A review of the week’s plant-based nutrition news 24th October 2021
This week I cover vegan diets and type 2 diabetes, the impact of a plant-based meal on oxidative stress, healthy eating and aging, dairy and the increased risk of prostate cancer and the new UK undergraduate medical school nutrition curriculum.
VEGAN DIETS FOR PREVENTING AND TREATING TYPE 2 DIABETES: Vegan diets have a number of potential health benefits when they are composed of minimally processed whole plant foods. This systematic review assesses the impact of a vegan diet for the prevention and treatment of type 2 diabetes, including glycaemic control and diabetes-related complications. The analysis included 7 observational studies (2 cross-sectional and 5 prospective cohort studies) and 8 randomised controlled studies.
The most robust observational data supporting a vegan diet for prevention of type 2 diabetes comes from the Adventist Health Study-2, both the cross-sectional analysis, which assesses diabetes prevalence, and the prospective follow-up, which assesses incidence. Both types of analyses demonstrate a significant protective effect of a vegan diet when compared to an omnivorous diet and this was independent of body weight, with similar protection seen in people of different ethnicities. Prevalence of type 2 diabetes was 2.6 times lower in vegans compared to omnivores and incidence was 32% lower. Although the EPIC Oxford study showed a significant reduction in type 2 diabetes incidence amongst vegans, the significance of this finding was lost once the results were adjusted for BMI (body mass index), meaning that the benefits of a vegan diet are really down to maintaining a healthy weight. In the one study including Indian participants there was no protective impact of a vegan diet. There may be a number of reasons for this finding, including very little difference in BMI between vegans and non-vegans, self-reported vegans may still continue to consume dairy products such as ghee, omnivores in India eat significantly less meat than in higher income countries and there is a heavy reliance on white rice, which may limit the effectiveness of a vegan diet.
In addition to studies that include vegans, dietary data from prospective cohort studies have been analysed using the plant-based diet index, which shows a large and significant protective effect of a healthy plant-based diet for the prevention of type 2 diabetes, in the order of 18–34% reduction in risk. Of note, unhealthy plant-based diets are not protective and may increase the risk of type 2 diabetes.
The 8 randomised studies investigating the effect of a vegan diet in people with established type 2 diabetes used a variety of comparator diets, including usual diet, conventional low-fat diet, conventional diabetes diet and portion controlled diet. All studies reported a reduction in HbA1c in participants on a vegan diet in the order of -0.3% to -1.4%. In addition, some of these studies reported additional benefits for weight loss, reduction in waist circumference, blood pressure and cholesterol with variable results in triglyceride levels. Impact on renal function was variable and not consistent. One study demonstrated a positive impact on diabetic neuropathy.
The reason for these positive findings include the beneficial impact of dietary fibre, which promotes satiety, helps maintain a healthy weight and is beneficial to the gut microbiome, an important factor in regulating blood glucose. The lower fat consumption on a plant-based diet results in lower levels of intramyocellular fat and hence improved insulin sensitivity. Excessive intramyocellular fat storage has a cytotoxic effect on mitochondria through generation of reactive oxygen species and increased metabolic stress, which promotes the insulin resistance.
The authors conclude that a vegan diet can be considered comparable to other healthy eating patterns such as the DASH and Mediterranean diet and when appropriately planned, paying attention to obtaining all the necessary nutrients, can be protective against the development of type 2 diabetes and useful as a treatment modality. These findings are consistent with national and international guidelines. For example the American College of Lifestyle Medicine recommends a whole food plant-based diet for remission induction of type 2 diabetes and the American College of Endocrinology a primarily plant-based eating plan for diabetes management.
OXIDATIVE STRESS AFTER A VEGAN OR MEAT-BASED MEAL: Oxidative stress is a form of cellular stress that leads to inflammation. When inflammation is chronic it leads to a number of different chronic diseases. Oxidative stress is a key mechanism at play in the development of insulin resistance and type 2 diabetes. The food we eat can either create oxidative stress or help prevent it. High glucose and cholesterol levels after a meal results in oxidative stress. In type 2 diabetes, it is thought that the formation of dicarbonyl compounds (molecules containing two carbonyl (C=O) groups), toxic reactive metabolites formed from glucose and lipids, may be involved in the development of vascular complications of diabetes. Dicarbonyls interact with proteins to form advanced glycation end products, which can damage the endothelium and impair vascular function.
