Joe Page, James Aggett, Shane M Heffernan, Applied Sports Science Technology and Medicine Research Centre (A-STEM), Faculty of Science and Engineering, Swansea University, Swansea, UK.
Recent scientific and public interest in plant-based diets has fuelled and reignited the debate surrounding the effectiveness of habitual dietary patterns (patterns adopted consistently over time) for optimal health and athletic performance. Public engagement from high-profile elite athletes (and celebrities) have generated significant interest in the message that plant-based diets, in particular, are favourable for improved long-term health and physical performance. Whilst the former is certainly supported by scientific evidence, in a number of contexts, what is the evidence for the latter?
In recent years, documentaries such as ‘What the Health’ (2017) and ‘Eating You Alive’ (2018) have presented seemingly simplistic and overwhelming evidence for the health benefits of plant-based diets, but not accounting for the complexities within the nutrition literature. The 2019 documentary ‘The Game Changers’ presented a case that athletes, from a variety of sports, may benefit from following a plant-based diet. As interesting as this potential is, the scientific literature supporting the benefit of a plant-based diet on the athletic phenotype is lacking, particularly the potential impact on muscle[1]. It is important that if an individual should choose to adopt a plant-based diet for a particular outcome (athletic performance for example) that they do so informed by the best available, high-quality, peer-reviewed scientific evidence, regardless of inspirational media productions.
So, what does the science tell us so far? From an overall health perspective, the scientific literature provides good evidence that plant-based or plant-predominant diets (including vegetarian, i.e. diets in which the main components are plants) can elicit cardio-protective benefits by reducing the risk of cardiovascular diseases, some by ~25 %[2], as well as related factors such as body mass index[3] and circulating cholesterol levels[4]. These dietary patterns have previously been shown to be effective for the management of type-II diabetes[5], typically through improving insulin sensitivity and blood glucose regulation, potentially as a result of a more favourable dietary fat profile[6]. Additionally, vegetarian (defined as excluding any meat products, poultry or seafood) and vegan diets (defined as omitting all animal-derived products) have been associated with an 8% and 15% reduction in the overall risk of developing all-cause cancer (not site-specific), respectively[2]. That said, we still don’t fully understand the effects of plant-based diets on specific markers of cardiovascular health, the direct mechanisms by which these health benefits occur or how these health-protective mechanisms may impact athletic performance.
The debate surrounding bone health and plant-based/vegetarian diets is more controversial, with evidence to support and refute the impact of these dietary patterns compared with omnivores. For example, one recent meta-analysis (combining the results from many studies) showed that vegan dieters had lower spinal, upper leg and whole-body bone mineral density (BMD), and a subsequent ~44% greater risk of bone fracture compared with omnivores[7]. It is important to note, however, that further subgroup analyses revealed that these differences in BMD were only significant in individuals over 50 years, and fracture risk was significant in Asian populations and in men, but not women. Not all studies reported the duration of adherence to a vegan diet and some did not report a number of known risk factors for BMD, including body mass index (BMI)[8], physical activity levels[9] and other lifestyle factors such as alcohol consumption[10] and smoking[11]. This study is supported, however, by a recent EPIC-Oxford study that showed a ~43% greater risk of total, as well as site-specific (hip, leg and vertebral) bone fractures in “vegans” (reported as ~85-90% avoiding any meat or animal products for five years prior to recruitment), compared with omnivores[12]. A more recent study, using quantitative ultrasound technology showed that vegans (defined as those not consuming any animal products for at least one year) had ~5% lower bone health, compared with omnivores[13]. However, there was no adjustment for BMI (a main risk factor for bone health/risk of fracture)[14] which was particularly likely to impact BMD at the specific site that was assessed (the heel of the foot). It is also important to consider that an earlier study, which highlighted a 4% lower BMD in vegans, suggested that these reported differences in bone health may not result in a ‘clinically significant’ increased risk of fracture[15]. There is also evidence to suggest that BMD does not differ between omnivores, vegetarians and vegans[16], therefore contradicting the previously mentioned studies, suggesting no negative impact of adopting a plant-based/plant-predominant diet on bone health. This is supported by a breadth of evidence to suggest that foods typically consumed in high amounts by plant-based dieters, namely fruit and vegetables, are associated with greater BMD and reduced risk of bone-related diseases later in life (such as osteoporosis), in omnivores[17][18][19]. These discrepancies are likely explained by differences in methodology and design, for example, cohort specific results (biological sex, religious groups, geographical ancestry etc.) as well as different definitions of plant-based/vegetarian diets and time that participants had chosen to adopt their dietary pattern. There has been little scientific consideration for the effects of healthy vs unhealthy plant-based diets on bone health, which could potentially explain part of the discrepancies in the findings thus far, as already suggested in cardiovascular disease[20]. It is clear that the bone and dietary pattern question requires more work with more detailed and well-defined homogenous dietary groups. This is important for the present discussion because of the intrinsic link between bone and muscle health[21] and athletic performance[22].
