Dietary intakes and nutritional adequacy of Australians consuming plant-based diets compared to a regular meat-eating diet

This cross-sectional study found that compared to RMEs, individuals following vegan, and LOV dietary patterns had a more favourable nutrient composition including lower intakes of saturated fat, trans fat, sodium and cholesterol, higher intakes of dietary fibre, vitamin E, folate, magnesium, iron, ALA, and n-6PUFA, whereas PVs and SVs were intermediate. Those adhering to a PBD consumed significantly more daily serves of vegetables, fruit (vegans only), legumes/nuts and less discretionary choices and sugar sweetened beverages in comparison to the regular-meat diet. Evaluating nutrient intakes against the dietary guideline recommendations revealed all dietary patterns met adequate intake for protein (EN%), exceeded for fat (EN%) were below for carbohydrate (EN%), had adequate serves of fruit and vegetables, but not grains. Moreover, vegans, LOVs, and SVs had inadequate serves of protein-rich foods, and SVs and RMEs had inadequate serves of dairy. Regarding micronutrients, vegans and LOVs had inadequate intake of LCn-3PUFA, vitamin B₁₂, iodine, and in addition, calcium in vegans, PVs in iodine, and SVs and RMEs in LCn-3PUFA.

Studies that investigate nutritional intake of PBDs are generally scarce, especially within the Australian context, therefore our current understanding is largely from secondary analyses of overseas prospective cohort studies including the ‘EPIC Oxford Study’ (n = 65,429) [23] and the ‘Adventist Health Study-2 (AHS-2)’ (n = 71,751) [25], and moderate sized cross-sectional studies [44, 45]. Protein-rich foods are predominantly of animal origin including meat, poultry and fish, although relatively high levels of protein can also be found from plant-based sources such as legumes and nuts [46]. Our findings suggest that those following meat-free or low meat PBDs consume adequate amounts of protein (15–17 EN% vs the AMDR of 15–25 EN%), consistent with a similar cross-sectional cohort study which found that vegans, LOVs and SVs consume 13–14 EN% from protein. These findings also align with results from the larger ‘EPIC oxford’ cohort [23, 44] which states LOVs and vegans typically consume sufficient (or borderline) amounts of protein to meet daily requirements, provided a variety of protein-rich foods are consumed and energy requirements are met [45, 47]. Moreover, the ‘Nurses Health Study’ (n = 48,762) prospective cohort found that plant-based protein, are associated with higher odds of healthy ageing and lower risk of chronic illness [48].

Vegans and LOVs are known to consume higher amounts of carbohydrates compared to dietary patterns inclusive of animal products [45, 49], likely driven by the higher intake of carbohydrate-rich plant foods namely legumes/pulses, fruit and grains [49]. This was reflected in the current study as vegans and LOVs were observed to have a higher carbohydrate intake (EN%), although none of the dietary patterns reached the recommended carbohydrate AMDR (40 EN% vs 45–65 EN%). This was akin to the larger cohort studies including the ‘EPIC Oxford Study’ and ‘AHS-2’ which detailed vegans to consume the highest amount of carbohydrates (54–61 EN%) in comparison to LOVs (49–57 EN%) and RMEs (46–57 EN%) [23, 25, 44]. In the present study intake of dietary fibre among vegans was over twice the NRV (25–30 g/day), although all dietary patterns reached adequate intake. These results are supported by previous cross-sectional and prospective cohort studies which demonstrated vegans to consume 20–70% above the recommended 30 g/day, however, RMEs have previously been reported to not meet recommendations [25, 44, 45]. The most recent 2011–2012 National Nutrition and Physical Activity Survey reported 7 in 10 adults did not meet the adequate dietary fibre intake, further establishing general levels of inadequacy among those following a RME diet [50]. Above adequate dietary fibre among RMEs within this sample could be attributed towards the ‘healthy user effect’ which explains health conscious individuals who become involved in research may have healthier lifestyle behaviours and dietary patterns [51].

