Should we reduce consumption of red meat?

Lars T. Fadnes; Erik Kristoffer Arnesen; Dagfinn Aune

doi:10.4045/tidsskr.19.0786

Perspectives

Should we reduce consumption of red meat?

Norwegian

Lars T. Fadnes, Erik Kristoffer Arnesen, Dagfinn Aune

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Lars T. Fadnes

Orcid

E-mail: lars.fadnes@cih.uib.no

Lars T. Fadnes, MD, PhD, professor at the Department for Global Health and Community Medicine, University of Bergen, and research group leader at Haukeland University Hospital. He is also a member of the National Council for Nutrition in Norway.

The author has completed the ICMJE form and declares no conflicts of interest.

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Erik Kristoffer Arnesen

Erik Kristoffer Arnesen has a master in public health nutrition and is a PhD candidate at the University of Oslo, Department for Nutrition. He is also a member of the National Council for Nutrition in Norway.

The author has completed the ICMJE form and declares no conflicts of interest.

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Dagfinn Aune

Dagfinn Aune, PhD, dietitian and researcher at the Department of Epidemiology and Biostatistics, Imperial College London and associate professor at Bjørknes University College.

The author has completed the ICMJE form and declares no conflicts of interest.

Article

Recently, a series of systematic reviews on red and processed meat was published. To what degree do these add new knowledge and the direction for how we regard the health effect of red meat?

The new systematic reviews on red and processed meat show that a reduction in consumption of processed meat of 21 g per day is associated with a relative risk reduction of 8 % for premature deaths (1). However, mean consumption in Norway is three times as high (approx. 70 g/day) (2), and a reduction that corresponds to the mean consumption is associated with a relative risk reduction of around 35 % for premature deaths (1, 3). For each 50 g per day reduction in unprocessed red meat intake, a 7 % risk reduction in premature mortality was found.

An optimal diet has been estimated to prevent 11 million deaths per year globally, or 255 million years of life lost (4). However, the field of nutrition research is a jungle of published material that can overwhelm clinicians and even researchers, not to mention journalists and consumers. In 2018 alone, more than 60 000 scientific articles on diet and nutrition were indexed in Medline/PubMed. Fortunately, meta-analyses have summarised research on how different food groups impact on the risk of various diseases and early mortality. The effects of fruits and vegetables, whole and refined grains, nuts, legumes, fish, eggs, dairy, red meat and sugar-sweetened beverages, have been summarised (1, 3, 5)-(7). However, sometimes meta-analyses fraught with substantial methodological weaknesses and problematic interpretations are published and these can have negative health consequences if translated into health policy. A recent series of systematic reviews on red and processed meat from the group Nutritional Recommendations Consortium (NutriRecs) is a good example of this, and ignited a debate on how nutrition research should be interpreted (1).

Many excluded studies

The meta-analyses from NutriRecs excluded several of the most recognised studies that have been included in earlier meta-analyses with the same outcomes (8). Especially in the analysis of unprocessed meat, many large-scale cohort studies have been erroneously excluded in the analyses of the effect on mortality, cardiovascular outcomes, colorectal cancers and type 2 diabetes, but also for other outcomes. Two examples are the European Prospective Investigation into Nutrition and Cancer and UK Biobank.

The average intake in Norway is therefore three times higher than the basis for comparison used in the new meta-analyses

Further, NutriRecs presented no forest plots, which are seen as essential for meta-analyses. This makes it time-consuming to find which studies are included in each analysis and makes the articles less transparent. They graded the quality of evidence using GRADE, and concluded that there was a low degree of evidence (1). Based on this, and because a small reduction in consumption of meat was considered only a small risk reduction, an 'expert panel' consisting of the authors made the surprising recommendation that most people do not need to reduce their current meat consumption. This contrasts with most nutritional guidelines from other organisations and countries (9).

Inappropriate use of GRADE

GRADE was developed to evaluate the quality of interventional studies, and all observational studies will initially be graded as low or very low degree of evidence (10). For studies on dietary patterns, activity and smoking, however, blinded randomised controlled trials are rarely possible, or even desirable. Does that mean that we cannot advise against high alcohol consumption, smoking, inactivity, or unhealthy diets?

