Let them eat steak: Hold the shame, red meat is not bad for you or climate change
Plant-based meat may enjoy the perception of being healthier than real meat, but it has more sodium and calories and can cause weight gain.
Will Coggin
Opinion contributor
Imagine ordering dinner at your favorite restaurant. You know what you want without hesitation: a perfectly marbled 8-ounce steak cooked medium rare. Just before you order, your date tells you they’ve read that cows cause climate change and that meat might be unhealthy. Suddenly, the Caesar salad seems like a better option.
We’ve all been steak-shamed before. Ever since Sen. George McGovern’s 1977 Dietary Goals report declared red meat a health villain, Americans have been chided out of eating red meat. According to the U.S. Department of Agriculture, red meat consumption has fallen more than 24% since 1976. During that time, study after study has attempted to tie red meat to a laundry list of health problems.
Until now.
So many studies, so many flaws
Three studies published recently in the Annals of Internal Medicine did something too few papers do: Ask whether the previous studies had any meat on their bones. The researchers who wrote the report analyzed 61 past studies consisting of over 4 million participants to see whether red meat affected the risk of developing heart disease and cancer.
All three came to the same conclusion: Decreasing red meat consumption had little to no effect on reducing risk of heart disease, cancer or stroke.
How can so many studies be wrong?
Nutritional research often relies on survey-based observational studies. These track groups of people and the food they eat, or try to tie a person’s past eating habits to a person’s current state of health. The result is something akin to a crime chart from a mob movie with a random red string connecting random suspects trying to figure out “who dunnit.”Observational studies rely on participants to recall past meals, sometimes as far back as a month. Even when eating habits are tracked in real time using food diaries, issues arise. Research has shown that participants don’t give honest answers and often pad food diaries with typically “good” foods like vegetables while leaving out things like meat, sweets and alcohol. There’s also the matter of having to accurately report portion sizes and knowing the ingredients of the food eaten in restaurants.
Some companies like Impossible Foods and Beyond Meat have tried to cash in on the misconception about meat’s healthfulness. According to the market research firm Mintel, 46% of Americans believe that plant-based meat is better for you than real meat. Ironically, the anti-meat messages could be leading people to less healthful options.
Science on your side: Don't let vegetarian environmentalists shame you on meat
Plant-based meat might enjoy the perception of being healthier, but that perception is far from reality. A lean beef burger has an average of nearly 20% fewer calories and 80% less sodium than the two most popularfake-meat burgers, the Impossible Burger and the Beyond Burger.
Fake meat is also an “ultra-processed” food, filled with unpronounceable ingredients. The National Institutes of Health released a study in May finding that ultra-processed foods cause weight gain. Unlike observational studies, this research was a controlled, randomized study.
Earth will survive your meat-eating
It’s not just the flawed health claims about red meat that deserve a second look. In recent years, we’ve been told reducing meat consumption is essential to saving the planet. But despite what critics say, even if everyone in America went vegan overnight, total greenhouse gas emissions (GHG) in the United States would only be reduced 2.6%.
Eat better meat:Don't go vegan to save the planet. You can help by being a better meat-eater.
Since the early 1960s, America has shrank GHG emissions from livestock by 11.3% while doubling the production of animal farming. Meat production is a relatively minor contributor to our overall GHG levels. In other countries, it may have a higher impact. The solution is not lecturing everyone else to go meat-free. Sharing our advancements would prove to be a more likely and efficient way to reduce emissions than cutting out meat or replacing it with an ultra-processed analogue.
Those who enjoy a good steak now have a good retort the next time they’re criticized for their choice: Don’t have a cow.
Will Coggin is the managing director at the Center for Consumer Freedom, which is funded by restaurants, food companies and other interests.
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REVIEWS |1 OCTOBER 2019
Red and Processed Meat Consumption and Risk for All-Cause Mortality and Cardiometabolic Outcomes: A Systematic Review and Meta-analysis of Cohort Studies
Abstract
Dietary guidelines generally recommend limiting intake of red and processed meat. However, the quality of evidence implicating red and processed meat in adverse health outcomes remains unclear.
