Plant-based diets as therapy: Do they work, and if so, why?


The aim of this article is to present evidence for the use of plant-based diets as therapy, so that practitioners can appreciate the value of adding plant-based dietary therapy to their Lifestyle Medicine toolkit and offering this as an alternative or adjunct to other medical treatment.  

Back in the early 1990’s, when I was a post-graduate nutrition student, I came across a paper in The Lancet about the “Lifestyle Heart Trial”; a study led by cardiologist Dean Ornish1. This study reported that putting cardiac patients on a very low fat vegetarian diet, along with encouraging regular exercise, smoking cessation and stress management training, resulted in over 82% of subjects achieving regression of atherosclerosis. This was achieved in only 12 months, without the use of lipid-lowering medication. The “usual care” control group, who had been given the usual advice (such as making moderate dietary changes etc.), showed progression of their atherosclerosis.

The study results did not surprise me as it was well-accepted that the consumption of cholesterol and saturated fatty acids (which are found in all types of fat, not just “saturated” fats) was a major contributor to cardiovascular disease. Excitedly, I showed the paper to one of my lecturers, believing that this would be a real game-changer in giving hope to those afflicted with atherosclerosis (which by all accounts is probably most adults living in Australia). I thought that it would be no longer standard practice (nor ethical) to continue with the “trim your bacon and choose low fat cheese” type of advice that we’d been taught to give cardiac patients. Sadly, my enthusiasm was not contagious, and I was told that there were too many confounding variables (such as exercise) to support the use of such a diet, and that in any case, nobody would want to adopt such an “extreme” diet. 20 years later, cardiologist Dr Caldwell B Esselstyn articulated my thoughts at that time:


“Some people think the plant-based whole foods diet is extreme. Half a million a year will have their chests opened up and a vein taken from their leg and sewn into their coronary artery. Some people would call that extreme.”

(Caldwell B Esselstyn in the documentary, “Forks Over Knives”, 2011)

My experience has been that thousands of patients have been elated at the results of adopting an “extreme” plant-based diet. Indeed, many have expressed anger at not being given this option by other health or medical professionals, who were perhaps unaware of the potential benefits or had made a decision (on behalf of their patients) that uptake or compliance would be low.

There can be confusion over the term “plant-based”, as it’s been used broadly and imprecisely to include various types of eating patterns such as those that are completely free of animal products (such as vegan diets), as well as diets that regularly include animal products, such as a traditional Mediterranean diet2. The discussion here focuses on studies on plant-based diets that do not contain meat of any kind, but in some cases do contain some dairy products and/or eggs. The most compelling evidence for the use of this type of diet as therapy is for the treatment of cardiovascular disease, diabetes and obesity.

Cardiovascular disease

Since the landmark “Lifestyle Heart Trial”, not only Ornish but others have published research on the effect of plant-based diet on cardiovascular disease. This has included documented examples of reversal of atherosclerosis in cardiac patients by cardiologists such as Esselstyn, who has been prescribing a very low fat vegan diet to his patients since the mid 1990’s, with impressive results3-5.

It was as long ago as the 1980’s that Australian researchers demonstrated in a randomized controlled crossover trial that adopting an ovo-lacto-vegetarian diet resulted in a significant fall in systolic blood pressure7. In the same decade, another study found that a vegan diet was able to significantly reduce both systolic and diastolic blood pressure in medicated hypertensives, enabling cessation or drastic reduction of medication in most subjects8.  A 2014 review of both intervention trials and epidemiological studies concluded that vegetarian diets are associated with lower systolic and diastolic blood pressure, and offer an alternative to pharmacological control of hypertension9.

Many studies published since have added to the evidence for the benefits of plant-based eating for cardiovascular health. A substantial body of evidence is supplied by the “Adventist Health Study 2”; a long-running longitudinal study of nearly 100,000 participants, divided into five dietary groups based on the types and amounts of animal products consumed. This study revealed that vegetarian diets are associated with lower body mass index (BMI), lower prevalence of diabetes, metabolic syndrome and hypertension, lower all-cause mortality, and in some instances, lower risk of cancer10. Further analysis of this data along with other studies on Adventists found that vegan diets offer additional protection over lacto-ovo-vegetarian diets not only for hypertension and cardiovascular mortality, but also for diabetes and obesity11.

