December 2, 2020

expert reaction to study looking at red meat, plant foods and heart disease

A study published in the BMJ looks at red meat intake and risk of coronary heart disease among US men.

Aygul Dagbasi, Registered Dietitian and researcher within the Centre for Translational Nutrition and Food Research, Imperial College London, said:

“The paper named ‘Red meat intake and risk of coronary heart disease among US men: prospective cohort study’ is a powerful piece of work which included over 40,000 participants that were followed up for 30 years.  The number of participants included increases the statistical power of the study.  The dietary intakes of participants were collected by food frequency questionnaires every four years.  Although this is a validated method, it relies of participants’ memory and honesty, implementing a potential inaccuracy.  The authors however minimised this error by using a cumulative average of total intake over the 30 years.  Other components of diet and lifestyle including total energy (calorie) intake, body weight (body mass index), smoking, physical activity etc. have all been shown to affect CHD risk.  The authors took these into account and used a statistical model to adjust for these variables and also adjusted for an overall health score of the dietary intake.  The authors did not adjust for salt (sodium) intake.  Even after adjustment, total red meat, processed red meat and unprocessed red meat intakes were associated with an increased risk of CHD.  Another strength of the study is the comparisons it provided between different dietary protein sources such as substitution of plant protein sources for red meat which showed a significant reduction in the risk of CHD.  The substitution of fish for total red meat was not associated with a reduction in risk however the authors commented that this could be due to fish including fried and processed alternatives such as fish cakes.  Substitution of dark fish (including oily fish like salmon) was associated with a reduction in the CHD risk.

“The results of this study is in line with the findings of many other studies which is assuring and supports the causality between red meat intake and increased CHD risk.  However; due to the nature of the study being observational, it is not possible to fully determine a causal relationship.  It should also be noted that the study cohort only included white males who were all health care professionals (all belonging to one socio economic group).  This limits the generalisability of the findings to other ethnicities, females and socio economic groups.  The same group however, showed similar results in the Nurses’ Health Study which included females.  In general, this is a well-controlled, powerful study which support the reduction of red meat consumption and replacement of these with plant alternatives such as beans, legumes, nuts, soya.”

Dr Ian Johnson, Nutrition researcher and Emeritus Fellow, Quadram Institute Bioscience, said:

“As with much contemporary human nutrition research this is an observational study, rather than an experimental intervention, and so it cannot establish the precise causes of the effects observed.  However, it has been very well conducted, and it has made use of a large well-established American study-population, whose health and dietary habits have now been followed up for several decades.

“The fact that the diets of these subjects have been re-examined repeatedly over this period is a particular strength.  The other important point is that the complexity of human diets is taken into account.  Instead of simply measuring absolute meat intake, the researchers have looked at the types of protein foods that are being eaten instead of meat.

“The principal observation, that a diet with more plant proteins and whole-grain cereals and less red and processed meat is associated with a lower risk of coronary heart disease is consistent with recent dietary recommendations.  One other interesting observation is that having eggs and dairy products rather than meat has also been shown to be associated with a lower risk of heart disease, which suggests that the health benefits of reducing meat consumption can be obtained without the need to give up animal products entirely.”

Prof Tim Chico, Professor of Cardiovascular Medicine and Honorary Consultant Cardiologist, University of Sheffield, said:

“This study highlights the challenges in trying to work out exactly what diet is most healthy for the heart.  It involved over 43 thousand men who filled out a questionnaire on their diet every four years.  This is a common approach but it is very hard to accurately measure exactly what people eat and questionnaires such as this provide only a rough estimate.  However, since the same issues apply for all participants, this approach is able to detect differences between groups of people who indicate they have different diets, although the exact amount of each food type, and the effect of this on health, can only be an estimate.

“The study asks two questions; is red meat consumption associated with a higher risk of heart attack, and if so, what is the healthiest option to replace the meat with?

“In line with several other studies, people who consumed more red meat had around a 1.1 times higher risk of heart attack (about a 10% increase in relative risk).  This means that if someone’s risk of having a heart attack would normally be 10% in ten years, eating red meat once a day would increase it to 11%, if the relationship is causal.

“This study also suggests that if someone eats a portion of plant based protein (soy, nuts, legumes, etc), their risk may be reduced by 0.8 times; if their risk was 10% normally, this would be reduced to 8%, if the relationship is causal.

“The study findings are consistent with other evidence showing that people who eat red meat are somewhat more likely to have heart attacks, and in my view justifies a recommendation to reduce the amount of red meat we eat, and replace this with plant-based proteins especially given other benefits (such as on the environment) by such alterations.”

Prof Tom Sanders, Professor Emeritus of Nutrition and Dietetics, King’s College London, said:

“This is a carefully conducted observational study US health professionals by a group with great experience.  Total red meat (e.g. beef, pork, lamb and meat products made from them) was associated with a greater risk of getting coronary heart disease (CHD).  The finding was replicated for unprocessed and processed meat and remained significant after statistical adjustment for known risk factors (e.g. smoking habit, family history etc).  Further modelling was undertaken and showed that replacing red meat with dairy products (both high and low-fat varieties), nuts, legumes including soy, and plant-based proteins was associated with a significantly lower risk.  Interestingly, replacing red meat with poultry or fish (except for oily fish) was not significantly associated with lower risk.  Neither was their any clear effect of replacement with eggs.

“It is a modelling exercise looking at substitution of red meat.  It is not a trial.  Individuals did have multiple assessments of diet over the 30 year follow-up.  The caveat is that it was a modelling exercise: they look at what individuals are instead of red meat as well as other aspects of diet e.g. trans fatty acids.

“It is uncertain why red meat consumption was associated with increased risk of CHD.  The authors suggest it may be due to effects on LDL cholesterol.  However, high blood cholesterol was less prevalent in the men with the highest intake of red meat than in those with the lowest intake.  Furthermore, the effect of red meat on blood LDL cholesterol levels in controlled feeding trials is small and unlikely to explain the size of the risk reduction reported.  The authors, however, do suggest that the overall dietary pattern may be important i.e. high red meat intake is associated with lower intakes of other foods such as fruit and vegetables and whole grains.  There is growing evidence for another mechanism by which red meat may increase risk of CHD, especially when the intake of fibre is low; high red meat intake increases the production of trimethylamine oxide by the gut microflora which when absorbed causes damage to the lining of blood vessels.

“The main limitation of this study is that this was a health-conscious cohort of predominantly white men who were not obese.  The findings provide support for dietary advice to reducing red meat consumption and replacing it with plant protein sources as well as milk products.”

‘Red meat intake and risk of coronary heart disease among US men: prospective cohort study’ by Laila Al-Shaar et al. was published in the BMJ at 23:30 UK time on Wednesday 2 December 2020.

DOI: 10.1136/bmj.m4141

Declared interests

Aygul Dagbasi: “No COI to declare.”

Dr Ian Johnson: “No conflicts to declare.”

Prof Tim Chico: “No conflicts.”

Prof Tom Sanders: “Member Science Committee British Nutrition Foundation, Honorary Nutritional Director HEART UK.”