So far, there has been a lack of scientific attention devoted to skeletal muscle health/performance through the lens of plant-based diets, and many questions remain unanswered. Given that plant-based diets are typically lower in protein intake[23] it has been posited that individuals following a plant-based diet would have less muscle mass compared to omnivores. There was some early evidence to support this, from a small number of vegetarians (n = 19) showing a ~22% lower muscle mass (measured via urinary creatinine), compared to omnivores[24]. However, other studies in larger populations (n < 986) have shown that protein source may not be relevant to prevent muscle disease (sarcopenia), rather protein amount is more important[25]. Or using isocaloric substitution modelling (computer generated estimations) and dual-energy X-ray imaging of muscle mass, plant protein intake may be more beneficial in preventing muscle disease[26]. But experimental support is needed for this hypothesis. Recent work has demonstrated no differences in skeletal muscle architectural parameters and subsequent maximal strength, between habitual vegan and omnivorous dieters, albeit with a small sample of recent vegans (n=9; 6 months) and omnivores (n=16)[27]. Nonetheless, more work is needed in larger samples of well-defined plant-based dieters, as well as including a more detailed assessment of skeletal muscle structure/architecture and size.
Muscle size is directly linked to force producing-capacity (or strength). If omnivorous dieters did have greater muscle mass (potentially due to additional protein intake as mentioned above), it could be hypothesized that they may also possess stronger muscles. However, regardless of the limited data, the currently available evidence suggests that there are no differences between plant-based/vegetarian dieters and omnivores regarding muscular strength[28], [29]. This could be the result of similar muscle architectural parameters (muscle fibre characteristics which directly affect muscle strength, examples in Figure 1)[27], [30] between diet groups, or could be explained, in part, by greater muscle quality in plant-based dieters compared with omnivores. Although this is yet to be empirically shown. Here, muscle quality specifically refers to fat accumulation and presence of fibrous tissue within the muscle that can significantly disrupt the electrical signals that initiate muscle contractions[31]. Considering this, there may be a ‘trade-off’ between larger muscles in omnivores (if this proves to be the case) and higher muscle quality (e.g. less intramuscular fat) in plant-based dieters. Subsequently resulting in similar muscular strength outputs. However, further evidence is required, both for muscle architecture and quality in plant-based dieters before this concept could be accepted.
To address some of these gaps, we have brought together a team of experienced researchers in nutrition, dietetics, muscle physiology, bone health, strength and conditioning, and molecular exercise physiology to design a research project to build on, and add to, the currently available evidence. The aim is to further our understanding of some of the physiological impacts that plant-based diets have on human physiology vital for athletic performance. The OMnivorous & Non-meat eater Integrative PhysioLogy and NutriTion (OMNIPLaNT) Study is based at the Applied Sports Science Technology and Medicine Research Centre (A-STEM), Swansea University, UK. The study will combine detailed dietary analysis with physiological assessments of skeletal muscle size and architecture, vascular structure and function, measures of body composition, bone health and blood biomarkers in a range of dietary patterns, from plants only to omnivores.
We plan to use ultrasound (such as used in Figure 1) and dual X-ray imaging to analyse the size, volume, architectural structure and composition of the quadriceps muscles (Figure 1) to gain an insight into the physiology and strength capacity of muscles. We will also take ultrasound measures of echo intensity (an estimate of the fat accumulation inside a muscle), in order to assess the potential impact of, hypothetically, larger muscles (in omnivores) and, hypothetically, better quality muscles (in plant-based dieters) on maximal strength, measured via maximal knee extension contractions – as proposed above.
Given the cardiovascular health benefits linked to plant-based diets, and by extension to athletic performance, we will also use ultrasound technology to assess brachial artery (upper arm) and carotid artery (neck) function and structure. This will allow us to investigate vascular and arterial function as well as plaque build-up inside arteries, which may provide evidence to suggest better cardiovascular health with particular dietary patterns.
The findings from this study will build on current evidence, to further inform the scientific and health communities, as well as athletes and the general public that are interested in the physiological health benefits associated with their chosen dietary patterns. Our first study is now live and is comprised of two online questionnaires aimed to give us the first insight into people’s diet and eating behaviours with different dietary patterns. The OMNIPLaNT Questionnaire can be accessed at:
https://survey.swan.ac.uk/index.php/788865?lang=en or by scanning the QR code here: 
If you are interested in taking part in our research, please click this link and read the participant information sheet linked at the beginning of the questionnaire (or you can contact Joe or Shane at omniplantresearch[at]swansea.ac[dot]uk). To keep up to date with our findings, follow us on Twitter @omniplantres or visit our, developing, webpage.
Joe Page, James Aggett, Shane M Heffernan, Applied Sports Science Technology and Medicine Research Centre (A-STEM), Faculty of Science and Engineering, Swansea University, Swansea, UK.
References
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The views expressed by our Research News contributors are not necessarily the views of The Vegan Society.