All groups marginally exceeding AMDR for dietary fat intake (37% vs 20–35 EN%). This agrees with previous observational studies which have reported vegans to consume 26–28 EN%, LOVs 30–33 EN% and RMEs 32–35 EN% from dietary fats [23, 25, 44, 45]. Moreover, such studies describe vegans, LOVs, PVs, and SVs to have the lowest intake of saturated fat, trans fats and cholesterol and highest intakes of PUFAs compared to RMEs, reflective of the current study [23, 25, 44, 45]. Vegans achieved <10 EN% (recommended level) from saturated fat and as expected, had a notably lower intake of dietary cholesterol, seven times less than RMEs, similar to the a cross-sectional study in healthy adults which demonstrated a ten times difference [44]. Dietary fat profiles may be partially explained by protein sources, for example plant-based protein such as legumes/pulses, nuts, seeds are predominantly being rich in unsaturated fats, whilst consuming minimal/nil animal-based foods which are inherently a rich source of saturated and trans fats [52]. Our findings demonstrated that all dietary patterns except for PVs did not meet the recommended intake for LCn-3PUFA. Vegans and LOVs consumed <10% of the recommended NRV, therefore could be considered most at risk of LCn-3PUFA inadequacy, and essential nutrient. These findings are also in accordance with moderate size cross-sectional studies from Germany which reported inadequate intake amongst individuals adhering to both PBDs and RME diets [44, 45]. Noteworthy, this is not necessarily a characteristic specific only to individuals following PBDs, since the 2011–2012 National Nutrition and Physical Activity Survey (the most recent at the time) found 80% of the Australian population did not meet recommendations for LCn-3PUFA [53]. LCn-3PUFA intake continues to be observed at critically low level of consumption for both PBDs and meat-eating diets within the current study population but represents a broader issue among the Australian adult population.

Despite the healthful nature of PBDs, there is potential for nutrient inadequacies to occur given the restriction of key food groups such as dairy, eggs and animal meats. Of most concern is iron, zinc, iodine, calcium, and vitamin B₁₂ which are essential for human health including growth and development, immune health, production of hormones, bone health and neurological functioning [20, 54]. Calcium intake was inadequate in vegans by 16%. Previous studies report inconclusive outcomes whereby similar cross-sectional studies reported 25–40% of vegans to not meet recommendations for calcium [44, 45]. On the other hand, alike the current study, the ‘AHS-2’ cohort detailed most PBDs (SV, PV, and LOV) reached recommendations with only 7% of vegans to fall short [25]. The inclusion of vitamin and mineral-fortified food products in the current study may explain the higher intake levels of calcium as well as vitamin B₁₂ when compared to previous studies. Low dietary intake of calcium among the vegans in this study may be explained by the fact that over half of the Australian population aged ≥2 years have inadequate intake of calcium intake from foods, with females reporting a higher prevalence of inadequacy than male [55]. Moreover, while calcium is found in plant foods such as leafy greens [56], milk and milk-based products are the riches sources of dietary calcium among Australians [57]. Given the omission of dairy products as inherent to the vegan dietary pattern, and the majority of individuals following a vegan diet in this study were females, it is no surprise that dietary calcium was observed to be low in this group [56].

Vitamin B₁₂ intake was observed in a descending order parallel to restrictiveness of animal products since vitamin B₁₂ is usually derived from animal-based foods [58]. It is for this reason that the Australian dietary guidelines recommend strict vegans to supplement with vitamin B₁₂ [42]. Those adhering to a PBD also rely on B₁₂-enriched food products [58], such as plant-based alternatives and nutritional yeast which were frequently consumed by individuals within this cohort and are commonly spread in the current food supply globally [59]. The NRV for vitamin B₁₂ was met by meat-eating groups (RME, SV, PV), although intakes of vegans and LOVs were 42% and 26% below recommendation, respectively. Other previously mentioned cross-sectional and prospective studies mirror results and found LOVs, SVs and PVs to meet recommendations, however, vegans fell below [23, 25, 44, 45]. Of interest, around 50% of the total sample of vegans in this study were taking vitamin B₁₂ supplements which would contribute to improved nutritional adequacy.

