I think there are very good ethical reasons to avoid animal-based food (particularly the low-welfare / high-torture majority). And I think there are very good environmental reasons to avoid animal-based food. But on top of all of that, I’m frequently surprised by just how damning the evidence is that processed and/or red meat is bad for the people eating it (and arguably bad for the NHS). There have been enough headlines on this topic that people should have a vague sense of the conclusions, but I don’t think it’s common knowledge to the degree that it should be, and there is probably a misplaced sense that it’s fat in particular that should be avoided and not something more intrinsic to red meat, or at least processed meat such as bacon, sausages, ham and similar. So this post – which may grow over time – collects some of the papers I’ve seen (and suggestions are welcome). As a vegan I could be a biased source, and few people strive to eat the perfect diet above all else, but I have not set out to cherry pick the evidence, and if these findings were about tofu or coffee rather than red meat then personally I would make changes to my diet.
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September 2024: C Li et al., ‘Meat consumption and incident type 2 diabetes: an individual-participant federated meta-analysis of 1·97 million adults with 100 000 incident cases from 31 cohorts in 20 countries’, The Lancet Diabetes & Endocrinology. “To our knowledge, we provide the most comprehensive evidence to date on the associations of unprocessed red meat and processed meat with type 2 diabetes” “Greater consumption of each of the three types of meat was associated with increased incidence of type 2 diabetes, with HRs of 1·10 (95% CI 1·06–1·15) per 100 g/day of unprocessed red meat (I2=61%), 1·15 (1·11–1·20) per 50 g/day of processed meat (I2=59%), and 1·08 (1·02–1·14) per 100 g/day of poultry (I2=68%).” Note that this is after accounting for BMI. “Illustrative models without BMI adjustment showed HRs of 1·18 (95% CI 1·07–1·29) for unprocessed red meat, 1·23 (1·14–1·34) for processed meat, and 1·21 (1·12–1·31) for poultry.” “The true magnitude of the association could have been underestimated owing to the use of baseline-only dietary exposure data in most cohorts, as well as our inability to conduct any correction for potentially varying degrees of measurement error in dietary assessment across cohorts.” “The underlying mechanisms that link meat intake with the development of type 2 diabetes are not fully established. […] Meat consumption could affect type 2 diabetes risk through different causal mechanisms that worsen insulin sensitivity, pancreatic β-cell function, or both. For example, red meat is rich in saturated fatty acids but low in polyunsaturated fatty acids, and switching from a diet rich in saturated fatty acids to one rich in polyunsaturated fatty acids was found to be associated with improved insulin resistance in a meta-analysis of short-term trials. Additionally, meat is characterised by its high protein content, and some research has indicated a potential association between a high intake of animal proteins and increased risk of type 2 diabetes. Another potential mechanism could be via trimethylamine N-oxide, a gut microbiota-dependent metabolite generated during the digestion of choline and l-carnitine, which are abundant in red meat, although the exact mechanism is yet to be established. Nitrate or nitrite additives and the formation of N-nitroso compounds during meat processing are associated with a higher risk of type 2 diabetes. Small-scale trials have indicated that advanced glycation end products—compounds generated when cooking meat products at high temperatures, such as frying or grilling—could contribute to oxidative stress, pro-inflammatory response, and subsequently insulin resistance. Meat can be a major source of iron in many populations, but long-term iron intake has been implicated in an increased risk of type 2 diabetes in observational studies and in Mendelian randomisation analysis.”
February 2024: Y Wang et al., ‘Grilling the data: application of specification curve analysis to red meat and all-cause mortality’, Journal of Clinical Epidemiology. This is an interesting paper, but note the big caveats. “In this study, we applied specification curve analysis—a method that involves defining and implementing all plausible and valid analytic approaches for addressing a research question—to estimate the effect of unprocessed red meat on all-cause mortality… We performed 1208 unique analyses and found considerable variability in results, with HRs ranging from 0.51 to 1.75. Our results suggest that findings in nutritional epidemiology studies may be contingent on analytic methods. In contrast to previous studies addressing red meat, we found few of our analytic specifications to yield statistically significant effects. …[But] The continuous 2007–2014 NHANES data [used in this paper] are likely suboptimal for investigating the effect of red meat and other nutritional exposures on health outcomes, due to it including few deaths and only collecting data on diet at a single point in time. Nevertheless, our primary objective is not to provide conclusive answers about the health effects of red meat but to demonstrate a proof-of-concept application of specification curve analysis to nutritional epidemiology.”
