The aims of this review are to systematically analyze the available literature on plant-based diet interventions targeting and/or including adults with diabetes and to clearly define the benefits on well-being of such interventions.

A meta-analysis 29 focusing on vegetarian diets (defined as those excluding meat, poultry and fish, and including eggs and dairy) in diabetes revealed a significantly improved glycated hemoglobin (HbA1c) in people who followed a vegetarian diet pattern. No systematic review was found in the literature solely focusing on the psychological and medical outcomes of plant-based diet interventions in adults with T2D.

The term plant-based diet refers to eating habits that avoid the consumption of most or all animal products and support high consumption of fruits, vegetables, legumes, seeds, whole grains and nuts. 21 Satija et al 22 reviewed three prospective cohort studies of a total sample of 200 727 participants and concluded that a plant-based diet is associated with significantly lower risk of T2D. Tonstad et al 23 state that plant-based diets could effectively and substantially decrease diabetes incidents. Furthermore, high consumption of fiber, whole grains, fruits and vegetables is associated with a lower risk of T2D. 24–27 A plant-based diet pattern seems to offer high protection against the development of diabetes as it contains antioxidants, fiber, micronutrients and unsaturated fatty acids, which are considered to act as protective factors against diabetes. 22 In addition, Mayo 28 states that the management of diabetes is mostly based on a behavioral approach in which individuals could establish nearly complete control of their condition as long as they have been equipped with the right knowledge and skills. This suggests that effective management of diabetes could result in better glycemic control and psychological well-being.

There is a large body of research that suggests the association between high meat consumption and T2D. 16–19 The European Prospective into Cancer and Nutrition (EPIC) -InterAct study 20 is a large prospective cohort study which explored the role of lifestyle and genetics on the risk of developing T2D in approximately 340 000 adults from eight countries in Europe during a mean period of 11 years. The results of the EPIC-InterAct study 20 show a high risk of T2D among individuals with high meat consumption, specifically red and processed meat. Moreover, after controlling and managing other risk factors for T2D (eg, smoking, physical activity, alcohol intake), the association between meat consumption and incidence of T2D remained statistically significant. This suggests the importance of healthy diet behavior in the management of T2D.

The IDF reports that the most influential factor for the development of T2D is lifestyle behavior commonly associated with poor diet (eg, processed and high fat content foods). 2 WHO underlines the importance of achieving a standardized and consistent management approach by promoting interventions that support healthy diets. 1

In January 2018 the American Association of Clinical Endocrinologists and the American College of Endocrinology released their new guidelines in which they suggest that plant-based diet is the optimal nutrition plan for people with diabetes as it promotes the well-being and the better management of diabetes. 15

T2D is considered to be one of the most psychologically demanding chronic conditions, 8 and people with diabetes often have poor psychological well-being. 9 Diabetes is often comorbid with depression, which has an impact on its management and control. 1 A systematic review 10 shows high rates of comorbidity between diabetes and depression and suggests that depression is three times as common among people with diabetes. The psychological stress associated with the management of diabetes could lead to elevated symptoms of depression. 11 Depression comorbid with T2D has been associated with poorer glycemic control and poorer management of the condition. 12–14

Diabetes can cause a number of health complications if not well managed and treated and has the potential to have a huge impact on people’s physical and psychological well-being. The WHO and the IDF suggest that diabetes considerably increases the risk of cardiovascular disease (CVD), nephropathy, loss of vision due to diabetic retinopathy and lower extremity amputation. 1 2 Diabetes (and its health complications) is also associated with an increased risk of mortality in most countries. 2 7 The IDF estimates the number of people who died from diabetes in 2015 before the age of 60 was approximately 5 million; 14.5% of all-cause mortality globally is attributed to diabetes. 2 The countries with the highest number of people with diabetes are the ones with the highest number of deaths associated with diabetes: China, Russian Federation, India and the USA. In 2015 T2D was the seventh leading cause of death in the USA. 4

Diabetes UK states that 90% of people with diabetes have type 2 diabetes (T2D), while at the same time the percentage of people with T2D is on the rise and increasing. 3 The increasing levels of obesity in many countries nowadays have underlined a very concerning, newly introduced aspect: the number of T2D in children is growing. 4 The IDF raises the concern by stating that T2D in children has the potential to become a global public health issue which will lead to serious health outcomes and underlines the need for more research in this aspect of diabetes. 2

