A review of the week’s plant-based nutrition news 27th February 2022

This week I highlight the benefits of avoiding meat consumption for cancer prevention and include studies on gut health, brain health and a report that puts the final nail in the coffin for alcohol consumption.

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LOWER RISK OF CANCER IN THOSE AVOIDING MEAT CONSUMPTION: In case you needed anymore reasons to eliminate meat from the diet, this study will help. It included 472,377 adult participants of the UK Biobank study with a mean age in the 50’s at recruitment and followed for 11.4 years. Participants were categorised into regular meat-eaters (n = 247,571), low meat-eaters (n = 205,385), fish-eaters (n = 10,696), and vegetarians (n = 8685).Regular meat eaters consumed meat more than 5 times per week. Low meat eaters, less than 5 times per week. The vegetarian group included 466 vegans. Participants also had blood measurements of insulin-like growth factor 1 (IGF-1), testosterone and sex hormone binding globulin. During the follow-up period 54,961 cancers were diagnosed.

The data analysis showed that vegetarians and fish-eaters had a lower body mass index, were younger, more likely to be never smokers, have a university/college degree, and report consuming less alcohol at recruitment compared to regular meat-eaters

The results showed a step-wise reduction in cancer incidence as meat and fish were reduced/eliminate from the diet. Compared with regular meat eaters, vegetarians has a 14% reduction in the risk of all cancers and fish-eaters had a 10% lower risk. Low meat-eaters had an 11% reduction in the risk of colorectal cancer. There was no significant difference in risk of colorectal cancer for fish-eaters and vegetarians, potentially due to lack of power (not enough participants) as the estimates suggested lower risks in both these non-meat-eating diet groups.

Post menopausal women who were vegetarian had an 18% lower risk of breast cancer but this was likely due to being a lower body mass index. In men, not eating meat was associated with a 20% reduction in the risk of prostate cancer and being vegetarian reduced the risk by 31%.

Regarding blood biomarkers, there did not seem to be a strong interaction between diet, IGF-1 levels and cancer risk in this cohort. Prior analysis of this cohort has shown IGF-1 levels to be slightly lower in vegetarians. In the current study, vegetarian women did have slightly lower circulating IGF-I concentrations, which may have benefited their risk of breast cancer. However, for prostate cancer the analysis did not suggest that IGF-1 levels were mediating the lower risk in vegetarians. Of note, prior studies have documented lower IGF-1 level in vegans but given the low number of vegans in this studies the difference between diet groups was not that significant.

There are so many plausible reasons for these results, including reduced exposure to carcinogens in meat and increased consumption of anti-cancer compounds in plant foods. These data are in line results from the EPIC-Oxford and Adventist Health Studies that show vegetarians and vegans have a lower risk of cancer, with a greater benefit in vegans. The current study did not have enough vegan participants to draw any conclusions but a meta-analysis from 2017 demonstrated a 15% reduction in the risk of cancer in vegans.

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DIETARY MACRONUTRIENTS AND THE GUT MICROBIOME: This review summarises knowledge to date on the interaction between diet, gut microbial composition and health outcomes. Whether changes in the gut microbiome composition is causal or merely a bystander effect remains an open question. In addition, it appears that individuals differ in their response to dietary interventions and this difference may be determined by the baseline gut microbiome composition. This has led to the field of precision nutrition, whereby knowledge of the gut microbial composition is used to personalise dietary recommendations. This review summarises the literature on the impact of fibre and protein consumption on the composition of the gut microbiome, the production of metabolites and the impact on metabolic health. Gut-derived metabolites may be either beneficial or detrimental. The balance between carbohydrate and protein fermentation by the gut microbiota, as well as the site of fermentation in the colon, seems important determinants of host metabolism.

The review finds that fermentation of dietary fibre by bacteria in the large intestine produces microbial products such as short chain fatty acids (SCFAs — predominantly acetate, propionate and butyrate). SCFAs are formed primarily in the proximal colon, in which approximately 95% are subsequently absorbed. Butyrate is largely used as the major energy source by colonocytes, while propionate and acetate travel to the liver via the portal vein. In particular, acetate can also reach the peripheral tissues after entering the systemic circulation, inducing a diversity of metabolic and satiety-related effects. SCFAs are involved in centrally regulating food intake and energy expenditure by affecting the secretion of the hormones such as GLP-1 and PYY. SCFAs also promote fatty acid oxidation, insulin sensitivity, beta-cell function and maintain the health of the gut barrier.

In contrast, fermentation of protein (particularly animal protein) by bacteria in the distal colon produces gaseous products such as hydrogen, methane, carbon dioxide and hydrogen sulfide and also branched chain fatty acids (isobutyrate, 2-methylbutyrate and isovalerate) derived from fermentation of branched chain amino acids and phenolic and indolic compounds derived from fermentation of aromatic amino acids. In addition, choline and carnitine from meat is converted to trimethylamine (TMA) by gut bacteria.This TMA reaches the liver and is then coverted into trimethylamine N-oxide (TMAO), which is strongly associated with the development of cardiovascular diseases, non-alcoholic fatty liver disease and systemic inflammation. The authors state that ‘most products derived from protein fermentation are considered detrimental for gut and metabolic health’.

Although further research is required, the review finds that saturated fat consumption is associated with a decrease in gut microbial richness and diversity whereas polyphenol-rich foods promote both gut and metabolic health.

I was listening to Prof Tim Spector speak last weekend and he was asked what his top diet tips were for looking after gut health. He said the following: eat at least 30 different plants per week, include polyphenol-rich foods and drink (I do not endorse red wine as you will see below), eat a daily portion of fermented food and avoid ultraprocessed foods, including artificially sweetened beverages. This sounds to me like a healthy plant-based diet.