January 2024 (preprint): N Henderson & C Sampson, ‘The impact of higher uptake of plant-based diets in England: model-based estimates of health care resource use and health-related quality of life’, medRxiv. This is a paper about the societal implications of eating healthier, rather than adding to the evidence about what is healthy, but provides a useful summary of that evidence and a sense of whether these results are important.
e.g. Most importantly, it uses earlier work that suggests “the risk of diabetes in vegans was 47% lower than in meat-eaters” – though in this work that is basically all down to vegans having healthier BMIs.
The combination of this relationship, together with the high prevalance and cost of diabetes, leads to an estimate that a 100% vegan England would have 1.3 million fewer cases of type 2 diabetes at any given time, saving the NHS £4.6 billion a year. There would also be savings from lower rates of cancer and CHD, though tentatively with more spending related to strokes (possibly due to low saturated fat or B12), adding up to a saving of £6.7 billion a year.
November 2023: L Fadnes et al., ‘Life expectancy can increase by up to 10 years following sustained shifts towards healthier diets in the United Kingdom’, Nature Food. “sustained dietary change from unhealthy to longevity-associated dietary patterns is associated with 10.8 and 10.4 years gain in life expectancy in males and females, respectively. The largest gains are obtained from consuming more whole grains, nuts and fruits and less sugar-sweetened beverages and processed meats.” For red and processed meat, your best bet is to be in the lowest fifth of consumption. Accounting for lots of other factors, those in the top fifth of processed meat consumption were 1.47x as likely to die as those in the bottom fifth.
November 2023: J Goode et al., ‘Modelling the replacement of red and processed meat with plant-based alternatives and the estimated effect on insulin sensitivity in a cohort of Australian adults’, British Journal of Nutrition. “We modelled the replacement of red and processed meat with plant-based alternatives and the estimated effect on insulin sensitivity. We included 783 participants (55 % female) from the Childhood Determinants of Adult Health study, a population-based cohort of Australians… Replacing red meat with a combination of plant-based alternatives was associated with higher insulin sensitivity (β = 10·5 percentage points, 95 % CI (4·1, 17·4)). [Adam: This is a good thing!] Adjustment for waist circumference attenuated this association by 61·7 %. Replacing red meat with either legumes, nuts/seeds or wholegrains was likewise associated with higher insulin sensitivity. Point estimates were similar but less precise when replacing processed meat with plant-based alternatives. Our modelling suggests that regularly replacing red meat, and possibly processed meat, with plant-based alternatives may associate with higher insulin sensitivity. Further, abdominal adiposity may be an important mediator in this relationship. Our findings support advice to prioritise plant-based sources of protein at the expense of red meat consumption.“
November 2023: R Cordova et al., ‘Consumption of ultra-processed foods and risk of multimorbidity of cancer and cardiometabolic diseases: a multinational cohort study’, The Lancet. It turns out the only ‘ultra-processed foods’ associated with ‘multimorbidity of cancer and cardiometabolic diseases’ are animal-based foods and sweetened drinks. “Among UPF subgroups, associations were most notable for animal-based products (HR: 1.09, 95% CI: 1.05, 1.12), and artificially and sugar-sweetened beverages (HR: 1.09, 95% CI: 1.06, 1.12). Other subgroups such as ultra-processed breads and cereals (HR: 0.97, 95% CI: 0.94, 1.00) or plant-based alternatives (HR: 0.97, 95% CI: 0.91, 1.02) were not associated with risk.”
October 2023: X Gu et al., ‘Red meat intake and risk of type 2 diabetes in a prospective cohort study of United States females and males’, The American Journal of Clinical Nutrition. “Compared with people who reported eating the least red meat each week, people who reported eating the most had a 62% higher risk of developing diabetes.” “every 1 serving/d increment in total red meat was associated with a 1.28 times higher hazard of [type 2 diabetes] (95% CI: 1.24, 1.31). Moreover, every 1 serving/d increment of processed red meat was associated with 1.46 times higher hazard of [type 2 diabetes] (95% CI: 1.40, 1.53), and that increment of unprocessed red meat was associated with 1.24 times higher hazard of [type 2 diabetes] (95% CI: 1.20, 1.29).” “with adjustment for BMI, the positive association was partially attenuated but still highly significant.”
Editorial: “All in all, the study by Gu et al. may arguably be the best evidence to date on the relation between red meat intake and type 2 diabetes.”
NB: 1 in 4 people age 75+ in England have diabetes, so diabetes risk is something we should care about!
