Plant-based diets for the prevention and treatment of type 2 diabetes
Worldwide, the prevalence of diabetes among adults over 18 years of age has risen from 4.7% in 1980 to 8.5% in 2014. In 2016, an estimated 1.6 million deaths were directly caused by diabetes and diabetes was the 7th leading cause of death. Diabetes is also a leading cause of blindness, renal failure, heart disease, stroke and leg amputation.
Type 2 diabetes is characterised by insulin resistance, where the body can no longer respond to increasing levels of insulin. This results in elevation of blood glucose. Persistently high levels of glucose can damage cells and organs. It must be remembered though that high blood glucose is the symptom and not the cause of type 2 diabetes. Most medications lower blood glucose without reversing insulin resistance. Insulin resistance is implicated in a number of other chronic diseases, including atherosclerosis and cardiovascular disease, dementia, polycytic ovary syndrome and even cancer.
Type 2 diabetes is predominantly caused by lifestyle factors. A Western-style diet pattern, high in animal-derived and processed foods, is the leading risk factor. We know that plant-based diets (including vegetarian and vegan) can significantly reduce the risk of diabetes. In the Adventist Health Study-2, vegans had a 49% decreased risk of diabetes compared to non-vegetarians and this represented the lowest risk of all the diet patterns compared (image below). In the EPIC-Oxford study, those avoiding meat had around a 50% reduced risk of diabetes when compared with regular meat eaters. However, the effect was reduced once results were adjusted for body weight. The fish eaters had the lowest incidence of diabetes, but it should be noted that this group was small and results in the literature have been inconsistent.
Combined results from the Nurses’ Health Study and the Health Professionals follow-up study have shown that those eating a healthy plant-based diet (not necessarily vegan or vegetarian) composed of predominantly whole grain, fruits, vegetables, nuts, seeds and unsaturated fats, have a significantly reduced risk of type 2 diabetes. A diet that emphasised plant foods and was low in animal foods was associated with a 20% reduction in the risk of diabetes. Consumption of a plant-based diet that included specifically healthy whole plant foods was associated with a larger decrease (34%) in diabetes risk, while consumption of a plant-based diet high in less healthy plant foods (processed foods and refined grains, potatoes and fruit juice) was associated with a 16% increased risk of diabetes. There was a dose relationship between the consumption of healthy whole plant foods and the reduction in risk of diabetes — the more plant foods consumed the lower the risk. The study suggests that increasing intake of healthy plant foods while moderately reducing intake of some animal foods, especially red and processed meats, could be beneficial for diabetes prevention, without the need to become fully vegetarian or vegan.
Data from Nurses’ Health Study and the Health Professionals follow-up study have shown a significant association between the consumption of red and processed meat and the risk of developing type 2 diabetes. In the study, a standard serving size was 85g for unprocessed red meat, 45g for one hot dog, 28g for 2 slices of bacon, or 45g for one piece of other processed red meat. For a one serving/day increase in unprocessed, processed, and total red meat consumption, the risk of diabetes was increased by 12%, 32% and 14% respectively. When food substitutions were examined, one serving of nuts, low-fat dairy, and whole grains per day substituted for one serving of red meat per day was associated with a 16–35% lower risk of type 2 diabetes.
In the EPIC study, during 10 year follow-up of 38,094 participants, 918 cases of type 2 diabetes were documented. The consumption of animal protein, but not vegetable protein, was associated with an increased risk of type 2 diabetes. For every 5% of calories derived from animal protein there was a 30% increased risk of developing type 2 diabetes.
There are a number of reasons why a diet high in animal-derived and processed foods promote the development of type 2 diabetes.
1) Nitrites and nitrates in processed meat are converted in the intestine to nitrosamines which have been shown to be toxic to the beta cells of the pancreas and impair insulin response through generation of reactive oxygen species and pro-inflammatory cytokines.
2) Advanced glycation end products — AGEs; These are a group of compounds that induce oxidative stress. They are formed by a spontaneous chemical reaction between an amino acid (protein) and a monosaccharide (glucose). Some AGEs are produced in the body every day. However, diet is the biggest contributor to AGE formation (along with tobacco products). AGEs from food are generated more readily from protein rich foods, when cooking at high temperatures and for longer and with dry heat cooking (lower AGE formation when there is water/moisture present). Foods that generate the most AGEs are fried and processed foods and also animal-derived foods. Unprocessed whole plant foods such as fruits, vegetables, whole grains and legumes, generate the least amount of AGEs. AGE’s are associated with insulin resistance, inflammation and oxidative stress and hence implicated in the pathogenesis of type 2 diabetes.