Meta-analyses of these and other epidemiological studies have echoed the findings6,12-14 that people avoiding meat have lower blood pressure12, lower total cholesterol and LDL cholesterol6,12, and reductions in inflammatory and endothelial markers12. A 2017 review of the cardiometabolic effects of plant-based diets estimated that vegetarian diets can reduce the risk of coronary heart disease events by 40%, the risk of cerebral vascular disease events by 29%, and the risk of metabolic syndrome by about 50%13.


Several intervention studies on the use of a vegetarian diet for diabetes have been conducted, with significant benefits observed15-21. Systematic reviews and meta-analyses indicate that vegetarian diets improve glycaemic control, with significant reduction in HbA1c levels22, and cardiometabolic risk factors in diabetes23.

A recent review found that consuming a vegetarian diet reduced the risk of developing type 2 diabetes by about 50%13, with prevalence being even lower in vegans than in other types of vegetarians11.

In 2013 the Canadian Diabetes Association Clinical Practice Guidelines recommended the use of plant-based diets for management of Type 2 diabetes, and the evidence for this has been comprehensively reviewed24.



Observational studies such as the “Adventist Health Study 2” have found that reduced consumption of animal products is associated with lower body weight25, and higher intake of plant foods has been associated with lower BMI and incidence of obesity, and smaller waist circumference12.

Interventions such as the “BROAD study” have found that a low fat whole foods plant-based diet can result in significant weight loss in obese individuals, even without specific advice on calorie restriction26. A review of intervention studies has found that adopting a vegetarian diet resulted in a mean weight change of -3.4 kg, with greater losses when weight loss was a goal27.

Why do plant-based diets confer health benefits?

While it isn’t clear to what extent the health benefits of plant-based diets are due to the absence of animal products or to the presence of plant products, there are many reasons why eating plants rather than animals can assist in the prevention and treatment of common chronic health conditions. Plant-derived foods are devoid of cholesterol, are almost all low in saturated fat (with the exceptions being coconut and palm oil), and whole plant foods are rich in fibre, antioxidants and other beneficial phytonutrients. Animal products, on the other hand, all contain cholesterol, tend to be rich in saturated fat, and all are devoid of fibre. Being higher in the food chain, meat, fish, eggs and milk accumulate toxic substances such as arsenic, cadmium, lead, mercury28 and persistent lipophilic organic pollutants29 which can negatively impact human health. The use of antibiotics and sex hormones in modern farming practices and their presence in animal products is also a concern30,31.

Evidence on the strong relationship between tri-methylamine N-oxide (TMAO) levels and cardiovascular mortality32 is another problematic aspect of animal product consumption. Interestingly, the microbiome of vegetarians has been found to be distinct to that of omnivores33, with vegan gut profile being unique in lacking the intestinal microbiota for converting dietary l-carnitine into TMAO, as well has having a greater abundance of protective species and a reduced abundance of pathobionts34. The World Health Organisation has classified processed meats (e.g. ham, bacon, sausages, hot dogs) as Group 1 carcinogens (carcinogenic to humans) and red meats as Group 2A (probably carcinogenic), with carcinogenicity attributed to both endogenous and exogenous components of these foods35.

Are all plant-based diets “healthy”?

It is important to note that not all plant-based diets are the same, and poorly-planned diets can contain inadequate amounts of essential nutrients and/or excessive amounts of potentially deleterious food components. While plant-based diets can be powerful therapeutic tools in the treatment of cardiovascular disease, diabetes, and obesity, patients need clear guidelines on what foods to exclude, what to include, and advice on supplementation where indicated.

The next article in this series will focus on the planning of plant-based diets to ensure nutritional adequacy and to maximise health benefits.

Amanda Benham is an Accredited Practitioner Dietitian, Fellow of the Australasian Society of Lifestyle Medicine, and owner of The Human Herbivore

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This article has been written for the Australasian Society of Lifestyle Medicine (ASLM) by the documented original author. The views and opinions expressed in this article are solely those of the original author and do not necessarily represent the views and opinions of the ASLM or its Board.