Are plant-based diets nutritionally adequate?

It is well established that a balanced plant-based diet, predominantly based on healthful plant foods with reduced amounts of animal-based foods, is beneficial to both planetary and human health. In order to prevent further climate change and eco-system degradation and help mitigate the continued rise in non-communicable diseases, a worldwide shift towards more plant-based diets is urgently called for.

Nutritional adequacy of plant-based diets

The Academy of Nutrition and Dietetics state that plant-based diets containing a variety of fruits, vegetables, cereals, pulses, nuts and seeds are more environmentally sustainable and can help to reduce risk of non-communicable diseases.(1)

“Appropriately planned vegetarian, including vegan, diets are healthful, nutritionally adequate, and may provide health benefits for the prevention and treatment of certain diseases. These diets are appropriate for all stages of the life cycle, including pregnancy, lactation, infancy, childhood, adolescence, older adulthood, and for athletes.”

A common concern about plant-based diets is that they lack or have a lower bioavailability of key nutrients which humans need to thrive such as protein, calcium, iron and vitamin B12.(2) However, international research and dietary guidelines show that it is possible to eat a plant-based diet that is both environmentally beneficial and nutritionally adequate.

In this review, we explore the key nutrients of importance on a plant-based diet and how they can be addressed. As with all dietary patterns, the key is to ensure variety, balance and portion control.

What is a plant-based diet?

The term 'plant-based' is now used extensively to include a variety of dietary patterns from food based dietary guidelines/flexitarian, vegetarian and vegan diets. For the purposes of this report, the term plant-based diets will reflect a flexitarian type dietary pattern where healthful plant foods dominate the diet whilst animal products are significantly reduced but not necessarily omitted.

Read more about plant-based eating

Macro-nutrients on a plant-based diet

Compared to animal-based omnivorous diets, healthful plant-based diets tend to include more carbohydrates and fibre, less saturated fat and less but adequate protein. Overall, this is a healthier balance of macro-nutrients. Diets that eliminate oil-rich fish are lower in long-chain omega-3 fatty acids but do tend to provide more of the essential plant omega-3 fatty acid – alpha-linolenic acid (ALA).

Protein

There is a misconception that reducing or excluding animal protein foods will compromise protein status. Protein is an important macro-nutrient that we need for growth and repair. Requirements are different for each individual, are impacted by lifestyle factors and change throughout the life cycle.

Key plant-based sources of protein include soya, pulses, legumes, nuts and mycoproteins as well as smaller contributions from cereals. Although the amount of protein in plant-based foods is typically lower than in animal-based foods, this is not a concern in Western populations where we continue to exceed recommended intakes. Plant-based diets often provide protein quantities closer to current guidelines compared to current eating habits.(2)

How much protein?

The European Food Safety Authority (EFSA) recommend a minimum of 0.83 grams of protein per kilogram body weight for overall good health.(3) The recommended protein intake for children, pregnant or lactating women, older adults and athletes is higher. Clarys et al. compared the nutrient intakes of omnivores, pescatarians, vegetarians and vegans and found that intakes of protein were adequate across the spectrum of diets.(4)

Protein quality

There is also a concern that plant-based proteins are of a lower quality compared to animal-based proteins. It is important to note that it is not protein per se that is needed in the diet but rather the amino acids and nitrogen that make up proteins. The protein quality of individual foods is often assessed by comparing the amino acid profile to that of human protein requirements, as in the Protein Digestibility-Corrected Amino Acid Score (PDCAAS) and the Digestible Indispensable Amino Acid Score (DIAAS).(2,6)

Animal-based proteins such as meat, eggs and dairy have high scores, close to the maximum of one. Plant-based protein sources typically score lower, with the exception of soya which is comparable to cow’s milk.(2,6) Although plant-based proteins do contain all essential amino acids, some are present in smaller quantities such as leucine, isoleucine, and valine.(5) This has led to the concern that plant-based diets do not provide adequate high-quality protein.