When evaluating observational studies with GRADE, there are criteria for upgrading the strength of the evidence, such as dose-response effects and strong effect estimates. For nutritional studies an adapted version, NutriGrade, has been recommended, whereby evidence is upgraded for risk differences of 20 % or more (11). Many of the analyses by NutriRecs showed dose-response effects that were strongly significant and far beyond 20 % for moderately increased consumption (1). However, they did not upgrade the evidence, as the authors considered that red and processed meat may have been correlated with other dietary factors that could have confounded the relationship with morbidities and mortality.

The average Norwegian intake of processed meat compared with a diet without processed meat is associated with an increase in the relative risk of early mortality of about 35 %

The authors could nonetheless have investigated this by performing analyses stratified by whether studies adjusted for various dietary factors, but no such subgroup analyses were undertaken. Both measurement errors and changes in meat consumption during the follow-up period can affect the strength of observed associations (12, 13). For example, when using repeated measurements instead of only one baseline measurement and when also correcting for measurement errors, the relative risk for all-cause mortality per portion per day for red and processed meat increased from 1.05 to 1.11 to 1.25 in the Nurses' Health Study and from 1.08 to 1.14 to 1.83 in the Health Professionals Follow-up Study (and for type 2 diabetes from 1.10 to 1.14 to 1.44). To date, relatively few studies have used repeated measurements of diet and published risk estimates corrected for measurement errors. It is therefore likely that the effect estimates reported in most observational studies are conservative estimates of the true underlying effect.

The use of the GRADE criteria in the new meta-analyses has been criticised by several nutrition researchers. Other systems for classifying strength of evidence, which for example are used in the Nordic nutrition recommendations, also take into account biological plausibility, mechanisms and experimental data. When grading the evidence of the effect of red and processed meat on early mortality with the diet-adapted version NutriGrade or the evidence grading developed by the World Cancer Research Fund, which also incorporates experimental data in the evaluation of evidence (3, 14), the evidence is considered moderate to strong.

Most of the studies included in the analyses of red and processed meat and all-cause mortality had low risk of selection bias and scored high on most of the quality criteria by which they were evaluated (1). Studies which scored high on all quality criteria showed clearly that a 50 g per day increase in consumption of processed meat was associated with a 20 % increase in risk of all-cause mortality (12). The studies which the authors judged to have the highest quality came to opposite conclusions to those of the NutriRecs authors. Previous meta-analyses have found that high red and processed meat intake increases the risk of all-cause mortality (3), type 2 diabetes (13), colorectal cancer (15, 16), and cardiovascular disease (17).

Not adapted for the Norwegian context

The effects which are described in figures and tables of the new meta-analyses are partly consistent with previous meta-analyses, but the interpretation is different (1, 3). An important difference between NutriRecs' and previous meta-analyses is that previous meta-analyses presented results per 50 g of processed meat and per 100 g of unprocessed red meat per day (3, 13), while NutriRecs instead used 21 g per day (or 3 portions of 50 g per week), as a comparison for processed meat. It is not unexpected that this will show a weaker effect than if they had used the same frames of reference as used in previous meta-analyses.

But how do these amounts fit with the Norwegian diet? In the report 'Trends in the Norwegian diet 2018' by the Norwegian Directorate of Health (2), it is estimated that the intake of meat in Norway is 76 kg per person per year, of which red meat accounts for 51 kg and white meat for 21 kg (in addition to some by-products of meat). If additional data from the national nutrition survey Norkost 3 (2010–2011) is considered, one can assume that the intake of processed meat in Norway is around 70 g per day on average, while the intake of white meat is probably around 30–60 g/day and unprocessed red meat is around 50–70 g per day (18). This equates to around 10 servings of processed meat per week. The average intake in Norway is therefore three times higher than the basis for comparison used in the new meta-analyses. In addition, many people eat more than these averages. In addition, many people consume far more than the average amount.

