Studies examining the health risks of consuming red and processed meats have yielded conflicting results, yet current US dietary guidelines recommend limiting the intake of these foods to one serving per week.
A new meta-analysis supports that recommendation, suggesting that greater consumption of processed meat, unprocessed red meat, and, unexpectedly, poultry — but not fish — is significantly associated with a small increased risk of incident cardiovascular disease (CVD), which included cardiovascular deaths.
Moreover, the large six-cohort study finds that a higher intake of processed meat and unprocessed red meat, but not poultry or fish, significantly correlates with a small increase in the risk of all-cause mortality.
The analysis, published online today in JAMA Internal Medicine, reports increased relative risks for these associations ranging from approximately 3% to 7% and increased absolute risks of less than 2% during a follow-up period that lasted up to 30 years.
"The findings of this study appear to have critical public health implications given that dietary behaviors are modifiable and most people consume these four food types [processed or unprocessed red meat, poultry, and fish] on a daily or weekly basis," Victor W. Zhong, PhD, an assistant professor in the Division of Nutritional Sciences at Cornell University in Ithaca, New York, and colleagues write.
Larger Sample, Longer Follow-Up Than Most Published Studies.
This study comes several months after the Annals of Internal Medicine published a controversial guideline stating that eating red and processed meat at current US levels carries little risk.
In the meantime, an earlier study in BMJ had reported that consuming more red meat, especially the processed type, may increase mortality risk over 8 years.
Against this backdrop of disparity, Zhong told Medscape Medical News, "Our study was designed to provide high-quality observational data to help address inconsistent recommendations and controversies around animal protein foods — in particular, unprocessed red meat and processed meat. It is also based on a larger and more diverse sample and longer follow-up than most published studies.
One unexpected finding, Zhong pointed out, is that poultry intake was positively associated with CVD.
"Literature evidence suggests an inverse association or no association with poultry. The positive association found in this study may be related to the method of cooking — for example, frying chicken and consumption of the skin," he suggested.
Offering his perspective on the analysis to Medscape Medical News, Qi Sun, MD, MMS, ScD, an associate professor in the Department of Nutrition at Harvard T.H, Chan School of Public Health in Boston, Massachusetts, called it timely, especially after the publication of the aforementioned Annals guideline essentially placing its approval on current levels of red and processed meat consumption.
"There is already abundant evidence that these foods increase the risk of cardiovascular disease, obesity, diabetes, and certain cancers," said Sun, who was not involved with the current research.
Mercedes R. Carnethon, PhD, a coauthor of the study and vice-chair of the Department of Preventive Medicine at Northwestern University Feinberg School of Medicine in Chicago, Illinois, hopes these large-sample findings will offer a broad perspective on the impact of these dietary patterns and enable future studies to test whether changing these elements can influence health outcomes.
However, according to Sun, the best path forward is not to pursue more trials but to use modern techniques such as metabolomics and microbiome analysis to understand why these meats promote disease and mortality.
"It’s just not feasible to randomize 5000 people for long periods to red meat or no red meat," he said.
Individual Advice vs Public Health Message?
Zhong and colleagues do acknowledge that the studies included in their new analysis are heterogeneous, differing in highest and lowest intake categories, covariate adjustments, outcome definitions, and dietary assessments.
"Nevertheless, the robust, significant positive association between processed meat intake and health outcomes was retained despite the challenges introduced by these heterogeneities," they note.
Meanwhile, a British statistician offered this perspective on the seemingly conflicting recommendations.
"I think that a major reason for the differing conclusions is that the 2019 Annals researchers specifically stated that, 'The target audience for our guidance statement was individuals who consume unprocessed red meat or processed meat as part of their diet.' The [Annals] panel took the perspective of individual decision making rather than a public health perspective," explained Kevin McConway, PhD, The Open University, United Kingdom, in a statement published by the UK Science Media Centre.
"In other words, they took the line that the risk differences were small, and that many individuals who eat meat would consider that such small differences in disease risk are unimportant when looked at beside the enjoyment they get from what they eat or the awkwardness of changing eating habits.
"The researchers on the new study are taking a public health perspective; they note that people can choose to eat less meat, and if they do so and the relationship between meat eating and disease risk is indeed causal, then fewer people would have heart attacks and strokes, and on average people would live a bit longer.
"Individual trade-offs between small risk increases and personal preferences don’t really come into that view — but it is individuals who decide how much meat they are going to eat, if any. And of course, none of this takes into account animal welfare, or environmental effects of livestock raising," McConway observed.
Small but Significant Associations With CVD, All-Cause Mortality
Overall, Zhong and colleagues observed the following small but significant associations with incident CVD events (a composite endpoint of coronary heart disease, stroke, heart failure and CVD deaths):
• According to the most fully adjusted model comparing two vs zero servings per week, processed meat intake had a hazard ratio (HR) of 1.07 (95% confidence interval [CI], 1.04 - 1.11) and a 30-year absolute risk difference (ARD) of 1.74% (95% CI, 0.85% - 2.63%) for incident CVD.
• Consumption of each additional two servings of unprocessed red meat per week had an HR for incident CVD of 1.03 (95% CI, 1.01 - 1.06) and a 30-year absolute risk difference of 0.62% (95% CI, 0.07% - 1.16%).