WHO estimates that in 2014, 422 million adults were living with diabetes globally, while according to the International Diabetes Federation (IDF) this number will rise to 642 million by 2040. 1 2 In the UK the number of people who have diabetes is over 4.5 million, and in the USA it is more than 30 million. 2–4 In 2010/2011 in the UK, diabetes costed approximately £10 billion in direct costs and £14 billion in indirect costs, totaling £24 billion (10% of the National Health Service budget), with the estimation being £40 billion in total in 2035. 5 In the USA the annual economic burden is $245 billion, $176 and $69 billion in direct and indirect costs, respectively. 6

The Quality Assessment Tool for Quantitative Studies is a standardized quality appraisal instrument that was developed by the Effective Public Health Practice Project (EPHPP) of McMaster University for assessing the quality of public health. 32 The instrument has been evaluated for inter-rater reliability, content validity and construct validity and has received high scores. 33 34 This quality assessment tool and its dictionary are provided at www.ephpp.ca. Two researchers carried out the quality assessment independently. The identified relevant studies were critically assessed according to the EPHPP’s tool and rated as ‘strong’, ‘moderate’ or ‘weak’ in six sections: (1) selection bias; (2) study design; (3) confounders; (4) blinding; (5) data collection methods; and (6) withdrawals and dropouts. The aim of the quality assessment was to extract results from studies with an overall ‘strong’ or ‘moderate’ methodological rating. Studies that were rated as ‘poor’ have been excluded from the systematic review. Please see table 1 for the results of quality assessment of included studies.

The search results from the databases were combined and then the identified duplicates were removed. The described inclusion criteria were applied in a standardized manner, and relevant studies were screened using the title, abstract and full text of the article.

The following data was collected from each study as available: quality of life, self-esteem, depression,perceived pain and neuropathy symptoms, foot conductance, nutritional efficacy,general efficacy, dietary adherence, dietary restraint, dietary disinhibition,diet acceptability, quality of life HbA1c, weight, fasting blood glucose,total cholesterol, low-density lipoprotein cholesterol, triglycerides anddiabetes-related medication. In addition, baseline characteristics of each study’s sample size, mean age, gender (proportion of men), design and duration were collected. Mean values for baseline age, proportion of male population and HbA1c were also calculated.

A computerized systematic search was performed on 10 November 2017 in the following electronic databases limited to studies published in English language since the inception of each database: Allied and Complementary Medicine, Cochrane Central Register of Controlled Trials, Cumulative Index to Nursing and Allied Health Literature, E-Journals, Excerpta Medica Database, MEDLINE, Health Management Information Consortium, PsycARTICLES, PsycINFO, PubMed, SocINDEX and Web of Science. In order to search for further potentially eligible research material, the reference lists of studies with relevant topic were searched and reviewed. The following were the search terms used to locate/find studies: (1) plant based OR plant-based OR “plant based” OR plant food OR “plant food” OR vegan* OR vegetarian* OR “Daniel fast”; (2) diabet* OR T2D OR DT2; and (3) intervention* OR program* OR “controlled trial*” OR “randomised controlled trial*” OR “randomized controlled trial*” OR RCT.

The incorporation of the Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines was used in order to conduct this review. 30 31 The following criteria were considered for inclusion in the systematic review: (1) plant-based diet interventions targeting or including adults with T2D; (2) age of participants ≥ 18 years; (3) clearly defined health outcomes; (4) controlled trials with length of ≥ 3 weeks; and (5) peer-reviewed studies. The exclusion criteria were (1) diet interventions for diabetes other than type 2; (2) age of participants <18 years; (3) uncontrolled studies; (4) diet interventions that included more than 10% of daily calories on animal products; and (5) duplicate results.

