming dairy products during pregnancy was strongly associated with healthy birth weight and length. Meat consumption has been associated with lower risk of iron deficiency among breastfed infants with low iron intakes and stores, and fish and seafood consumption with better child health outcomes. Among adolescents, dairy intake has been associated with improved bone health and height, reduced weight and degree of obesity (Kouvelioti, et al.; de Beer). A systematic review found convincing evidence that consuming lean red meat contributes to muscle health and protection from sarcopenia, with important implications for the elderly; and other studies suggested that dairy may also reduce the risk of sarcopenia and frailty, as well as dementia and Alzheimer’s. It goes without saying that the same argument on confounding factors applies to both the epidemiological evidence on the risks and benefits of ASFs, making it impossible to draw definitive conclusions on either and imperative to consider both aspects.

I hope I have effectively addressed your queries on our paper and shed some light on the health benefits of a balanced, plant-rich diet including small amounts of minimally processed nutrient-dense ASFs. I look forward to continuing our discussion on this topic.

Best wishes,
Flaminia Ortenzi

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