This study, which is a secondary analysis of previously published data, tested the postprandial (aftermeal) response to a plant-based meal compared with an energy- and macronutrient-matched conventional meat-based meal on oxidative and dicarbonyl stress, inflammation and appetite hormones in type 2 diabetes and obesity.
This randomised crossover study group included 20 men with diabetes, 20 obese men with normal glucose tolerance, and 20 healthy men. All individuals consumed two energy- and macronutrient-matched test meals in random order. One meal was a vegan burger (V-meal) and the other was a meat burger (M-meal). The meals were consumed after an overnight fast and a 1 week washout period between the two different meals. Blood tests were taken before and after the meal to measure markers of oxidative stress, dicarbonyl stress, inflammation and appetite hormones.
The main finding of the study was that the V-meal was able to improve markers of oxidative stress and dicarbonyl stress, especially in the men with type 2 diabetes. That is, there was less of a rise in markers of oxidative stress after the V-meal compared to the M-meal. This suggests that a vegan meal may offer better protection against the generation of toxic metabolites involved in the development of complications of diabetes. This is likely to be due to the higher concentration of antioxidants compounds in plant-based foods. Dietary polyphenols for example can activate the transcription factor Nrf2, which is important in the defence against oxidative stress. The other main findings was that the V-meal resulted in higher secretion of leptin in men with type 2 diabetes, which may result in improved satiety and has been shown in previous studies of a vegan diet.
Overall, this study adds to our understanding of why a plant-based diet is beneficial for diabetes and other metabolic conditions. Of course, longer term studies with repeated V versus M-meals would be good to see, but in the meantime a plant-based diet is the best recommendation we can make for prevention and treatment of type 2 diabetes.
DAIRY AND PROSTATE CANCER: The link between dairy consumption and prostate cancer strengthens as more concordant studies are published. The World Cancer Research Fund guidelines avoid making recommendations on dairy consumption because of the decreased risk of colorectal cancer with higher intakes of dairy. Nonetheless, it fully acknowledges the data showing an association of higher dairy consumption with an increased risk of prostate cancer.
The current paper reports findings from the The Japan Collaborative Cohort study, which included 26,464 men (age range: 40–79 years) from 24 Japanese districts who completed a a questionnaire survey during 1988–1990. The survey collected data on the consumption of dairy, including milk, cheese, butter, and yogurt. Frequency was classified into 5 groups, “seldom,” “once or twice a month,” “once or twice a week,” “three or four times a week,” and “almost every day.”
During the 697,777 person-years of follow-up, there were 412 cases of prostate cancer. The risk of prostate cancer in men consuming the most milk (almost every day) was 37% higher compared to men consuming the least (seldom or once or twice a month) with a dose-response apparent. Yogurt cheese and butter consumption when comparing high and low consumers did not show a significant impact on prostate cancer risk although for butter consumption there was a significant dose–response relationship. However, after adjusting for both BMI and total energy intake the consumption of milk and yogurt was associated with an even more significant positive risk, 48% and 68% increase respectively, with a dose-response apparent. This suggests that being overweight may mask the negative impact of dairy consumption.
Most prior studies on dairy consumption and prostate cancer have been reported from Caucasian populations. Prostate cancer incidence used to be low in Japan but has recently increased and this appears to in line with an increase in dairy consumption. According to The National Health and Nutrition Survey in Japan, intake of dairy products increased from 103.5g/day in 1975 to 122.2g/day in 1988 and increased 125.1g/day in 2009. Prior studies on dairy consumption and prostate cancer in Japanese people have yielded conflicting results. However, the prospective nature of the current study and the dose-response finding lends weight to the current results. A number of mechanisms have been hypothesised with the most consistent theory being the rise in IGF-1 levels from dairy consumption which is a risk factor for prostate, and other cancers.
A recently published systematic review on milk consumption and prostate cancer concludes ‘The overwhelming majority of the studies included in this systematic review were suggestive of a link between milk consumption and increased risk of developing prostate cancer’. In addition, dairy consumption after a diagnosis of prostate cancer may increase the risk of death.