To evaluate the association between red and processed meat consumption and all-cause mortality, cardiometabolic outcomes, quality of life, and satisfaction with diet among adults.
EMBASE (Elsevier), Cochrane Central Register of Controlled Trials (Wiley), Web of Science (Clarivate Analytics), CINAHL (EBSCO), and ProQuest from inception until July 2018 and MEDLINE from inception until April 2019, without language restrictions, as well as bibliographies of relevant articles.
Cohort studies with at least 1000 participants that reported an association between unprocessed red or processed meat intake and outcomes of interest.
Teams of 2 reviewers independently extracted data and assessed risk of bias. One investigator assessed certainty of evidence, and the senior investigator confirmed the assessments.
Of 61 articles reporting on 55 cohorts with more than 4 million participants, none addressed quality of life or satisfaction with diet. Low-certainty evidence was found that a reduction in unprocessed red meat intake of 3 servings per week is associated with a very small reduction in risk for cardiovascular mortality, stroke, myocardial infarction (MI), and type 2 diabetes. Likewise, low-certainty evidence was found that a reduction in processed meat intake of 3 servings per week is associated with a very small decrease in risk for all-cause mortality, cardiovascular mortality, stroke, MI, and type 2 diabetes.
Inadequate adjustment for known confounders, residual confounding due to observational design, and recall bias associated with dietary measurement.
The magnitude of association between red and processed meat consumption and all-cause mortality and adverse cardiometabolic outcomes is very small, and the evidence is of low certainty.
None. (PROSPERO: CRD42017074074)
Growing evidence shows an increased risk for cardiometabolic disease associated with the consumption of red and processed meat. Although previous systematic reviews reported positive associations between red meat intake and all-cause mortality (1), cardiovascular mortality (2), and stroke (3) and between processed meat consumption and all-cause mortality (1, 4), cardiovascular mortality (2), stroke (3), coronary heart disease (5), and type 2 diabetes (5), results have not been consistent. One review did not find an association between unprocessed red meat and all-cause mortality (4), and another found no association with cardiovascular disease (5). Although Aune and colleagues (6) reported a relationship between red meat intake and type 2 diabetes, Micha and colleagues (5) did not detect this association in a review published 1 year later.
Methodological limitations in previous reviews included failure to address risk of bias of primary studies (for example, references 3 and 6), lack of evaluation of certainty of evidence (for example, references 2 to 6), and failure to consider the magnitude of observed effect (for example, references 2 to 6). These limitations may have affected the credibility of recommendations issued by governments and authoritative organizations regarding red and processed meats.
As part of NutriRECS (Nutritional Recommendations and accessible Evidence summaries Composed of Systematic reviews), a new initiative to establish trustworthy dietary recommendations that meet internationally accepted standards for guideline development, we developed guidelines addressing red and processed meat consumption (7). To inform these recommendations, we conducted 5 systematic reviews of the evidence (8–11). Here, we present results from a systematic review of cohort studies addressing the association between red and processed meat consumption and all-cause mortality, cardiometabolic outcomes, quality of life, and satisfaction with diet among adults.
Methods
We registered a protocol for this review at PROSPERO (CRD42017074074) in August 2017.
Data Sources and Search Strategy
An experienced research librarian developed the search strategy, which was used across all supporting reviews except the one addressing public values and preferences (Supplement 1). We searched MEDLINE, EMBASE (Elsevier), Cochrane Central Register of Controlled Trials (Wiley), Web of Science (Clarivate Analytics), CINAHL (EBSCO), and ProQuest from inception. We also reviewed reference lists of relevant systematic reviews. The final search of all databases included references up to July 2018, except for the MEDLINE search, which included references up to April 2019.
