Introduction
Overweight and obesity are a worldwide epidemic with over 2 billion people overweight or obese (OWO) globally; posing one of the greatest public health challenges of our time (Baker, 2018; Dreher, 2018). According to figures released by the National Health Service (2017) in England as of 2015, over 50% of women and more than 60% of men are OWO. Obesity is a serious and chronic disease associated with many types of cancer, cardiovascular disease (CVD), type 2 diabetes (T2D) fatty liver disease, osteoarthritis, oxidative stress and inflammation (Sharma & Kanwar, 2018). Preventable, non-communicable diseases (NCDs) associated with OWO such as diabetes are a gargantuan economic burden; accounting for 10% of the NHS annual budget (Diabetes UK, 2016). Modest weight loss of just a 5% decrease in body weight has been shown to lower the risk of chronic disease (Turner-McGrievy, Mandes, & Crimarco, 2017), but reviews of dietary interventions for weight loss have failed to identify a gold standard diet (Wright, Wilson, Smith, Duncan, & McHugh, 2017). Traditionally, dietary interventions targeting weight-loss have championed energy restriction, though more literature examining the role of plant-based diets (PBDs) in weight-loss are emerging (Turner-McGrievy et al., 2017). The term PBDs encompasses all diets which are centred around fruit, vegetables, grains, legumes, nuts and seeds, including vegan and vegetarian diets with the latter typically including eggs and dairy products. In recent years there has been a growing interest in PBDs due to their ability to not only support good health (Harland & Garton, 2016), but their inherent sustainability and benefit to the planet (Harland & Garton, 2016).
There exists a variety of ways in which OWO can be identified and assessed over time; by body mass index (BMI), waist circumference (WC) waist-to-hip ratio (WHR) and waist-to-height ration (WHtR) (Brownlee, 2016; Sharma & Kanwar, 2017). There is now good evidence from data collected over 4 years of the UK National Diet and Nutrition Survey (2008-2012), that central obesity carries more health risks compared with total obesity assessed by BMI (Ashwell & Gibson, 2016). The World Health Organization guidelines (WHO, 2008) state that alternative measures that reflect abdominal obesity, such as WC, WHR, and WHtR have been found to be superior to BMI. A WHtR (using a boundary of 0.5) may be a simpler and more predictive indicator of the health risks associated with central obesity (Ashwell & Gibson, 2016). Therefore, dietary interventions which are effective at reducing or preventing an increase in these measures of adiposity may be useful tools to alleviate the burden of NCDs. This review aims to assess the evidence relating to the effect of a plant-based dietary pattern on obesity and weight-loss from both observational and intervention studies.
Observational studies
The link between various dietary patterns and body weight has been investigated in several epidemiological studies, including vegan, vegetarian, pescatarian, flexitarian and omnivorous dietary patterns. Whilst seeking to evaluate the link between dietary patterns and diabetes, the Adventist Health Study-2 (AHS), a prospective study of 15,200 men and 26,187 women, (grouped as vegan, lacto-ovo vegetarian, pesco-vegetarian, semi-vegetarian or non-vegetarian) found that as the amount of animal products in the diet increases, so too does BMI (lowest BMI seen in vegans, highest in omnivores) (Tonstad et al., 2013). A study by Spencer, Appleby, Davey and Key (2003), sought to compare the BMI in four diet groups (omnivores, pescatarians, vegetarians and vegans) in the Oxford cohort of the European Prospective Investigation into Cancer and Nutrition (EPIC-Oxford). A total of 37,875 healthy men and women participated in EPIC-Oxford. The age-adjusted mean BMI was significantly different between the four diet groups (Figure 1), with the highest BMI in the omnivores (24.41 kg/m2 in men, 23.52 kg/m2 in women) and lowest in the vegans (22.49 kg/m2 in men, 21.98 kg/m2 in women) (Spencer et al., 2003).
Three years later, research by Rosell, Appleby, Spencer and Key (2006) investigated weight gain over 5 years in 21,966 omnivorous, pescatarian, vegetarian, and vegan men and women in the EPIC-Oxford cohort; a study prompted by the observation that vegetarians and vegans are leaner than omnivores (Rosell et al., 2006). The study found a step-wise reduction in mean annual weight gain of men and women who changed their diet in one or more steps in the direction, omnivore > pescatarian > vegetarian > vegan (Figure 2). Multivariable-adjusted mean weight gain was smaller in vegans (284 g in men and 303 g in women, P<0.05) compared to omnivores (Rosell et al., 2006), meaning that vegans show significantly less weight gain as they age, compared to omnivores.
