This article discusses the environmental benefits of plant-based diets, citing a study led by Prof. Peter Scarborough at Oxford. It emphasises the reduction in diabetes risk associated with plant-based diets, detailing a research paper published in Diabetes and Metabolism on potential mechanisms. 

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Researchers know that plant-based diets are becoming increasingly popular because they have very favourable environmental footprints.

According to Peter Scarborough, a professor of population health at Oxford,

“If meat eaters in the United Kingdom who consumed more than 3.5 ounces of a meat a day (slightly less than the size of a quarter pound burger) cut their intake to less than 1.7 ounces a day (roughly the amount of a single McDonald’s meat patty) it would be the equivalent of taking 8 million cars off the road”.

These researchers showed that a vegan diet causes significantly less harm to land, water and biodiversity, (Nature Food, 20 July 2023)

Those who consume a plant-based diet have sound reasons to rejoice! Their diets have been associated with a lower risk of type 2 diabetes mellitus(T2DM). No one knew fully the mechanisms which help to achieve this status. A paper published in Diabetes and Metabolism (Print Edition) on 1st January 2024 by researchers from the UK, Austria, Spain, Denmark and Australia tried to find the potential mechanisms to explain the lower diabetes risk seen among persons eating plant-based diets

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The study

They analysed the prospective data from the UK Biobank by using appropriate statistical methods to identify which cardiometabolic risk factors correctly explained the observed linkages.

The UK Biobank, was set up in 2007 by the Well come Trust, the Medical Research Council, the UK Department of Health, and the Scottish Government.

It has also received funding from the Welsh Assembly Government, the British Heart Foundation, Diabetes United Kingdom, Northwest Regional Development Agency, Scottish Government.

This large-scale biomedical database and research resource contains de-identified genetic, lifestyle and health information and biological samples

Researchers have been using this resource for large, long-term biobank studies in the United Kingdom which is investigating the respective contributions of genetic predisposition and environmental exposure to the development of disease.

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Background

Researchers noted that in 2021 the global prevalence of diabetes was 6.1 %; it was projected to increase to over 10 % by 2050, with Type 2 Diabetes Mellitus (T2DM) accounting for approximately 95 % of cases. In the UK alone, it costs the National Health Service around £10 billion to take care of approximately 4.7 million individuals who are living with type 2 diabetes (T2DM)

Based on many studies in the field, the present researchers found the following:

1. A healthy lifestyle could prevent at least 75 % of all T2DM cases.
2. Adopting a healthy lifestyle can reduce the risk of cause-specific and total diabetes mortality by up to 56 %.
3. High consumption of fruits, vegetables, whole grains, dairy, and coffee has been associated with lower T2DM risk.
4. On the other hand, intakes of sugary drinks and red and processed meat have been associated with increased T2DM risk.

These findings on food groups and T2DM risk are in agreement with many studies which suggest that vegan and vegetarian dietary patterns are associated with lower T2DM risk and better diabetes control.

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Popularity of plant-based diet

The researchers noted that presently, plant-based diets are becoming popular. For instance, in 2021, 13–15 % of Germans, Swiss and UK citizens stated that they were following a meat-free diet.

The researchers realised that operationalisations of plant-based diets merely based on the exclusion of animal foods do not offer insights into the quality of the consumed plant-based foods.

Therefore, they established healthful and unhealthful Plant-Based Diet Indexes (hPDI and uPDI), as measures of plant-based diet quality.

Higher hPDI scores, reflecting a plant-based diet low in sweets, desserts, refined grains, potatoes, and sugary drinks, have been associated with a lower risk of T2DM, while an unfavourable plant-based dietary pattern characterised by high uPDI values has been associated with a higher risk.

Let me hasten to add that these are associations mostly based on observational studies. Because of the potential frailties of such studies, we cannot accept them as causational studies.

The researchers investigated the biological mechanisms that underlie the associations between the healthful and unhealthful PDI with T2DM risk using data from a large-scale perspective cohort study, the UK Biobank.

They hypothesised that obesity/glucose metabolism, inflammation, kidney function, liver function, and hormonal and lipid pathways could be potential mediators, as many previous studies have shown; these factors were associated with both plant-based diets and T2DM risk.