Study Selection
We included cohort studies in any language that enrolled at least 1000 adults, compared participants consuming different amounts of unprocessed red meat or processed meat, and reported on 1 or more of our outcomes of interest. Red meat and processed meat were defined, respectively, as mammalian meat and white or red meat preserved by smoking, curing, salting, or adding chemical compounds (for example, hot dogs, charcuterie, sausage, ham, and deli meats) (12). We also included studies comparing vegetarians with nonvegetarians for sensitivity analyses. Our outcomes of interest were determined in consultation with our guideline panel—which comprised members of the public, clinicians, epidemiologists, and methodologists—and include all-cause mortality, cardiovascular mortality (or fatal coronary heart disease or fatal myocardial infarction [MI]), cardiovascular disease (or coronary heart disease), stroke, MI, type 2 diabetes, anemia, quality of life, and satisfaction with diet. For studies reporting on ischemic and hemorrhagic stroke separately, we included results only for ischemic stroke in our meta-analyses (13).
Cohorts in which more than 20% of the sample was younger than 18 years, had a noncardiometabolic disease (such as cancer), or was pregnant at baseline were excluded. We also excluded studies in which diet was assessed before adulthood, participants were asked to recall their diet before adulthood, or dietary assessments were completed by proxies, as well as studies that reported on specific components of red meat (such as iron or fat) or specific types of red meat (such as lamb). However, we did include studies reporting on beef–pork combinations because beef and pork account for most red meat intake in most Western populations (14, 15). If we encountered more than 1 eligible article on the same exposure and cohort and addressing the same outcome, we included results only from the study with the longest follow-up. If the follow-up was the same, we chose the study with the most participants.
Pairs of reviewers completed calibration exercises, after which they performed screening independently and in duplicate, with disagreements resolved by discussion or through third-party adjudication by an expert research methodologist. Screening was done in 2 stages: First, the reviewers assessed titles and abstracts; then, for those deemed potentially eligible, they evaluated the full-text articles.
Data Extraction and Quality Assessment
Using standardized, pilot-tested forms, reviewers completed calibration exercises and worked in pairs to independently extract the following information from eligible studies: cohort characteristics (such as cohort name and country), participant characteristics (including age and proportion who were female), diet characteristics (such as frequency and quantity of consumption of unprocessed red meat or processed meat), and outcomes (including absolute and relative effect measures for outcomes of interest and measures of variability). Disagreements between pairs of extractors were resolved through discussion or by third-party adjudication by an expert research methodologist.
Reviewers, working independently and in duplicate, assessed each study's risk of bias by using the CLARITY (Clinical Advances Through Research and Information Translation) risk-of-bias instrument for cohort studies, omitting an item related to co-interventions that was not relevant to our review (16). Disagreements were resolved through discussion or by third-party adjudication. Research methodologists and nutrition researchers were consulted to confirm the appropriateness of the CLARITY instrument and to advise us regarding criteria for evaluating each of its items. The instrument and detailed guidance are presented in Supplement Table 1. Studies rated as high risk of bias on 2 or more of the 7 domains were considered to have a high overall risk of bias. This threshold, although somewhat arbitrary, represents a compromise between excessive stringency and leniency.
Data Synthesis and Analysis
We conducted separate analyses for unprocessed red meat, processed meat, and mixed unprocessed red and processed meat. If an article reported on red meat and did not specify whether it was processed or unprocessed, we assumed that it included both unprocessed and processed red meat. We included such studies in the analysis of mixed unprocessed red and processed meat because most processed meat is typically consumed as red meat (17, 18).