All included studies were controlled trials with a duration range of between 3 and 74 weeks (mean duration 23.2 weeks). Of the nine controlled trials, seven were randomized controlled trials (RCTs), of which four reported the randomization process 38 41 44 45 and three failed to describe it. 36 42 43 Moreover, two studies used a parallel design 36 44 and one used a cluster study design. 39 Eight of the studies examined vegan diets in the intervention groups, while one study examined plant-based diet with an option of one portion of low-fat yogurt a day, which accounted for approximately 8% of the total daily intake of calories. 36 A full description of the diet intervention was described in all of the articles. All studies reported HbA1c as their primary or secondary outcome, which was measured at baseline and at the end of each study in both groups. From the 11 articles included, 3 reported psychological well-being outcomes in their results. The studies were dietary interventions and therefore none included blinding of participants. Five controlled trials provided nutritional education in both groups 36 38 42 43 45 and four provided education only to the intervention group. 39–41 44 All studies reported both baseline and end-line results and used validated measures. Highly specialized professionals (medical doctors, registered dietitians) delivered the dietary interventions in all of the studies.

The majority of the studies (n=6) 37–42 were conducted in the USA, while the rest took place in Czech Republic (n=2), 35 36 Italy (n=1), 43 New Zealand (n=1) 44 and South Korea (n=1). 45 One study had been conducted in a supervised residential environment, 43 while the rest had been conducted in community-based settings. The total sample size was 433 participants (219 in the intervention groups/214 in the control groups; mean sample size 48), aged between 27 and 80 years (mean age 54.8 years). The number of withdrawals was reported in all studies, with the rates being between 3.5% and 29.1%. All studies targeted populations with T2D except for three. 39 40 44 Of these three studies, two 39 40 included individuals with body mass index (BMI) ≥25 kg/m 2 and/or T2D, and one 44 also included individuals with BMI ≥25 kg/m 2 and at least one diagnosis of T2D or ischemic heart disease or hypertension or hypercholesterolemia. Therefore not all of the participants of these three studies had T2D, but in the results specific outcomes related to the individuals with T2D were described.

The 11 included studies were published between 1999 and 2017 (please see table 2 ). Two of the studies produced two articles from the same sample highlighting different aspects of research; these articles are included:

The included studies relied on different tools in order to extract psychological data outcomes, and therefore a meta-analysis was not possible in this regard. For the other outcomes of our study, we considered that because the overall pooled N in the study is small, the CIs would be quite wide due to imprecision, which might cause our pooled estimate to cross the null hypothesis, which would make it difficult to draw a conclusion in either direction. Dietary interventions are difficult to engage a large number of individuals as it is hard for participants to alter their dietary patterns for a medium/long period of time. Our aim was to present as clear as possible the physical and psychological outcomes of plant-based interventions.

The search strategy retrieved 1240 articles. After removing the duplicates, the title/abstract screening process identified 41 studies. After a further full-text assessment, 30 articles were excluded from the systematic review and 11 met the inclusion criteria (please see figure 1 ). No additional publications were found through reference lists and hand searching.

Two studies reported an increase in diabetes-related medication at end-line, 38 41 and one 45 reported that 14% (n=7) of the participants in the intervention group and 23% (n=12) participants of the control group increased the diabetes medication doses. Bunner et al 41 showed that two participants in the intervention group and two in the control group increased the glucose-lowering medication. Also, the lipid-lowering medication increased in one intervention group participant and in three control group participants. 41

Six studies reported a reduction or discontinuation of diabetes-related medications among participants at end-line. 36 38 41–44 Interestingly, Nicholson et al 42 mentioned that two intervention group participants on insulin decreased their insulin doses from baseline to end-line. Moreover, three intervention group participants decreased and one discontinued the oral hypoglycemic treatment. 42 Additionally, two participants in the intervention group discontinued the antihypertensive medication at end-line. 42 In contrast, participants in the control group did not have changes on their medication. 42 Barnard et al 38 reported that 35% (n=17) of the participants in the intervention group reduced their T2D medication in contrast to 20% (n=10) of the participants in the control group. Kahleova et al 36 showed that the diabetic medication reduced in 43% of participants in the intervention group and in 5% of participants in the control group. Soare et al 43 mentioned that from a total of seven participants on oral hypoglycemic medication in the intervention group, five of them discontinued their medication. In the control group one participant discontinued the oral hypoglycemic medication treatment. 43 Similarly, Bunner et al 41 reported that 10 participants in the intervention group reduced the glucose-lowering treatment, while in the control group one participant reduced the glucose-lowering medication. Moreover, four intervention group participants decreased the lipid-lowering medication, while no participant reduced it in the control group. 41 Wright et al 44 reported that two participants with T2D in the intervention group did no longer meet the criteria for T2D diagnosis at end-line.