Read this excellent article by plant-based dietitian Lisa Simon on why we can and should make better choices for health than choosing dairy.
HEALTHY EATING AND AGEING: We know that nutrition has a huge influence on ageing. Ageing can be determined at a cellular level by looking at the expression of genes (epigenetics) and by measuring the length of telomeres (the caps at the end of the chromosomes which shorten with age). This study examined the impact of healthy diet patterns on DNA methylation, individual cytosine-phosphate-guanine (CpG) sites in candidate gene regions, using two different methods.
The study included 2684 women from the Sister Study. Food frequency questionnaires were scored using 4 dietary indices: the Dietary Approaches to Stop Hypertension (DASH) diet, the Healthy Eating Index (HEI) 2015, the Alternative Healthy Eating Index (aHEI) 2010, and the Alternative Mediterranean (aMed). All these diet indices share in common an emphasis on fruits, vegetables, whole grains, beans nuts and seeds, whilst limiting red and processed meat, processed foods and sugar-sweetened beverages. They including varying amounts of animal flesh (poultry and fish), dairy and olive oil.
The analysis is quite complex but in essence shows that all four healthy diet patterns were inversely associated with measures of biological ageing. However, the impact was not that large. The greatest benefit was seen in women who had lower levels of physical activity. As would be predicted, higher BMI and smoking adversely affected markers of ageing and associations with healthy diet were stronger in never smokers.
May be the reason for the very moderate results was that the diet in the participants was not plant-based enough. We know from the seminal studies by Dr Dean Ornish that a whole food plant-based diet can favourable alter gene expression in the setting of cancer and cardiovascular disease and can also result in lengthening of telomeres. These studies are muddied by the fact that participants adopted other healthy lifestyle factors as part of the overall Ornish programme, however, a plant-based diet is central to the disease reversal programme.
FIRST UK NUTRITION CURRICULUM FOR MEDICAL DOCTORS: This has been a long time coming so a massive congratulations to everyone involved. The UK now has a curriculum in nutrition designed to be incorporated within the core curriculum of undergraduate medical students. The document covers the key competencies around healthy diets for all stages of life, their role in disease and health, assessment of nutritional status, knowledge of managing undernutrition and overweight and the consequent metabolic syndrome. Dietary recommendations are based around then UK dietary guidelines. The goal is to embed nutrition into teaching by connecting it with relevant disease-based modules rather than teaching it separately.
Although a major step forward, I am disappointed that the impact of diet choice on the climate and biodiversity crises is not addressed and there is no mention about the co-benefits of a sustainable diet for personal and planetary health. A paper just published from the EPIC study cohort, which I will cover in more details next week, analysed the co-benefits of a sustainable diet for human and planetary health. Planetary impacts were assessed by considering diet-related greenhouse gas (GHG) emissions and land use. The results are obvious and clear. Diets heavy in meat and dairy and thus contributing more to GHG emissions and land use were shown to be associated with an increased risk of all-cause, coronary heart disease, cardiovascular disease and cancer death. Remarkably, diets that are worst for the planet also increase the risk of developing up to 14 of 20 specific cancers investigated. Overall, by choosing a climate-friendly diet such as the Eat-Lancet planetary health diet, up to 19–63% of all deaths and up to 10–40% of cancer could be prevented whilst at the same time reducing GHG emissions by 50% and land use by 62%.
In addition, the only mention of vegan and vegetarian diets in the curriculum are in relation to restrictive diets and deemed to pose ‘nutritional risks’. This is despite the fact that the BDA consider plant-based diets as nutritionally adequate and healthy for all stages of life. In addition, plant-based diets (be it vegan, vegetarian or whole food plant-based) are central to a number of evidence-based treatment guidelines including the 2019 American College of Cardiology guidelines for prevention of cardiovascular disease the American College of Lifestyle Medicine position statement of remission induction for type 2 diabetes.
Dr Hayley Tait, on behalf of PBHP UK, will be teaching in two medical schools in UK this year on climate friendly plant-based diets and how to incorporate them into personal and clinical practice. This provides some hope.