For our primary analyses, we conducted a random-effects dose–response meta-analysis using methods proposed by Greenland and Longnecker (19) and Orsini and colleagues (20). These methods require knowledge of the distribution of events and number of participants or person-years and mean or median quantity of intake across categories of exposure. When results from studies were analyzed across quantiles of intake but person-years or number of participants was not reported within each quantile, we estimated these values by using figures reported for the total population and dividing the total person-years or total number of participants by the number of quantiles. For studies reporting effect estimates stratified by participant characteristics (such as sex), we meta-analyzed across subgroups by using the fixed-effects model. For studies that treated the exposure as a continuous predictor in a logistic regression and did not present categorical analyses, we calculated a regression coefficient based on the relative effect reported and meta-analyzed these regression coefficients with effects from other studies obtained via the estimation method described by Greenland and Longnecker (19). These studies were excluded from the nonlinear analyses. For analyses including 5 or more studies, we tested for nonlinearity by using restricted cubic splines with knots at 10%, 50%, and 90% and a Wald-type test. For analyses in which we observed statistically significant nonlinear associations, we present results from the nonlinear model.
For studies reporting the intake of red meat or processed meat as a range of values, we assigned the midpoint of upper and lower boundaries in each category as the average intake. If the highest or lowest category was open ended, we assumed that the open-ended interval was the same size as the adjacent interval. For studies reporting exposure as number of servings, we assumed that each serving of unprocessed red meat was equal to 120 g; processed meat, 50 g; and mixed unprocessed red and processed meat, 100 g. These serving sizes were selected for comparability with those used in other systematic reviews, as well as to reflect serving sizes used by the U.S. Department of Agriculture and United Kingdom Food Agency (1–3, 21–25). We report results corresponding to the effects of a reduction in unprocessed red or processed meat intake of 3 servings per week.
We used the dosresmeta package in R, version 3.5.1 (R Foundation for Statistical Computing), for our dose–response meta-analyses (26). Further details about these meta-analyses, including sample code, are presented in Supplement 2.
As a secondary analysis, we used the Hartung–Knapp–Sidik–Jonkman approach to calculate pooled relative effects, comparing the lowest category of exposure in each study with the highest one (27, 28). We also present results using a random-effects meta-analysis with the restricted maximum likelihood estimator. In these analyses, we also included studies comparing vegetarians with nonvegetarians. For studies that treated the exposure as a continuous predictor in logistic regression models and did not present categorical analyses, we converted relative effect estimates from the logistic regression model to correspond to a difference in intake of 1 serving per day—which was the difference observed most often between lowest and highest categories of consumption across studies—and used them in our meta-analyses. We used the metafor package in R (version 3.5.1) for these secondary analyses (29).
Because all outcomes of interest were rare (<10% event rate) within included studies for all analyses, we assumed that odds ratios and hazard ratios were similar to estimates of relative risk. We quantified heterogeneity using the I2 statistic and interpreted the magnitude of heterogeneity according to guidelines from the Cochrane Handbook for Systematic Reviews of Interventions (0% to 40%, low; 30% to 60%, moderate; 50% to 90%, substantial; 75% to 100%, considerable) (30). We also visually inspected forest plots for consistency, given that I2 statistics may be artificially inflated when effect estimates from primary studies are very precise—as was the case in many of our analyses (31). For all meta-analyses with at least 10 studies, we used the Egger test to look for small study effects (32).
We conducted a priori specified meta-regressions to test for differences among studies at higher versus lower risk of bias. For analyses with a statistically significant subgroup effect based on risk of bias, we present results only for studies at lower risk of bias. We had also planned to conduct subgroup analyses on the effects of red versus white processed meat and the effects of red meat consumption in iron-deficient populations, as well as a sensitivity analysis on the robustness of results to incomplete outcome data (33). However, we could not complete these additional analyses because of insufficient information reported in the primary studies.
Certainty of Evidence
One investigator assessed certainty of evidence by using the GRADE (Grading of Recommendations Assessment, Development and Evaluation) approach for each outcome, and the senior investigator confirmed the assessments (34). According to GRADE, observational studies start at low certainty and may be downgraded for risk of bias, inconsistency, indirectness, imprecision, or publication bias and may be upgraded for large effect, if suspected biases work against the observed direction of effect, or for dose–response gradient. To calculate absolute effects presented in summary-of-findings tables, we used population risks from the Emerging Risk Factors Collaboration to calculate risk differences associated with a reduction in red meat intake of 3 servings per week (35). The Emerging Risk Factors Collaboration is a consortium of 102 international cohorts, primarily from North America and western Europe, including mostly middle-aged to older adults who are omnivores.
