31. Dec, 2017

An update on meat consumption and chronic disease

Vegetarianism is on the increase in Australia. According to Roy Morgan, between 2012 and 2016, there was a massive 30% increase in NSW alone. The national average is now 11.2%. The state with the highest level is Tasmania at 12.7%, followed closely by NSW with 12.4%, and the lowest is Queensland with 9.2% - the only state with less than 10%.

Why are we seeing this decrease in meat consumption? I think consumers, and particularly young people, are concerned about their health, and as a nation I think we are far better informed than we have been in the past. There are two main chronic diseases associated with consumption of red meat (beef, pork, lamb, veal & mutton) - bowel cancer and cardiovascular disease (CVD).

 The increased risk of bowel cancer we see could be due to the displacement of fibre-containing foods by meat. We do tend to eat very large portions of meat in Australia, and there's not much room left on our plates for 'rabbit food' as I've heard male friends refer to vegies, so there is definitely merit in this theory. Adequate fibre is crucial for bowel health, and the average Aussie doesn't even come close to the recommended intake of 25-30g/day thanks to our love of hyper-processed food.

 Specific compounds currently implicated in bowel cancer include:

  1. N-nitroso compounds (NOCs)made by the haem iron contained in meat, and/or
  1. heterocyclic amines (HAA) & polycyclic aromatic hydrocarbons (PAHs) created when meat is cooked at high temps (eg BBQ, grilling, frying).

 Processed meat contains both nitrates and nitrites. The curing and smoking process can both lead to the formation of carcinogenic substances such as NOCs & PAHs.

The link with CVD has been shown causally in animal studies, but it would be unethical to induce the disease in humans, so observational studies have been conducted to show the association. They show an increased (double) CVD risk, in particular thrombotic events due to atherosclerosis - such as stroke or heart attack. This is the case even in those with low 'bad' blood cholesterol levels, normal blood pressure and even non-smokers. 

A compound recently implicated in this increased CVD risk is called trimethylamine-N-oxide (TMAO). Choline (found in liver, eggs, and red meat) and carnitine (found in red meat) are made into trimethylamine (TMA) by some of our gut microbiota (mostly Firmicutes, also Actinobacteria & Proteobacteria). This then travels to the liver, where it is converted to TMAO.  High levels of TMAO in the blood have been proposed as a marker for (ie it shows us, without invasive measures) impaired kidney function (the kidneys filter out excess TMAO) and atherosclerosis. 

The evidence of choline from eggs & TMAO production is currently conflicting, but that for red meat is compelling in individuals with specific microbiome compositions. As current faecal tests can only scratch the surface of our true microbiome composition, I'm happy to err on the side of caution to avoid atherosclerosis. This is even more important for those with a family history. 

So what can you do if you want to keep eating red meat? Well the good news is that studies show those eating a Mediterranean diet, where meat is a much smaller proportion of the plate and is often missing altogether, have very low levels of TMAO in their blood. This could be because they lack the gut bacteria required to create it's precursor (TMA), or it could be due to blockage of TMA production by a compound called 3,3-dimethyl-1-butonol (DMB) and/or by resveratrol.

DMB is found in some olive & grapeseed oils, balsamic vinegars and red wines. Resveratrol is a polyphenol found in berries, grapes and red wine. The Mediterranean diet also includes seafood though, and there is a paradox with marine fish, which contain very high amounts of dietary TMAO, actually being cardio-protective. This might have been due to the effects of Omega 3s in oily fish, but we also see it in white fish consumption, so it's unlikely.

It suggests that the TMAO pathway is important, and we must consider that gut dysbiosis (microbial imbalance) may ultimately be responsible for the increased CVD risk. This poses the question of whether red meat consumption is causing the dysbiosis. I look forward to what further studies tell us about this, but in the meantime I happily remain pescatarian (a seafood-eating vegetarian). 

The Australian Dietary Guidelines tell us that adult men need to decrease portion sizes to what fits into the palm of their hand (65g cooked weight), and replace some meat-based dishes with healthier protein sources such as poultry, seafood and legumes.

If you'd like help getting the balance in your diet right as a means of preventing chronic disease, you can make an appointment through the Individual Consults page for personalised advice.

References

Aykan, NF 2015, 'Red Meat and Colorectal Cancer', Oncology Reviews, vol. 9, no. 1, p. 288,  <https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4698595/>

Eat for Health 2017, The Five Food Groups, Lean Meat and poultry, fish, eggs, tofu, nuts and seeds and legumes/beans, <https://www.eatforhealth.gov.au/food-essentials/five-food-groups/lean-meat-and-poultry-fish-eggs-tofu-nuts-and-seeds-and>

Fennema, D, Phillips, IR & Shephard, EA 2016, 'Trimethylamine and Trimethylamine N-Oxide, a Flavin-Containing Monooxygenase 3 (FMO3)-Mediated Host-Microbiome Metabolic Axis Implicated in Health and Disease', Drug Metabolism and Disposition, vol. 44, no. 11, pp. 1839-1850, <https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5074467/>

Landfald, B, Valeur, J, Berstad, A & Raa, J 2017, 'Microbial trimethylamine-N-oxide as a disease marker: something fishy?', Microbial Ecology in Health and Disease, vol. 28, no. 1, p. 1327309, https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5444358/#CIT0008

NHMRC 2014, Nutrient Reference Values, Dietary Fibre, <https://www.nrv.gov.au/nutrients/dietary-fibre>

Roy Morgan 2016, The slow but steady rise of vegetarianism in Australia,

<http://www.roymorgan.com/findings/vegetarianisms-slow-but-steady-rise-in-australia-201608151105>

The Nutrition Source 2015, WHO report says eating processed meat is carcinogenic: Understanding the findings,  <https://www.hsph.harvard.edu/nutritionsource/2015/11/03/report-says-eating-processed-meat-is-carcinogenic-understanding-the-findings/>

Velasquez, MT, Ramezani, A, Manal, A & Raj, DS 2016, 'Trimethylamine N-Oxide: The Good, the Bad and the Unknown', Toxins (Basel), vol. 8, no. 11, <https://www.ncbi.nlm.nih.gov/pubmed/27834801>

World Cancer Research Fund International 2015, Red meat and bowel cancer risk – how strong is the evidence?, <http://www.wcrf.org/int/blog/articles/2015/10/red-meat-and-bowel-cancer-risk-how-strong-evidence