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expert reaction to study looking at red meat intake and risk of death

Research published in the BMJ shows that increasing red meat intake, particularly processed red meat, is associated with a heightened risk of death.

 

Catherine Collins, Registered Dietitian, Fellow of The British Dietetic Association, and NHS Dietitian, said:

“This paper pulls on data extracted from two ongoing studies on diet, lifestyle and health –  the Nurses’ Health Study (of 121,700 married, US registered female nurses aged 30-55 at enrolment in 1976) and the Health Professionals Follow-up Study (recruiting 51,529 US male health professionals aged 40-75 in 1986).  Both studies returned a baseline questionnaire detailing medical history, lifestyle, and usual diet.  Dietary intake was estimated from the return of a ‘food frequency’ questionnaire completed every two years.

“This study evaluated typical dietary intakes obtained in 1986 and compared them to data collected 8 years later, in 1994.  Estimated total meat intake, and relative changes in dietary intake, including red and processed meats over the 1986-1994 period were compared with mortality data over the subsequent 8 years (from 1994-2002) – although the tables in the published report provide data up to 2010, some 16 years after the 1994 baseline assessment – which adds confusion to the interpretation.

“From the original studies, the overall number of participants analysed in this paper reflected half the subjects initially recruited, representing data from 54% of the Health Professionals Follow-up study, but only 44% of subjects recruited to the original Nurses study in this dataset.  The average age in 1994 was between 59-61 years of age.

“What can be determined definitively from this study is that we change our diet as we age (whether that is by choice, health issues or interest in food trends is not measurable from this data), and that increasing age is significantly associated with risk of death.

 “What this population study also showed was a correlation between increasing intake of red and processed meats with increasing age and risk of mortality, that wasn’t attenuated by substituting more ‘heath halo’ foods like dairy or legumes.  Including fish, eggs, or nuts as a red meat alternative, consistent with the healthy eating message of the UK Eatwell guide did moderate the risk in the middle aged.” * 

“The paper shows that red and processed meats were a key part of daily diet in both men and women in 1986, but by 1994 there was around a 30% reduction in all red/processed meats by women, whereas men reduced meat intake more modestly.  Whether this reduction was due to women paying greater heed to public health messages on diet and nutrition is unknown, but the timing of the study covered the age range of typical menopause.

“Given that women of child-bearing age were (and are) often recommended to increase intake of iron rich foods (especially haem iron – that from red meat sources) to compensate for iron loss from menses, this reduction noted over the 8 years may have reflected menopausal status and a deliberate choice not to focus on iron-rich foods.  With women representing 66% of this 81,469 sample, this confounding variable may have an important role in determining iron status and its impact on health risks.  Too little iron (from all dietary sources, including red meat) is associated with anaemia, early fatigue and possible reduction in activity levels.  Low activity levels are independently associated with cardiovascular mortality.  Excessive dietary iron intake is associated with increased risk of bowel cancer, and a higher mortality in those unable to regulate iron absorption effectively.  The authors have not correlated blood sample data from these cohorts with iron status, which would have been useful to identify a biologically plausible rationale for their findings.

“Swapping a portion of red meat for chicken or dairy protein once a week seems beneficial for men, but less so for women – perhaps because they’d already lowered their red meat intake over time.  Wholegrains, legumes, eggs, fish and nuts are all recognised as healthy food choices, but swapping them for a portion of red or processed meat didn’t influence mortality risk, which isn’t surprising as red meat and all these food groups have different nutrient profiles and contribute in different ways towards a healthy balanced diet.

“In summary, this population study shows that it’s likely beneficial for us to reduce a generous red meat intake slightly as we get beyond middle age – especially if you’re a woman.  It also suggests, not definitively, that we probably shouldn’t increase our red meat intake above that which we typically ate in middle age.  Increasing meat intake with increased age seemed to increase mortality risk by some 10-13%.

“How relevant is this work to the current UK population?  It’s worth noting that dietary trends have continue to evolve over the last 15 years since that 1994 assessment.  Average red meat intake has fallen, according to the National Diet and Nutrition survey, both in terms of frequency and portion size.  However, the rise in popularity of LCHF (low carb, high fat) or ‘keto’ diets typically relying on high intakes of meat or eggs for protein and energy source as a means of losing and maintaining weight loss may not be conducive to longevity, if the findings in this study are accepted.”

* Quote edited at authors request at 12:36 12/06/2019

 

Prof Kevin McConway, Emeritus Professor of Applied Statistics, The Open University, said:

“This is a major new study that adds something useful to what was previously known about associations between the consumption of red meat and death rates.  But the size of the associations that were found is not huge.  The kind of data used in the study means that we can’t be sure whether it’s the red meat consumption that is causing differences in death risks, and in particular the findings dealing with reductions in red meat consumption are rather difficult to interpret.

