Research Commentary

Yan Zheng, Yanping Li, Ambika Satija, An Pan, Mercedes Sotos-Prieto, Eric Rimm, Walter C Willett, Frank B Hu

https://doi.org/10.1136/bmj.l2110

Abstract

Objective 

To evaluate the association of changes in red meat consumption with total and cause specific mortality in women and men.

Design 

Two prospective cohort studies with repeated measures of diet and lifestyle factors.

Setting 

Nurses’ Health Study and the Health Professionals Follow-up Study, United States.

Participants 

53 553 women and 27 916 men without cardiovascular disease or cancer at baseline.

Main outcome measure 

Death confirmed by state vital statistics records, the national death index, or reported by families and the postal system.

Results 

14 019 deaths occurred during 1.2 million person-years of follow-up. Increases in red meat consumption over eight years were associated with higher mortality risk in the subsequent eight years among women and men (both P for trend<0.05, P for heterogeneity=0.97). An increase in total red meat consumption of at least half a serving per day was associated with a 10% higher mortality risk (pooled hazard ratio 1.10, 95% confidence interval 1.04 to 1.17). For processed and unprocessed red meat consumption, an increase of at least half a serving per day was associated with a 13% higher mortality risk (1.13, 1.04 to 1.23) and a 9% higher mortality risk (1.09, 1.02 to 1.17), respectively. A decrease in consumption of processed or unprocessed red meat of at least half a serving per day was not associated with mortality risk. The association between increased red meat consumption and mortality risk was consistent across subgroups defined by age, physical activity, dietary quality, smoking status, or alcohol consumption.

Conclusion 

Increases in red meat consumption, especially processed meat, were associated with higher overall mortality rates.¹

Comment

The association shown between increased red meat intake of ½ serving per day and the risk of early death is actually quite low. A hazard ratio of 1.1 is equivalent to a 10% greater risk of death over the 8-year study period. This might seem compelling but is actually very low when we consider the 95% confidence intervals (1.04-1.17) and that small hazard ratios are inaccurate for showing causation because of the large array of other factors that could affect the outcome. For example, in this study attempts were made to adjust for baseline and change in factors such as smoking, alcohol use, diabetes, heart disease, cholesterol, total energy intake, consumption of food groups (sugar, fruit, vegetables, whole grains) and exercise. However, it is extremely difficult to effectively model for all of these and when the hazard ratio is low, the result is typically considered to be statistical noise.

This ‘noise’ is further exacerbated when lifestyle factors that could affect the outcome accumulate.

What factors could have affected this outcome?

In this cohort, the group that reduced red meat intake the most over the 8-years (and had the lowest risk of mortality) also reduced alcohol, and increased activity the most, with the smallest increase in weight. (Figure 1.)

The groups also had a linear association between energy intake (calories consumed per day) and red meat decrease/increase and, given that energy intake is one of the key factors for obesity, metabolic syndrome and future health risk (see Can You Be Healthy at Every Size) this is a major flaw in this study. (Figure 2.)

What does this all mean?

There are a number of factors to ‘unpick’ here. There are a large number of other lifestyle factors, as shown above, that might have affected the outcome. Perhaps more importantly, the lifestyle factors above suggest that those with baseline markers like high cholesterol were advised to reduce red meat…AND alcohol, and to exercise, and reduce portions.

This is consistent with what is shown above and that those with high cholesterol at baseline tended to reduce meat intake. This is consistent with the advice provided at the time (in the low-fat crazed 1980s from which this data was drawn). Those who did reduce meat intake also reduced those factors indicated above and this also suggests that they were more active in their health.

Notwithstanding that we would now say that a reduction in fat and animal products does not positively impact health, an increase in natural unprocessed foods, reduced red-meat containing convenience foods (such as hamburgers and pizza which are typically found with high prevalence in these types of cohorts), reduced alcohol, and better energy balance and reduced weight gain are all powerful impactors of health. Again, those who take positive health steps are typically more motivated, more active in their health, and achieve better results. This is a clear and undeniable fact that we have seen replicated thousands of times in the literature.

Take home message

The small percent increase in mortality from increased red meat consumption is not robust enough to warrant concern, especially when we see the range of other factors that have a higher likelihood of being responsible (in concert) for improving outcomes. Most importantly, it is highly likely that those who took control of their health by eating ‘better’ overall, regulated their energy intake, drank less alcohol, and were more active, achieved better results. So, eat natural foods, move, drink little alcohol (a little is fine!) and make sure that your diet allows both freedoms to live, AND an appropriate energy balance for you.

References

35.       Zheng Y, Li Y, Satija A, Pan A, Sotos-Prieto M, Rimm E, et al. Association of changes in red meat consumption with total and cause specific mortality among US women and men: two prospective cohort studies. BMJ. 2019;365:l2110.