However, assessing protein quality in this way does not accurately represent the overall protein quality of the diet. Nitrogen balance is achieved over the course of a day as the body is able to accumulate and store amino acids over this period. Thus, there is no need to rely on one food or one meal to provide all essential amino acids. Consuming a variety of plant-based protein sources over the course of a day and as long as energy requirements are met, all amino acids requirements (including essential) will be met.(8,9)

Omega-3 fatty acids

Dietary guidelines such as the UK Eatwell Guide (10) the US MyPlate (11) as well as organisations such as the European Society of Cardiology and the European Atherosclerosis Society (12) recommend that we consume oil-rich fish as a source of omega-3 fatty acids eicosapentaenoic (EPA) and docosahexaenoic acid (DHA) and plant sources of omega-3, alpha-linolenic acid (ALA).

Omega-3 fatty acids support heart, brain and vascular health as well as foetal development during pregnancy. Adequate omega-3 intake, in particularly EPA and DHA are associated with 10-16% reduced risk of cardiovascular disease.(13) And a 2018 extensive review of the evidence identified cardiovascular benefits from the consumption of ALA.(14)

Plant-based sources of omega-3 fatty acids include hemp, chia and linseeds (flaxseeds) – seeds or their oils, walnuts, and rapeseed oil. These are good sources of ALA which can be converted in small quantities to EPA and DHA within the body. Algae supplements are an alternative option to ensure adequate intakes of EPA and DHA for those who decide not to consume fish.(8,15)

There is space for consumption of ethically sourced seafood in a sustainable plant-based diet. Consuming oil-rich fish such as salmon, mackerel and herring weekly is therefore an excellent way to meet recommended omega-3 intakes on a plant-based diet. Oil-rich fish also provides protein as well as vitamin D and iodine, two other key nutrients needed for optimal health.(8)

Micro-nutrients on a plant-based diet

Balanced and varied plant-based diets rich in fruits and vegetables, wholegrains, legumes, nuts and seeds provide a good source of many vitamins and minerals. Individuals following a plant-based diet with small amounts of animal-based products are unlikely to experience nutrient deficiencies.(8) However, there are several micro-nutrients to be conscious of for those who choose to eliminate meat, dairy and seafood entirely, or for specific population groups.

Vitamin B12

Vitamin B12 or cobalamin is one of the only nutrients not found naturally in plant-based foods. This water-soluble nutrient is critical for red blood cell development and maintenance of the nervous system.(16) Deficiency can occur due to problems absorbing vitamin B12 or by inadequate intake from the diet. It is especially important for pregnant and breastfeeding women as well as young children and elderly adults to ensure adequate intakes of this vitamin.(17)

Plant-based diets which include smaller quantities of meat and dairy should still provide sufficient quantities of vitamin B12. For example, a 200ml glass of dairy milk or 70g portion of beef provides more than the daily recommended amount of vitamin B12.(17)

It is only those who wish to exclude meat and dairy altogether from their diet (less than 5% of the population in 2020 according to New Nutrition data).(18) For these individuals, it is important that they include 2-3 servings daily of vitamin B12 fortified foods or eventually take a vitamin B12 supplement. Plant food sources which can be fortified with vitamin B12 include breakfast cereal, plant-based drinks and alternatives to yogurt.(8,17) Nutrition labels should always be checked.

Vitamin D

Vitamin D plays a key role in bone and dental health and may be associated with prevention of other chronic diseases.(19) Dietary sources of vitamin D are few on both omnivorous and vegetarian diets. Oil-rich fish and liver are rich sources whilst other animal foods provide significantly smaller quantities such as meat and eggs.

Some countries like the US, mandatorily fortify dairy products with vitamin D. However, this is not the case for most European countries and omission of dairy in the diet has little impact on vitamin D intakes.(8) Interestingly, many mainstream non-organic plant-based drinks are fortified with vitamin D.