June 2023: The Nordic Council’s Nutrition Recommendations 2023 (summarised here), building on “approximately 100 qualified systematic reviews”. “For health reasons, it is recommended that consumption of red meat (including red meat in products and processed foods) should be low and not exceed 350 gram/week ready-to-eat (cooked) weight. Processed red meat should be as low as possible.” “The WCRF/AICR demonstrated strong evidence for a significant, largely linear relationship between red meat and risk of colon cancer. For colorectal cancer, stratified analyses by geographic location showed especially a significant increased risk in studies with European populations. The report also concluded that intake of processed meat is a convincing cause of colorectal cancer.” “…a diet high in red meat is the fourth-highest dietary risk factor for Disability Adjusted Life Years (DALYs) in the Nordic and Baltic countries” “numerous qSRs on dietary patterns have found that adhering to diets characterized by lower amounts of red and processed meats are compatible with beneficial health effects, including strong and consistent evidence for lower risk of all-cause mortality (Boushey et al., 2020), cardiovascular disease (2020 Dietary Guidelines Advisory Committee, 2020), and moderate evidence for lower risk of type 2 diabetes (Boushey et al., 2020f) and favourable body weight-related outcomes (Boushey et al., 2020a).” “The Committee notes that a clear-cut level of intake that minimizes risk is difficult to set, as the associations are often linear“
October 2022: C Murray et al., ‘Health effects associated with consumption of unprocessed red meat: a Burden of Proof study’, Nature Medicine (with broader paper here). “We found weak evidence of association between unprocessed red meat consumption and colorectal cancer, breast cancer, type 2 diabetes and ischemic heart disease. Moreover, we found no evidence of an association between unprocessed red meat and ischemic stroke or hemorrhagic stroke. We also found that while risk for the six outcomes in our analysis combined was minimized at 0 g unprocessed red meat intake per day, the 95% uncertainty interval that incorporated between-study heterogeneity was very wide: from 0–200 g d−1. While there is some evidence that eating unprocessed red meat is associated with increased risk of disease incidence and mortality, it is weak and insufficient to make stronger or more conclusive recommendations. More rigorous, well-powered research is needed to better understand and quantify the relationship between consumption of unprocessed red meat and chronic disease.” “In light of these findings, we contend that consuming no unprocessed red meat likely minimizes the risk of health consequences compared to consuming any”
Note: the method in this paper has been criticised for creating a very high and “misleading” burden of proof (e.g. even for smoking risks).
September 2022: E Hill et al., ‘Red and processed meat intakes and cardiovascular disease and type 2 diabetes mellitus: An umbrella systematic review and assessment of causal relations using Bradford Hill’s criteria’, Critical Reviews in Food Science and Nutrition. “Our umbrella systematic review builds upon this evidence by using long-established criteria for causal inference in which we additionally incorporated experimental evidence from short-term randomized controlled feeding trials of relevant clinical risk factors… We infer that a causal relationship is not likely present between red and processed meat intakes and CVD risk. However, we infer a potential for a causal relationship between processed meat and Mixed unprocessed red meat + processed meat and type 2 diabetes mellitus… Most T2DM assessments had RR’s ≥1.2, some as large as 1.41 (95%CI: 1.25, 1.60) (Aune, Ursin, and Veierød Citation2009) for processed meat and T2DM incidence. These consistently large effect sizes that reach statistical significance are not commonly seen in nutrition epidemiology (Potischman and Weed Citation1999)… More randomized controlled feeding trials are needed, specifically for processed meat, to support or refute these inferences.”
September 2022: K Hidayat et al., ‘Is replacing red meat with other protein sources associated with lower risks of coronary heart disease and all-cause mortality? A meta-analysis of prospective studies’, Nutrition Reviews. “Keeping red meat, particularly processed red meat, consumption to a minimum along with increasing healthier alternative protein sources to replace red meat in the diet may contribute to the prevention of Coronary Heart Disease and premature death.”
February 2022: L Fadnes et al., ‘Estimating impact of food choices on life expectancy: A modeling study’, PLOS Medicine. “A sustained change from a typical Western diet to the optimal diet from age 20 years would increase life expectancy by more than a decade for women from the United States (10.7 [95% UI 8.4 to 12.3] years) and men (13.0 [95% UI 9.4 to 14.3] years). The largest gains would be made by eating more legumes (females: 2.2 [95% UI 1.1 to 3.4]; males: 2.5 [95% UI 1.1 to 3.9]), whole grains (females: 2.0 [95% UI 1.3 to 2.7]; males: 2.3 [95% UI 1.6 to 3.0]), and nuts (females: 1.7 [95% UI 1.5 to 2.0]; males: 2.0 [95% UI 1.7 to 2.3]), and less red meat (females: 1.6 [95% UI 1.5 to 1.8]; males: 1.9 [95% UI 1.7 to 2.1]) and processed meat (females: 1.6 [95% UI 1.5 to 1.8]; males: 1.9 [95% UI 1.7 to 2.1]).” i.e. Eating less meat could add around 2 years to life expectancy. The “typical Western diet” here includes 100g/day of red meat and 50g/day of processed meat, while the “optimized diet” was 0g of these (see S Lukas et al. 2017).