Iron intake has been shown to be higher in PBDs compared to RME diets as 80–90% of dietary iron intake is non-haem [25]. Non-haem iron is derived from plant-sources such as beans, legumes, nuts and leafy greens, whereas haem iron is derived from animal sources such as meats and poultry [60]. The amount of iron derived from non-haem food sources is often much higher than that of haem food sources [25]. In this study we observed vegans and LOVs to consume 1–1.5 more daily serves of vegetables 0.5–1 additional serves of legumes and nuts which explains the higher overall iron intake among PBDs. The food structure and bioavailability play a key role when analysing iron adequacy. Haem iron from animal-based food is readily absorbed (20–30% rate), while the absorption of non-haem iron from plant-based foods is less available (1–10% rate) [61]. It is for this reason that the Institute of Medicine states vegetarians and vegans require 1.8 times higher dietary iron compared to non-vegetarians [61]. Those following PBDs (vegan, LOV, PV, and SV) in this sample reached this additional requirement which is in accordance with previous literature [44, 62]. When comparing this samples iron intake to the Australian dietary guidelines all groups met the recommended intake for iron and vegans consumed two times the NRV. At current the Australian dietary guidelines do not recommend any additional iron intake for those following PBDs [42]. Bioavailability can also strongly depend on dietary factors and an individual’s current iron status [61]. Inhibitors such as phytates, oxalates, fibre, calcium and polyphenols can bind and interfere with absorption of non-heam iron, while vitamin C rich food such a citrus fruits and vegetables can simultaneously enhance absorption [61]. High fibre dietary patterns such as PBDs may lead to an increase consumption of inhibitors which compromise absorption rates [61]. Despite vitamin C intake being comparable across groups, vegans and LOVs tended to have higher intakes compared to RMEs which may play a role in increasing bioavailability of non-haem iron. This may be amplified by use of vitamin C supplements which was reported at a comparable level (13–27% being users). Therefore, it is pivotal to use biochemical measures to ascertain the true iron status of individuals adhering to a PBD.

Zinc and iodine are two trace minerals that are typically consumed in lower amounts when following vegan and LOV dietary patterns and their bioavailability is also affected by the presence of inhibitors such as phytic acid [44]. Intake of zinc was adequate and comparable across groups, aligning with a previous cross-sectional study and large observational studies [23, 25, 45]. Noteworthy, in one study LOVs, vegans and PVs males were slightly below recommendations [25] and LOVs males in another study [45]. Additional analyses not shown in results tables of the current study also show all males following PBDs fell just below (5–20%) recommended intakes. The Australian dietary guidelines recommend vegetarians to consume 50% more zinc as majority of plant-based protein-rich foods (also rich in zinc) are absorbed at a lower rate than animal proteins, and this combined with higher consumption of legumes and grains may exceed the phytate zinc ratio (15:1) [63]. None of the individuals adhering to a PBD met the additional 50% requirement for zinc.

Iodine consumption was markedly different between vegans and RMEs equating to a 70% difference and most PBDs fell below recommendations by 37% for vegans, 17% for LOVs and 7% for PVs. Literature on dietary iodine intake is scarce, however, one similar cross sectional cohort study reported 60% of vegans and LOVs did not reach requirements [44]. Like other nutrients, iodine content in plant-based foods are much lower compared to that of animal origin [64]. Therefore, urinary iodine concentrations are more commonly used to assess iodine levels as it provides an objective and more accurate reading. Previous studies examining urinary iodine have found vegans to be moderately iodine deficient and LOVs and PVs mildly deficient [65], however further research is warranted.

This study is the first to measure daily food group intake of Australians following various PBDs in accordance with the national recommendations as per the AGTHE. Those following a PBD did not reach the recommended serves per day for ‘protein-rich foods’ and dairy (without inclusion of plant-based alternatives). However, when including intake of PBDAs, intake of dairy was adequate across all PBDs, except SVs (7% below). This could explain why those adhering to PBDs (excluding vegans) met the recommended AMDR for protein and calcium. Moreover, the combined category of PBMA with ‘protein-rich foods’ evaluate adequacy of individuals following PBDs by 7–10%, enabling PVs to reach recommended serves and all other PBDs to reach >70% adequacy. This points out the necessity for dietary collection tools to encompass the abundance of plant-based alternative food products available within society to better evaluate daily food group intake amongst those following PBDs. It was found that individuals adhering to a PBD consumed greater quantities of food groups such as fruit, vegetables and grains compared to RMEs and lower intakes of sugar sweetened beverages and discretionary choices. This may suggest PBDs obtain energy intake predominantly from wholefood food groups. Moreover, PBDs consumed significantly more daily serves of legumes, nuts and seeds compared to other RMEs, which may be a key driver for the higher levels of PUFA. A similar cross-sectional pilot study in athletic adults corroborated results reporting that vegans and LOVs had a higher vegetable, fruit, wholegrain, and legume intake compared to RME [45]. In addition, the intake of legumes among vegans was 18 times more than RMEs, higher than the current study which was two times greater. More research surrounding PBDs and food group intake using validated FFQs inclusive of plant-based alternatives is warranted to substantiate trends observed in the current study [66].