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A VEGAN DIET IS GOOD FOR GUT HEALTH: This paper specifically asked whether the health benefits associated with a healthy vegan diet are due to favourable changes in the gut microbiome. The cross-sectional study included healthy participants of normal body weight. 62 were vegan and 33 omnivores. Researchers assessed the participants glucose and lipid metabolism and compared their dietary intake, metabolic health, gut microbiome, and faecal, serum, and urine metabolites.

The results showed that vegans had a better lipid profile with significantly lower levels of total and LDL-cholesterol, lower levels of inflammation as evidenced by a lower CRP level. There were more favourable indicators of glucose regulation with a lower concentration of glycated haemoglobin and lower secretion of insulin during an oral glucose tolerance test in the vegans.

There was not much of a difference in the composition of the gut microbiota between vegans and omnivores, however, there were significant differences in the metabolites that these bacteria produced and thus detected in faeces, urine and blood. This was undoubtedly due to the fact that vegans were consuming significantly more fibre than omnivores (33g vs 18g per day) and less protein (69g vs 81g per day). The higher fibre intake in the vegans in this study resulted in higher levels of beneficial metabolites in blood and faeces, mainly SCFAs and their derivatives and lower levels of of potentially harmful metabolites (such as p-cresol, lithocholic acid, branched chain amino acids, aromatic compounds, etc.), mainly a reflection of protein fermentation.

The authors conclude ‘Our study clearly demonstrated that a vegan dietary pattern is associated with distinct metabolomic profiles compared to a meat-containing diet, even when comparing vegan vs. non-vegan individuals with otherwise similar characteristics’. The authors go on to suggest that a vegan diet may be a means of personalised manipulation of the microbiome to improve health outcomes. YES YES YES!

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THE GREEN-MED DIET AND BRAIN HEALTH: I have discussed the Green-Mediterranean (Med) diet several times over the last few months. Researchers from Israel have been trying to improve upon the standard Med diet by reducing the consumption of red meat and increasing the consumption of polyphenols by adding Mankai (duckweed) and green tea. Both versions of the Med diets include a daily portion of walnuts. Prior reports have shown that this Green-Med diet outperforms the standard Med diet for improving cardiovascular risk factors, hunger hormone profile and fatty liver.

The current paper reports further outcomes from the 18-month DIRECT-PLUS (Dietary Intervention Randomized Controlled Trial Polyphenols Unprocessed) study conducted in participants who work in a remote area of Israel in a nuclear research centre, are were overweight with dyslipidaemia. The 284 participants with a median age of 51 years were randomised to a healthy eating control group, a standard, calorie restricted MED diet group or the calorie-restricted Green-Med diet. The calorie restriction was 1500–1800 kcal/day for men, 1200–1400 kcal/day for women. All groups received instruction to undertake regular physical activity.

This part of the study investigated the impact of the intervention on the brain, specifically on brain atrophy (shrinking of the brain) as measured by brain MRI (hippocampal occupancy score and lateral-ventricle-volume and expansion scores). Remarkably, changes in the brain were identified before and after the 18-month intervention such that those consuming the standard Med and the Green-Med diet had less evidence of brain atrophy compared to the control good, with the Green-Med diet most protective for brain health. The components of diet that were most associated with this benefit were the Mankai and walnuts and the lower consumption of red/processed red meat. In addition, those participants that showed the most improvement in insulin sensitivity had less of a decline in the measured parameters of brain health. There was no association of APOE‐ε4 gene status (known risk factor for Alzheimer disease) with brain volumes at baseline or with trajectories of brain volumes after 18-months of the intervention.

This study supports several learnings already available from the literature. Diet and lifestyle interventions are effective at promoting and preserving brain health, managing cardiovascular risk factors are important for preventing/delaying the onset of dementia, polyphenols and nuts are beneficial in the diet whereas red and processed red meat are best avoided. A healthy plant-based diet meets all the criteria for a brain healthy diet.

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ALCOHOL HAS NO HEALTH BENEFITS: Finally an honest, evidence-based assessment of the impact of alcohol consumption on human health, specifically cardiovascular health. Over the last few decades, researchers, the alcohol industry and the media have supported the erroneous message that alcohol consumption, in moderation, is beneficial for heart health. Yet the reality is that during this same time period the prevalence of cardiovascular disease has actually doubled and alcohol has directly contributed to the increasing burden of these diseases (hypertensive heart disease, cardiomyopathy, atrial fibrillation and flutter, and stroke). In addition, alcohol consumption has contributed to deaths from infectious diseases, injuries, digestive diseases and cancer.

This report from the World Heart Federation summarises that in 2019, 2.4 million deaths were attributable to alcohol consumption. It states that contrary to popular belief, alcohol is not good for the heart. The report also highlights the fact alcohol exhibits socio-economic inequity. Individuals with low socio-economic status experience a disproportionately greater alcohol-associated harm than individuals with high socio-economic status from similar or lower amounts of alcohol consumption. ‘Recent evidence has found that no level of alcohol consumption is safe for health. Alcohol consumption negatively affects mental and physical health and is also linked with poorer quality of life and poverty. Even in smaller quantities, alcohol consumption can increase the risk of breast cancer. It can cause more severe motor and cognitive dysfunction in women at much lower levels of consumption than men’.

The report is a call to action for health professionals, health organisations, policy makers and governments to join forces to stop the immense harm caused by alcohol consumption. This starts with being clear about the fact that there is no safe level of consumption.

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