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expert reaction to study on meat-eating, fish-eating and vegetarian diets and risk of heart disease and stroke

Research, published in The BMJ, reports that those who have a vegetarian or fish eating diet have lower risk of heart disease than meat eaters, however those with vegetarian diets reportedly have a higher risk of stroke than the other categories. 

 

Dr Malcolm Finlay, Consultant Cardiologist, Barts Heart Centre, Queen Mary University of London (QMUL), said:

“This paper is the result of a decades-long effort to investigate the effects of diet on health and wellbeing, really seeking to show whether there are health effects of sticking to a vegetarian, or meat-free diet. The excellent data they present show that people who follow a vegetarian diet, or just not-eating meat, were very significantly healthier than meat eaters. This stretches right across the board. Not only did vegetarians have less heart attacks and strokes, but also less high blood pressure, fewer medications, and fewer were diabetic. It is also interesting to note that vegetarian post-menopausal women had less HRT. People who ate fish were moreover slightly less healthy than vegetarians, but all-in-all pretty healthy too.

“Unfortunately, the claim that vegetarians have a higher risk of strokes isn’t really supported by the data, and on my reading, this is putting too much weight on a complex statistical method to try and correct for the fact that the vegetarians were very much healthier than meat eaters. This is almost certainly because their conclusions are based on adjusted hazard ratios: where the researchers basically put the question “what if the meat eaters didn’t have higher blood pressure” and use moderately complicated statistics to adjust the likelihood of stroke and heart attack and so on. This only works if there isn’t an interaction between the “baseline characteristics” – what the people were like when they enrolled into this observational study – and the thing that’s being investigated, in this case what they eat. But there is a clear and well-known effect on diet and blood pressure and diabetes, so correcting for these things and assuming they aren’t caused by diet might be inappropriate. So, while this method can say the risk of stroke isn’t as low as one might expect it to be in vegetarians considering how much healthier they are in general compared to meat-eaters, their overall risk of a major life-changing cardiovascular event happening still appears much lower.”

 

Prof Tom Sanders, Professor emeritus of Nutrition and Dietetics, King’s College London (KCL), said:

“This is a high quality observational study of middle aged British men and women. The lower incidence of ischaemic heart disease in the vegetarians and vegans compared with meat eaters is expected from their lower body mass index and lower blood cholesterol levels. However, the higher risk of stroke, which is more debilitating, is unexpected as plant based diets tend to have lower blood pressure (high blood pressure is the main risk factor for stroke). Observational studies in the Far East find very low cholesterol levels are associated with haemorrhagic stroke especially in the presence of hypertension. However, the blood cholesterol levels were not so low in this study so unlikely to be the reason for this observation. I don’t think the low cholesterol levels in the vegans should raise concern about increasing risk of stroke. The important thing is to identify hypertension when it occurs and effectively treat it. If vitamin B12 deficiency was the cause of the increased risk of stroke, one would expect stroke risk to be lower in the pescatarians. It may well be that people who follow alternative diets are less likely to take blood pressure lowering medication for hypertension and as a consequence suffer a stroke.”

 

Dr Stephen Burgess, Group Leader at the MRC Biostatistics Unit, University of Cambridge, said:

“Nutritional epidemiology is notoriously difficult, as it is hard to measure what people eat. As observational studies in nutritional epidemiology go, this is a reasonably high-quality one, with relatively detailed dietary information and a long follow-up period. Still though, this is an observational study, and so it tells us about differences between people who choose to be vegetarian compared with those who choose to eat meat, but it does not tell us what would happen if those who currently choose to eat meat instead decided to eat a vegetarian diet. That would require performing a randomized trial and intervening on people’s diet directly, which is not practical to do for large numbers of people, particularly for such a long time period.

 “That said, if taking up a vegetarian diet were beneficial for reducing risk of cardiovascular disease, we would expect cardiovascular risk to be lower in vegetarians, particularly as on average vegetarians tend to lead healthier lives than meat eaters. However, in this study, after accounting for alternative risk factors, while risk of heart disease was lower in vegetarians, risk of haemorrhagic stroke was higher in vegetarians. Heart disease is more common than haemorrhagic stroke, so vegetarians did have better cardiovascular health outcomes overall despite having higher risk of haemorrhagic stroke.

 “While the differences observed were small in magnitude, this study suggests that taking up a vegetarian diet may not be universally beneficial for all health outcomes. When considering cardiovascular health, switching to a vegetarian diet should not be seen as an end in itself, but should be considered alongside additional dietary and lifestyle changes. In isolation, the benefit of switching to a meat-free diet is not likely to be substantial.”

 

‘Risks of ischaemic heart disease and stroke in meat eaters, fish eaters, and vegetarians over 18 years of follow-up: results from the prospective EPIC-Oxford study’ by Tammy Y N Tong et al. was published in The BMJ at 23:30 UK time on Wednesday 4th September. 

DOI: 10.1136/bmj.l4897

 

Declared interests

Dr Malcolm Finlay :

Chief Medical Officer of Echopoint Ltd.

Founder and Director of Rhythm AI Ltd.

Founder and shareholder of Epicardio Ltd.

Prof Tom Sanders:  Tom was one of the reviewers of this paper. Honorary Nutritional Director of HEART UK.  Scientific Governor of the British Nutrition Foundation.  He is now emeritus but when he was doing research at King’s College London, the following applied: Tom does not hold any grants or have any consultancies with companies involved in the production or marketing of sugar-sweetened drinks.  In reference to previous funding to Tom’s institution: £4.5 million was donated to King’s College London by Tate & Lyle in 2006; this funding finished in 2011.  This money was given to the College and was in recognition of the discovery of the artificial sweetener sucralose by Prof Hough at the Queen Elizabeth College (QEC), which merged with King’s College London.  The Tate & Lyle grant paid for the Clinical Research Centre at St Thomas’ that is run by the Guy’s & St Thomas’ Trust, it was not used to fund research on sugar.  Tate & Lyle sold their sugar interests to American Sugar so the brand Tate & Lyle still exists but it is no longer linked to the company Tate & Lyle PLC, which gave the money to King’s College London in 2006.  Tom also used to work for Ajinomoto on aspartame about 8 years ago.  Tom was a member of the FAO/WHO Joint Expert Committee that recommended that trans fatty acids be removed from the human food chain.  Tom has previously acted as a member of the Global Dairy Platform Scientific Advisory Panel and Tom is a member of the Programme Advisory Committee of the Malaysian Palm Oil Board.  In the past Tom has acted as a consultant to Archer Daniel Midland Company and received honoraria for meetings sponsored by Unilever PLC.  Tom’s research on fats was funded by Public Health England/Food Standards Agency.

Dr Stephen Burgess:  No conflict of interest to declare.

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