These findings suggest that transitioning to a more PBD appears to offer protection from weight gain and is supported by the findings of an off-shoot of the EPIC-Oxford study, EPIC-PANACEA, which found a positive correlation between meat consumption and weight gain (an increase of 250g/day of meat produced a weight gained of 2kg after 5 years), even when energy intake was adjusted for (Turner-McGrievy et al., 2017).
The findings of the large AHS and EPIC-Oxford studies are supplemented by the findings in Swedish and Taiwanese cohorts. A cross-sectional analysis of data from The Swedish Mammography Cohort (questionnaire completed by 6,651 over 3 years) found that the highest prevalence of overweight and obesity existed in omnivores compared to individuals with dietary patterns containing less meat (Newby, Tucker, & Wolk, 2005). In a cohort of 49,098 Taiwanese adults Chiu et al., (2015) determined that the proportion of overweight individuals (BMI> 27kg/m2) was significantly lower among those following a vegetarian diet, also observing that for every year on a vegan diet the risk of developing obesity was reduced by 7%.
An investigation of 3 cohorts from the Nurse’s Health Studies (NHS) and Health Professionals Follow-up (HPFU), including 120,877 participants over 4 years, found whole plant foods (wholegrains, fruit, vegetables and nuts) to be inversely associated with weight gain (Dreher, 2018). The HPFU tracked 27, 082 participants over 8 years and determined that for each 40g daily increase in whole grains, there was a reduction in weight gain of half a kilogram (Koh-Banerjee et al., 2004).
Intervention studies
An abundance of intervention trials and review articles consistently show that an unrestricted PBDs are significantly more effective in lowering measures of OWO, compared to the typical Western diets (Dreher, 2018). A randomised control trial (RCT) in New Zealand, The BROAD study followed 65 participants over 12 months, concluding that a PBD produced a greater reduction in BMI at 6 and 12 months, compared to the control diet (Wright et al., 2017). A RCT following 250 young adults over 9 months in Australia, found that meeting the recommendations for daily fruit and vegetable intake increased the likelihood of reduced weight by more than 200% (Allman-Farinelli, Partridge, & Roy, 2016).
Two recent meta-analyses examined clinical trials which assessed PBDs and weight-loss, both of which reported a significant weight-loss amongst those prescribed a PBD (Barnard, Levin, & Yokoyama, 2015; R. Huang, C. Huang, Hu, & Chavarro, 2016). The analysis by Barnard et al., (2015) reviewed 15 clinical trials which used PBDs for a minimum of 4 weeks with no energy restriction, finding PBDs to be associated with a mean weight-loss of 3.4kg (in an intention-to-treat analysis) and 4.6kg (computer-only analysis). The meta-analysis of PBD clinical trials by Huang et al., (2016) reviewed 12 studies, with participants randomised to a PBD losing more weight than those assigned to non-PBDs. Of the 12 studies 6 contained an element of energy restriction. Perhaps not surprisingly, PBDs with energy restriction, produced greater weight loss compared to unrestricted energy PBDs.
Other more recent trials, not included in the meta-analyses are The New DIETs Study, The HER Health Study and The VA Beach Diet Study. The New DIETs study was one of the first randomised control trials to examine the difference between PBDs and omnivorous diets, finding that participants allocated to a PBD lost significantly more weight at 6 months than omnivore participants (Moore, McGrievy, & Turner-McGrievy, 2015). Another randomised control trial, the HER Health Study, compared a vegan and low-calorie diet in overweight women with polycystic ovary syndrome, demonstrating greater weight loss amongst the vegan participants compared to the low-calorie group at 3 months (though no difference in weight loss observed at 6 months) (Turner-McGrievy, Davidson, Wingard, & Billings, 2014). The VA BEACH Diet Study followed 199 participants over 6 months to examine the impact of 4 diets (Vegan, Mediterranean, Palaeolithic and DASH diets) on cardiovascular risk factors of overweight individuals, with the greatest weight loss and improvement in lipid risk factors in the Vegan and Palaeolithic groups (A. Talreja, S. Talreja, R. Talreja, & D. Talreja 2015).