The average Norwegian intake of processed meat compared with a diet without processed meat is associated with an increase in the relative risk of early mortality of about 35 % based on both the new and previous meta-analyses (1, 3). For an average Norwegian man aged 45 years, such an increased risk will be associated with a reduction in estimated life expectancy of around 3 years (19). In addition, the intake of unprocessed red meat will be associated with loss of one additional year of estimated life expectancy. This is unlikely to be trivial from a public health perspective. Even though NTB's press release on the 1 October 2019 claimed that red meat is 'not so dangerous after all', these new meta-analyses do not cast doubt on the importance of the current dietary recommendations to reduce the intake of red and processed meat (20).

Comments ( 2 )

Dette kommentarfeltet modereres, men kommentarer blir ikke redaksjonelt behandlet ut over å sikre at de følger retningslinjer for vårt kommentarfelt.

23.11.2020:

I Tidsskriftet nr. 10, 2020 har Lars T. Fadnes, Erik K. Arnesen og Dagfinn Aune skrevet kronikken «Bør vi spise mindre kjøtt?» (1). De imøtegår bruk av «Nutritional Recommendations guideline development process» (NutriRECS) (2) som ut fra fem utvalgte, systematiske oversiktsartikler rangeres sammenhengen mellom bevis og anbefalinger for bruk av rødt og bearbeidet kjøtt mot livsstilsrelaterte sykdommer og død. Ved å rangere sikkerheten i bevis for hvert utfall, er funnene ikke sterke nok for å anbefale moderasjon av kjøttforbruket. De fant imidlertid signifikant sammenheng mellom stort kjøttforbruk og alle typer livsstilssykdommer, men dette ble oppveiet av uønskede effekter på livskvalitet, matvaner, preferanser og motviljen til forandringer (2). Ved å endre på metodebruk finner Fadnes og medarbeidere at evidensgrunnlaget er godt nok for å anbefale redusert bruk av kjøtt og i henhold til nasjonale og internasjonale retningslinjer. Forskjellene blir derfor ikke store.

Hva er årsaken til denne usikkerheten? Det er vanskelig, kanskje umulig, å gjennomføre troverdige studier på effektene av ulikt inntak av makronæringsstoffer når studiepopulasjonen inkluderer et bredt spekter av livsstilsfaktorer. Observasjonsstudier kan heller ikke si hva som er årsak og virkning - om fisk i kosten gjør oss friske, eller om det bare er slik at friske folk spiser mer fisk. De fleste undersøkelser er gjort i USA og Europa og viser stort sett det samme, mens metaanalyser fra asiatiske land med andre kulturer viser en motsatt sammenheng (3).

Det foreligger kompliserte assosiasjoner mellom spiseatferd, kultur og andre atferdsmessige faktorer som påvirker helsen (4). Alle tilgjengelige kohorter har gitt forvirrende ernæringsmessige assosiasjoner. Vi vet for eksempel ikke i dag om isokaloriske kostregimer gir ulik overlevelse. Dette forskningsfeltet krever en radikal reform. For de fleste matvaregrupper er det høye nivåer av menneskelig statistisk heterogenitet.

Kjøtt vil være en naturlig og nødvendig del av kosten og i tråd med våre evolusjonære røtter, men høyt inntak er relatert til overspising bør derfor reduseres i henhold til nasjonale og internasjonale anbefalinger. Dagens forskning på makronæringsstoffer er ikke god nok for sikre anbefalinger.

Litteratur:

1. Fadnes LT, Arnesen EK, Aune D. Bør vi spise mindre kjøtt? Tidsskr Nor Legeforen 2020; 10: 990-992
2. Zeraatkar D, Han MA, Guyatt GH et al. Red and processed meat consumption and risk for all-cause mortality and cardiometabolic outcomes: A systematic review and meta-analysis of cohort studies. Ann Intern Med 2019; 171: 703–10.
3. Lee JE, McLerran DF, Rolland B et al. Meat intake and cause-specific mortality: a pooled analysis of Asian prospective cohort studies. Am J Clin Nutr 2013; 98: 1032–
4. Ioannidis JPA, The Challenge of Reforming Nutritional Epidemiologic Research. JAMA 2018; 320: 969-970

27.11.2020:

