Plant-based diets and type 2 diabetes

Plant-based diets and diabetes risk

Observational studies have shown that compared to non-vegetarians, those following a vegetarian or vegan diet have a significantly lower risk of T2D.  A 2017 systematic review and meta-analysis found a pooled odds ratio for diabetes in vegetarians vs. non-vegetarians of 0.73.¹

The original Adventist Health Study (AHS), involving 25 698 Seventh Day Adventist church members who were followed up for 21 years from recruitment in 1960, found that non-vegetarian women had a 1.4-fold higher risk and non-vegetarian men a 1.8-fold higher risk of developing T2D compared to those following a vegetarian diet.²

In the Adventist Health Study 2 (AHS-2), a cross-sectional analysis of baseline data from 60 903 Seventh Day Adventist Church members from North America found that those following a vegan diet had a diabetes prevalence approximately one-third that of non-vegetarians (2.9% versus 7.6%), while the lacto-ovo vegetarians, pesco-vegetarians and semi-vegetarians had an intermediate diabetes prevalence of 3.2%, 4.8% and 6.1% respectively.³ Even after adjusting for confounding factors, those following a vegan diet had almost a 50% reduction in the risk of developing T2DM compared to non-vegetarians.  The reduction in risk of diabetes became incrementally smaller as more animal products were consumed –lacto-ovo vegetarians had a 46% reduction in risk, pesco-vegetarians a 30% reduction in risk and semi-vegetarians a 24% reduction in risk compared to those following an omnivorous diet.³

In a subset of 15 200 men and 26 187 women without diabetes at baseline who were followed for two years, vegans were found to have a 62% lower risk of developing T2D and lacto-ovo vegetarians a 38% lower risk compared to non-vegetarians, after adjustment for confounding factors.⁴

Plant-based diets for managing diabetes

Early studies of low-fat vegan diets in people with T2D demonstrated significant improvements in blood glucose levels and blood lipids, as well as considerable reductions in medication requirements, although some of these studies were small and did not include a control group.  However, a 2018 systematic review of nine randomised controlled trials (RCTs) found that, compared to control diets (including those of several diabetes associations), plant-based diets were associated with significant improvement in emotional well-being, physical well-being, depression, quality of life, general health, HbA1c levels, weight and total and LDL cholesterol levels.⁵

The largest RCT, involving 99 individuals with T2D, compared a low-fat vegan diet with a diet based on the American Diabetes Association (ADA) guidelines.  After 22 weeks, the vegan diet reduced HbA1c, weight, LDL cholesterol and urinary albumin levels significantly more than the ADA diet and 43% of subjects were able to reduce their medication compared to only 26% in the ADA group.⁶  Subjects were then followed up until 74 weeks  and while weight loss was no longer significantly different, the vegan diet group still had significantly greater reductions in HbA1c, total and LDL cholesterol (before any medication adjustment), and diabetes medication doses were reduced in 35% of the vegan diet group compared to 20% in the ADA group.⁷

Plant-based diets and weight management

It is well accepted that excess weight is a significant contributor to insulin resistance and T2D risk, and that weight loss is a key component of the management of T2D.  Despite the current popularity of lower carbohydrate, higher fat diets for weight loss, evidence suggests that plant-based diets (usually low in fat and higher in carbohydrate) are an effective approach to weight management.

A 2015 review and meta-analysis of 15 RCTs of at least 4 weeks duration examining the effect of vegetarian diets, without energy restriction, versus conventional weight loss diets found that vegetarian diets were associated with a greater weight loss than non-vegetarian diets.⁸  Similarly, a 2016 meta-analysis of 12 RCTs found that individuals assigned to the vegetarian diet groups lost significantly more weight than those assigned to the non-vegetarian diet groups, with greater weight loss seen in those following a completely plant-based (vegan) diet.⁹  A third systematic review and meta-analysis of 29 studies demonstrated an improvement in obesity-related inflammatory profiles associated with plant-based diets.¹⁰

Mechanisms

Aside from the impact on weight, there are a number of possible explanations for the benefits of plant-based diets for diabetes prevention and management.  Compared to most western diets, vegetarian and vegan diets are generally higher in dietary fibre and are likely to include more wholegrains, legumes and nuts, all of which have been associated with a reduce risk of T2D.

There is also increasing evidence for the role of the gut microbiome in chronic disease risk.  Plant-based diets, due to their high content of dietary fibre and fermentable carbohydrates, may promote a more favourable gut microbial profile.¹¹ For example, one study found that those following a vegan diet had a unique gut microbiota, with a reduced number of pathobionts, implicated in low grade inflammation, and a greater abundance of protective species.¹²

A key difference between vegetarian and omnivorous diets, however, is the absence or limited intake of animal protein and red meat.  At least 25 studies have been published assessing the relationship between meat intake and T2D risk, with the majority showing a positive association between red meat and/or processed meat intakes. A 2013 meta-analysis found an association between higher intakes of total meat, unprocessed red meat and processed meat and T2D risk.¹³  There is also consistent evidence for an association between total dietary haem iron intake and haem iron intake from red meat and risk of T2D , and high serum ferritin levels are associated with insulin resistance and T2D risk.¹⁴

Finally, animal protein intake itself has also been positively associated with T2D risk.  A 2019 meta-analysis of prospective cohort studies looking at dietary protein intake and subsequent risk of T2D found high total protein and animal protein intakes to be associated with an increased risk of T2D while a moderate plant protein intake was associated with a decreased risk.¹⁵  An earlier systematic review and meta-analysis of 13 RCTs in people with diabetes found that replacing animal protein with plant protein (around 35% of total protein/day) resulted in significant reductions in HbA1c, fasting glucose and fasting insulin levels compared to control groups.¹⁶

Acceptability

Concerns about acceptability are often cited as a barrier to individuals adopting a vegetarian or vegan diet and may be one reason why more health professionals are not recommending this type of eating plan, despite evidence of the benefits. However, this is not supported by research. In fact, several studies, including those discussed above, have found that low-fat vegetarian or vegan diets were well accepted, similar to acceptance levels standard low-fat diets.^{7, 17, 18}

Nutritional concerns

While concerns about meeting nutritional needs on a vegetarian diet are also often cited, well planned vegetarian diets, including vegan diets, have been shown to be nutritionally adequate and are appropriate for individuals during all stages of life.¹⁹ Some nutrients can be more difficult to obtain on a vegetarian diet, but careful planning, and in some cases the use of fortified foods or supplements, can ensure that an individual’s nutrition needs are met while maximising the health benefits of a vegetarian or vegan diet.²⁰ Those following a vegan diet will need to ensure a reliable source of vitamin B12 from supplements or fortified foods, and this is particularly important for those on metformin, which can reduce vitamin B12 absorption.²¹

Not just diabetes

Plant-based diets don’t just benefit diabetes and weight management.  Dietary patterns which are rich in plant foods and low in animal foods (particularly red meat) have repeatedly been associated with a reduction in risk of other chronic conditions including cardiovascular disease and some types of cancer.^{22, 23} These include vegetarian and vegan diets, Mediterranean diets and the Dietary Approaches to Stop Hypertension (DASH) diet.