In addition, an analysis among participants who reported high adherence showed a greater difference of HbA1c levels between the two groups 45 : a significant within-group decrease of 0.9% in the intervention group (p<0.01) and a significant within-group decrease of 0.3% in the control group (p<0.05). The reported decrease between the two groups was significantly greater in the intervention group compared with the control group among participants of high adherence (p=0.01). 45

Further analyses of HbA1c levels in two studies 36 40 among participants whose medication remained unchanged showed a greater decrease of 1% in the intervention group compared with a decrease of 0.2% in the control group in one study 40 and significantly decreased by 0.9% only in the intervention group (p=0.002) in the other. 36

All nine studies provided baseline and end-line measures for HbA1c. In eight studies the decrease of HbA1c was greater among participants in the intervention groups compared with the control groups, while in one study 40 the decrease was similar in both groups. After excluding Wright et al 44 for reasons of not reporting HbA1c levels specifically for participants with T2D and after performing weighting statistical adjustment, the mean difference among the eight studies that provided specific HbA1c levels for participants with T2D between baseline and end-line measurements shows a decrease of 0.55% in the intervention groups and 0.19% in the control groups. The results suggest that participants in the intervention groups managed to better control their diabetes via the plant-based diet compared with participants in the control groups.

Overall, the results showed that even though the plant-based diets were slightly more demanding in initial effort, adherence was greater among those in the intervention groups compared with the control groups, suggesting high rates of acceptability of the plant-based diet among the participants of the intervention groups. In addition, control group diets were more likely to be described as constraining, and as a result dietary restraint was higher in the control groups.

Dietary restraint increased to a greater extent only in the control group, 35 37 suggesting that the control group felt more constrained by their assigned diet than the participants of the plant-based diet group. Dietary disinhibition decreased significantly only in the intervention group (p=0.01) in one study, 35 while Barnard et al 37 reported that it declined in both groups, suggesting that individuals in both groups were feeling less hungry compared with baseline and they were less likely to overeat as a response to stressful stimuli. Also, no significant differences between the two groups were reported with regard to food acceptability. 37 Within-group analysis demonstrated significant improvement in levels of energy only in the intervention group (p<0.001), 37 while the control group reported significant improvement in digestion (p<0.05). 37

Five studies 36 37 39 41 44 reported greater adherence among the participants of the intervention group compared with the control group, while one study 43 showed high adherence among both groups. Also, one study 40 reported complete adherence to the plant-based intervention diet in approximately half of participants (44%). In contrast, Lee et al 45 reported that the control group reported higher adherence (p=0.002) compared with the intervention group, while the percentage of participants who reported high adherence was significantly higher in the control group compared with the intervention group (p<0.001).

Overall, the results suggest that a plant-based diet could improve the overall quality of life, psychological well-being and chronic diabetic neuropathy in people with T2D without changes in food enjoyment and diet costs. In addition, the slight increase in electrochemical skin conductance in the intervention group and the significant decline of the control group 41 make plant-based diet intervention promising for further testing in people with painful T2D neuropathy.

Quality of life significantly improved only in the intervention group 35 41 (p=0.01; p<0.05), and significant improvements in the intervention group in both psychological and physical components summary 44 (p<0.001; p<0.0001) were also reported. The control group showed a significant improvement only in the physical component summary 44 (p=0.03). Significant differences between the two groups in favor of the intervention group were reported for both the psychological and physical component summary 44 (p<0.01; p=0.03). Depression levels dropped and reached statistical significance only in the intervention group 35 (p=0.03). Changes in perceived pain and neuropathy symptoms were reported as pain significantly decreased between groups favoring the intervention group 41 (p=0.04). Also, a significant decline in foot conductance was reported in the control group 41 (p=0.03), suggesting that the intervention might have stopped or slowed down the progress of nerve impairment. Furthermore, statistically significant differences were reported in favor of the intervention group for self-esteem 44 (p<0.01), nutritional efficacy 44 (p<0.0001) and general efficacy 44 (p=0.01).