December 2021: S Hazen et al., ‘The microbial gbu gene cluster links cardiovascular disease risk associated with red meat consumption to microbiota L-carnitine catabolism’, Nature Microbiology (and article here). “The heightened cardiovascular disease (CVD) risk observed among omnivores is thought to be linked, in part, to gut microbiota-dependent generation of trimethylamine-N-oxide (TMAO) from L-carnitine, a nutrient abundant in red meat.” “Blood levels of TMAO are a predictor of future risks of heart attack, stroke, and death” “These new studies identify the gut microbial gene cluster responsible for the second step of the process that links a red meat-rich diet to elevated cardiac disease risks” “long-term vegetarians and vegans have very low levels of this microbe in their gut and therefore have minimal to no capacity to convert carnitine into TMAO.” “Patients who transitioned to a non-meat diet went on to exhibit reduced gut microbial levels of gbuA. This suggests that dietary modifications may help reduce diet- and TMAO- associated cardiovascular disease risk”
December 2021: O Vincze et al., ‘Cancer risk across mammals’, Nature. “We show that the phylogenetic distribution of cancer mortality is associated with diet, with carnivorous mammals (especially mammal-consuming ones) facing the highest cancer-related mortality.” Note this has some heavy caveats, stressing that we don’t really know why this is.
August 2021: M Farvid et al., ‘Consumption of red meat and processed meat and cancer incidence: a systematic review and meta-analysis of prospective studies’, European Journal of Epidemiology. “This comprehensive systematic review and meta-analysis study showed that high red meat intake was positively associated with risk of breast cancer, endometrial cancer, colorectal cancer, colon cancer, rectal cancer, lung cancer, and hepatocellular carcinoma, and high processed meat intake was positively associated with risk of breast, colorectal, colon, rectal, and lung cancers. Higher risk of colorectal, colon, rectal, lung, and renal cell cancers were also observed with high total red and processed meat consumption.”
February 2019: Food in the Anthropocene: the EAT-Lancet Commission on healthy diets from sustainable food systems. “in a meta-analysis of prospective studies, consumption of processed red meat (beef, pork, or lamb) was associated with increased risk of death from any cause and cardiovascular disease; unprocessed red meat was also weakly associated with cardiovascular disease mortality.” “consumption of red meat was associated with increased risk of stroke and type 2 diabetes. In two large studies, red meat (processed and unprocessed) was linearly associated with total mortality, and without a threshold, suggesting that optimal intake would be low. In a pooled analysis of three large cohorts, an increment of about 35 g/day of red meat was associated with a significant increase (6%) in risk of type 2 diabetes, and this association was also approximately linear.” “red meat was clearly associated with increased risk of coronary heart disease when specifically compared with consumption of poultry and fish, and especially nuts and legumes. Similar associations have been seen in analyses with type 2 diabetes, stroke, and total mortality. Low intake of red meat is consistent with traditional Mediterranean diets that have been associated with exceptional longevity” “Based on evidence related to colorectal cancer, processed red meat (eg, treated with salt or other preservatives) was determined by the International Agency for Research on Cancer review to be a group 1 carcinogen, and because data were less consistent, unprocessed red meat was classified as a group 2 carcinogen.” “Because intake of red meat is not essential and appears to be linearly related to total mortality and risks of other health outcomes in populations that have consumed it for many years, optimal intake might be 0 g/day, especially if replaced by plant sources of protein.”
June 2017: S Lukas et al., Food groups and risk of all-cause mortality: a systematic review and meta-analysis of prospective studies, The American Journal of Clinical Nutrition. “Each additional daily 50 g of processed meat was associated with a risk of all-cause mortality (RR: 1.23; 95% CI: 1.12, 1.36)” “Each additional daily 100 g of red meat was positively associated with risk of all-cause mortality (RR: 1.10; 95% CI: 1.04, 1.18)” See charts below (from Figure 2) for the dose-response relations between daily intakes of these and risk of all-cause mortality.