Role of the Funding Source
This review received no external funding or other support.
Results
Study Selection
Supplement Figure 1 presents study selection details. A total of 62 articles including 56 cohorts proved eligible. One article did not provide sufficient quantitative information for meta-analysis (36). The quantitative analysis included 61 reports of 55 cohorts (4.2 million participants). Thirty-one cohort studies (2.2 million participants) were eligible for inclusion in the dose–response meta-analyses.
Study Characteristics
We found 20 articles (30 cohorts) addressing all-cause mortality; 18 (28 cohorts), cardiovascular mortality; 9 (7 cohorts), cardiovascular disease; 6 (7 cohorts), fatal and nonfatal stroke; 8 (11 cohorts), fatal stroke; 1 (1 cohort), fatal and nonfatal MI; 1 (1 cohort), nonfatal MI; 24 (25 cohorts), type 2 diabetes; and 1 (1 cohort), anemia (Supplement Table 2). We found no publications reporting on nonfatal stroke, fatal MI, quality of life, or satisfaction with diet.
Eighteen cohorts were from North America (United States and Canada), 21 from Europe, 15 from Asia, and 1 from the Middle East. The number of participants in each cohort ranged from 1757 to 536 969. Participants ranged in age from 17 to 92 years, with most cohorts recruiting those aged 40 to 50 years. Follow-up ranged from 2 to 28 years. Authors of 8 articles disclosed intellectual, financial, or personal conflicts of interest. All studies were funded by governmental bodies, with some receiving additional support from not-for-profit organizations.
Risk of Bias
Supplement Tables 3 through 11 present risk-of-bias assessments. The proportion of studies with high overall risk of bias varied on the basis of outcome: 10 of 31 studies for all-cause mortality, 17 of 22 for cardiovascular mortality, 3 of 8 for cardiovascular disease, 3 of 7 for fatal and nonfatal stroke, 10 of 13 for fatal stroke, 1 of 1 for fatal and nonfatal MI, 0 of 1 for nonfatal MI, 15 of 27 for type 2 diabetes, and 0 of 1 for anemia. The most common limitations in the studies were a lack of periodic repeated evaluation of dietary intake with a measure validated for red and processed meat and inadequate adjustment for potential confounders.
Reduction of 3 Servings per Week of Unprocessed Red Meat
Table 1 presents results of the possible effect of a reduction in unprocessed red meat intake of 3 servings per week. Details are presented in Supplement Table 12. Results showed a very small apparent effect on cardiovascular mortality, fatal and nonfatal stroke, fatal stroke, fatal and nonfatal MI, and type 2 diabetes, but not all-cause mortality or cardiovascular disease. We found evidence of a subgroup difference between studies at higher and those at lower risk of bias for type 2 diabetes (P < 0.001), so we present results from studies with a lower risk of bias. We did not find evidence of publication bias for type 2 diabetes.
Table 1. Summary of Findings for Unprocessed Red Meat Intake (Reduction of 3 Servings per Week) and Risk for Cardiometabolic Outcomes
The certainty of evidence was downgraded from low to very low for all-cause mortality and cardiovascular disease because CIs around absolute effect estimates included appreciable benefit as well as no effect or appreciable harm. The certainty of evidence for cardiovascular mortality, fatal stroke, and fatal and nonfatal MI was downgraded to very low because of the lack of periodic repeated measurement of diet and inadequate adjustment for potential confounders in the primary studies.