• Intake of each additional two servings of poultry per week was associated with an HR of 1.04 (95% CI, 1.01 - 1.06); 30-year absolute risk difference 1.03% (95% CI, 0.36% - 1.70%).
• Each additional two servings of fish, however, was not significantly associated with incident CVD: HR, 1.00 (95% CI 0.98 - 1.02) and 30-year absolute risk difference 0.12% (95% CI, –0.40% to 0.65%).
In terms of all-cause mortality, the following associations were observed:
• Based on the most fully adjusted model, the consumption of each additional two servings of processed meat per week was statistically significantly associated with all-cause mortality: HR, 1.03 (95% CI, 1.02 - 1.05); 30-year absolute risk difference 0.90% (95% CI, 0.43% - 1.38%).
• Each additional two servings of unprocessed red meat consumed per week correlated significantly with all-cause mortality: HR, 1.03 (95% CI, 1.01 - 1.05); 30-year absolute risk difference 0.76% (95% CI, 0.19% - 1.33%).
•Each additional two servings of poultry per week did not significantly correlate with all-cause mortality: HR, 0.99 (95% CI, 0.97 - 1.02); 30-year absolute risk difference –0.28% (95% CI, –1.00% to 0.44%).
• Nor was there an association with every additional two servings of fish per week: HR 0.99 (95% CI, 0.97 - 1.01); 30-year absolute risk difference –0.34% (95% CI, –0.88% to 0.20%).
Analysis Pools Six Large Cohorts With Average of 19 Years Follow-up
The six cohorts, pooling results from 29,682 US adults followed for a mean of 19 years, were drawn from the Lifetime Risk Pooling Project, comprising the Atherosclerosis Risk in Communities (ARIC) study, the Coronary Artery Risk Development in Young Adults (CARDIA) study, the Cardiovascular Health Study (CHS), the Framingham Heart Study (FHS), the Framingham Offspring Study (FOS), and the Multi-Ethnic Study of Atherosclerosis (MESA).
Free of CVD at baseline, participants had a mean age of 53.7 years; 44.4% were male, and 30.7% identified as non-white. With dietary data collected at baseline during the period 1985 to 2002 and a median follow-up of 19 years, outcomes were analyzed in 2019.
The median number of servings per week was 1.5 for processed meat, three for unprocessed red meat, two for poultry, and 1.6 for fish. One serving was equivalent to 4 oz of unprocessed red meat or poultry or 3 oz of fish. A serving of processed meat equated to two slices of bacon, two small links of sausage, or one hot dog.
Participants with a higher total intake of these items tended to be younger, male, non-Hispanic black, and current smokers. They were more likely to have diabetes, a higher body mass index (BMI), lower high-density lipoprotein (HDL) cholesterol levels, and higher energy and alcohol intakes. Furthermore, the quality of their diets was lower and they were less likely to use lipid-lowering drugs and hormone therapy.
During follow-up, there were 6963 incident CVD events, 2366 heart failure events, 170 other CVD deaths, and 8875 all-cause deaths.
Unexpected: Poultry Was Associated With CVD; Is Cooking Method Key?
Zhong said it is difficult to tease out whether the method of cooking poultry played any role in outcomes since food preparation methods were not consistently and universally assessed across the six cohorts; therefore, it was not possible to separate the effect of fried chicken and skin-on chicken from total poultry intake.
He added that the new findings support current dietary guidelines recommending low or no intake of processed meat and unprocessed red meat.
"People can choose other sources of protein, including fish, seafood, and plant-based sources such as nuts and legumes, including beans and peas," he advised.
Another surprise finding, according to Carnethon, is the counterintuitive observation that the influence of processed meats appeared strongest in those with higher-quality dietary patterns.
"Our hypothesis is that if these processed meats do convey added risks, having an overall healthy diet cannot mitigate those risks," she said.
However, she cautioned, "This is a subgroup finding that warrants further exploration and replication in other studies."
Also meriting further investigation, added Zhong, is the impact of food preparation methods and the biological mechanisms underlying the associations between animal protein intake and chronic diseases.
And, he concluded, "Whether replacing animal protein with plant protein sources is differentially associated with cardiovascular disease and premature death warrants further investigations."
This study was funded in part by a postdoctoral fellowship to Zhong from the American Heart Association Strategically Focused Research Networks. The Lifetime Risk Pooling Project was funded by National Institutes of Health (NIH) and the National Heart, Lung, and Blood Institute (NHLBI) and by the Northwestern University Feinberg School of Medicine.
Coauthor Philip Greenland reports grants from the NIH and the American Heart Association during the conduct of the study. Coauthor John Wilkins reports grants from the NHLBI and a consulting fee from NGM Biopharmaceuticals outside the submitted work. Coauthor Donald Lloyd-Jones reports grants from the NIH. Sun has disclosed no relevant financial relationships.
JAMA Intern Med. Published online February 3, 2020. Abstract
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Cite this: Red Meat, Poultry Linked to Slightly Higher Risk for CVD Events, CV Death - Medscape - Feb 03, 2020.