Strengths of this study include its novel study design that specifically recruited individuals habitually following a variety of PBDs and RME diets from the Australian population. It utilised a variety of validated tools including the AES FFQ^® and experienced APDs to collect quantitative dietary intake data from interviewer-administered diet histories, thus providing a greater level of accuracy. In the absence of a validated population-based FFQ specific for PBDs, this study design enabled a better canvas of the current plant-based food and beverage (including fortified items) supply and thus resulted in a truer representation of dietary intake in the current climate. Moreover, it was the first Australian-based study to our knowledge to compare nutrient and daily food group intake of PBDs to the NRVs/AGTHE recommendations.

Several limitations must be addressed. First, vitamin and mineral supplementation use were not included in the final analysis due to irregular usage, variations in brand and bioavailability and possibility of skewing the data. Second, vegans were significantly younger than the other diet groups which may have affected usual eating habits and nutrient intakes, although age was adjusted for in all analyses. Third, most data collected was self-reported, however, the data collection tools implemented in this study have been validated in the Australian population [58], and interviewer-administered research methods by APDs were used to strengthen collection of subjective measures. Fourth, the dietary analysis software used was unable to quantify data on animal versus plant-derived iron and vitamin D intake. Analysis of nutritional status indicated by biochemical measures were outside the scope of this study, however, the researchers acknowledge that this would verify the findings. More information on plasma blood lipids has been published elsewhere [67]. Fifth, this study was a secondary analysis on a modest sample size from the same geographical region with low reported prevalence of chronic disease so results should be interpreted with caution and may not be generalised to all Australians. Lastly FFQs can be prone to some measurement error including under or over reporting due to memory, inability to capture all foods consumed, day-to-day variation and food preparation methods [68]. Conversely, validated FFQs are the most accurate form of measurement of the intake of food items relating to a food category over a long period of time, therefore such measures were used to measure qualitative dietary intake [35]. Diet histories were used to collect further detail on dietary intake as well as measure quantitative dietary data. Diet histories have been considered more reliable than food weighed records [35, 41], however can be prone to some limitations such as recall bias, interviewer bias and errors in portion size estimations [69]. Efforts were made to mitigate these limitations such as administration by a trained APDs, taking an average of two diet histories to inform analyses, and completing each diet history with a dietary checklist to ensure no food category or eating occasion were missed. This study could provide guidance for the design and conduct, including its consistent method of categorising and tailored dietary collection of PBDs, for forthcoming larger observational and/or interventional studies.

In summary, findings from this cross-sectional cohort which purposefully sampled a variety of Australians habitually following PBDs, revealed vegan and LOV dietary patterns offer some advantages in nutrient composition including lower saturated fats, trans fats, cholesterol and sodium and higher intakes of dietary fibre, n-6PUFA, ALA, vitamin E, iron, folate, and magnesium, whereas PVs and SVs were intermediate in comparison to RMEs. Regarding levels of nutritional adequacy all dietary patterns met adequate intake for protein, exceeded for fat, and were below for carbohydrate (EN%), had adequate serves of fruit and vegetables, but not grains. Vegans, LOVs, and SVs had inadequate serves of protein-rich foods, and SVs and RMEs had inadequate serves of dairy. Rates of nutritional inadequacies varied across groups, vegan and LOVs were deficient in vitamin B₁₂, LCn-3PUFA, iodine, and in addition calcium among vegans, PVs in iodine, and RMEs and SVs in LCn-3PUFA. Our study contributes to current population-based evidence on the nutritional intake of PBDs in middle-aged adults in Australia which is consistent with findings from similar overseas cohorts [25, 44]. Findings may aid the development of national dietary strategies and guidelines to achieve nutritional adequacy for those following plant-based eating patterns, in congruence with the updated dietary guidelines around the globe. Nutrients of concern among PBDs and RMEs as well as suitability of supplementation should be carefully considered by individuals alongside tailored nutrition interventions with their healthcare professionals when adjusting dietary intakes to meet nutritional needs. Moreover, nutrients found to be in deficit should be considered when designing plant-based alternatives. Larger primary population-based longitudinal studies investigating nutrient intakes and objective biomarkers are warranted to substantiate nutritional status of various PBDs.