Reduction of 3 Servings per Week of Processed Meat
Table 2 presents results of the possible effect of a reduction in processed meat intake of 3 servings per week. Details are presented in Supplement Table 13. Results show a very small apparent effect on all-cause mortality, cardiovascular mortality, fatal and nonfatal stroke, fatal stroke, fatal and nonfatal MI, and type 2 diabetes, but not cardiovascular disease. We found evidence of a nonlinear association between processed meat intake and type 2 diabetes (P < 0.001), with a decrease from 3 to 0 servings per week associated with a very small reduced risk for type 2 diabetes (Figure). We found no evidence of publication bias for type 2 diabetes.
Table 2. Summary of Findings for Processed Red Meat Intake (Reduction of 3 Servings per Week) and Risk for Cardiometabolic Outcomes
FIGURE.
Nonlinear association between processed meat intake and type 2 diabetes.
The solid black line represents the point estimate, the shaded region represents the 95% CIs, and tick marks represent the positions of the study-specific estimates.
The certainty of evidence was downgraded to very low for cardiovascular mortality, fatal stroke, fatal and nonfatal MI, and type 2 diabetes because of a lack of periodic repeated measurement of diet and inadequate adjustment for potential confounders in the primary studies, as well as for type 2 diabetes because of substantial statistical heterogeneity.
Reduction of 3 Servings per Week of Mixed Unprocessed Red and Processed Meat
Supplement Table 14 presents results of the possible effect of a reduction in intake of mixed unprocessed red and processed meat of 3 servings per week. Details are presented in Supplement Table 15. Results show a small to very small apparent effect on all-cause mortality, cardiovascular mortality, cardiovascular disease, fatal and nonfatal stroke, fatal stroke, fatal and nonfatal MI, and type 2 diabetes, but not on nonfatal MI or anemia. We found evidence of a subgroup difference between studies at higher and those at lower risk of bias for all-cause mortality (P = 0.002) and type 2 diabetes (P = 0.027), so we present results only from studies at lower risk of bias. We found evidence of a nonlinear association between intake of mixed unprocessed red and processed meat and all-cause mortality (P = 0.037), with a reduction from 3 to 0 servings per week associated with a small decrease in risk (Supplement Figure 2. We found no evidence of publication bias for type 2 diabetes.
The certainty of evidence was downgraded to very low for cardiovascular mortality, fatal stroke, and fatal and nonfatal MI because of a lack of periodic repeated measurement of diet and inadequate adjustment for potential confounders in the primary studies.
Comparison of Extreme Categories of Intake
Results from meta-analyses comparing extreme categories of intake were generally consistent with the findings from our dose–response meta-analyses, although effect sizes typically were smaller than those from dose–response meta-analyses (Supplement Tables 16 to 18).
Discussion
We found low- to very-low-certainty evidence that reducing unprocessed red meat intake by 3 servings per week is associated with a very small reduction in risk for cardiovascular mortality, stroke, MI, and type 2 diabetes. Likewise, we found low- to very-low-certainty evidence that a reduction in processed meat intake is associated with a small to very small reduction in risk for all-cause mortality, cardiovascular mortality, stroke, MI, and type 2 diabetes. The magnitude of apparent effect of processed meat consumption on adverse cardiometabolic outcomes was somewhat greater than that observed for unprocessed red meat.
According to the GRADE system, the certainty of evidence may be upgraded if evidence suggests a dose–response relationship between the exposure and the outcomes of interest. Although we found evidence for dose–response relationships, we did not upgrade the certainty of evidence because of the possibility that red and processed meat consumption may be correlated with other dietary components, which may then confound their relationship to health outcomes (37). Support for this concern comes from a parallel systematic review in which we found the magnitude of association between dietary patterns lower versus higher in red and processed meat and adverse cardiometabolic outcomes to be very similar to the estimates found in this review (10). If red meat and processed meat were indeed the primary drivers of the association between diet and adverse cardiometabolic outcomes, we would anticipate stronger associations in our analyses of red and processed meat compared with dietary patterns (7).