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The study population

As stated earlier, the researchers based the present analyses on data from the UK Biobank, a large-scale prospective study among over 500,000 volunteers between the ages of 40 to 69 recruited from 2006 to 2010 across centres in England, Scotland and Wales, and included several comprehensive baseline assessments.

The researchers have used certain exclusion criteria. Participants who had missing dietary data or covariate information (n = 372,173), implausible energy intakes (> 17,573KJ or 〈 3,347KJ for men and 〉 14,644KJ or < 2,092KJ for women (n = 3953), prevalent diabetes (all types) (n = 5009), a diabetes diagnosis (all types) between baseline and the last dietary assessment (n = 301), were pregnant at baseline assessment (n = 46) or had prevalent cancer (n = 5920), or prevalent cardiovascular disease (n = 1868) resulting in a study-cohort of 113,097

Researchers found that in the study cohort, 2628 developed type 2 diabetes mellitus over 12 years of follow-up. In these cases, they estimated healthful plant-based indices (hPDI) and unhealthful plant-based indices (uPDI)

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Dietary assessment and plant-based diet indices

The researchers used the validated Oxford WebQ online questionnaire to assess information on dietary habits. The Oxford data are humongous. The Oxford WebQ captures dietary information on up to 206 different food types and 32 different beverages consumed within the previous 24 hours.

Completing their first 24-hour dietary assessment in April 2009, participants followed by up to four further assessments until June 2012 (February/11 – April/11; June/11 - September /11; October/11 – December/11 and April/12 - June/12). To analyse the quality of plant-based diets, they calculated the hPDI and uPDI

Initially, Satija et al.(doi:10.1371/journal.pmed.1002039) developed these indices. They categorised 18 different food groups based on intake. Fruits, legumes, nuts, tea and coffee, vegetables, vegetable oils and whole grain.

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Key biomarkers identified

The researchers found that the reasons behind the anti-diabetic effect of a healthy plant-based diet go beyond the well-known lower body fat percentage and waist circumference.

"Our study is the first to identify biomarkers of central metabolic processes and organ functions as mediators of the health effects of a plant-based diet,” a press release from the University of Vienna quoted Tilman Kühn, Professor of Public Health Nutrition at the University who led the study in close collaboration with other researchers.

“The study confirmed that normal values for blood lipids (triglycerides), blood sugar (HbA1c), inflammatory parameters (CRP) and the insulin-like growth factor (IGF1) are associated with a low risk of diabetes.” The press release added.

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Further benefits

The study revealed how important the full function of the liver and kidneys is in preventing diabetes. These organs play a major role in people who already have diabetes.

"However, our research has now shown that a healthy plant-based diet can improve liver and kidney function and thus reduce the risk of diabetes," Kühn asserted highlighting a previously underestimated benefit of a conscious plant-based diet.

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Conclusions

The researchers found that:

1. The participants with the highest hDPI score had a 24% lower risk than those with the lowest score mediated by a lower BMI (proportion mediated: 28 %).
2. Lower waist circumference (28 %), and
3. Lower concentrations of HBA1c (11 %), triglycerides (9 %), alanine aminotransferase (5 %), gamma-glutamyl transferase (4 %), C-reactive protein (4 %), insulin-like growth factor 1 (4 %), cystatin C (4 %) and urate (4 %).
4. Higher uPDI scores were associated with a 37 % higher T2DM risk [HR: 1.37], with higher waist circumference (proportion mediated: 17 %), BMI (7 %), and higher concentrations of triglycerides (13 %) potentially playing mediating roles.

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Expert comments

Dr Duane Mellor, Registered Dietitian and Senior Lecturer, Aston Medical School, Aston University:

“This is an interesting analysis of the UK Biobank which used limited dietary data from only 206 different food types from single-day food intakes. This has led the authors to claim high-quality plant-based diets are beneficial for type 2 diabetes prevention. This study was observational and cannot be certain of causality, only that this dietary pattern high in fruit, vegetables, legumes and wholegrain was associated with a reduced risk of developing type 2 diabetes.” Dr Mellor asserted in a press release from Science Media Centre, London that the journal published the paper online on 20 December 2023.