Additionally, it is well accepted that the main source of vitamin D is the endogenous production from skin exposure to sunlight, which can only occur during certain months of the year – normally spring and summer, and in the winter months we are reliant on stores. Therefore, it is not surprising that vitamin D intakes from food are often insufficient amongst both meat eaters and vegans alike.(19)

Plant-based sources of vitamin D include fortified products such as a few breakfast cereals, plant-based drinks and alternatives to yogurt as well as mushrooms grown under UV light. Adequate and safe exposure of skin to sunlight during summer and spring months as well as consuming oil-rich fish and/or fortified foods e.g. dairy in the US and some breakfast cereals is very important.

In some countries like the UK, due to significantly high prevalence of sub-optimal vitamin D status and inadequate dietary intakes, supplements are recommended for all individuals during autumn and winter and throughout the year for at risk groups such as under 5-year-olds, individuals who cover their skin, pregnant and breastfeeding women, those with little exposure to sunlight and the elderly.(20)

Minerals on a plant-based diet

Key sources of minerals in plant-based diets are legumes, pulses, nuts and seeds. These contain a variety of minerals which would otherwise be found in meat and dairy. The benefit of plant-based sources is that alongside their mineral content, they also provide a good source of fibre and less saturated fat than animal-based sources.

Iron

Iron is an important nutrient to preserve brain and immune function and to prevent anaemia. Menstruating women have higher iron requirements than male adults and low iron stores are more common in women and teenage girls. In Western diets, the majority of iron intakes are from fortified cereals, followed by red meat.(21) Although red meat is a good, bioavailable source of iron, individuals following plant-based diets have been shown to have adequate iron intakes.(4)

Adopting a plant-based diet with reduced amounts of meat is unlikely to have a significant impact on iron status. For those who wish to completely eliminate meat from their diet, plant-based sources of iron include soya, fortified cereals, nuts and seeds, pulses, beans and dark green leafy vegetables.(8) Despite having reduced iron intakes and lower ferritin stores compared to meat-eaters, vegetarians do not have a higher prevalence of iron deficiency anaemia.(21)

Some plant-based iron sources contain phytates which may limit iron absorption. Preparation methods and cooking can help minimise the impact phytates have on iron absorption.(21) Plant-based iron is also of the non-haem variety which is less bioavailable than haem iron found in meat. However, consuming iron-rich foods with vitamin C enables conversion of non-haem to haem iron and increases absorption.(8)

Additionally, the evidence is clear that the body becomes more efficient at absorbing and utilising iron when stores or dietary intakes are low or when there are greater demands e.g. during pregnancy.(8,21) There is some evidence to demonstrate that this is the case for individuals consuming a plant-based diet with a higher phytate content – over the long-term the body adapts by upregulating non-haem iron uptake and more efficient utilisation.(22)

Zinc

Zinc is a mineral which is needed for immune function and respiratory health. Low zinc status is estimated to affect one-third of the global and 8.9% of European population.(23) Red meat is a major source of zinc in current Western diets, for example providing around 30% of adults’ intake in the UK.(8)

Plant-based sources of zinc include fortified cereals, soya, beans and lentils, sunflower seeds, pumpkin seeds and nuts. It is possible to consume adequate zinc from plant food sources.(8,24)

Additionally, there is some evidence to suggest that zinc status is adequate in vegetarians, despite lower intakes than those following an omnivorous diet.(25) As with iron, zinc absorption can be influenced by the presence of phytates and other anti-nutrients, therefore preparation such as soaking, cooking, sprouting and fermenting are recommended to improve absorption.(24)

Calcium

Dairy is the major provider of calcium in current Western diets. However, dairy is also one of the main contributors to our dietary environmental footprint and cheese is a significant contributor to salt and saturated fat in the diet. Calcium is ubiquitous in the diet and there is no reason why someone on a varied and healthful vegan diet cannot meet recommendations.(8,26,27)