Strengths of this review include the prespecification of our methods in the review protocol and the inclusion of a large number of cohorts and participants. We conducted both linear and nonlinear dose–response meta-analyses, which provide the most compelling evidence for the association between red and processed meat consumption and health outcomes, in addition to secondary analyses comparing extreme categories of intake. Results from our dose–response analyses are presented for a realistic reduction of 3 servings per week, which corresponds to the elimination of red and processed meat from the typical North American and western European diet based on the average intake of these foods in these populations (38–40). We assessed risk of bias and, when results differed, based our estimates on studies with lower versus higher risk of bias. Finally, we used the GRADE approach to rate the certainty of evidence.
In evaluating risk of bias of the primary studies, we assessed whether studies adjusted for a set of important potential confounders for each outcome. However, our results are limited by the potential for residual confounding or measurement error in confounders. In addition, studies varied in their choice of adjustment variables. All included studies measured diet via recall-based methods, primarily food-frequency questionnaires, which are subject to measurement error that can both attenuate and overestimate observed associations (41, 42). Although food-frequency questionnaires may provide reliable information on relative intake, substantial error regarding absolute intake may compromise dose–response meta-analyses that rely on these estimates (41). We could not assess the effects of reduced intake of red meat and processed meat on the basis which foods were consumed in their place, and the associated health effects of these alternative food choices may differ.
Half the studies in our review did not report sufficient information to be included in the dose–response meta-analyses (19, 20). Nonetheless, we are more confident in our results from these meta-analyses because they account for differences in gradients of intake across cohorts (43). In secondary analyses comparing extreme categories of intake, studies omitted from dose–response meta-analyses produced smaller effect estimates. The reason may be that studies that could not be included in dose–response meta-analyses had a higher risk of bias and typically measured diet with methods not validated for red and processed meat and did not repeat diet measurements throughout the study; hence, they may have underestimated the association between red and processed meat and adverse cardiometabolic health outcomes.
We could not conduct 3 additional analyses that we had planned—a subgroup analysis on the effects of red versus white processed meat, a subgroup analysis on the effects of red meat intake in iron-deficient populations, and a sensitivity analysis to assess the robustness of results to loss to follow-up—because the primary studies did not report sufficient information (33). We converted effect estimates reported in grams to servings. Although we used typical serving sizes in our conversions, our estimates may have been unreliable (1–3, 21, 23–25).
Although we found no evidence of publication bias, given the lack of standard registration practices for observational studies, publication bias is possible. In addition, none of the included studies had a priori specified statistical analysis plans (44); therefore, analysts' modeling decisions may have been guided by the possibility of obtaining interesting results.
Previous reviews reported similar positive associations between red and processed meat intake and all-cause mortality, cardiovascular disease, stroke, MI, and type 2 diabetes (1, 3–6). Similar to our work, other reviews reported slightly stronger associations between processed meat versus unprocessed red meat and adverse health outcomes. We believe our review provides the most up-to-date evidence on the topic and adds to the existing literature by using a more rigorous evaluation of risk of bias and by providing an assessment of certainty of evidence. Our results, as well as those of other reviews of observational studies, contrast with findings from randomized trials, which have failed to demonstrate an effect of lower red and processed meat consumption on cardiometabolic outcomes (8).
Current dietary guidelines recommend limiting red and processed meat consumption (25, 45). Our results, however, demonstrate that the evidence implicating red and processed meat in adverse cardiometabolic outcomes is of low quality; thus, considerable uncertainty remains regarding a causal relationship. Moreover, even if a causal relationship exists, the magnitude of association between red and processed meat consumption and cardiometabolic outcomes is very small.
Reducing the consumption of unprocessed red and processed meat may result in a decrease in risk for cardiometabolic disease and mortality. The magnitude of absolute effect, if indeed it exists, is very small, and the certainty of evidence is low. Findings from our review raise questions regarding whether—on the basis of possible adverse effects on cardiometabolic outcomes—the evidence is sufficient to recommend decreasing consumption of red and processed meat.
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