“Previous studies have found an association between consumption of red meat, particularly of processed red meat like bacon and sausages, and the risk of death in a given period of time – though it’s impossible to be completely sure, from these studies, whether the meat consumption actually caused the increased risk.  But that doesn’t tell you what difference it would make to the risks if people change their consumption of meat.  The previous studies compared death rates between people who ate low and high amounts of meat.  But that doesn’t tell you whether, if someone changes from low to high meat consumption, or from high to low, their risk of disease and early death will change.  It could be that their risks are fixed by their previous eating habits and won’t change much.  But the new study was able to look at changes in meat consumption over time.  Looking at changes is important if we want to have rational public health advice on whether our diets should change.

“The new study found an association between changes in red and processed meat consumption over an eight-year period and the risk of death in the following eight years.  People who had increased their red meat consumption (processed or unprocessed) had a higher risk of dying in the following eight years.  People who had decreased their red meat consumption had a slightly lower risk of dying, but the decrease was not statistically significant – that is, it’s plausible that the decrease was due only to random variability between people.  A very important point to make about these findings is that, as the research report makes clear, it’s impossible to conclude that the changes in death rates are caused by the changes in meat consumption.  There are some important differences between the people who changed their meat consumption and the people who didn’t, and these other factors could be the real explanation of the differences in death rates.  It’s possible to make statistical adjustments to try to allow for such differences between people, and the researchers made a good job of this, but it’s impossible to allow for differences that weren’t recorded, and one can never be sure that the adjustments allowed for everything in an adequate way.  It’s also the case that the populations that were studied were rather limited – nurses and health professionals in the USA, mostly white, mostly reasonably well off, and it’s possible that different patterns of risk would be found in other countries or in other population groups in the USA.

“But, just to get a handle on the size of the associations that were found, let’s assume for now that the differences in risk are caused by the changes in meat consumption, and that the associations found in this study do apply in the UK too.  Those are big assumptions!  On average, the people in the study were about 60 years old at the start of the eight years over which their death risk was measured.  In the UK, of 100 women aged 60, about 5 will die in the following eight years, and of 100 men aged 60, about 8 will die.  If they had all increased their consumption of processed red meat by seven slices of bacon a week between ages 52 and 60, then about 1 more of the 100 women would die, and less than that (somewhere between 0 and 1) more men would die.  So in total somewhere between 1 and 2 extra deaths out of the 200 men and women.  That’s an increase, but it isn’t immense, and an extra 7 slices of bacon a week is quite a substantial change.  And this is what would happen if the US results apply here, and if the changes in death risk are entirely caused by the meat consumption – those are two big ifs.

“If it really is true that (from previous studies) eating more red meat increases the chances of disease and earlier death, and (from this study) increasing your red meat consumption increased your change of earlier death, what about reducing consumption?  A big issue with any study of what we eat is that people who eat less of one particular type of food will tend to eat more of some other types, and people who reduce how much they consume of one type of food will tend to increase consumption of something else.  That’s something else that makes it difficult to be sure what’s causing any differences in risk.  In this new study, the change in death risk over eight years for people who reduced their consumption of red meat was not statistically significant.  That could be because reducing red meat consumption really did have no effect on death risks.  Or maybe the effect of the reduction in red meat really was beneficial but it was lost in the statistical noise.  It could also be because the people who reduced red meat consumption also increased the consumption of some other food types, and that those increases were associated with higher risks, so that any lowering of risk from eating less meat was balanced by an increase from the other food types.  The researchers did make statistical adjustments to try to allow for this possibility, but it’s possible that they did not take all the right things into account.  So they did another statistical analysis, which looked at the associations between making more than one change in diet, at the same time, and the risk of death over 8 years.  That did produce some statistically significant associations between the diet change and death risk.  In statistical terms, reducing consumption of red meat and at the same time increasing consumption of something else, such as nuts or fish or eggs or whole grains, was associated with a lower risk of death over 8 years.  We have to be rather careful with the resulting figures – in addition to all the provisos about the difficulty of drawing conclusions and the untypical populations, these results come from a statistical model.  That is, they were not found by picking out the individuals who had really increased their consumption of, say, fish by a particular amount and at the same time reduced their consumption of red meat by a certain amount, and comparing them with the individuals who had not changed consumption of either fish or meat.  There would not have been enough people in the groups that they studied to pick out people who had changed consumption like that to make a good comparison.  So that’s yet another reason to be cautious about these particular results.