Limitations and future direction
Although evidence from observational studies cannot infer cause and effect, a benefit inherent to such studies is the long length of follow-up over which the studies are typically conducted, which better mimic the development trajectory of weight gain and non-communicable diseases (Brownlee, 2016).
Despite the plentiful amount of research studies investigating PBD for weight loss, the need for more studies which better define the term PBD, include more diverse populations and are conducted over longer periods is apparent. Within the existing literature there is wide variation in what is considered a PBD, typically being defined by the foods they exclude, rather than those foods which are eaten (fruit, vegetables, whole grains and legumes). The lack of a satisfactory definition of a PBD is likely due to the relative recency of the PBD concept (Harland & Garton, 2016). With so few authors including the term ‘plant-based’ in the title of their publications and a lack of a precise definition of a PBDs, identifying relevant data is a challenge.
Defining a diet in terms of what foods are excluded misses an opportunity to identify what foods are best to include when designing a diet to achieve positive health outcomes. Further compounded by the lack of distinction drawn between heavily processed plant foods, which may not promote health to the same degree that their whole unprocessed counterparts may. To address these limitations a Plant-based Diet Index (PDI) has been developed by Satija et al. (2016), ascribing positive scores to plant foods and negative scores to animal products allowing the distinction between healthy whole plant foods and processed or unhealthy foods to be made. As such, the PDI may serve as a useful tool in the assessment of dietary quality and help better define PBDs in future research (Turner-McGrievy et al., 2017).
The effects of PBDs on weight loss in more diverse groups is needed, to better determine how effective its application may be within different ethnic groups, children, young adults and older adults (Turner-McGrievy et al., 2017). As are intervention studies over a greater follow-up period (>12 months), to allow the long-term health effects and sustainability of a PBD to be properly assessed.
Another major limitation of the existing studies, is the failure of many of the PBD weight loss trials to report the physical activity levels (PAL) of the participants. In the review by Barnard et al. (2015), only 3 of the 15 clinical trials included, considered PAL, 2 of which reporting an increase in the PAL of participants during the intervention.
A limitation common to all the PBD weight loss studies, is the consistent underreporting of intervention methods and materials (Turner-McGrievy et al., 2017). This limited description, perhaps the result of restrictive journal page limits, will greatly hinder reproducibility in future studies, and clouds which components of the study were responsible for eliciting the desirable health outcomes. In future, studies investigating PBD and weight loss should use a standardised reporting structure, to allow findings to be disseminated and replicated.
Population based studies indicate that individuals following a vegetarian diet have lower mean intakes of zinc and vitamin B12 (Farmer, 2014). Therefore, care should be taken when advising or designing PBDs to optimise the intake of foods to provide adequate intake of nutrients of concern.
More research on specific foods and mechanisms by which they exude their effects are also needed, so that dietary advice may be better tailored to achieve optimal weight loss and desirable health outcomes. Future studies may also seek to investigate the effect of PBD on proxies of adiposity other than BMI, such as WHtR; given its greater sensitivity in predicting health risks (Ashwell & Gibson, 2016).
Conclusions
Those who consume a predominately PBD tend to have lower BMI and lower weight gain over time than those consuming non-PBDs. Any movement along the continuum toward a more PBD appears to be safe and effective for weight loss, a major advantage being the ability to eat to satiation without restricting the amount of food eaten (Wright et al., 2017). Plant foods which are lower in energy density, higher in fibre and rich in phytonutrients can confer protection from weight gain and promote weight loss or support weight management following weight loss (Dreher, 2018). Further research is needed to elucidate the usefulness of PBDs for weight management as assessed by different metrics (WHtR), in broader groups, over greater periods of time. Future research could also focus on how to identify participants who are likely to succeed with a PBD intervention; reducing drop outs and increasing effectiveness (Wright et al., 2017). To date, observational and intervention studies indicate that PBDs may be a viable option for individuals who could benefit from weight loss and improved dietary quality. Therefore, promotion of the consumption of diets rich in fruit and vegetables possessing a wide variety of potentially anti-lipogenic phytochemicals, may assist in the dietary control of OWO and related chronic diseases.
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