Discussion

This systematic review demonstrates evidence that a plant-based diet can significantly improve psychological well-being, quality of life, control of T2D measured by HbA1c and a number of physical characteristics (weight loss, FBG, total cholesterol, LDL cholesterol, triglycerides) in people with T2D.

Three studies reported results with regard to psychosocial and quality of life outcomes.35 41 44 More specifically, there were significant improvements in the intervention group compared with the control group in depression,35 emotional well-being,44 physical well-being44 and general quality of life.35 41 44 Further outcomes showed a significant decrease of reported pain among participants in the intervention group.41 The participants of the intervention groups reported greater psychosocial results and an improved quality of life in all three studies.

Participants’ adherence was reported in eight of the studies.36 37 39–41 43–45 Reported adherence was higher among participants of the intervention groups compared with the control groups in four studies.36 37 39 44 In contrast Lee et al 45 reported that the control group showed significantly higher adherence compared with the intervention group. A potential explanation for this difference might be the absence of weekly workshops, as Lee et al 45 used only phone consultations, while the four studies with high adherence results in the intervention groups carried out weekly educational sessions. One study reported high adherence in both the intervention and the control groups,43 potentially because education was provided on a daily basis and the RCT was conducted in a supervised environment. Moreover, two of the studies monitored the adherence only in the intervention group and reported high41 and medium levels of adherence.40 These results of the systematic review suggest that providing consistent nutrition education can better support people to adapt more effectively to a plant-based diet, as high dietary adherence and compliance is linked with educational interventions.46–48

Three studies monitored the diet behavior and diet acceptability among participants in both groups.35 37 39 Dietary disinhibition significantly decreased in the intervention group, while dietary restraint significantly increased only in the control group.35 Barnard et al 37 reported that both diets were equally well accepted by participants in both groups. Kahleova et al 35 results suggested that the plant-based diet was highly accepted by the participants as the dietary restraint and dietary disinhibition were significantly improved in the intervention group.

With regard to diabetes control, this systematic review demonstrates that a plant-based diet intervention can significantly reduce HbA1c in adults with T2D and potentially reduce diabetes medication. Eight of the included studies reported baseline and end-line HbA1c levels of participants with T2D.36 38–43 45 All eight studies reported a greater reduction of HbA1c levels among participants in the intervention groups compared with the control groups, with an average decrease of 0.55% in the intervention groups and 0.19% in the control groups. This result is consistent with the findings of previous research.29 Furthermore, findings from six studies reported a reduction or discontinuation of diabetes-related medication in the intervention groups.36 38 41–44

Five studies reported significant greater weight loss in the intervention group (mean weight loss 5.23 kg) compared with the control group (mean weight loss 2.83 kg) from baseline to end-line.36 38 41–43 Three studies reported significant improvements in FBG levels.36 38 42 Two studies demonstrated a significant decrease in LDL cholesterol levels in the intervention groups,36 38 while one study showed a decrease in total cholesterol and triglycerides levels.38 There are studies that have demonstrated that a plant-based diet can significantly lower total cholesterol, LDL cholesterol, triglycerides and the overall risk for CVD.49–52 This is noteworthy because CVD is one of the main comorbidities of T2D53 54 and one of the main causes of premature mortality in the diabetic population.55–59

The present systematic review has several strengths. It is the first review, to our knowledge, that attempted to summarize the significant psychological outcomes of a plant-based diet intervention in adults with T2D. Furthermore, the included CTs were conducted in several different countries (Czech Republic, Italy, New Zealand, South Korea and USA), which suggests that generalizability of the findings in the UK setting is likely. Also, reviewing CTs which focus on dietary patterns and not isolated nutrients makes the findings of the review easier to be applied to the general population. Finally, when plant-based diets were compared with official guidelines from a number of countries, the plant-based diet was found to be superior. Although this is a preliminary review of evidence specific to people with T2D, other research which includes but is not specific to people with diabetes supports the psychological findings.60 61

This systematic review also has limitations. The included studies had rather small sample sizes, although they were adequate to estimate significance levels. Another limitation is that two of the studies were not randomized.39 40 In addition, adherence in control trials regarding diet is difficult to be followed and measured by researchers because it is highly dependent on participants’ recalls, which might be inaccurate.