Takk for kommentar fra Tor-Erik Widerøe til vår kronikk [1]. Som Widerøe skriver, er det vanskelig å utelukke forvekslingsfaktorer og bias i observasjonsstudier. Heldigvis er det også gjort en rekke studier med eksperimentelt design, og disse støtter funnene vi oppsummerte. Randomiserte kontrollerte studier vil sjelden ha like lang oppfølgingstid som en del av observasjonsstudiene, men ved å måle såkalte intermediatorer/risikofaktorer som igjen kan forårsake sykdom, kan dette supplere observasjonsstudiene. Eksempel på slike er glykosylert hemoglobin (HbA1c), blodtrykk og LDL-kolesterol. En systematisk oversiktsartikkel fra 2018 over randomisert kontrollerte studier [2], viste at det var gjort 66 randomisert kontrollerte studier som så på ulike matvaregrupper på slike faktorer. Disse inkluderte LDL-, HDL- og total-kolesterol, triglysider, fastende glukose, HbA1c, og insulinresistensmålet HOMA-IR, systolisk og diastolisk blodtrykk og C-reaktivt protein. Både for mange av disse faktorene enkeltvis og når en summerte disse sammen, kom kjøtt og sukret drikke verst ut av alle matvaregruppene mens nøtter, belgvekster og fullkorn kom best ut.

Nylig ble det også publisert en liten randomisert kontrollert studie som så på effekten av å bytte kjøttprodukter ut med plantebaserte alternativer til kjøtt (SWAP-MEAT-studien) [3]. Studien var en overkrysningsstudie med 2x8 ukers oppfølging. Studien konkluderte med at plantebaserte alternativer bedret flere kardiovaskulære risikofaktorer inkludert markøren trimetylamin-N-oksid. Noe parallelt er det også gjort et kostbytte-eksperiment over 2+2 uker mellom et plantedominert sør-afrikansk kosthold og et animalsk dominert vestlig kosthold [4]. Også denne konkluderte med at det animalsk-dominerte kostholdet resulterte i begynnende endringer som er forbundet med tykktarmskreft. I tillegg er det gjort en rekke dyrestudier som viser biologiske mekanismer blant annet med dyreforsøk. Dette har World Cancer Research Fund & American Institute for Cancer Research tatt hensyn til når de vurderer evidensgraden mellom for eksempel rødt og bearbeidet kjøtt og tykktarmskreft som sterk [5]. Det er derfor grunn til å kunne formidle informasjon om risiko – ikke bare om tobakk og alkohol, men også om kostholdsmønstre.

Litteratur:
1. Fadnes LT, Arnesen EK, and Aune D. Should we reduce consumption of red meat? Tidsskr Nor Laegeforen, 2020; 140(10).
2. Schwingshackl L., et al., Food groups and intermediate disease markers: a systematic review and network meta-analysis of randomized trials. American Journal of Clinical Nutrition, 2018. 108(3): p. 576-586.
3. Crimarco A et al. A randomized crossover trial on the effect of plant-based compared with animal-based meat on trimethylamine-N-oxide and cardiovascular disease risk factors in generally healthy adults: Study With Appetizing Plantfood-Meat Eating Alternative Trial (SWAP-MEAT). Am J Clin Nutr, 2020.
4. O'Keefe SJ et al. Fat, fibre and cancer risk in African Americans and rural Africans. Nat Commun, 2015. 6: p. 6342.
5. World Cancer Research Fund/American Institute for Cancer Research (WCRF/AICR), Meat, fish and dairy products and the risk of cancer. 2018, WCRF/AICR.

This article was published more than 12 months ago and we have therefore closed it for new comments.

Published: 24 June 2020

Tidsskr Nor Legeforen 24 June 2020 Vol. 140.

doi:

10.4045/tidsskr.19.0786

Received 5.12.2019, first revision submitted 25.2.2020, accepted 2.3.2020.

Published: 24 June 2020

Tidsskr Nor Legeforen 2020 Vol. 140.

doi: 10.4045/tidsskr.19.0786

Received 5.12.2019, first revision submitted 25.2.2020, accepted 2.3.2020.

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Should we reduce consumption of red meat?

Many excluded studies

Inappropriate use of GRADE

Not adapted for the Norwegian context

Kjøttinntak og helse

Svar til Widerøe

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