“But, with yet another big IF involved, let’s see how these results would apply in the UK.  For example, across men and women taken together the study found a reduction of 17% in death risk (over 8 years) for people who had reduced their total red meat consumption by 14 slices of bacon a week or 600g of unprocessed red meat (or some equivalent combination of the two) and increased their consumption of fish by somewhere round 600g a week.  That’s quite a change in consumption of both.  If 100 men and 100 women in the UK, aged 60, had all made that change in the previous eight years, that would reduce the total number of deaths in the following eight years from about 13 to about 11 (out of the 200).  That’s a slightly bigger change than the extra 1 or 2 deaths that were associated with increasing bacon consumption by 7 slices a week, but it’s associated with a considerably bigger change in diet, across two food types rather than one.  And don’t forget there are serious statistical concerns about whether the changes in risk are really caused by the diet changes, whether the US data apply here, and whether the statistical model used to combine the associations of a reduction in one type of food with an increase in another is really appropriate.”

 

Prof Louis Levy, Head of Nutrition Science, Public Health England, said:

“We should all limit the amount of red and processed meat we eat – following a healthy, balanced diet based on the Eatwell Guide is best for long-term health.”

 

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

“This is an important and powerful analysis of two large observational studies in US nurses and health professionals.  Its strength is that several measurement of diet were made allowing estimation of changes in risk from substituting red and processed meat with other foods.  They found a fall in all cause mortality from decreasing red and processed meat consumption but an increase in those who increased consumption.  Modelling exchanges suggest that replacing red and processed meat with lean chicken meat, fish, nuts, eggs, legumes, vegetables and wholegrain all reduced mortality rates.  The biggest benefits were seen when nuts and fish replaced meat.  This could be owing to the healthier fats provided by these foods.

“This report comes hot on the heels of another big study1 of 409,885 man and women in nine European countries that found red and processed meat consumption was associated with a 19% increased risk of ischemic heart disease, which is a leading cause of premature mortality.  That study also found milk, fish, eggs and poultry were not associated with risk.  These findings taken together challenge the popular myth that high protein diets containing lots of red meat are good for health, but support current dietary guidelines that advocate a shift to a Mediterranean style diet which contains plenty of vegetables, nuts, wholegrain, some fish, poultry and milk but very small amounts of red and processed meat.”

1 Key TJA et al. Consumption of Meat, Fish, Dairy Products, Eggs and Risk of Ischemic Heart Disease: A Prospective Study of 7198 Incident Cases Among 409,885 Participants in the Pan-European EPIC Cohort. Circulation

 

Dr Gunter Kuhnle, Associate Professor in Nutrition and Health, University of Reading, said:

“This is a well conducted observational study – but with results that are difficult to interpret.  While it is known that high red and processed meat consumption can increase the risk of a number of diseases, including heart disease and cancer, there are however not many data showing how changes in meat consumption can affect health.  Overall, the authors found a modest increase in all-cause mortality in participants who increased their meat intake, with a stronger effect observed for processed meat.  A reduction in meat intake was only found to be modestly beneficial when the meat was replaced with certain foods – in particular nuts, fish, whole grains and vegetables (without legumes).

“The study is mainly of interest for long-term dietary recommendations, as it shows that on a population scale, even small increases in meat intake were associated with increased mortality, although only slightly – and that replacing meat with nuts, fish, whole grains or vegetables was associated with reduced mortality.  However, the study has been conducted in a US population, where dietary habits are different from the UK and therefore more data are needed to investigate whether the results can be easily translated into a UK population.

“Meat – processed and unprocessed – is an important source of essential nutrients such as iron and Vitamin B12, but high intake has also been linked to increased risk of heart disease and cancer.  This highlights the difficulties of nutritional research and the development of dietary recommendations: many foods are ‘healthy’ when consumed in the right amounts and it is excessive (or insufficient) consumption that has an adverse effect on health.”

 

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

“Although this is an observational investigation, these results come from a large, well established cohort study and the results are consistent with current public health advice.  The important new point is that adults seem to be able to significantly improve their chances of a longer healthier life by adjusting their diets toward what can be broadly described as a more ‘Mediterranean’ pattern.”

 

‘Association of changes in red meat consumption with total and cause specific mortality among US women and men: two prospective cohort studies’ by Yan Zheng et al. was published in the BMJ at 23:30 UK time on Wednesday 12 June 2019.

 

Declared interests

Catherine Collins: “No conflict of interest.”

Prof Kevin McConway: “Prof McConway is a member of the SMC Advisory Committee, but his quote above is in his capacity as a professional statistician.”

Prof Tom Sanders: “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 Gunter Kuhnle: “None.”

Dr Ian Johnson: “No interests.”

None others received.

 

 

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