3) Higher blood levels of branched chain amino acids (BCAA) — leucine, isoleucine and valine — are associated with insulin resistance and an increased risk of type 2 diabetes. BCAA are found in higher quantities in animal derived foods — particularly red meat and dairy.
4) High consumption of saturated fat results in the accumulation of fat in muscle and liver cells, which causes insulin resistance. Saturated fat consumption also leads to oxidative stress and mitochondrial dysfunction, which contributes to insulin resistance. The main sources of saturated fat are meat, dairy and processed foods.
5) Haem iron in red meat (in haemoglobin and myoglobin) is a pro-oxidant that contributes to cellular oxidative stress and can result in insulin resistance.
6) Choline and carnitine present in meat and eggs is converted by gut microbiota into trimethylamine (TMA), which the liver subsequently converts to trimethylamine oxide (TMAO). TMAO has been implicated in the development of insulin resistance.
7) A diet high in meat and processed foods and low in fibre results in an unhealthy gut microbiome. This contributes to insulin resistance and glucose dysregulation.
How do plant-based diets prevent diabetes
The benefits of plant-based foods for diabetes prevention are due to the high fibre content, the low glycaemic index of certain plant-foods, high levels of phytonutrients and antioxidants and low levels of saturated fat. Antioxidants such as polyphenols may inhibit glucose absorption, stimulate insulin secretion, reduce hepatic glucose output, and enhance glucose uptake. Fibre maintains a more consistent blood sugar level and is fermented by intestinal bacteria to produce short-chain fatty acids, which also improves the glucose response, insulin signalling, and insulin sensitivity. Fibre reduces the energy density of foods, promotes satiety, and has been associated with maintaining a more healthy weight, which in turn promotes insulin sensitivity. In addition, plant-based meals result in increased post-prandial incretin and insulin secretion when compared to a meat-based meal, suggesting that plant-based diets may be able to improve beta cell pancreatic function in patient with type 2 diabetes. Incretin hormones are a group of gut hormones that are released after eating and promote the effects of insulin, thus decreasing blood glucose levels. In people with type 2 diabetes, this incretin effect is reduced or absent.
Treating type 2 diabetes with a plant-based diet
We now know that there are a number of dietary approaches that can reverse type 2 diabetes. Most of these approaches rely on weight loss, with the more weight lost acheived the greater the chance of reversing type 2 diabetes. Remarkably, a high carbohydrate, high fibre diet can reverse type 2 diabetes even in the absence of weight loss. When you centre your diet around whole plant foods, you reverse the root cause — insulin resistance — which is caused predominately by the accumulation of fat in the muscle and liver cells. The trouble with the low-carbohydrate dietary approach is that blood glucose levels will be lowered through the avoidance of carbohydrates, but this does not reverse insulin resistance, unless there is significant weight loss. A very low calorie approach as investigated in the DIRECT trial can be effective but this is ultimately only going to be successful in the longer term if patients are transitioned to healthy, sustainable, plant-predominate diet.
There are now a number of studies demonstrating the effectiveness of a healthy vegan diet (whole food plant-based diet) for the treatment of type 2 diabetes. Many of the studies have been conducted by the Physicians Committee for Responsible Medicine.
A randomised study of a low-fat vegan diet diet, without portion or calorie control, resulted in better glycaemic control, with reduction in the need for medication and insulin, when compared to the American Diabetes Association diet. Vegetarian diets have been shown to reverse insulin resistance. In a systematic review, a plant-based diet was shown not only to improve weight and glycaemic control, but was associated with improved psychological health and quality of life.
The evidence for plant-based diets is strong enough that the American Association of Clinical Endocrinologists and the American College of Endocrinology consensus statement on the management of diabetes, recommends a plant-based diet as the first and foremost nutrition intervention for patients with type 2 diabetes. The Canadian Diabetes Association also support the use of plant-based diets for the management of diabetes.
Listen to this testimonial by Eric Adams, Borough President of Brooklyn, New Your City. He is speaking to the American 2020 dietary guideline committee.
The data supporting a predominately plant-based diet composed of minimally processed whole plant foods for prevention and treatment of type 2 diabetes is now overwhelming. Of course, other lifestyle factors are important in preventing type 2 diabetes including regular physical activity, good quality sleep and stress management. For more information check out our factsheet shown below.
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