Non-organic plant-based alternatives to dairy are also typically fortified with calcium in equivalent amounts and with a comparable bioavailability to dairy. It is often stated that calcium fortificants such as tri-calcium phosphate and calcium carbonate have a lower bioavailability compared to dairy calcium. However, the difference is physiologically insignificant. A study comparing the bioavailability of calcium in dairy milk with two soya drinks fortified with either calcium carbonate or tri-calcium phosphate, showed a bioavailability of 21.7%, 21.1% and 18.1% respectively.(28) Based on typical calcium levels of cow’s milk, soya and other plant-based drinks of 120mg per 100ml, this would provide 52mg, 50mg and 44mg of absorbable calcium per 200ml glass serving respectively. An 8mg difference in intake, would not compromise calcium status. Other plant-based sources of calcium include cereals, sesame seeds and green vegetables. Oxalates found in some vegetables e.g., spinach can reduce the bioavailability of calcium. However, there are low oxalate variants that can provide significant quantities of calcium such as okra (lady’s fingers), pak choi and kale which have almost double the calcium bioavailability compared to dairy and are therefore excellent plant-based calcium sources.(26)

Iodine

Iodine is critical for thyroid function and metabolism. Low iodine status exists amongst adult populations across the European Union, particularly young and pregnant women and in Northern and Western countries where iodine fortification of salt is not mandatory.(29) Ensuring adequate iodine intake is especially important during pregnancy as deficiency can have severe impacts on the developing foetus.

The main sources of iodine in Western diets are dairy products and seafood as well as iodised salt in certain countries such as the US. Iodine is not naturally occurring in milk but is present in varying quantities, depending on farming practices such as addition of iodine to cattle feed and use of iodine-based cleaning products.(8)

White fish and shellfish are by far the richest natural sources of iodine and can be incorporated in moderate amounts within a varied plant-based diet. As plant-based foods are not generally good sources of iodine, those following a vegan diet may be at risk of deficiency. For individuals in countries which have not adopted the WHO iodised salt programme and who choose to avoid or with poor intakes of white fish, shellfish and potentially dairy, an iodine supplement is recommended of no more than 150mcg per day. This is especially important for pregnant and breastfeeding women.(8)

Selenium

Selenium is another nutrient which supports thyroid function. Research in the dietary patterns of free living vegans show that selenium intakes are lower than recommended.(30) The primary source of selenium in an omnivorous diet is organ meat and seafood as well as other muscle meats.

However, selenium is also found in plant-based foods such as cereals and nuts and it is easy to consume adequate selenium on a vegetarian or vegan diet. Brazil nuts are an exceptional plant-based source of selenium. Just a few Brazil nuts per day is enough to meet selenium recommendations for adults.(8)

It is important to note that selenium content of both animal and plant-based foods varies widely. This is because it is dependent on the selenium content of the soil in which plant-based foods or animal feed crops are grown. This is influenced by environmental factors such as soil pH and climate. Such variation makes it difficult to estimate selenium intakes in populations as food databases are less reliable.(31)

Plant-based dietary guidelines

International guidelines for sustainable and nutritionally adequate plant-based diets have been suggested by a number of health organisations. The EAT-Lancet Planetary Health diet launched in 2019 is now the benchmark for all countries to adopt.(32) National dietary guidelines have also been proposed for countries including Denmark and The Netherlands which are more sustainable and meet the recommended nutrient intakes.(33,34) These guidelines demonstrate how to consume a healthy plant-based diet which reduces risk of many health conditions, is nutritionally adequate and also has a lower environmental footprint.

Conclusion

In general, a varied and balanced plant-based diet provides an excellent source of many nutrients necessary for good health, whilst limiting less favourable nutrients such as saturated fat. It is the quality and variety of the overall diet that will determine nutrient adequacy.

For individuals wishing to follow an environmentally beneficial and healthful plant-based diet which includes sustainably sourced fish, and smaller quantities of meat and dairy, there is no reason why nutrient intakes should be compromised. For the small percentage of individuals who have chosen to exclude all animal products, focus is needed to ensure daily intakes of plant sources of key nutrients and to consider supplements specifically of vitamins B12 and iodine under the supervision of a health professional.