The analysis looked at aspects of liver health and other measures of inflammation and explored how they might be linked to diet and the risk of developing type 2 diabetes. Mellor conceded that this suggests many possible designs for future research to assess if this type of plant-based diet can reduce the risk of developing type 2 diabetes.

“This study also does not provide evidence that switching to this type of diet will directly reduce your risk of developing type 2 diabetes – it has been shown in previous diabetes prevention studies that these types of healthy diet with physical activity and weight management can reduce an individual’s risk of developing type 2 diabetes.” Dr Mellor clarified.

Association with diet and diseases: a complex issue

Establishing a causal relation between any diet and any disease is an incredibly complex issue. On January 9, 2024, just over a week after Diabetes and Metabolism published the paper we are discussing in this article on the importance of a healthy plant-based diet in lowering T2DM risk, another group of researchers published a paper which looked at vegetarian and plant-based diets and the incidence of COVID-19 in BMJ Nutrition, Prevention & Health.

In a critical appraisal of the latter paper Prof Margaret Rayman, Professor of Nutritional Medicine, University of Surrey disagreed with the conclusion that “predominantly plant-based diets provide more nutrients that boost the immune system and help to fight viral infections”. (Press release from Science Media Centre. 9 January 2024).

In that context, Prof Rayman referred to a recent study which assessed 141 observational/intervention studies published since 2000 that looked at nutrient intake and status in adult populations consuming vegetarian, vegan or meat-eating diets.

“Similar data come from the German Nutritional Evaluation (NuEva) study of similar numbers of omnivores, flexitarians, vegetarians, and vegans (total n=258) which revealed an insufficient dietary intake of selenium, zinc, potassium, iron (women), calcium, vitamin B12, n-3 LC-PUFA, and vitamin D particularly in vegetarians and vegans. Analysis of biomarkers showed a deficiency of vitamin B12, selenium, zinc and iodine and low ferritin (i.e. low iron stores)”. She added

Prof Rayman explained the issue with metal ions, thus:

1. Food factors such as phytate (myo-inositol hex phosphate), soybean protein, polyphenols, calcium, oxalic acid and dietary fibre inhibit their absorption.
2. Phytate is the primary storage form of phosphorus in plants and is found in cereals, legumes, oil seeds and nuts. It can chelate iron, zinc, calcium and magnesium, reducing their absorption. Polyphenols (e.g. tannins) and oxalates in food matrices also bind metal ions.

Dietary studies are controversial. In August 2018, University of Freiburg professor Karin Michels (she then held a joint appointment at the Harvard T H Chan School of Public Health) claimed that coconut oil is “pure poison”. It was not a study, but only a lecture! She did not authenticate references. The news story got wide traction in the media.

Initially, I thought that she was an “activist”. However, on further review, I realised that the American Heart Association (AHA) has included coconut oil in a list of foods to be avoided. I also found that there are scientists who believe that coconut oil is a good food.

I was intrigued to note how dietary guidelines change over time.

The roles of saturated fats, carbohydrates, salt, sugar, vitamins, etc., are all under the scanner. Based on studies published in premier journals such as The Lancet, I have argued that dietary studies are inconclusive, and confusing even for specialists. (Deccan Herald, 24 October 2018). Discerning readers may find them unimpressive and incomprehensible if not useless.

What is the way forward?

“It is not necessary that the whole world should adopt the Mediterranean diet. Local cuisines can incorporate the principles of a healthy composite diet, combining easily accessible and relatively inexpensive ingredients.”, K. Srinath Reddy, formerly President of the Public Health Foundation of India and formerly head of the Department of Cardiology at All India Institute of Medical Sciences asserted (Deccan Herald, 3 September 2023)

“The key guiding principles of a healthy dietary habit are like the four legs of the dining table. They are variety, balance, moderation and regularity. These are the legs on which a health-promoting diet will need to stand.” Food policy planners must compulsorily read his brief, thought-provoking article in the Deccan Herald.

Disclaimer- The views and opinions expressed in this article are those of the author and do not necessarily reflect the official policy or position of M3 India.

About the author of this article: Dr K S Parthasarathy is a former Secretary of the Atomic Energy Regulatory Board and is a medical physicist with a specialisation in radiation safety and regulatory matters.